Meat, Poultry, Fish, Dry Beans, Eggs, and Nuts

Meat is an excellent source of protein. Its digestibility relates to the amount of muscle protein versus connective tissue, collagen, and fat. The collagen may vary from 2.5 to 23.6% and fat from 5 to 50%. Tenderness is associated with fat (marbling). Lean meat contains about 20% protein and 5-10% fat; the protein is of high biological value. Pork and chicken have a higher protein-to-fat ratio than either beef or lamb. Meats are usually rich in iron and zinc, contain little calcium, and are important sources of nicotinic acid and riboflavin. Muscle provides moderate amounts of vitamin B₁₂, but little vitamin A or ascorbic acid. Current recommendations limit meat to no more than 6 ounces per day.

Fish is an important source of protein of high biological value. Lean fish, such as cod, haddock, and sole, contain less than 1 % fat and about 10% protein. They are relatively low in calories and are easily digested. Fatty fish, such as herring, salmon, and sardines, contain 8-15% fish oil, doubling the calories. Halibut, mackerel, and trout have an intermediate fat content. Fish roe contains 20-30% protein and 20% fat. Fish oils are rich sources of vitamins A and D and long-chain, polyunsaturated o»3 fatty acids. Iodine and fluoride are ample in marine fish, and small whole fish are high in calcium.

Shellfish have little fat and calories. The protein content of oysters, mussels, and other molluscs is about 15%. Oysters are the richest food source of zinc.

The average egg (60 g) contains 6 g protein and 6 g fat and yields 80 calories. Egg proteins are mostly albumin, with the highest biological value of all food proteins for human adults. The yolk is a fair source of vitamin A and contains significant amounts of B vitamins. The average large egg contains about 215 mg of cholesterol.

Textured vegetable protein derived from soybeans is flavored to resemble meat. The natural ingredients contain no vitamin B₁₂. Vegetable proteins have less of the amino acid methionine than do animal proteins (Table V) and may be lower in iron, thiamin, and riboflavin than meat.

Legumes are seeds of the family that includes peas, beans, peanuts, and lentils. Their high protein content (20 g per 100 g dry weight) qualifies legumes as meat substitutes. Their low content of sulfur-containing amino acids reduces the biological value of the protein; they are rich in lysine, which is deficient in many cereals (Table V). A combination of legumes and cereal proteins may have a nutritive value as good as animal proteins and is an excellent source of fiber.

Legumes are a good source of B vitamins, except for riboflavin. Legumes lack ascorbic acid but sprouted legumes will prevent scurvy. Soybeans are high in protein; the whole dry grain contains 40% protein and up to 20% fat. Soya also provides B vitamins. Peanuts contain about 20% fat. Other legumes include a variety of beans, and although their digestion and absorption are virtually complete, flatulence may be a by-product.

3. Fruits. Fruits have many pleasing flavors and can serve as desserts without excessive calories. Ascorbic acid is an essential nutrient abundant in fruits. Fruits are sources of dietary fiber and contain carotene and small quantities of B vitamins. Most fruits have little or no protein or fat and contain 5-20% carbohydrate. Fructose and glucose are the major sugars in ripe fruits. Bananas may serve as a useful energy source, but provide no protein. Fruits are high in potassium.

4. Vegetables. Vegetables include leaves, roots, flowers, stalks, and gourds. Their chief nutritional value is for carotene, ascorbic acid, folate, and dietary fiber. Calcium and iron may be present in significant amounts but absorption is variable. Leafy vegetables may provide some B vitamins (riboflavin), but vegetables are poor sources of energy and protein. Potatoes are the inexpensive food that is best capable of supporting life as the sole diet, with starch supplying most of the calories. Although low in protein, it is of relatively high biological value. Potatoes are high in fiber and a good source of potassium; they are easily digested and well absorbed. The quality and nutritional value of canned and frozen vegetables compare favorably with those of fresh produce.

A single serving of fruit or vegetable is one-half cup, or one medium-size piece. The National Research Council report on “Diet and Health” (1989) recommends five or more servings every day of vegetables and fruits, especially green and yellow vegetables and citrus fruits.

5. Breads, Cereals, Rice, and Pasta. Cereal grains are the most important single food in many countries and are consumed as bread and in flour products. Corn, wheat, barley, oats, and rye are the principal cereals of North America.

Whole-grain cereals provide energy, good-quality protein, and appreciable amounts of calcium and iron. Cereals contain no ascorbic acid and practically no vitamin A; yellow corn contains significant amounts of carotene. Whole-grain cereals contain adequate amounts of B vitamins except for corn, in which the bound nicotinic acid is not biologically available.

Milling and discarding the outer portion of the seed diminishes the B vitamin content of wheat and rice. Wheat may contain 10-20 g protein per 100 g, with lysine the limiting amino acid. In the average flour used to make white bread, protein provides about 13% of the energy. Whole-what flour contains three times as much dietary fiber as white flour. Whole wheat is also high in phytate, which binds minerals, making them unavailable. Thus, although whole-wheat flour contains appreciable calcium, iron, and zinc, absorption may be limited. In the United States, 100 g of white flour is enriched with up to 0.44 mg thiamin, 0.26 mg riboflavin, 3.5 mg nicotinamide, and 2.9 mg iron. In some states, calcium and vitamin D may also be added. Addition of folic acid has been approved.

Pasta is made from a hard variety of wheat but utilizes that portion relatively poor in B vitamins; it is frequently enriched. Highly refined rice is almost devoid of vitamins. Parboiling fixes the vitamins so that they are not removed with milling and is the simplest preventive measure against beriberi (thiamin deficiency). Most rice contains 6.5-8.0 g of good-quality protein per 100 g. The principal protein in corn is incomplete, lacking the amino acids lysine and tryptophan.

The preparation of tortillas makes nicotinic acid biologically available from corn by heating the grains in lime water to soften them. Oatmeal contains more protein (12 g per 100 g) and more oil (8.5 g per 100 g) than other common cereals and is rich in soluble fiber. Barley produces the malt for brewers and is the basis of the best beers and some whiskey. Bread made from rye flour is rich in B vitamins and also contains fiber. The chief nutritive value of most dry breakfast cereals is derived from the addition of milk. Some cereals are fortified with B vitamins, iron, and, most recently, calcium.