- Decline in the glomerular filtration rate.
- Diminished appetite or anorexia.
- Abnormalities in intestinal absorption of some minerals and vitamins.
- Abnormalities in urinary, intestinal, and dermal excretion of nutrients.
- Disorders of nutrient metabolism.
In patients with chronic kidney disease (CKD), as the glomerular filtration rate (GFR) declines, numerous nutritional and metabolic disorders develop, and the dietary requirements for many nutrients are altered. These disorders and alterations include (1) diminished appetite or anorexia; (2) abnormalities in intestinal absorption of certain minerals (eg, calcium) and other nutrients including some trace elements (eg, iron and possibly zinc) and vitamins (eg, riboflavin); (3) abnormalities in urinary, intestinal, and dermal excretion of nutrients; and (4) disorders of nutrient metabolism.
Patients with renal insufficiency also are prone to accumulate toxins that normally are eaten in small amounts and would readily be excreted by the kidneys, such as aluminum. There are alterations in the concentrations and/or composition of certain lipoproteins, with an abnormal proportion of individual lipids and altered structure of some apolipoproteins. Potentially toxic oxidants and reactive carbonyl compounds accumulate in plasma and tissues. Deficiencies of antioxidants, including vitamins C and E and possibly selenium, may increase oxidative stress. Oxidative stress, along with the occurrence of inflammation in renal insufficiency, increases the risk of endothelial injury and atherosclerosis, leading to cardiovascular disease and higher death rates usually observed in patients with advanced CKD.
Patients with advanced CKD (stages 4 and 5) frequently suffer from protein–energy malnutrition. Protein–energy malnutrition is defined as a state of decreased body protein mass with or without fat depletion or a state of diminished functional capacity due to protein–energy depletion, which is usually caused at least partly by inadequate nutrient intake relative to nutrient demand and/or which is improved by nutritional repletion. In CKD, several conditions may contribute to protein–energy malnutrition or wasting. Because these conditions may be caused by factors in addition to inadequate nutrient intake, the term “wasting” (or protein–energy wasting) can also be used instead of protein–energy malnutrition. It is important to recognize that in advanced chronic renal insufficiency (CRI), ie, stages 4–5 CKD, there are other types of wasting or malnutrition. This is particularly likely to occur for calcium, iron, zinc, and vitamins C, B6, folic acid, and 1,25-dihydroxycholecalciferol.
Approximately one-third to one-half of patients with advanced CKD including those undergoing maintenance dialysis have mild to moderate malnutrition and 5–10% more have severe malnutrition. In malnourished CKD patients, decreased relative body weight or body mass index (BMI, body weight (kg) divided by the square of body height in meters2), skinfold thickness (an estimate of body fat), arm muscle diameter area (a reflection of muscle mass), total body nitrogen and potassium, and increased total body water and extracellular water (Table 21–1) are usually observed. Malnutrition is also manifested by decreased concentrations of many serum proteins including albumin, prealbumin, and transferrin. Serum lipoprotein ...