Macronutrients in Health and Disease
Macronutrients "carbohydrate, protein, and fat" are essential for health, growth, healing, and immune function. Too little or too much of any of these macronutrients may result in poor health and a variety of diseases.
In the past, research on nutrition and disease frequently focused on the problems caused by diets that provided too little nutrition. However, eating too much has become a far greater threat to health in developed countries and in many developing nations as well.
Protein supports the growth and maintenance of the body. The amino acids that make up proteins are used for building DNA, cell membranes, hormones, receptors, brain chemicals, and many other molecules in the body. Protein is also the second largest source of stored energy (second to fat cells) because of the large amount of muscle that is a steady source of amino acids.
The Food and Nutrition Board of the National Academy of Sciences has determined that nine amino acids are essential for health. These cannot be manufactured in significant quantities by the body and must be obtained from the diet. The essential amino acids are:
During growth and in various diseases, several other amino acids (including arginine, cysteine, glutamine, glycine, proline, and tyrosine) are also considered essential because the body cannot produce enough of them on its own. For example, normal individuals can produce enough of the amino acid cysteine. However, individuals with liver disease can no longer manufacture enough cysteine to meet the body's needs. Thus, they must consume additional cysteine from their diets or take an amino acid supplement.
While some plant foods do not contain all the essential amino acids, the amino acids provided by various plant foods tend to complement each other. The natural combinations of foods in typical vegetarian diets provide more than adequate amounts of complete protein. And plant sources of protein have many advantages over animal protein: they are free of cholesterol and low in saturated fat, and provide dietary fiber and various antioxidants.
- Protein needs are influenced by life stage: Protein requirements are highest in the growing years. Up until 3 years of age, we require more protein (about 1 to 1.5 grams of protein per kilogram of body weight) than do older children and adults (about 0.85 to 0.95 grams of protein per kilogram of body weight). Pregnancy and lactation also increase protein needs (1.1 to 1.3 grams of protein per kilogram of pre-pregnancy weight).
Most adults in Western countries consume protein far in excess of recommended amounts. In fact, the Continuing Survey of Food Intakes indicated that most people eat double the required amount. Excessive intakes may contribute to risk for certain chronic diseases.
- Too much protein can be harmful: Although a deficiency of dietary protein is clearly harmful, many chronic diseases may be caused or worsened by too much protein, particularly animal protein. These diseases include: osteoporosis, kidney stones, kidney failure, gout, and possibly certain cancers. Food from plant sources supply protein in the amount and quality appropriate for all ages.
- Protein requirements are increased in certain conditions: As noted above, severe illness may cause protein deficiency. These illnesses include liver and kidney diseases, burns, severe infections, and major surgery.