Surprisingly, little is known about protein and health. We know that adults need a minimum of 1 gram of protein for every kilogram of body weight per day to keep from slowly breaking down their own tissues. That’s about 8 grams of protein for every 20 pounds. For athletes who work out very intensely, theirs a somewhat higher requirement, but how much more and whether or not it can be gained from a good diet, is a hot topic. Beyond that, there’s relatively little solid information on the ideal amount of protein in the diet, a healthy target for calories contributed by protein, or the best kinds of protein.

Worldwide, millions of people don’t get enough protein. This protein malnutrition leads to the condition known as kwashiorkor. Lack of protein can cause growth failure, loss of muscle mass, decreased immunity, weakening of the heart and respiratory system, and death.

In the United States and other developed countries, it’s usually easy to get the minimum daily requirement of protein. A bowl of cereal with milk for breakfast or a piece of fish with a side of beans for dinner adds up to about 70 grams of protein, which is adequate for the average adult.

Too little protein is clearly a problem. And what about too much? The digestion of protein releases acids that the body usually neutralizes with calcium and other buffering agents in the blood. Eating lots of protein, such as the amounts recommended in the so-called no-carb diets, requires lots of calcium. Some of this may be pulled from bone.

Following a high-protein diet for a few weeks probably won’t have much effect on bone strength. Doing it for the long term could weaken bone. In a recent study, for example, women who ate more than 95 grams of protein a day were 20 percent more likely to have broken a wrist over a 12-year period when compared to those who ate an average amount of protein (less than 68 grams a day). While additional research is needed to define the optimal amount of daily protein, these results suggest that high-protein diets should be used with caution, if at all.

All proteins are not the same

Some of the protein you eat contains all the amino acids needed to construct new proteins. This type protein is called a complete protein. Animal sources of protein tend to be complete. Other protein lacks one or more amino acids that the body can’t make from scratch or create by modifying another amino acid. Termed incomplete proteins, these usually come from fruits, vegetables, grains, and nuts.

In regards to the many types of diets and methods in nutrition, vegetarians need to be aware of this difference. To obtain all the amino acids needed to make new protein and to keep the body’s systems in good shape, people who don’t eat meat, fish, poultry, eggs, or dairy products should eat a variety of protein-containing foods each day as we have stressed elsewhere.

The protein source

Animal protein and vegetable protein probably have the same effects on health. It’s the protein package that is likely to make a difference. A 6-ounce broiled Porterhouse steak is a great source of complete protein because it contains 38 grams of it. But, it also contains 44 grams of fat, 16 of which are saturated. For a 2,000 calorie per day diet, that’s almost three-fourths of the recommended daily intake for saturated fat. The same amount of salmon yields 34 grams of protein and 18 grams of fat, 4 of them saturated. A cup of cooked lentils also has 34 grams of protein but under 1 gram of fat.

Consequently, it’s important to pay attention to what comes with the protein in your food choices. If you are partial to beef, stick with the leanest cuts. Fish or poultry are excellent alternatives. Even better options are vegetable sources of protein, such as beans, nuts, and whole grains.

Protein and chronic disease

The most solid connection between proteins and health has to do with a common disorder of the immune system. Proteins in food and the environment are responsible for a variety of allergies. These are basically overreactions of the immune system to what should be harmless proteins. Beyond that, relatively little evidence has been gathered regarding the effect of protein on the development of chronic diseases.

Cardiovascular disease: To date, only one large, prospective study has investigated the association between dietary protein and heart disease or stroke. In a recent study, women who ate the most protein (about 110 grams per day) were 25 percent less likely to have a heart attack or to die of heart disease than the women who ate the least protein (about 68 grams per day) over a 14-year period. The source of protein, i.e., animals or grains didn’t seem to matter. These results offer reassurance that eating a lot of protein doesn’t harm the heart. In fact, it is possible that eating more protein while cutting back on easily digested carbohydrates may be benefit the heart.

Diabetes: Proteins found in cow’s milk may play a role in the development of type 1 diabetes (formerly called juvenile or insulin-dependent diabetes), which is one reason why cow’s milk isn’t recommended for infants. Later in life, the amount of protein in the diet doesn’t seem to adversely affect the development of type 2 (adult-onset) diabetes, although research in this area is ongoing.

Weight control: In short-term studies, a lower-calorie diet that includes more protein and less carbohydrate is more effective for losing weight or keeping weight steady than a lower-calorie, high-carbohydrate diet. Eating high-protein foods such as beef, chicken, fish, or beans will make you feel full longer because they slow the movement of food from the stomach to the intestine. This strategy may also delay hunger signals. The digestion of protein, when compared to that of carbohydrates results in smaller, steadier increases in blood sugar. This helps avoid the steep climbs and drops in blood sugar, which trigger hunger pangs and occur after eating rapidly digested carbohydrates. Unfortunately, little data have been collected on the long-term effects of a high-protein diet on weight control.

Cancer: There’s no good evidence that eating a little protein or a lot of it influences cancer risk.

What about soy as a protein source?

The U.S. Food and Drug Administration now allows food makers to claim on the labels of low-fat foods containing at least 6.25 grams of soy protein that soy can help reduce the risk of heart disease.

Some research suggests that regularly ingested soy-based foods lower cholesterol, chill hot flashes, prevent breast and prostate cancer, aid weight loss, and ward off osteoporosis. These effects may be due to a unique characteristic of soybeans, their high concentrations of isoflavones, a type of plant-made estrogen.

This research has prompted many media reports touting the joys of soy. It also has food makers churning out new soy products that are beginning to move into the mainstream. In Boston, for example, soymilk is now advertised on the radio during Boston Red Sox games, alongside donuts, oil additives, and beer.

Often, many of the claims made for soy go far beyond the available evidence.

Heart disease: There’s good evidence that soy lowers cholesterol levels. A 1995 meta-analysis of 38 controlled clinical trials showed that eating approximately 50 grams of soy protein a day in place of animal protein reduced total cholesterol levels by 9.3 percent, LDL cholesterol by 12.9 percent, and triglycerides by 10.5 percent. Such reductions, if sustained over time, could yield a 20 percent reduction in the risk of myocardial infarction (heart attacks) or other forms of cardiovascular disease. Individuals with very high cholesterol levels, in the vicinity of 300 mg/dL, appeared to benefit most from eating soy-based foods. Keep in mind that 50 grams of soy protein is the equivalent of 1 pounds of tofu or eight 8-ounce glasses of soymilk a day. The American Heart Association now recommends including soy-based foods as part of a heart-healthy diet.

Hot Flashes: Soy has also been investigated as a treatment for hot flashes and other problems that often accompany menopause. In theory, this makes sense. Soybeans are rich in plant estrogens, also called phytoestrogens. In some tissues, these substances mimic the action of estrogen. Thus, they could cool hot flashes by giving a woman an estrogen-like boost during a time of decreasing estrogen levels. Yet carefully controlled studies haven’t found this to be the case. Such studies do not negate the effects, but may not show such results because of too low number of participants in the study.

Breast cancer: In some tissues, phytoestrogens block the action of estrogen. If this occurs in breast tissue, for example then, eating soy could reduce the risk of breast cancer because estrogen stimulates the growth and multiplication of breast and breast cancer cells. However, studies to date haven’t provided a clear answer, with some showing a benefit and others showing no association between soy consumption and breast cancer. Large prospective studies now underway should offer better information regarding soy and breast cancer risk.

Unsettling reports suggest that concentrated supplements of soy proteins may stimulate the growth of breast cancer cells. Too much soy could also lead to memory problems. Among older women of Japanese ancestry living in Hawaii, those who relied on the traditional soy-based diet were more likely to have cognitive problems than those who switched to a more Western diet. These preliminary findings suggest that too much anti-estrogen in the wrong place at the wrong time could be harmful.

The Best, Current Recommendations for Protein Intake:

Get a good mix of proteins. Almost any reasonable diet will give you enough protein each day. Eating a variety of foods will ensure that you get all of the amino acids you need.
Pay attention to the protein package. You rarely eat pure protein. Some comes packaged with lots of unhealthy fat, such as when you eat marbled beef or drink whole milk. If you eat meat, choose the leanest cuts. If you like dairy products, skim or low-fat versions are healthier choices. Beans, soy, nuts, and whole grains offer protein without much saturated fat and with plenty of healthful fiber and micro-nutrients.

Balance carbohydrates and protein. Cutting back on highly processed carbohydrates and increasing protein improves levels of blood triglycerides and HDL, and so may reduce your chances of having a heart attack, stroke, or other form of cardiovascular disease. It may also make you feel full longer, and stave off hunger pangs. Too much protein, though, could weaken bones.
Eat soy in moderation. Soybeans, tofu, and other soy-based foods are an excellent alternative to red meat. But don’t go overboard. Two to four servings a week are a good target. And stay away from supplements that contain concentrated soy protein or soy extracts, such as isoflavones. Larger amounts of soy may soothe hot flashes and other menopause-associated problems, but the evidence for this is weak.

Various Dietary Sources and Amounts of Protein

Proteins are complex organic compounds. The basic structure of protein is a chain of amino acids that contain carbon, hydrogen, oxygen, and nitrogen. The presence of nitrogen differentiates protein from carbohydrate and fat.

Proteins are described as essential and nonessential proteins or amino acids. The human body requires approximately 20 amino acids for the synthesis of its proteins. The body can make only 13 of the amino acids – these are known as the nonessential amino acids. They are called non-essential because the body can make them and does not need to get them from the diet. There are 9 essential amino acids that are obtained only from food, and not made in the body.

If the protein in a food supplies enough of the essential amino acids, it is called a complete protein. If the protein of a food does not supply all the essential amino acids, it is called an incomplete protein. Most of us are familiar with the general foods that provide protein in our diet, but here’s a brief review.

All meat and other animal products are sources of complete proteins. These include beef, lamb, pork, poultry, fish, shellfish, eggs, milk, and milk products.

Protein in foods (such as grains, fruits, and vegetables) are either low, incomplete protein or lack one of the essential amino acids. These food sources are considered incomplete proteins.

Plant proteins can be combined to include all of the essential amino acids and form a complete protein. Examples of combined, complete plant proteins are rice and beans, milk and wheat cereal, and corn and beans.

Protein is the main component of muscles, organs, and glands. Every living cell and all body fluids, except bile and urine, contain protein. The cells of muscles, tendons, and ligaments are maintained with protein. Children and adolescents require protein for growth and development.

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General Intake Recommendations For Protein:
The body does not make more protein than usual when dietary protein is increased, nor do the individual requirements of protein vary much. Extra protein in the diet is broken down in the body and the amino acids from this breakdown are used for energy; the excess is turned into fat. Typically, a nutritionally balanced diet provides adequate protein. Vegetarians are able to get enough protein if they eat the proper combination of plant proteins.

The amount of recommended daily protein depends upon age, medical conditions, and the type of diet one is following. Generally, ½ to ¾ gram of protein per pound of body weight is adequate. Even for athletes, the recommendation will not exceed 1 gram per pound of body weight. Two to three servings of protein-rich food will generally meet the daily needs of most adults. Additional needs can be met by consuming larger portions of recommended foods or with a dietary supplement.

The following are the recommended serving sizes for protein:

• 2 to 3 ounces of cooked lean meat, poultry, and fish (a portion about the size of a deck of playing cards)
• 1/2 cup of cooked dry beans, lentils, or legumes
• 1 egg or 2 tablespoons of peanut butter, which count as 1 ounce of lean meat

Select lean meat, poultry without skin, fish, and dry beans, lentils, and legumes often. These are the protein choices that are the lowest in fat. If you’re not at all familiar with diet or nutrition, please consult the food guide pyramid below (source: USDA).