World-class gymnast Christy Henrich died of complications of the self-starvation eating disorder anorexia nervosa in July 1994. In this photo, taken 11 months before her death, she weighed under 60 pounds. Concern over weight gain propelled her down the path of this deadly nutritional illness.

The human body requires fuel as well as materials to develop, grow, and heal. Nutrients from food fulfill these requirements. However, like many physiological processes, nutrition is very much a matter of balance. Too few nutrients, and disorders associated with malnutrition result. Too many nutrients, and obesity is the consequence.

Why We Eat

Nutrients (nu'tre-ents) are chemical substances supplied from the environment that an organism requires for survival. The macronutrients, needed in bulk, include carbohydrates, proteins, and fats. Micronutrients are essential in small daily doses and include vitamins and minerals. The body also requires water.

In countries with adequate food supplies, most healthy individuals can obtain nourishment by eating a variety of foods and limiting fat intake. People who do not eat meat products can also receive adequate nutrition but must pay more attention to food choices to avoid develop ing nutrient deficiencies. For example, eliminating red meat also means eliminating an excellent source of iron, copper, zinc, and vitamin B12. The fiber that often makes up much of a vegetarian's diet, although very healthful in many ways, also decreases absorption of iron. Therefore, a vegetarian must be careful to obtain sufficient iron from nonmeat sources. This is easily done, providing proper nutrition (adequate nutrients) when sources, actions, and interactions of nutrients are considered. Fortified foods, green leafy vegetables, and especially whole grains provide many of the nutrients also present in meat. Table 18.1 lists the different types of vegetarian diets.

Digestion breaks down nutrients to sizes that can be absorbed and transported in the bloodstream. Metabolism refers to the ways that nutrients are altered chemically and used in anabolism (building up or synthesis) and catabolism (breaking down) of chemical compounds to support the activities of life. (Chapter 4, pp. 114-121, introduced metabolism of carbohydrates.) Nutrients that human cells cannot synthesize, such as certain amino acids, are particularly important and are therefore called essential nutrients.


Food Restrictions


No animal foods


Eggs allowed; no dairy or meat


Dairy allowed; no eggs or meat


Dairy and eggs allowed; no meat


Dairy, eggs, and fish allowed; no other meat


Dairy, eggs, chicken, and fish allowed; no other meat

In an overall sense, we eat to obtain the nutrients that power the activities of life. In a more literal sense, eating is a complex, finely tuned homeostatic mechanism that balances nutrient utilization with nutrient procurement. At the center of this control are parts of the hypothalamus called the arcuate and paraventricular nuclei. Here, a molecule called neuropeptide Y links biochemical messages concerning nutrient use (levels of ¡eptin, insulin, cholecystokinin, and glucocorticoids) to regulators of food intake. These regulators include release of pep-tides that control appetite, pituitary hormones that oversee endocrine control of nutrient use and storage, and insulin release.

Leptin is a protein hormone discovered in mice in 1994, and soon afterwards, in humans. Eating stimulates adipocytes to secrete leptin, which travels to the hypothalamus, where it signals appetite suppression and increases metabolic rate—a negative feedback response to a meal. Low leptin levels signal starvation, and appetite increases as metabolic rate decreases to conserve energy. At least two other proteins produced in the hypothalamus mediate leptin's effects. The melanocortin-4 receptor is activated when weight is gained, and it suppresses appetite. Neuropeptide Y is synthesized in response to low leptin levels. Sensing starvation, it increases appetite. Leptin given as a drug can only cause weight loss, however, if a person has an inborn leptin deficiency, or lacks receptors for leptin on cells in the hypothalamus — both extremely rare conditions. Drug developers are focusing on all of these weight-control proteins in the never-ending search for anti-obesity drugs.


Carbohydrates are organic compounds and include the sugars and starches. The energy held in their chemical bonds is used to power cellular processes.

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Lose 10 Pounds Naturally

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