Protein: Moving Closer to Center Stage
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. 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 9 grams of protein for every 20 pounds. 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.
Can you get too much protein? Digesting it 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 to those who ate an average amount of protein (less than 68 grams a day).(1) Although more research is clearly needed to define the optimal amount of daily protein, these results suggest that long-term high-protein diets should be used with caution, if at all.
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.
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.(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. 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.
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.(5-9) 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.(10)
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."(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.
Finally, there's no evidence that pills containing isoflavones extracted
from soybeans offer benefits, and some studies raise concerns about harmful
The Bottom Line: Recommendations for Protein Intake:
Nutritive Value of Foods, USDA
1. Feskanich D, Willett WC, Stampfer MJ, Colditz GA. Protein consumption and bone fractures in women. Am J Epidemiol 1996; 143:472-9.
2. USDA National Nutrient Database for Standard Reference, Release 17. U.S. Department of Agriculture, Agricultural Research Service: 2004.
3. Hu FB, Stampfer MJ, Manson JE, et al. Dietary protein and risk of ischemic heart disease in women. Am J Clin Nutr 1999; 70:221-7.
4. 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.
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7. 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.
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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, Harris 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. Anderson JW, Johnstone 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, MacDonald 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. McMichael-Phillips DF, Harding C, Morton M, et al. Effects of soy-protein supplementation on epithelial proliferation in the histologically normal human breast. Am J Clin Nutr 1998; 68:1431S-1435S.
17. de Lemos ML. Effects of soy phytoestrogens genistein and daidzein on breast cancer growth. Ann Pharmacother 2001; 35:1118-21.
18. 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.
19. 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.
20. White LR, Petrovitch H, Ross GW, et al. Brain aging and midlife tofu consumption. J Am Coll Nutr 2000; 19:242-55.
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