1 Nutrition and Digestion
1.1 Nutrition and Energy Requirements
1.1.1 Nutrients
Nutrients are organic and inorganic molecules that support the growth and survival of a living organism. Those utilized by humans can be divided into six major classes: carbohydrates, lipids, proteins, vitamins, water, and minerals. Nutrients are used for three major purposes:
1. To supply the energy needed for the body to perform work
2. To provide the building blocks for the synthesis of other important molecules
3. To support the function of metabolic pathways
Energy content of nutrients
The energy content of nutrients is an expression of the amount of heat released upon burning 1 g of a nutrient. This energy is measured in kilocalories (kcal); however, it is conventionally expressed as "calories" on the Nutrition Facts labels of foods. The energy content for the major energy nutrients are as follows:
Carbohydrates: 4.1 kcal/g
Proteins: 4.1 kcal/g
Lipids: 9.3 kcal/g
Foundations
Caloric content of alcohol
Ethanol (alcohol) can serve as a source of energy because its energy content is 7 kcal/g, which is higher than that of carbohydrates and proteins.
1.1.2 Diet
The daily energy requirements are met through the consumption of food. The Food and Nutrition Board (Institute of Medicine, National Academies) has defined the following terms as part of the Dietary Reference Intake (DRI) values for the six major groups of nutrients (? Fig. 1.1):
Fig. 1.1 Measures of nutrient intake. The risk of adverse effects (due to inadequate or excessive amounts of a specific nutrient) increases when nutrient intake is below the recommended dietary allowance (RDA) or above the upper level intake (UL). AI, adequate intake; EAR, estimated average requirement.
Adequate intake (AI) is a recommended average daily nutrient intake level assumed to be adequate based on observed or experimentally determined estimates of daily nutrient intake by a group (or groups) of healthy people. AI is used when an RDA cannot be determined (e.g., for certain minerals).
Tolerable upper intake level (UL) is the highest average daily nutrient intake level likely to pose no risk of adverse health effects to almost all individuals in a particular life stage and gender group. As intake increases above the UL, the potential risk of adverse health effects increases (see Section "Lipid-soluble Vitamins).
Estimated average requirement (EAR) is the average daily nutrient intake level estimated to meet the requirement of 50% of the healthy individuals in a particular life stage and gender group.
Recommended dietary allowance (RDA) is a value that represents the amount of a nutrient that, if consumed by every member of a population, will keep 98% of individuals (of a particular life stage and gender group) in good health.
? The respective RDA for carbohydrates, lipids, and proteins is 60%, 30%, and 10% of an individual's daily caloric intake, which is further described in ? Table 1.1.
? Consumption of a diet that is high in carbohydrates but deficient in protein causes a type of malnutrition called kwashiorkor; consumption of a diet that is deficient in both carbohydrates and proteins leads to another type of malnutrition called marasmus.
Therapeutics
Kwashiorkor
Kwashiorkor is a form of malnutrition caused by consumption of a diet that lacks sufficient protein but is high in carbohydrates. In such cases, the total caloric intake is adequate, but not enough of it comes from protein. Individuals with this condition appear emaciated and irritable. They present with edema of the feet and hands, a swollen "moon" face, a distended abdomen, skin discolorations and lesions, and an enlarged fatty liver. Kwashiorkor is prevalent in weaning children from developing countries in which the food supply is limited.
Therapeutics
Marasmus
Marasmus is a form of malnutrition caused by consumption of a calorie-deficient diet that lacks adequate amounts of carbohydrates and proteins. Patients with this condition appear emaciated with decreased subcutaneous fat. Unlike kwashiorkor, individuals with marasmus do not present with a distended abdomen.
Table 1.1 Nutrients' recommended dietary allowance (RDA)
Nutrient
% of Kcal
Kcal Based on a 2,000 kcal/d Diet (130-150 lb person)
Carbohydrates
60
1,200
Lipids
30
600
Proteins
10
200
Table 1.2 Factors that influence basal metabolic rate and its use in calculating the total energy expenditure
Factor
Effects on basal metabolic rate (BMR)
Gender
Male > female
Age
Child > adult
Health
Elevated by fever
Elevated in individuals with hyperthyroidism (e.g., Graves disease)
Low in individuals with hypothyroidism (e.g., Hashimoto thyroiditis)
Hormones
Elevated by high levels of thyroid hormones (e.g., thyroxine), growth hormones, sex hormones, epinephrine, and cortisol
Calculating BMR and total energy expenditure:
Men: BMR = 66 + (6.23 × weight in lb) + (12.7 × height in in.) - (6.8 × age in years)
Women: BMR = 655 + (4.35 × weight in lb) + (4.7 × height in in.) - (4.7 × age in years)
Total energy expenditure = BMR × physical activity level (PAL)
1.1.3 Total Energy Expenditure
Total energy expenditure is the amount of calories needed to meet the body's energy demands. It is dependent on age, weight, gender, diet, physical activity, and overall health, which are expressed in the following terms:
Basal metabolic rate (BMR) is the amount of calories required to maintain normal physiological functions when the body is at rest and accounts for 60 to 70% of total energy expenditure. BMR varies with gender, age, health, and hormone levels (? Table 1.2) and is estimated using a formula that takes these factors into account.
Thermic effect of food (TEF) represents the amount of calories expended during the digestion, absorption, and metabolism of nutrients. It accounts for ~10% of total energy expenditure.
Physical activity level (PAL) accounts for the amount of calories required to support certain levels of physical exertion. It is expressed in values that range from 1.2 to 2.4. The lowest value represents the PAL of individuals engaged in no physical activity (e.g., bedridden persons); the highest value represents that of individuals engaged in strenuous forms of physical activity (e.g., performance athletes).
Therapeutics
Hashimoto thyroiditis
Another type of autoimmune disease that affects the thyroid is Hashimoto thyroiditis. In this case, the antibodies attack the thyroid gland and destroy its ability to synthesize thyroid hormones. This results in hypothyroidism, which is marked by fatigue, depression, constipation, abnormal sensitivity to cold, and modest weight gain. Treatment involves the administration of thyroid hormone.
Therapeutics
Graves disease
Graves disease is an autoimmune disorder in which antibodies bind to the thyroid-stimulating hormone (TSH) receptor on the surface of the thyroid gland and mimic the actions of TSH. This results in the production of excessive levels of thyroid hormones and hyperthyroidism, which is marked by goiter (enlarged thyroid gland), fatigue, restlessness, increased bowel movement, increased perspiration and heat intolerance, and weight loss. Treatment includes surgical resection of the thyroid gland, radioactive iodine to decrease the thyroid mass, or pharmacological intervention with antithyroid drugs such as propylthiouracil and methimazole.
Calculating total energy expenditure
Total energy expenditure is calculated by multiplying BMR and PAL. Weight gain occurs when caloric intake exceeds that of total energy expenditure. Likewise, weight loss occurs when caloric intake is less than that of total energy expenditure.
For instance, because 1 lb of fat is equivalent to 3,500 kcal,...
