Chapter 463: The Biology of Aging

Aging and old age are among the most significant challenges facing medicine this century. The aging process is the major risk factor underlying disease and disability in developed nations, while older people respond differently to therapies developed for younger adults (usually with less effectiveness and more adverse reactions). Modern medicine and healthier lifestyles have increased the likelihood that younger adults will now achieve old age. However, this has led to rapidly increasing numbers of older people, often encumbered with age-related disorders that are predicted to overwhelm health care systems. Improved health in old age and further extension of human health span are now likely to be generated primarily from increased understanding of the biology of aging, age-related susceptibility to disease, and modifiable factors that influence the aging process.

Definitions of Aging

Aging is easy to recognize but difficult to define. Most definitions of aging indicate that it is a progressive process associated with declines in structure and function, impaired maintenance and repair systems, increased susceptibility to disease and death, and reduced reproductive capacity. There are both statistical and phenotypic components to aging. As recognized by Gompertz in the nineteenth century, aging in humans is associated with an exponential risk of mortality with time (Fig. 463-1), although it is now realized that this plateaus in extreme old age because of healthy survivor bias. The phenotypic components of aging include structural and functional changes that are separated, somewhat artificially, into either primary aging changes (e.g., sarcopenia, grey hair, oxidative stress, increased peripheral vascular resistance) or age-related disease (e.g., dementia, osteoporosis, arthritis, insulin resistance, hypertension).

Definitions of aging rarely acknowledge the possibility that some of those biological and functional changes with aging might be adaptive or even reflect improvement and gain. Nor do they emphasize the impact of aging on responses to medical treatments. Old age is associated with increased vulnerability to many perturbations, including therapeutic interventions. This is a critical issue for clinicians—aging would be a less difficult problem if our disease-specific therapies retained their balance of risk to benefit into old age.

Aging and Disease Susceptibility

Old age is the major independent risk factor for chronic diseases (and associated mortality) that are most prevalent in developed countries such as cardiovascular disease, cancers, and neurodegenerative disorders (Fig. 463-2). Consequently, older people have multiple comorbidities, usually in the range of 5–10 illnesses per person.

Disease in older people is typically multifactorial with a strong component related to the underlying aging process. For example, in younger patients with dementia, Alzheimer’s disease is a single disorder confirmed by examining brain tissue for plaques and tangles containing amyloid and tau proteins. However, the vast majority of people with dementia are elderly, and here the association between typical Alzheimer’s neuropathology and dementia becomes less definitive. In the oldest-old, the prevalence of Alzheimer-type brain pathology is similar in people with and without clinical features of dementia. On the other hand, brains of older people with dementia usually show mixed pathology with evidence of Alzheimer’s pathology along with features of other dementias such as vascular lesions, Lewy bodies, and non-Alzheimer’s tauopathy. Many typical aging changes, such as microvascular dysfunction, oxidative injury, and mitochondrial impairment underlie many of the pathological changes.

The Longevity Dividend

“Compression of morbidity” refers to the concept that the burden of lifetime illness might be compressed by medical interventions into a shorter period before death without necessarily increasing longevity. However, continuing development of successful therapeutic and preventive interventions focusing on individual diseases is less effective in older people because of multiple comorbidities, complications of overtreatment, and competing causes of death. Therefore, it has been proposed that further gains in health span and life expectancy will be achieved by a single intervention that delays aging and age-related disease susceptibility, rather than multiple treatments each targeting different individual age-related illnesses. This is called the “longevity dividend” and is driving an explosion of research into aging biology and, more importantly, interventions—genetic, pharmaceutical, and nutritional—that influence the rate of aging and delay age-related disease.

At the most basic level, living things have only two approaches to maintain their existence: immortality or reproduction. In a changing environment, reproduction combined with a finite life span has proved to be the successful strategy. Of course, finite life span is not the same as aging—although aging, by definition, contributes to a finite life span.

Many evolutionary theories related to aging are linked by their attempts to explain this interaction between reproduction and longevity (Fig. 463-3). Most mainstream aging theories stem from the fact that evolution is driven by early reproductive success, whereas there is minimal selection pressure for late-life reproduction or postreproductive survival. Aging is seen as the random degeneration resulting from the inability of evolution to prevent it, i.e., the nonadaptive consequence of evolutionary “neglect.” This conclusion is supported by studies that restricted reproduction to later life in the fruit fly, Drosophila melanogaster, thus permitting natural selection to operate on later life traits and leading to an increase in longevity. Likewise, many of the interventions that delay aging are associated with reduced reproductive capacity.

There are some species of plants and animals that do not appear to age, or at least they undergo an extremely slow aging process, termed “negligible senescence.” The mortality rates of these species are relatively constant with time and they do not display any obvious phenotypic changes of aging. Conversely, some living things undergo programmed death immediately after reproduction such as annual plants and semelparous animals (Fig. 463-4). However, many other living things from yeast to humans undergo a gradual aging process leading to death that is surprisingly similar at the cellular and biochemical level across taxa.

Some of the major classical evolutionary theories of aging include:

• Programmed death. The first evolutionary theory of aging was proposed by Weismann in 1882. This theory states that aging and death are programmed and have evolved to remove older animals from the population so that environmental resources such as food and water are freed up for younger members of the species.

• Mutation accumulation. This theory was proposed by Medawar in 1952. Natural selection is most powerful for those traits that influence reproduction in early life, and therefore, the ability of evolution to shape our biology declines with age. Germline mutations that are deleterious in later life can accumulate simply because natural selection cannot act to prevent them.

• Antagonistic pleiotropy. George C. Williams extended Medawar’s theory when he proposed that evolution can allow for the selection of genes that are pleiotropic, i.e., beneficial for survival and reproduction in early life, but harmful in old age. For example, genes for sex hormones are necessary for reproduction in early life but contribute to the risk of cancer in old age.

• Life history theory. Evolution is influenced by the way that limited resources are allocated to all aspects of life including development, sexual maturation, reproduction, number of offspring, and senescence and death. Therefore “trade-offs” occur between these phases of life. For example, in a hostile environment, survival is highest for those species that have large numbers of offspring and short life span while in a safe and abundant environment, survival is highest for those species that invest resources in a smaller number of offspring and a longer life.

• Disposable soma theory. Kirkwood and Holliday in 1979 combined many of these ideas in the disposable soma theory of aging. There are finite resources available for the maintenance and repair of both germ and soma cells so there must be a trade-off between germ cells (i.e., reproduction) versus soma cells (i.e., longevity and aging). The soma cells are disposable from an evolutionary perspective, so they accumulate damage that causes aging while resources are preferentially diverted to the maintenance and repair of the germ cells. For example, the longevity of nematode worm, Caenorhabditis elegans is increased when its germ cells are ablated early in life.

All of these theories assume that natural selection has negligible or negative influences on aging. Some ideas propose that aspects of aging might be adaptive and raise the possibility that evolution can act on the aging process in a positive way. These include:

• Grandmother hypothesis. The grandmother hypothesis proposed by Hamilton in 1966 describes how evolution can enhance old age. In some animals, including humans, the survival of multiple, dependent offspring is beyond the capacity and resources of their mother. In this situation, the presence of a long-lived grandmother who shares in care of her grandchildren can have a major impact on their survival. These children share some of the genes of their grandmother including those that promoted their grandmother’s longevity.

• Mother’s curse. Mitochondrial dysfunction is a key component of the aging process. Mitochondria contain their own DNA and are only passed on from mother to child, because sperm cells contain almost no mitochondria. Therefore, natural selection can only act on the evolution of mitochondrial DNA in females. The “mother’s curse” of the maternal inheritance of mitochondrial DNA might explain why females live longer and age more slowly than males.

• Adaptive senectitude. Many traits that are harmful in younger humans such as obesity, hypertension, oxidative stress, and declines in growth hormone and insulin-like growth factor type I (IGF-1) paradoxically appear to be associated with greater survival and function in old age. Perhaps driven by the grandmother effect, this might represent “adaptive senectitude” or “reverse antagonistic pleiotropy” whereby some traits that are harmful in young people become beneficial in older people.

Many cellular processes change with aging. These are generally considered to be degenerative and stochastic or random changes that reflect some sort of time-dependent damage (Fig. 463-3) and have been called the hallmarks and pillars of aging. Whether any of these is the root cause of aging is unknown but they all contribute to the aging phenotype and disease susceptibility.

Oxidative Stress and the “Free Radical Theory of Aging”

Free radicals are chemical species that are highly reactive because they contain unpaired electrons. Oxidants are oxygen-based free radicals that include the hydroxyl free radical, superoxide, and hydrogen peroxide. Most cellular oxidants are waste products generated by mitochondria during the production of ATP from oxygen. More recently, the role of oxidants in cellular signaling and inflammatory responses has been recognized. Unchecked, oxidants can generate chain reactions leading to widespread damage to biological molecules. Cells contain numerous antioxidant defense mechanisms to prevent such oxidative stress including enzymes (superoxide dismutase, catalase, glutathione peroxidase) and chemicals (uric acid, ascorbate). In 1956, Harman proposed the “free radical theory of aging” whereby oxidants generated by metabolism or irradiation are responsible for age-related damage. It is now well established that old age in most species is associated with increased oxidative stress, for example to DNA (8-hydroxyguanosine derivatives), proteins (carbonyls), lipids (lipoperoxides, malondialdehydes), and prostaglandins (isoprostanes). Conversely, many of the cellular antioxidant defense mechanisms including the antioxidant enzymes decline in old age. The free radical theory of aging has spawned numerous studies of supplementation with antioxidants such as vitamin E to delay aging in animals and humans. However, meta-analyses of human clinical trials performed to treat and prevent various diseases with antioxidant supplements indicate that they have no effect on, or may even increase, mortality.

Mitochondrial Dysfunction

Aging is characterized by altered mitochondrial production of ATP and oxygen-derived free radicals. This leads to a vicious cycle mediated by accumulation of oxidative injury to mitochondrial proteins and DNA. With age, the number of mitochondria in cells decreases and there is an increase in their size (megamitochondria) associated with other structural changes including vacuolization and disrupted cristae. These morphological aging changes are linked with decreased activity of mitochondrial complexes I, II, and IV and decreased ATP production. Of all of the complexes involved in ATP production, the activity of complex IV (COX) is usually reported to be most impaired in old age. Reduced energy production is linked with generation of hydrogen peroxide and superoxide radicals leading to oxidative injury to mitochondrial DNA and accumulation of carbonylated mitochondrial proteins and mitochondrial lipoperoxides. As well as being implicated in the aging process, common geriatric syndromes including sarcopenia, frailty, and cognitive impairment are associated with mitochondrial dysfunction.

Telomere Shortening and Replicative Senescence

Cells that are isolated from animal tissue and grown in culture only divide for a certain number of times before entering a senescent phase. This number of divisions is known as the Hayflick limit and tends to be less in cells isolated from older animals compared to younger animals. It has been suggested that aging in vivo might in part be secondary to some cells ceasing to divide because they have reached their Hayflick limit. Senescent cells produce a variety of cytokines, chemokines, and proteases, termed the senescence-associated secretory phenotype (SASP), that are major drivers of age-related inflammation. Eliminating senescent cells delayed aging in mice. On the other hand, cellular senescence may have a role in preventing proliferation of cells at risk of malignant transformation. One mechanism for replicative senescence relates to telomeres. Telomeres are repeat sequences of DNA at the end of linear chromosomes that shorten by around 50–200 base pairs during each cell division by mitosis. Once telomeres become too short, cell division can no longer occur. This mechanism contributes to the Hayflick limit and has been called the cellular clock. There are some studies that suggest that the length of telomeres in circulating leukocytes (leukocyte telomere length, LTL) decreases with age in humans. However, the aging process also occurs in tissues that do not undergo repeated cell division such as neurons.

Altered Gene Expression, Epigenetics, and microRNA

The expression of many genes and proteins changes during the aging process. These changes are complicated and vary between species and tissue. Such heterogeneity reflects increasing dysregulation of gene expression with age while appearing to exclude a programmed and/or uniform response. With old age, reductions in the expression of genes and proteins associated with mitochondrial function and increased expression of those involved with inflammation, genome repair, and oxidative stress are noted. Several factors control the regulation of gene and protein expression that change with aging. These include the epigenetic state of the chromosomes (e.g., DNA methylation and histone acetylation) and microRNAs (miRNAs). DNA methylation correlates with age, although the pattern of change is complex. Histone acetylation is regulated by many enzymes including Sirtuin 1 (SIRT1), a protein that has marked effects on aging and the response to dietary restriction in many species. miRNAs are a very large group of noncoding lengths of RNA (18–25 nucleotides) that inhibit translation of multiple different mRNAs through binding their 3’ untranslated regions (3’UTRs). The expression of miRNAs usually decreases with aging and is altered in some age-related diseases. Specific miRNAs linked with aging pathways include miR-21 (associated with target of rapamycin pathway) and miR-1 (associated with insulin/insulin-like growth factor 1 pathway).

Impaired Autophagy and Proteostasis

Cells can remove damaged macromolecules and organelles in a number of ways, often generating cellular energy as a by-product. Intracellular degradation is undertaken by the lysosomal system and the ubiquitin proteasomal system. Both are impaired with aging, leading to the accumulation of waste products that alter cellular functions. Such waste products include lipofuscin, a brown auto-fluorescent pigment found within lysosomes of most cells in old age and often considered to be one of the most characteristic histological features of aging cells. Lysosomes are organelles that contain proteases, lipases, glycases, and nucleotidases that degrade intracellular macromolecules, membrane components, organelles, and some pathogens through a process called autophagy. The lysosomal process most impaired with aging is macroautophagy, which is regulated by numerous autophagy-related genes. Proteostasis refers to the maintenance of protein quality through regulation of protein folding and protein degradation. Chaperones orchestrate appropriate folding of proteins while degradation involves ubiquitin tagging, proteases, and the unfolded protein response. With aging damaged, aggregated and misfolded proteins increase because of age-related changes in proteostasis. This may contribute to the aggregation of proteins such as tau, β-amyloid, α-synuclein in age-related neurodegenerative diseases such as dementia and Parkinson’s disease.

Aging changes in some tissues increase susceptibility to age-related disease as a secondary or downstream phenomenon (Fig. 463-3). In humans, this includes, but is not limited to, the immune system (leading to increased infections and autoimmunity), hepatic detoxification (leading to increased exposure to disease-inducing endobiotics and xenobiotics), the endocrine system (leading to hypogonadism and bone disease), and the vascular system (leading to segmental or global ischemic changes in many tissues).

Inflamm-aging and Immunosenescence

Old age is associated with increased background levels of inflammation including blood measurements of C reactive protein (CRP), erythrocyte sedimentation rate (ESR), and cytokines such as interleukin 6 (IL-6) and tumor necrosis factor alpha (TNFα). This has been termed “inflamm-aging” and elevated IL-6 in particular has been associated with frailty and dementia. T cells are less numerous because of age-related atrophy of the thymus, while B cells overproduce autoantibodies, leading to the age-related increase in autoimmune diseases and gammopathies. Thus, older people are generally considered to be immunucompromised and have reduced responses to infection (fever, leukocytosis) with increased mortality.

Detoxification and the Liver

Old age is associated with impaired detoxification of various disease-causing endobiotics (e.g., lipoproteins) and xenobiotics (e.g., neurotoxins and carcinogens) leading to increased systemic exposure. In humans, the liver is the major organ for the clearance of such toxins. Hepatic clearance of many substrates is reduced in old age as a consequence of reduced hepatic blood flow, impaired hepatic microcirculation and, in some cases, reduced expression of xenobiotic metabolizing enzymes. These changes in hepatic detoxification also increase the likelihood of increased blood levels of, and adverse reactions to, medications.

Endocrine System

Hormonal changes with aging have been a focus for aging research for over a century, partly because of the erroneous belief that supplementation with sex hormones will delay aging and rejuvenate older people. There are age-related reductions in sex steroids, growth hormone, IGF-1, and dehydroepiandrosterone (DHEA). These hormonal changes may contribute to some features of aging such as sarcopenia and osteoporosis but also provide protection against cancer and cardiovascular disease. Adverse effects of long-term hormonal supplementation outweigh any potential beneficial effects on life span.

Vascular Changes

There is a continuum from vascular aging through to atherosclerotic disease, present in many, but not all, older people. Vascular aging changes overlap with the early stages of hypertension and atherosclerosis, with increasing arterial stiffness and vascular resistance. This contributes to myocardial ischemia and strokes but also appears to be associated with geriatric conditions such as dementia, sarcopenia, and osteoporosis. In these conditions, impaired exchange between blood and tissues is a common pathogenic factor. For example, the risk of Alzheimer’s disease and dementia is increased in patients with risk factors for vascular disease, and pathological evidence for microvascular changes is seen in postmortem studies of brains of people with established Alzheimer’s disease. Similarly, strong epidemiological links have been found between osteoporosis and standard vascular risk factors, while significant age-associated changes are in the microcirculation of osteoporotic bone. Sarcopenia might also be related to the effects of age on the muscle vasculature, which is altered in old age. The sinusoidal microcirculation of the liver becomes markedly altered during aging (pseudocapillarization), which influences hepatic uptake of lipoproteins, insulin, and other substrates. In fact, it has often been overlooked that in his original exposition of the free radical theory of aging, Harman proposed that the primary target of oxidative stress was the vasculature and that many aging changes were secondary to impaired exchange across the damaged blood vessels.

There is variability in aging and life span in populations of genetically identical species such as mice that are housed in the same environment. The heritability of life span in human twin studies is estimated to be only 25% (although there is stronger hereditary contribution to extreme longevity). These two observations indicate that the cause of aging is unlikely to lie only within the DNA code. On the other hand, genetic studies initially undertaken in the nematode worm C. elegans and, more recently, in models from yeast to mice have shown that manipulating genes can have profound effects on the rate of aging. Perhaps surprisingly, this can often be generated by variability in single genes, and for some genetic mechanisms, there is very strong evolutionary conservation.

Genetic Progeroid Syndromes

There are a few very rare, genetic premature aging conditions that are called progeroid syndromes. These conditions recapitulate some, but not all, age-related diseases and senescent phenotypes. They are mostly caused by impairment of genome and nuclear maintenance. These syndromes include the following:

• Werner’s syndrome. This is an autosomal recessive condition caused by a mutation in the Werner’s (WRN) gene. This gene codes for a RecQ helicase, which unwinds DNA for both repair and replication. It is typically diagnosed in teen years and is associated with premature onset of atherosclerosis, osteoporosis, cancers, and diabetes with death by age of 50 years.

• Hutchinson–Gilford progeria syndrome (HGPS). This usually occurs as a de novo, noninherited mutation in the lamin A gene (LMNA) leading to an abnormal protein called progerin. LMNA is required for the nuclear lamina, which provides structural support to the nucleus. Marked development changes are obvious in infancy with subsequent onset of atherosclerosis, kidney failure, and scleroderma-like features and death during the teen years. Lamin A-dependent nuclear defects have been reported in normal human aging.

• Cockayne syndrome. This includes a number of autosomal recessive disorders with features such as impaired neurological growth, photosensitivity (xeroderma pigmentosum), and death during childhood years. These are caused by mutations in the genes for DNA excision repair proteins, ERCC-6 and ERCC-8.

Gene Studies in Long-Lived Humans

The main genes that have been consistently associated with increased longevity in human candidate gene studies are APOE and FOXO3A. ApoE is an apoprotein found in chylomicrons while the ApoE4 isoform is a risk factor for Alzheimer’s disease and cardiovascular disease, which might explain its association with reduced life span. FOXO3A is a transcription factor involved in the insulin/IGF-1 pathway, and its homolog in C. elegans, daf16, has a marked impact on aging in these nematodes. Genome-wide association studies (GWASs) of centenarians have confirmed the association of longevity with APOE. GWAS has been used to identify a range of other single nucleotide polymorphisms (SNPs) that might be associated with longevity including SNPs in the sirtuin genes and the progeroid syndrome genes, LMNA and WRN. Gene set analysis of GWAS studies has shown that both the insulin/IGF-1 signaling pathway and the telomere maintenance pathway are associated with longevity.

One group of particular interest are people with Laron-type dwarfism. These people have mutations in the growth hormone receptor that causes severe growth hormone resistance. In mice, similar knockout of the growth hormone receptor (GHRKO mice, Methuselah mice) is associated with extremely long life. Therefore, subjects with Laron syndrome have been carefully studied and it was found that they have very low rates of cancer and diabetes mellitus, and possibly, longer lives.

Nutrient-Sensing Pathways

Many living things have evolved to respond to periods of nutritional shortage and famine by increasing cellular resilience and delaying reproduction until food supply becomes abundant once again. This increases the chances of reproductive success and survival of offspring. Lifelong food shortage, often termed “caloric restriction (CR)” (or “dietary restriction”), increases life span and delays aging in many animals, probably as a side effect of this famine response. Many of the genes and pathways that regulate the way that cells respond to nutritional undersupply have been identified, initially in yeast and C. elegans. In general, manipulation of these pathways (through genetic knockout or overexpression, or pharmacological agonists and antagonists) alters the aging benefits of CR, and in some cases, the life span of animals on normal diets. These pathways are all very influential cellular “switches” that control a wide range of key functions including protein translation, autophagy, mitochondrial function and bioenergetics, and the cellular metabolism of fats, proteins, and carbohydrates. The discovery of these nutrient-sensing pathways has led to targets for pharmacological extension of life span. The main nutrient-sensing pathways that influence aging and responses to CR include:

• SIRT1. The activity of SIRT1 is regulated by levels of reduced nicotinamide adenine dinucleotide (NAD⁺), which are increased when cellular energy stores are depleted. Important downstream targets include PGC-1α and Nrf2, which act on mitochondrial biogenesis.

• Mechanistic target of rapamycin (mTOR). mTOR is activated by branched chain amino acids, providing a link to dietary protein intake. It has two main components, mTORC1 and mTORC2, of which mTORC1 is most relevant to aging. Key downstream targets of mTOR of relevance to aging include the tuberous sclerosis protein (TSC) and 4EBP1, which influences protein production.

• 5’ adenosine monophosphate-activated protein kinase (AMPK). AMPK is activated by increased levels of AMP, which reflect cellular energy status.

• Insulin signaling and IGF-1/growth hormone (IIS). These two pathways are usually considered together because they overlap in lower animals and have diverged only in higher animals. They respond to carbohydrate and protein intake. An important downstream target for this pathway is a transcription factor called FOXO.

• Fibroblast growth factor 21 (FGF21). FGF21 is produced mostly by the liver in response to CR and reduced protein intake.

Mitochondrial Genes

Mitochondrial function is influenced by genes located both in the mitochondria (mtDNA) and the nucleus. mtDNA is considered to have a prokaryotic origin and is highly conserved across taxa. It forms a circular loop of 16,569 nucleotides in humans. Aging is associated with increased frequency of mutations in mtDNA as a consequence of its high exposure to oxygen-derived free radicals and relatively inefficient DNA repair machinery. Nuclear DNA encodes ~1000–1500 genes for mitochondrial function including genes involved with oxidative phosphorylation, mitochondrial metabolic pathways, and enzymes required for biogenesis. These genes are thought to have originated in mtDNA but subsequently translocated to the nucleus and, unlike mtDNA genes, their sequence is stable with aging.

Genetic manipulation of mitochondrial genes in animals influences aging and life span. In C. elegans, many mutants with defective electron transfer chain function have increased life span. The mtDNA “mutator” mice which lack the mtDNA proofreading enzyme have increased mtDNA mutations and premature aging, while overexpression of mitochondrial uncoupling proteins leads to longer life span. In humans, hereditary variability in mtDNA is associated with diseases (mitochondriopathies such as Leigh’s disease) and aging. For example, in Europeans, mitochondrial DNA haplogroup J (haplogroups are combinations of genetic variants that exist in specific populations) is associated with longevity, and haplogroup D is overrepresented in Asian centenarians.

Nuclear DNA

Genomic DNA damage accumulates in cells with aging while genetic progeroid conditions such as Werner’s syndrome and HGPS are associated with impaired DNA maintenance and repair. Age-related DNA changes include mutations, chromosomal aneuploidy, copy number variations, and telomere shortening. Apart from telomere attrition, these changes are random and vary between cells, and may contribute to age-related cancers.

Aging is an intrinsic feature of human life whose manipulation has fascinated humans ever since becoming conscious of their own existence. Several long-term experimental interventions (e.g., resveratrol, rapamycin, spermidine, and metformin) may open doors for corresponding pharmacological strategies. Surprisingly, most of the effective aging interventions proposed converge on only a few molecular pathways: nutrient signaling, mitochondrial proteostasis, and the autophagic machinery.

Life span is inevitably accompanied by functional decline, steady increase of a plethora of chronic diseases, and ultimately death. For millennia, it has been a dream of mankind to prolong both life span and health span. Developed countries have profited from the medical improvements and their transfer to public health care systems—as well as from better living conditions derived from their socioeconomic power—to achieve remarkable increases in life expectancy during the last century. In the United States, the percentage of the population aged ≥65 years is projected to increase from 13% in 2010 to 19.3% in 2030. However, old age remains the main risk factor for major life-threatening disorders, and the number of people suffering from age-related diseases is anticipated to almost double over the next two decades. The prevalence of age-related pathologies represents a major threat as well as an economic burden that urgently needs effective interventions.

Molecules, drugs, and other interventions that might decelerate aging processes continue to raise interest among both the general public and scientists of all biological and medical fields. Over the past two decades, this interest has taken root in the fact that many of the molecular mechanisms underlying aging are interconnected and linked with pathways that cause disease, including cancer, cardiovascular and neurodegenerative disorders. Unfortunately, among the many proposed aging interventions, only a few have reached a certain age themselves. Results often lack reproducibility because of a simple inherent problem: interventions in aging research take a lifetime to assess. Experiments lasting the lifetime of animal models are prone to develop artifacts, increasing the possibilities and time windows for experimental discrepancies. Some inconsistencies in the field arise from overinterpreting life span-shortening models and scenarios as being accelerated aging.

Many substances and interventions have been claimed to be antiaging throughout history and into the present. In the following sections, interventions will be restricted to those that meet the following highly selective criteria: (1) promotion of life span and/or health span, (2) validation in at least three model organisms, and (3) confirmation by at least three different laboratories. These include: (1) CR and fasting regimens, (2) some pharmacotherapies (resveratrol, rapamycin, spermidine, and metformin), and (3) exercise.

Caloric Restriction

One of the most important and robust interventions that delays aging is CR. This outcome has been recorded in rodents, dogs, worms, flies, yeasts, monkeys, and prokaryotes. CR is defined as a reduction in the total caloric intake, usually of about 30% and without malnutrition. CR reduces the release of growth factors such as growth hormone, insulin, and IGF-1, which are activated by nutrients and have been shown to accelerate aging and enhance the probability for mortality in many organisms. Yet the effects of CR on aging were first discovered by McCay in 1935 long before the effects of such hormones and growth factors on aging were recognized. The cellular pathways that mediate this remarkable response have been explored in many experimental models. These include the nutrient sensing pathways (mTOR, AMPK, insulin/IGF-1, and sirtuins) as well as transcription factors (FOXO in D. melanogaster and daf-16 in C. elegans). The transcription factor Nrf2 appears to confer most of the anticancer properties of CR in mice, even though it is dispensable for life span extension.

The effects of CR in monkeys have been assessed in two studies with different outcomes: one study observed prolonged life while the other did not. However, both studies confirmed that CR increases health span by reducing the risk for diabetes, cardiovascular disease, and cancer. In humans, CR is associated with increased life and health span. This is most convincingly demonstrated in Okinawa, Japan, where one of the most long-lived human populations resides. In comparison to the rest of the Japanese population, Okinawan people usually combine an above-average amount of daily exercise with a below-average food intake. However, when Okinawan families move to Brazil, they adopt a Western lifestyle that affects both exercise and nutrition, causing a rise in weight and a reduction in life expectancy by nearly two decades. In the Biosphere II project, where volunteers lived together for 24 months undergoing an unforeseen severe CR, there were improvements in insulin, blood sugar, glycated hemoglobin, cholesterol levels, and blood pressure—all outcomes that would be expected to benefit life span. CR changes many aspects of human aging that might influence life span such as the transcriptome, hormonal status (especially IGF-1 and thyroid hormones), oxidative stress, inflammation, mitochondrial function, glucose homeostasis, and cardiometabolic risk factors. Epigenetic modifications are an emerging target for CR.

It must be noted that maintaining CR and avoiding malnutrition is not only arduous in humans but is also linked with substantial side effects. For instance, prolonged reduction of calorie intake may decrease fertility and libido, impair wound healing, reduce the potential to combat infections, and lead to amenorrhea and osteoporosis.

While extreme obesity (body mass index [BMI] >35) leads to a 29% increased risk of dying, people with BMI in the overweight range seem to have reduced mortality, at least in population studies of middle-aged and older subjects. People with a BMI in the overweight range seem more able to counteract and respond to disease, trauma, and infection, whereas CR impairs healing and immune responses. On the other hand, BMI is an insufficient denominator of body and body fat composition. A well-trained athlete may have an equivalent BMI compared to a fat person because of the higher muscle mass density. The waist:hip ratio is a much better indicator for body fat and an excellent and stringent predictor for the risk to die from cardiovascular disease: the lower the waist:hip ratio, the lower the risk.

PERIODIC FASTING

How can CR be translated to humans in a socially and medically feasible way? A whole series of periodic fasting regimens are asserting themselves as suitable strategies, among them the alternate-day fasting diet, the “five:two” intermittent fasting diet, and a 48-h fast once or twice each month. Periodic fasting is psychologically more viable, lacks some of the negative side effects and is only accompanied by minimal weight loss.

It is striking that many cultures implement periodic fasting rituals, for example Buddhists, Christians, Hindus, Jews, Muslims, and some African animistic religions. It could be speculated that a selective advantage of fasting versus nonfasting populations is conferred by health-promoting attributes of religious routines that periodically limit caloric intake. Indeed, several lines of evidence indicate that intermittent fasting regimens exert antiaging effects. For example, improved morbidity and longevity were observed among Spanish home nursing residents who underwent alternate-day fasting. Even rats subjected to alternate-day fasting live up to 83% longer than normally fed control animals and one 24-h fasting period every 4 days is sufficient to generate life span extension.

Repeated fasting and eating cycles may circumvent the negative side effects of sustained CR. This strategy may even yield effects despite extreme overeating during the nonfasting periods. In a spectacular experiment, mice fed a high-fat diet in a time-restricted manner, i.e., with regular fasting breaks, showed reduced inflammation markers, no fatty liver and were slim in comparison to mice with equivalent total calorie consumption but ad libitum. From an evolutionary point of view, this kind of feeding pattern may reflect mammalian adaptation to food availability: overeating in times of nutrient availability (e.g., after a hunting success) and starvation in between. This is how some indigenous peoples who have avoided Western lifestyles live today; those who have been investigated show limited signs of age-induced diseases such as cancer, neurodegeneration, diabetes, cardiovascular disease, and hypertension.

Fasting exerts beneficial effects on health span by minimizing the risk of developing age-related diseases including hypertension, neurodegeneration, cancer, and cardiovascular diseases. The most effective and rapid repercussion of fasting is reduction in hypertension. Two weeks of water-only fasting resulted in a blood pressure below 120/80 mmHg in 82% of subjects with borderline hypertension. Ten days of fasting cured all hypertensive patients who had been taking antihypertensive medication previously.

Periodic fasting dampens the consequences of many age-related neurodegenerative diseases (Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, and frontotemporal dementia but not amyotrophic lateral sclerosis in mouse models). Fasting cycles are as effective as chemotherapy against certain tumors in mice. In a combination with chemotherapy, fasting protected mice against the negative side effects of chemotherapeutic drugs, while it enhances their efficacy against tumors. Combining fasting and chemotherapy rendered 20–60% mice cancer-free when inoculated with highly aggressive tumors like glioblastoma or pancreatic tumors, which have 100% mortality even with chemotherapy.

Pharmacological Interventions to Delay Aging and Increase Life Span

Virtually all obese people know that stable weight reduction will reduce their elevated risk of cardiometabolic disease and enhance their overall survival, yet only 20% of overweight individuals are able to lose 10% weight for a period of at least 1 year. Even in the most motivated people (such as the “Cronies” who deliberately attempt long-term CR in order to extend their lives), long-term CR is extremely difficult. Thus, focus has been directed at the possibility of developing medicines that replicate the beneficial effects of CR without the need for reducing food intake (“CR-mimetics,” Fig. 463-5):

• Resveratrol. Resveratrol, an agonist of SIRT1, is a polyphenol that is found in grapes and in red wine. The potential of resveratrol to promote life span was first identified in yeast, and it has gathered fame since, at least in part because it has been suggested to be responsible for the so-called French paradox whereby wine reduces some of the cardiometabolic risks of a high fat diet. Resveratrol has been reported to increase life span in many lower order species such as yeast, fruit flies, worms as well as mice on high-fat diets. In monkeys fed with a diet high in sugar and fat, resveratrol had beneficial outcomes related to inflammation and cardiometabolic parameters. Some studies in humans have also shown improvements in cardiometabolic function while others have not. Gene expression studies in animals and humans reveal that resveratrol mimics some of the metabolic and gene expression changes of CR.

• Rapamycin. Rapamycin, an inhibitor of mTOR, was originally discovered on the Easter Island (Rapa Nui, hence its name) as a bacterial secretion with antibiotic properties. Before its immersion in the antiaging field, rapamycin was known as an immunosuppressant and cancer chemotherapeutic in humans. Rapamycin extends life span in all organisms tested so far, including yeast, flies, worms, and mice. However, the potential utility of rapamycin for human life span extension is likely to be limited by adverse effects related to immunosuppression, wound healing, proteinuria, and hypercholesterolemia, among others. An alternative strategy may be intermittent rapamycin feeding, which was found to increase mouse life span.

• Spermidine. Spermidine is a physiological polyamine that induces autophagy-mediated life span extension in yeast, flies, and worms. Spermidine levels decrease during life of virtually all organisms including humans, with the stunning exception of centenarians. Oral administration of spermidine or upregulation of bacterial polyamine production in the gut both lead to life span extension in short-lived mouse models. Spermidine has also been found to have beneficial effects on neurodegeneration probably by increasing transcription of genes involved in autophagy.

• Metformin. Metformin, an activator of AMPK, is a biguanide first isolated from the French lilac that is widely used for the treatment of type 2 diabetes mellitus. Metformin decreases hepatic gluconeogenesis and increases insulin sensitivity. Metformin has other actions including inhibition of mTOR and mitochondrial complex I, and activation of the transcription factor SKN-1/Nrf2. Metformin increases life span in different mouse strains including female mouse strains predisposed to high incidence of mammary tumors. At a biochemical level, metformin supplementation is associated with reduced oxidative damage and inflammation and mimics the some of the gene expression changes seen with CR.

Exercise and Physical Activity

In humans and animals, regular exercise reduces the risk of morbidity and mortality. Given that cardiovascular diseases are the dominant cause of aging in humans but not in mice, the effects on human health may be even stronger than those seen in mouse experiments. An increase in aerobic exercise capacity, which declines during aging, is associated with favorable effects on blood pressure, lipids, glucose tolerance, bone density, and depression in older people. Likewise, exercise training protects against aging disorders such as cardiovascular diseases, diabetes mellitus, and osteoporosis. Exercise is the only treatment that can prevent or even reverse sarcopenia (age-related muscle wasting). Even moderate or low levels of exercise (30 min walking per day) have significant protective effects in obese subjects. In older people, regular physical activity has been found to increase the duration of independent living.

While clearly promoting health and quality of life, regular exercise does not extend life span. Furthermore, the combination of exercise with CR has no additive effect on maximal life span in rodents. On the other hand, alternate-day fasting with exercise is more beneficial for the muscle mass than single treatments alone. In nonobese humans, exercise combined with CR has synergistic effects on insulin sensitivity and inflammation. From the evolutionary perspective, the responses to hunger and exercise are linked: when food is scarce, increased activity is required to hunt and gather.

Hormesis

The term hormesis describes the, at first sight paradoxical, protective effects conferred by the exposure to low doses of stressors or toxins (or as Nietzsche stated “What does not kill me makes me stronger”). Adaptive stress responses elicited by noxious agents (chemical, thermal, or radioactive) precondition an organism rendering it resistant to subsequent higher and otherwise lethal doses of the same trigger. Hormetic stressors have been found to influence aging and life span presumably by increasing cellular resilience to factors that might contribute to aging such as oxidative stress.

Yeast cells that have been exposed to low doses oxidative stress exhibit a marked antistress response that inhibits death following exposure to lethal doses of oxidants. During ischemic preconditioning in humans, short periods of ischemia protect the brain and the heart against a more severe deprivation of oxygen and subsequent reperfusion-induced oxidative stress. Similarly, the lifelong and periodic exposure to various stressors can inhibit or retard the aging process. Consistent with this concept, heat or mild doses of oxidative stress can lead to life span extension in C. elegans. CR can also be considered as a type of hormetic stress that results in the activation of antistress transcription factors (Rim15, Gis1, and Msn2/Msn4 in yeast and FOXO in mammals) that enhance the expression of free radical-scavenging factors and heat shock proteins.

Clinicians need to understand aging biology in order to better manage those people who are elderly. Moreover, there is an urgent need to develop strategies based on aging biology that delay aging, reduce the onset of age-related disorders, and increase health span for future generations. Interventions related to nutrition and those drugs that act on nutrient-sensing pathways are being developed and, in some cases, are already being tested in humans.