Essential and nonessential proteins

To make all the proteins that your body needs, you require 22 different amino acids. Ten are considered essential, which means you can't synthesize them in your body and must obtain them from food (two of these, arginine and histidine, are essential only for children). Several more are nonessential: If you don't get them in food, you can manufacture them yourself from fats, carbohydrates, and other amino acids. Three — glutamine, ornithine, and taurine — are somewhere in between essential and nonessential for human beings: They're essential only under certain conditions, such as with injury or disease.

Essential Amino Acids

Nonessential Amino Acids

Arginine*

Alanine

Histidine*

Asparagine

Isoleucine

Aspartic acid

Leucine

Citrulline

Lysine

Cysteine

Methionine

Glutamic acid

Phenlyalanine

Glycine

Threonine

Hydroxyglutamic acid

Tryptophan

Norleucine

Valine

Proline

Serine

Tyrosine

  • Essential for children; nonessential for adults
  • Essential for children; nonessential for adults

High-quality and low-quality proteins

Because an animal's body is similar to yours, its proteins contain similar combinations of amino acids. That's why nutritionists call proteins from foods of animal origin — meat, fish, poultry, eggs, and dairy products — high-quality proteins. Your body absorbs these proteins more efficiently; they can be used without much waste to synthesize other proteins. The proteins from plants — grains, fruit, vegetables, legumes (beans), nuts, and seeds — often have limited amounts of some amino acids, which means their nutritional content is not as high as animal proteins.

Super soy: The special protein food

Nutrition fact No. 1: Food from animals has complete proteins. Nutrition fact No. 2: Vegetables, fruits, and grains have incomplete proteins. Nutrition fact No. 3: Nobody told the soybean.

Unlike other vegetables, including other beans, soybeans have complete proteins with sufficient amounts of all the amino acids essential to human health. In fact, food experts rank soy proteins on par with egg whites and casein (the protein in milk), the two proteins easiest for your body to absorb and use (see Table 6-1).

Some nutritionists think soy proteins are even better than the proteins in eggs and milk, because the proteins in soy come with no cholesterol and very little of the saturated fat known to clog your arteries and raise your risk of heart attack. Better yet, more than 20 recent studies suggest that adding soy foods to your diet can actually lower your cholesterol levels.

One-half cup of cooked soybeans has 14 grams of protein; 4 ounces of tofu has 13. Either serving gives you approximately twice the protein you get from one large egg or one 8-ounce glass of skim milk, or two-thirds the protein in 3 ounces of lean ground beef. Eight ounces of fat-free soy milk has 7 milligrams protein — a mere 1 milligram less than a similar serving of skim milk — and no cholesterol. Soybeans are also jam-packed with dietary fiber, which helps move food through your digestive tract.

In fact, soybeans are such a good source of food fiber that I feel obligated to add a cautionary note here. One day after I'd read through a bunch of studies about soy's effect on cholesterol levels, I decided to lower my cholesterol level right away. So I had a soy burger for lunch, a half cup of soybeans and no-fat cheese for an afternoon snack, and another half cup with tomato sauce at dinner. Delicacy prohibits me from explaining in detail how irritated and upset all that fiber made my digestive tract, but I'm sure you get the picture.

If you choose to use soybeans (or any other dry beans for that matter), take it slow — a little today, a little more tomorrow, and a little bit more the day after that.

The basic standard against which you measure the value of proteins in food is the egg. Nutrition scientists have arbitrarily given the egg a biological value of 100 percent, meaning that, gram for gram, it's the food with the best supply of complete proteins. Other foods that have proportionately more protein may not be as valuable as the egg because they lack sufficient amounts of one or more essential amino acids.

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For example, eggs are 11 percent protein, and dry beans are 22 percent protein. However, the proteins in beans don't provide sufficient amounts of all the essential amino acids, so they (the beans) are not as nutritionally complete as proteins from animal foods. The prime exception is the soybean, a legume that's packed with abundant amounts of all of the amino acids essential for adults. Soybeans are an excellent source of proteins for vegetarians, especially vegans, which are vegetarians who avoid all products of animal origin, including milk and eggs.

The term used to describe the value of the proteins in any one food is amino acid score. Because the egg contains all the essential amino acids, it scores 100. Table 6-1 shows the protein quality of representative foods relative to the egg.

Table 6-1

Scoring the Amino Acids in Food

Food

Protein Content (Grams)

Amino Acid Score (Compared to the Egg)

Egg

33

100

Fish

61

100

Beef

29

100

Milk (cow's whole)

23

100

Soybeans

29

100

Dry beans

22

75

Rice

7

62-66

Corn

7

47

Wheat

13

50

Wheat (white flour)

12

36

Nutritive Value of Foods (Washington, D.C.: U.S. Department of Agriculture, 1991); George M. Briggs and Doris Howes Calloway, Nutrition and Physical Fitness, 11th ed. (New York: Holt, Rinehart and Winston, 1984)

Nutritive Value of Foods (Washington, D.C.: U.S. Department of Agriculture, 1991); George M. Briggs and Doris Howes Calloway, Nutrition and Physical Fitness, 11th ed. (New York: Holt, Rinehart and Winston, 1984)

Homocysteine and your heart

Homocysteine is an intermediate, a chemical released when you metabolize (digest) protein. Unlike other amino acids, which are vital to your health, homocysteine can be hazardous to your heart, raising your risk of heart disease by attacking cells in the lining of your arteries by making them reproduce more quickly (the extra cells may block your coronary arteries) or by causing your blood to clot (ditto).

Years and years ago, before cholesterol moved to center stage, some smart heart researchers labeled homocysteine the major nutritional culprit in heart disease. Today, they've been vindicated. The American Heart Association cites high homocysteine levels as an independent probable (but not major) risk factor for heart disease, perhaps explaining why some people with low cholesterol have heart attacks.

But wait! The good news is that information from several studies, including the Harvard/ Brigham and Women's Hospital Nurses' Health Study in Boston, suggest that a diet rich in the B vitamin folate lowers blood levels of homocys-teine. Most fruits and vegetables have plentiful amounts of folate. Stocking up on them may protect your heart.

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