Protein Moving Closer to Center Stage

[Guest guest]

Harvard School of Public Health

Harvard School of Public Health

The Nutrition Source

Protein: Moving Closer to Center Stage

INTRODUCTION

Until recently, protein got little attention. Like a quiet child in a classroom of rowdies, it was often overshadowed by fat, carbohydrates, and vitamins. That's changing. Lately there's been an explosion of interest in protein, largely triggered by high-protein diets for weight loss.

Surprisingly little is known about protein and health. The Institute of Medicine recommends that adults get a minimum of 0.8 grams of protein for every kilogram of body weight per day to keep from slowly breaking down their own tissues (1). That's just about 8 grams of protein for every 20 pounds of body weight. 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.

Around the world, millions of people don't get enough protein. 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, getting the minimum daily requirement of protein is easy. Cereal with milk for breakfast, a peanut butter and jelly sandwich for lunch, and a piece of fish with a side of beans for dinner adds up to about 70 grams of protein, plenty for the average adult.

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 "essential" amino acids—that is, 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.

What Is Protein?

Protein is found throughout the body—in muscle, bone, skin, hair, and virtually every other body part or tissue. It makes up the enzymes that power many chemical reactions and the hemoglobin that carries oxygen in your blood. At least 10,000 different proteins make you what you are and keep you that way.

Twenty or so basic building blocks, called amino acids, provide the raw material for all proteins. Following genetic instructions, the body strings together amino acids. Some genes call for short chains of amino acids, others are blueprints for long chains that fold, origami-like, into intricate, three-dimensional structures.

Because the body doesn't store amino acids, as it does fats or carbohydrates, it needs a daily supply of amino acids to make new protein.

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. (2) 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. (2) A cup of cooked lentils has 18 grams of protein, but under 1 gram of fat. (2)

The bottom line is that it's important to pay attention to what comes along with the protein in your food choices. Vegetable sources of protein, such as beans, nuts, and whole grains, are excellent choices, and they offer healthy fiber, vitamins, and minerals; nuts are also a great source of healthy fat. The best animal protein choices are fish and poultry. If you are partial to red meat, such as beef, pork, or lamb, stick with the leanest cuts, choose moderate portion sizes, and make it only an occasional part of your diet: A major report on cancer prevention recommends consuming less than 18 ounces a week of red meat and avoiding processed meats (such as hot dogs, bacon, or ham) to lower the risk of colon cancer. (3)

Protein and Chronic Disease

The most solid connection between protein and health has to do with allergies. Proteins in food and the environment are responsible for these overreactions of the immune system. Beyond that, relatively little evidence has been gathered regarding the effect of protein on the development of chronic diseases.

Cardiovascular disease: One concern about the high-protein diet craze has been that eating diets high in protein and fat, and low in carbohydrate, would harm the heart. Recent research provides reassurance that eating a lot of protein doesn't harm the heart.

Many people think of nuts as just another junk food snack. In reality, nuts are excellent sources of protein and other healthful nutrients. Learn why nuts are healthy for the heart.

In fact, it is possible that eating more protein, especially vegetable protein, while cutting back on easily digested carbohydrates may benefit the heart. A 20-year prospective study of 82,802 women found that those who ate low-carbohydrate diets that were high in vegetable sources of fat or protein had a 30 percent lower risk of heart disease, compared to women who ate high-carbohydrate, low-fat diets. (4) But women who ate low-carbohydrate diets that were high in animal fats or proteins did not have a reduced risk of heart disease.

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. (5) The amount of protein in the diet doesn't seem to adversely affect the development of type 2 diabetes (formerly called adult-onset diabetes), although research in this area is ongoing. A recent 20-year prospective study in women suggests that eating a low-carbohydrate diet that is high in vegetable sources of fat and protein may modestly reduce the risk of type 2 diabetes.(26)

Cancer: There's no good evidence that eating a little protein or a lot of it influences cancer risk. Eating a lot of red meat is linked to an increased risk of colon cancer, however, as is eating processed meat. (3)

Osteoporosis: Digesting 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 low-carb or no-carb diets, takes 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 a long time, though, could weaken bone. In the Nurses' Health 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 with those who ate an average amount of protein (less than 68 grams a day). (6) But this area of research is still controversial, and findings have not been consistent. Some studies suggest increasing protein increases risk of fractures; others associate high-protein diets with increased bone -mineral density. The evidence is inconclusive, and more research is needed.

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 few years, head-to-head trials that pitted high-protein, low-carbohydrate diets against low-fat, high-carbohydrate diets have provided some evidence that a low-carbohydrate diet may help people lose weight more quickly than a low-fat diet, although so far, that evidence is short term.

In two short, head-to-head trials, low-carb approaches worked better than low-fat diets. (7, 8) A more-recent year-long study, published in 2007 in the Journal of the American Medical Association, showed the same thing. (9) In this study, overweight, premenopausal women went on one of four diets: Atkins, Zone, Ornish, or LEARN, a standard low-fat, moderately high-carbohydrate diet. The women in all four groups steadily lost weight for the first six months, with the most rapid weight loss occurring among the Atkins dieters. After that, most of the women started to regain weight. At the end of a year, it looked as though the women in the Atkins group had lost the most weight since the start of the study, about 10 pounds, compared with a loss of almost 6 pounds for the LEARN group, 5 pounds for the Ornish group, and 3½ pounds for the Zone group. Levels of harmful LDL, protective HDL, and other blood lipids were at least as good among women on the Atkins diet as those on the low-fat diet.

If you read the fine print of the study, though, it turns out that few of the women actually stuck with their assigned diets. Those on the Atkins diet were supposed to limit their carbohydrate intake to 50 grams a day, but they took in almost triple that amount. The Ornish dieters were supposed to limit their fat intake to under 10 percent of their daily calories, but they got about 30 percent from fat. There were similar deviations for the Zone and LEARN groups. What this and other diet comparisons tell us is that sticking with a diet is more important than the diet itself.

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. (10)

No one knows the long-term effects of eating high-protein diets with little or no carbohydrates. Equally worrisome is the inclusion of unhealthy fats in some of these diets. There's no need to go overboard on protein and eat it to the exclusion of everything else. Avoiding fruits 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 plant-based high-protein foods that are low in saturated fat will help the heart even as it helps the waistline. (4)

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 to a unique characteristic of soybeans—their high concentration of isoflavones, a type of plant-made estrogen (phytoestrogen).

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." (11) 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, (12) 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. (13) 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 (AHA's) nutrition committee, eating 50 grams of soy a day lowers LDL only about 3 percent. (12) 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 1½ pounds of tofu or eight 8-ounce glasses of soy milk a day.

All this doesn't mean you need to turn up your nose at tofu, tempeh, or 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, (14, 15) and the AHA committee concludes that soy hasn't been shown to ease hot flashes and other symptoms of menopause. (12)

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. (12, 16-18) In fact, a handful of unsettling reports suggests that concentrated supplements of soy proteins may stimulate the growth of breast cancer cells. (19, 20) 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. Recent trials have yielded contradictory results in this area, with one showing a benefit for soy, (21) and others showing no benefit. (22-24) Other studies 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. (25) This finding, which needs confirmation, could result from excessive intake of phytoestrogens or inadequate intake of something found in animal products, such as vitamin B-12.

Finally, there's no evidence that pills containing isoflavones extracted from soybeans offer benefits, and some studies raise concerns about harmful side effects. (12)

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 protein comes packaged with healthful fiber and micronutrients, such as beans, nuts, and whole grains. Some protein comes packaged with lots of unhealthy fat, like when you eat marbled beef or drink whole milk. Fish and poultry are the best choices for meat eaters; if you are partial to red meat, such as beef, pork or lamb, steer yourself toward the leanest cuts, and make it only an occasional part of your diet. If you like dairy products, skim or low-fat versions are healthier choices. Balance carbohydrates and protein. Cutting back on highly processed carbohydrates and increasing protein intake 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. 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.

References

1. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, and amino acids (macronutrients). National Academy Press.

2. USDA Nutrient Database for Standard Reference, Release 14. US Department of Agriculture.

3. World Cancer Research Fund, American Institute for Cancer Research. Food, Nutrition, Physical Activity, and the Prevention of Cancer: a Global Perspective. Washington DC: AICR, 2007.

4. Halton TL, Willett WC, Liu S, et al. Low-carbohydrate-diet score and the risk of coronary heart disease in women. N Engl J Med. 2006; 355:1991–2002.

5. Akerblom HK, Vaarala O, Hyoty H, Ilonen J, Knip M. Environmental factors in the etiology of type 1 diabetes. Am J Med Genet. 2002; 115:18–29.

6. Feskanich D, Willett WC, Stampfer MJ, Colditz GA. Protein consumption and bone fractures in women. Am J Epidemiol. 1996; 143:472–9.

7. GD, Wyatt HR, Hill JO, et al. A randomized trial of a low–carbohydrate diet for obesity. N Engl J Med. 2003; 348:2082–90.

8. Samaha FF, Iqbal N, Seshadri P, et al. A low-carbohydrate as compared with a low–fat diet in severe obesity. N Engl J Med. 2003; 348:2074–81.

9. Gardner CD, Kiazand A, Alhassan S, et al. Comparison of the Atkins, Zone, Ornish, and LEARN diets for change in weight and related risk factors among overweight premenopausal women: the A TO Z Weight Loss Study: a randomized trial. JAMA. 2007; 297:969–77.

10. Halton TL, Hu FB. The effects of high protein diets on thermogenesis, satiety and weight loss: a critical review. J Am Coll Nutr. 2004; 23:373–85.

11. Health claims: Soy protein and risk of coronary heart disease. Code of Federal Regulations 21CFR101.82 (2001).

12. Sacks FM, Lichtenstein A, Van Horn L, W, Kris–Etherton P, Winston M. Soy protein, isoflavones, and cardiovascular health. An American Heart Association science advisory for professionals from the nutrition committee. Circulation. 2006.

13. JW, stone BM, Cook–Newell ME. Meta–analysis of the effects of soy protein intake on serum lipids. N Engl J Med.1995; 333:276–82.

14. Krebs EE, Ensrud KE, Mac R, Wilt TJ. Phytoestrogens for treatment of menopausal symptoms: a systematic review. Obstet Gynecol. 2004; 104:824–36.

15. Kronenberg F, Fugh-Berman A. Complementary and alternative medicine for menopausal symptoms: a review of randomized, controlled trials. Ann Intern Med. 2002; 137:805–13.

16. Trock BJ, Hilakivi-e L, e R. Meta-analysis of soy intake and breast cancer risk. J Natl Cancer Inst. 2006; 98:459–71.

17. Michels KB, Mohllajee AP, Roset–Bahmanyar E, Beehler GP, Moysich KB. Diet and breast cancer: a review of the prospective observational studies. Cancer. 2007; 109:2712–49.

18. Linos E, Willett WC. Diet and breast cancer risk reduction. J Natl Compr Canc Netw. 2007; 5:711–718.

19. de Lemos ML. Effects of soy phytoestrogens genistein and daidzein on breast cancer growth. Ann Pharmacother 2001; 35:1118–21.

20. Allred CD, Allred KF, Ju YH, Virant SM, Helferich WG. Soy diets containing varying amounts of genistein stimulate growth of estrogen–dependent (MCF-7) tumors in a dose–dependent manner. Cancer Res. 2001; 61:5045–50.

21. Kritz–Silverstein D, Von Muhlen D, Barrett-Connor E, Bressel MA. Isoflavones and cognitive function in older women: the SOy and Postmenopausal Health In Aging (SOPHIA) Study. Menopause. 2003; 10:196–202.

22. Kreijkamp-Kaspers S, Kok L, Grobbee DE, et al. Effect of soy protein containing isoflavones on cognitive function, bone mineral density, and plasma lipids in postmenopausal women: a randomized controlled trial. JAMA. 2004; 292:65–74.

23. Fournier LR, Borchers TA, Robison LM, et al. The effects of soy milk and isoflavone supplements on cognitive performance in healthy, postmenopausal women. J Nutr Health Aging. 2007; 11:155–64.

24. Ho SC, Chan AS, Ho YP, et al. Effects of soy isoflavone supplementation on cognitive function in Chinese postmenopausal women: a double-blind, randomized, controlled trial. Menopause. 2007; 14:489–99.

25. White LR, Petrovitch H, Ross GW, et al. Brain aging and midlife tofu consumption. J Am Coll Nutr. 2000; 19:242–55.

26. Halton TL, Liu S, Manson JE, Hu FB. Low-carbohydrate-diet score and risk of type 2 diabetes in women. Am J Clin Nutr. 2008;87:339-46.

Terms of Use

The aim of the Harvard School of Public Health Nutrition Source is to provide timely information on diet and nutrition for clinicians, allied health professionals, and the public. The contents of this Web site are not intended to offer personal medical advice. You should seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. Never disregard professional medical advice or delay in seeking it because of something you have read on this Web site. The information does not mention brand names, nor does it endorse any particular products.

Harvard School of Public Health, 677 Huntington Avenue, Boston, MA 02115

Contact Us | Harvard University Home Page | Reporting Copyright Infringements | Privacy | Site Index

, President and Fellows of Harvard College