Although proteins act in virtually all life-sustaining processes in your body, too much of a good thing can be bad. As U.S. obesity rates skyrocket, many have turned to high-protein diets to lose weight. Athletes and bodybuilders also consume high-protein diets as a way of life for their performance needs. However, excessive protein intake has been linked to many health problems, from relatively benign and reversible conditions such as dehydration, constipation and nutritional deficiencies to obesity, heart and kidney diseases, insulin resistance and diabetes, prostate cancer, decreased thyroid function, metabolic acidosis and reduced immune function.
One of the major problems with high-protein diets is that they emphasize eating more meat, to the exclusion of other nutrients you need. Because of the saturated fat you consume eating large amounts of meat, you risk raising your blood cholesterol level, which sets you on a path toward heart disease. In addition, consuming too few carbohydrates, especially fiber, deprives you of a natural cholesterol-lowering nutrient. For this reason, the American Heart Association says it can't endorse high-protein diets.
Progression of Kidney Disease
High levels of protein in your diet can tax your kidneys and cause an acceleration of a decline in kidney function. High protein is associated with elevated uric acid levels, which is a waste product created as your body breaks down protein. High uric acid levels are often precursors to high blood pressure and chronic kidney disease. As part of the Nurses' Health Study, Harvard University researchers examined kidney health of 1,624 women over 11 years. In the March 18, 2003, "Annals of Internal Medicine," they reported that every 10 g increase in protein intake was strongly linked with an accelerated decline in kidney function --- but only in women who already had mild impairment. This would include women with diabetes.
A side effect of high-protein diets' action in the kidneys is that they lead to an increased loss of calcium through urine. Continuous loss of calcium due to protein consumption can increase your risk of osteoporosis. This happens because the excess protein causes you to shed water weight. While excreting fluids, you also lose nutrients. The Feminist Women's Health Center reported that for every extra 1 g of animal protein you eat, you can lose an average of 1.75 mg of calcium in urine. This passing of calcium through your kidneys can also cause kidney stones.
To say that high-protein diets cause cancer would be misleading, but scientists have linked the two in many ways. According to research published in the December 2006 "American Journal of Clinical Nutrition," high-protein diets are related to higher amounts of a substance called insulin-like growth factor, or IGF-1. Examining three groups of people eating varying amounts of protein, researchers found that those who ate greater than the recommend 0.4 g of protein per pound of body weight had more of this substance, which is linked to premenopausal breast cancer, prostate cancer, certain types of colon cancer and a shorter lifespan. The American Cancer Society says protein consumption is so wrapped up with saturated fat and cholesterol that it's difficult to disentangle and say which increases your risk of cancer. The society says meat, especially beef, pork and lamb, have been associated with colon and prostate cancer. In addition to the fat, dangerous compounds are created when meat is cooked at high temperatures. Fats alone also have been associated with cancers of the colon, rectum, prostate and endometrium, or uterus.
How Much Is Too Much?
The Dietary Guidelines for Americans says most healthy adults should get between 10 and 35 percent of their daily calories from protein sources. That works out to about 46 g for most women and 56 g for most men. The Institute of Medicine has not established what it calls a "tolerable upper limit" for protein. That's a level beyond which you could expect health problems from consuming too much of a nutrient. Given the concerns associated with too much protein, caution is advised, along with consultation with your health care provider. Here's another reason. The American Council on Exercise, or ACE, said your body can't store extra protein. When you consume more than you need, it doesn't go toward helping you build muscle. Instead, your body processes it so it can be stored as fat. Another helpful tip is to get your protein from a variety of sources. It doesn't all have to be meat and full-fat dairy. The American Cancer Society favors beans because of the vitamins, minerals and fiber that come along with the protein.
- "Journal of Nutrition"; "Metabolic Consequences of a High Dietary-Protein Intake in Adulthood: Assessment of the Available Evidence"; Cornelia Metges and Christian Barth; April 2000
- American Heart Association: High-Protein Diets
- "Annals of Internal Medicine"; "The Impact of Protein Intake on Renal Function Decline in Women with Normal Renal Function or Mild Renal Insufficiency"; Eric Knight et al; March 18, 2003
- "CA: A Cancer Journal for Clinicians"; "American Cancer Society Guidelines on Nutrition and Physical Activity for Cancer Prevention"; March/April 2002
- Feminist Women's Health Center; "High Protein Diets -- Are You Losing More Than Weight?"; Monique Gilbert
- "American Journal of Clinical Nutrition"; "Long-term low-protein, low-calorie diet and endurance exercise modulate metabolic factors associated with cancer risk"; Luigi Fontana et al; December 2006