Protein

The building blocks of human proteins are twenty amino acids that may be consumed from both plant and animal sources. Of these 20 amino acids, 9 are considered to be essential because their carbon skeletons cannot be synthesized by human enzymes. The remaining “nonessential” amino acids can be synthesized endogenously with transfer of amino groups to carbon compounds that are formed as intermediates of glucose (glucogenic amino acids) and lipid (ketogenic amino acids) metabolism.

Protein is the basic structural material of all cells. Biologically active proteins include enzymes, immunoglobulins, hormones, neurotransmitters, nutrient transport and storage compounds, and cell membrane receptors. Plasma proteins (e.g., albumin) contribute to oncotic pressure that directs the flow of fluid and metabolic waste from the intracellular compartment into the capillary venules. These proteins (e.g., hemoglobin) also contribute to plasma buffering capacity and oxygen-carbon dioxide transport (e.g., hemoglobin, myoglobin). Acute phase reactant proteins (e.g., ferritin, prealbumin) secreted by the liver bind minerals such as iron and zinc rendering them unavailable to support microbial proliferation.

Biological Value

Biological value of a dietary protein is determined by the amount and proportion of essential amino acids it provides. If any one of the essential amino acids is not available in sufficient amounts or is present in excessive amounts relative to other essential amino acids, protein synthesis will not be supported. Under these circumstances, labile body proteins such as plasma albumin will be catabolized to provide the limiting amino acid so that protein synthesis may continue.

Protein from animal sources (meat, fish, dairy products, egg white) is considered high biological value protein or a "complete" protein because all nine essential amino acids are present in these proteins. An exception to this rule is collagen-derived gelatin which is lacking in tryptophan. Plant sources of protein (grains, legumes, nuts, and seeds) generally do not contain sufficient amounts of one or more of the essential amino acids. Thus protein synthesis can occur only to the extent that the limiting amino acids are available. These proteins are considered to have intermediate biological value or to be partially complete because, although consumed alone they do not meet the requirements for essential amino acids, they can be combined to provide amounts and proportions of essential amino acids equivalent to high biological proteins from animal sources.

Plants that are entirely lacking in essential amino acids are considered incomplete proteins or sources of low biological value protein. These sources include most fruits and vegetables. A low biological value means that it is difficult or impossible to compensate for insufficient amounts of essential amino acids by combining different sources as with partially complete proteins.

Classification of Amino Acids

Essential Amino Acids

1. Histidine
2. Isoleucine
3. Leucine
4. Lysine
5. Methionine
6. Phenylalanine
7. Threonine
8. Tryptophan
9. Valine
10. Serine
11. Tyrosine

Nonessential Amino Acids

1. Alanine
2. Arginine*
3. Aspartic acid
4. Cysteine*
5. Cystine
6. Glutamic acid
7. Glutamine*
8. Glycine
9. Proline

*These amino acids, along with taurine, may be considered conditionally essential in that their requirements are increased during periods of catabolic stress.

Deficiency

If protein needs are not adequately met by dietary sources, an imbalance may develop. This imbalance is reflected by levels of urinary nitrogen which exceed the amounts being consumed from dietary protein. This increase in urinary nitrogen is due to the catabolism of visceral proteins and lean body mass to provide the essential amino acids that are not available in adequate amounts from dietary sources. Negative nitrogen balance may result from consumption of insufficient quantity of high biological protein, consumption of poor quality dietary protein of any quantity, or consumption of intermediate quality protein sources that are not appropriately mixed because the quantities of essential amino acids consumed will not be sufficient to support demand for synthesis of vital proteins. In addition to appropriate quantity and quality of protein consumed, sufficient energy must also be consumed to support protein metabolism or negative nitrogen balance will develop regardless of the quality or quantity of protein consumed.

Protein malnutrition or kwashiokor is the clinical consequence of uncorrected negative nitrogen balance. Protein deficiencies rarely occur when energy intake is adequate except in impoverished areas where adequate quality or quantity of protein is not consumed due to high costs of protein sources. The most common cause of protein deficiency insufficient energy intake, which is exacerbated when demand for both protein and energy is high. Protein-energy malnutrition (PEM) or marasmus may develop clinically from malabsorption syndrome, with excessive protein losses from burns, wound exudates, or fistula drainage, or with losses in urine from renal disease. Risk of PEM is also increased under conditions of metabolic stress, such as infection, trauma, burns, AIDS and surgery, where high levels of catabolic hormones increase protein catabolism. Clinical features of PEM include weight loss, diarrhea, loss of lean body mass, muscle weakness, depigmented hair and skin, pressure sores, and depressed immune function.

Requirements

Approximately 10-15% of total daily energy intake should be consumed as protein. Protein needs for sedentary adults average about 50 grams. Growth, pregnancy, lactation, and exercise increase protein needs as indicated in the table below.

Protein Requirements

Infants (0-6 months) = 1.0 grams per lb
Infants (6-12 months) = 0.72 grams per lb
Children (1-3 years) = 0.55 grams per lb
Children (4-6 years) = 0.50 grams per lb
Children (7-10 years) = 0.45 grams per lb
Adolescence (11-14 years), females = 46 grams per lb, males = 45 grams per lb
Adolescence (15-18 years), females = 44 grams per lb, males = 59 grams per lb
Young adults (19-24 years), females = 46 grams per lb, males = 58 grams per lb
Pregnancy total g/day = 60 grams per lb
Lactation total g/day = 65 grams per lb
Sedentary Adult = 0.4 grams per lb
Recreational Activity = 0.5-0.75 grams per lb
Competitive Athletics = 0.6-0.9 grams per lb
Muscle Building g/lb = 0.7-0.9 grams per lb
Maximum Usable Amount Adults = 1.0 grams per lb

All protein isn't alike

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

Vegetarians need to be aware of this. To get all the amino acids needed to make new protein - and thus 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.

The protein package

Animal protein and vegetable protein probably have the same effects on health. It's the protein package that's likely to make a difference. A 6-ounce broiled Porterhouse steak is a great source of complete protein - 38 grams worth. But it also delivers 44 grams of fat, 16 of them saturated. That's almost three-fourths of the recommended daily intake for saturated fat. The same amount of salmon gives you 34 grams of protein and 18 grams of fat, 4 of them saturated. A cup of cooked lentils has 18 grams of protein, but under 1 gram of fat.

The bottom line is that it's important to pay attention to what comes along 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 allergies. Proteins in food and the environment are responsible for these 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 - the Nurses' Health Study - has investigated the association between dietary protein and heart disease or stroke. In this study, women who ate the most protein (about 110 grams per day) were 25 percent less likely to have had a heart attack or to have died of heart disease than the women who ate the least protein (about 68 grams per day) over a 14-year period. Whether the protein came from animals or vegetables or whether it was part of low-fat or higher-fat diets 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: Although proteins found in cow's milk have been implicated in the development of type 1 diabetes (formerly called juvenile or insulin-dependent diabetes), ongoing research has yielded inconsistent results. 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.

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

Protein and weight control

The notion that you could lose weight by cutting out carbohydrates and eating plenty of protein was once tut-tutted by the medical establishment partly because such diets were based on little more than interesting ideas and speculation. In the past two years, head-to-head trials that pitted high-protein, low-carbohydrate diets against low-fat, high-carbohydrate diets have given them a scientific leg to stand on. These trials show that high-protein, low-carbohydrate diets may work more quickly than low-fat diets, at least in the first six months. After a year or so, though, weight loss is about equal. Compared with a low-fat, high-carbohydrate diet, a higher-protein diet that goes easy on saturated and trans fats may decrease the amount of triglycerides in the blood, which is also good for the heart.

Why do high-protein, low-carb diets seem to work more quickly than low-fat, high-carbohydrate diets? First, chicken, beef, fish, beans, or other high-protein foods slow the movement of food from the stomach to the intestine. Slower stomach emptying means you feel full for longer and get hungrier later. Second, protein's gentle, steady effect on blood sugar avoids the quick, steep rise in blood sugar and just as quick hunger-bell-ringing fall that occurs after eating a rapidly digested carbohydrate, like white bread or baked potato. Third, the body uses more energy to digest protein than it does to digest fat or carbohydrate.

There's no need to go overboard on protein and eat it to the exclusion of everything else. Avoiding fruits, vegetables, and whole grains means missing out on healthful fiber, vitamins, minerals, and other phytonutrients. It's also important to pay attention to what accompanies protein. Choosing high-protein foods that are low in saturated fat will help the heart even as it helps the waistline.

Straight talk about soy

One protein source that has been getting a lot of attention is soybeans. We've been told that regularly eating soy-based foods lowers cholesterol, chills hot flashes, prevents breast and prostate cancer, aids weight loss, and wards off osteoporosis. Some of these benefits have been attributed a unique characteristic of soybeans-their high concentrations of isoflavones, a type of plant-made estrogen (phytoestrogen).

Media reports tout the joys of soy, and food makers are churning out new soy products that are moving 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.

As is so often the case, some of the claims made for soy were based on preliminary evidence, while others go far beyond the available evidence. Back in 1999, the Food and Drug Administration let companies claim that foods containing soy protein "may reduce the risk of heart disease.” The claim was based on early research showing that soy protein lowered levels of LDL (bad) cholesterol. A number of solid studies done since then have tempered this finding, as well as those regarding soy's effects on other conditions.

Heart disease: 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 have meant a 20 percent reduction in the risk of heart attack, stroke, or other forms of cardiovascular disease. An updated look at the soy story, which includes several strong studies published since 2000, isn't so bullish on soy and cholesterol. According to this comprehensive update of soy research by the American Heart Association's nutrition committee, eating 50 grams of soy lowers LDL only about 3%. Keep in mind that 50 grams of soy protein is more than half the average person's daily protein requirement. It's the equivalent of 1_ pounds of tofu or eight 8-ounce glasses of soymilk a day.

All this doesn't mean you need to turn up your nose at tofu, tempeh, soy milk or ignore edamame (a fancy name for soybeans). The AHA committee says that even though soy protein itself has little direct effect on cholesterol, soy foods are good for the heart and blood vessels because they usually replace less healthful choices, like red meat, and because they deliver plenty of polyunsaturated fat, fiber, vitamins, and minerals, and are low in saturated fat.

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 phytoestrogens. In some tissues, these substances mimic the action of estrogen. So they could cool hot flashes by giving a woman an estrogen-like boost during a time of dwindling estrogen levels. Yet carefully controlled studies haven't found this to be the case, and the AHA committee concludes that soy hasn't been shown to ease hot flashes and other symptoms of menopause.

Breast cancer: Phytoestrogens don't always mimic estrogens. In some tissues they actually block the action of estrogen. If such estrogen-blocking action occurs in the breast, then eating soy could, in theory, reduce the risk of breast cancer because estrogen stimulates the growth and multiplication of breast and breast cancer cells. But studies so far haven't provided a clear answer, with some showing a benefit and others showing no association between soy consumption and breast cancer. In fact, a handful of unsettling reports suggest that concentrated supplements of soy proteins may stimulate the growth of breast cancer cells. Large prospective studies now underway should offer better information regarding soy and breast cancer risk.

Other cancers: Although substances in soy could conceivably protect against endometrial, ovarian, colorectal, prostate, and other cancers, there is no good evidence for this.

Memory and thinking ability: A few studies have raised the possibility that eating soy could help prevent the age-related loss of memory or decline in cognitive function. Two recent trials have yielded contradictory results in this area, with one showing a benefit for soy and another showing no benefit. Others go further, and suggest that too much soy could 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.

Finally, there's no evidence that pills containing isoflavones extracted from soybeans offer benefits, and some studies raise concerns about harmful side effects.

The Bottom Line: 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 straight protein. Some comes packaged with lots of unhealthy fat, like when you eat marbled beef or drink whole milk. If you eat meat, steer yourself toward 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 micronutrients.

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 is 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.

By Harvard School of Public Health, Northwestern University