High-protein diets for weight loss, disease prevention, and enhanced athletic performance have been greatly publicized over recent years. However, these diets are supported by little scientific research. Studies show that the healthiest diet is one that is high in carbohydrate, low in fat, and adequate in protein. Increased intake of whole grains, fruits, and vegetables is recommended for weight control and preventing diseases such as cancer (3) and heart disease (4). High-carbohydrate, low-fat, moderate- protein diets are also recommended for optimal athletic performance. (5) Contrary to the information on fad diets currently promoted by some popular books,
A diet that is high in protein can actually contribute to disease and other health problems.
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High protein intake is known to encourage urinary calcium losses. Plant-based diets, which provide adequate protein, can help protect against osteoporosis. Calcium-rich plant foods include leafy green vegetables, beans, and some nuts and seeds, as well as fortified fruit juices, cereals, and nondairy milks.
Although fat is the dietary substance most often singled out for increasing one’s risk for cancer, animal protein also plays a role. Specifically, certain proteins present in meat, fish, and poultry, cooked at high temperatures, especially grilling and frying, have been found to produce compounds called heterocyclic amines. These substances have been linked to various cancers including those of the colon and breast.(6-8)
Long-term high intake of meat, particularly red meat, is associated with significantly increased risk of colorectal cancer. The 2007 report of the World Cancer Research Fund and American Institute for Cancer Research, Food, Nutrition, and the Prevention of Cancer reported that, based on available evidence, diets high in red meat were considered probable contributors to colorectal cancer risk. In addition, high-protein diets are typically low in dietary fiber. Fiber appears to be protective against cancer.(3) A diet rich in whole grains, fruits, and vegetables is important in decreasing cancer risk,(3) not to mention adding more healthful sources of protein in the diet.
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Impaired Kidney Function.
When people eat too much protein, it releases nitrogen into the blood or is digested and metabolized. This places a strain on the kidneys, which must expel the waste through the urine. High-protein diets are associated with reduced kidney function. Over time, individuals who consume very large amounts of protein, particularly animal protein, risk permanent loss of kidney function. Harvard researchers reported that high-protein diets were associated with a significant decline in kidney function, based on observations in 1,624 women participating in the Nurses’ Health Study. The good news is that the damage was found only in those who already had reduced kidney function at the study’s outset. The bad news is that as many as one in four adults in the United States may already have reduced kidney function, suggesting that most people who have renal problems are unaware of that fact and do not realize that high-protein diets may put them at risk for further deterioration. The kidney-damaging effect was seen only with animal protein. Plant protein had no harmful effect.(9)
The American Academy of Family Physicians notes that high animal-protein intake is in part responsible for the high prevalence of kidney stones in the United States and other developed countries and recommends protein restriction for the prevention of recurrent kidney stones.(10)
Typical high-protein diets are extremely high in dietary cholesterol and saturated fat.
The effect of such diets on blood cholesterol levels is a matter of ongoing research. However, such diets pose additional risks to the heart, including increased risk for heart problems immediately following a meal. Evidence indicates that meals high in saturated fat adversely affect the compliance of arteries, increasing the risk of heart attacks.11Adequate protein can be consumed through a variety of plant products that are cholesterol-free and contain only small amounts of fat.
Many individuals see almost immediate weight loss as a result of following a high-protein diet. In fact, the weight loss is not a result of consuming more protein, but by simply consuming fewer calories. Over the long run, consumption of this type of diet is not practical as it can result in the aforementioned health problems. As with any temporary diet, weight gain is often seen when previous eating habits are resumed. To achieve permanent weight loss while promoting optimal health, the best strategy involves lifestyle changes including a low-fat diet of grains, legumes, fruits, and vegetables combined with regular physical activity.
High-protein diets are unhealthful. However, adequate but not excess amounts of protein to maintain body tissues, including muscle, are still important and can be easily achieved on a plant-based diet. If you are uncertain about the adequacy of protein in your diet, take inventory. Although all protein needs are individual, the following guidelines can help you to meet, but not exceed, your needs.
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- Aim for five or more servings of grains each day. This may include 1⁄2 cup of hot cereal, 1 ounce of dry cereal, or one slice of bread. Each serving contains roughly 3 grams of protein.
- Aim for three or more servings of vegetables each day. This may include 1 cup of raw vegetables, 1⁄2 cup of cooked vegetables, or 1⁄2 cup of vegetable juice. Each serving contains about 2 grams of protein.
- Aim for 2 to 3 servings of legumes each day. This may include 1⁄2 cup of cooked beans, 4 ounces of tofu or tempeh, 8 ounces of soy milk, and 1 ounce of nuts. Protein content can vary significantly, particularly with soy and rice milks, so be sure to check labels. Each serving may contain about 4 grams to 10 grams of protein. Meat analogues and substitutes are also great sources of protein that can be added to your daily diet.
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3. World Cancer Research Fund. Food, Nutrition and the Prevention of Cancer: A Global Perspective. American Institute for Cancer Research. Washington, D.C.: 2007.
4. Ornish D, Brown SE, Scherwitz LW. Can lifestyle changes reverse coronary heart disease? Lancet. 1990;336:129-133.
5. Rodriguez NR, Dimarco NM, Langley S. Position of the American Dietetic Association, Dietitians of Canada, and the American College of Sports Medicine: nutrition and athletic performance. J Am Diet Assoc. 2009;109:509-527.
6. Butler LM, Sinha R, Millikan RC, et al. Heterocyclic amines, meat intake, and association with colon cancer in a population-based study. Am J Epidemiol. 2003;157:434-445.
7. Sinha R. An epidemiologic approach to studying heterocyclic amines. Mutat Res. 2002;506:197.
8. Zheng W, Lee SA. Well-done meat intake, heterocyclic amine exposure, and cancer risk. Nutr and Cancer. 2009;61:437-446.
9. Knight EL, Stampfer MJ, Hankinson SE, Spiegelman D, Curhan GC. The impact of protein intake on renal function decline in women with normal renal function or mild renal insufficiency. Ann Int Med. 2003;138:460-467.
10. Romero V, Akpinar H, Assimos DG. Kidney stones: A global picture of prevalence, incidence, and associated risk factors. Rev Urol. 2010;12:e86–e96.
11. Nestel PJ, Shige H, Pomeroy S, Cehun M, Chin-Dusting J. Post-prandial remnant lipids impair arterial compliance. J Am Coll Cardiol. 2001;37:1929-1935.
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