INTRODUCTION TO NUTRIENTS
Food consists of nutrients, some of which are essential for the body to function on a daily basis. Digestion of food starts in our mouths, and food continues to break down as it travels from the mouth through the esophagus, stomach, and small intestine by a combination of mechanical forces and chemical reactions. Enzymes that are produced within the saliva, stomach, pancreas, liver, and small intestine target-specific chemical bonds holding complex nutrient structures together. Ultimately, those complex structures are broken down to simple nutrients that transporters along the stomach, small intestine, and large intestine can absorb into the body.
There are six main classes of nutrients existing in our dietary sources. They are categorized as (1) carbohydrates, (2) lipids, (3) proteins, (4) water, (5) vitamins, and (6) minerals. Carbohydrates, lipids proteins, and water are all considered macronutrients while vitamins and minerals are micronutrients (see following sections for more detail). Food also contains other bioactive substances that may not be essential for the human body to function but over the past decades have been found to have beneficial effects on human health. Such substances include phytochemicals, pre- or probiotics, and polyphenols. For example, phytochemicals contained in plant-based food products have been found to have protective effects on chronic disease such as hypertension, cancer and heart disease.1–3 This chapter will focus on the six main classes of nutrients by describing the different subclasses within each nutrient, defining their roles in the body, and relating their role to key health outcomes or development towards disease states.
Macronutrients encompass the nutrient classes of carbohydrates, lipids, proteins, and water. These nutrients are energy yielding, with the exception of water, and are needed in relatively large amounts (thus “macro-”). Macronutrients are essential for cells to function because of the energy they provide. The energy that macronutrients yield is measured in kilocalories, a unit representing the amount of heat required to raise the temperature of 1 kilogram of water by 1ºC. Kilocalorie is commonly presented as Calorie on food labels and other dietary resources (note the capital “C” represents “kilo-” while a lowercase “c” technically represents 1/1000 of a kilocalorie). Although water is not energy yielding it is also categorized as a macronutrient because it is needed in larger amounts on a daily basis and is essential for the body to function. The recommended amount of macronutrients required for normal body functioning are typically expressed in grams or kilograms per day. The amount of energy one requires from macronutrients depends on factors such as age, gender, weight, level of physical activity, and height. All of these things can this can vary even in people within the same life stage. A dietary guideline called the Acceptable Macronutrient Distribution Range provides a range of percent of Calories that one should get from each macronutrient per day. The recommendations include obtaining 45–65% of Calories from carbohydrates, ...