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High-Animal-Protein Diets in Middle Age May Shorten Life Span

A study following 6,800 people for 18 years shows that 50 to 65-year-olds who eat a high animal-protein diet (more than 20 percent of calories from protein, with two-thirds or more of the protein from meat, diary or eggs) have twice the risk of dying and four times the risk of dying from cancer or diabetes, compared to those who eat less than 20 percent protein (Cell Metabolism, March 4, 2014).

In this study, a low-protein diet for all ages was associated with reduced risk for cancer. There is a difference between protein from animals and protein from plants. The 50 to 65-year-olds who ate a diet high in protein mainly from plants had reduced risk for cancer. The authors then fed a low-protein diet for two months to mice at high risk for cancers and found smaller tumor growth and prolonged life spans.

High-Protein Diets May Harm
According to this study, diets such as Atkins and the various "paleo" diets could harm you because they recommend consuming large amounts of animal protein. The authors told the Manchester Guardian newspaper, "People need to switch to a diet in which only nine or ten percent of their calories come from protein, and the ideal sources are plant-based. We are not saying go and do some crazy diet we came up with. If we are wrong, there is no harm done, but if we are right you are looking at an incredible effect that in general is about as bad as smoking."

The people in the study took in an average of 1,820 calories a day. The high-protein group had more than 20 percent of calories from protein, the moderate-protein took in 10 to 19 percent, and the low group received less than 10 percent of calories from protein.

The same researchers then performed tests on mice and found that a high-protein diet led to more cancer and larger tumors than a low-protein diet. Blood tests in the people in the study showed that a high intake of protein caused the liver to increase production of Insulin-Like Growth Factor (IGF-1) and a low-protein diet reduced liver production of IGF-1. High levels of IGF-1 are associated with increased risk for cancer and diabetes, and with accelerated aging (Eur J Endocrinol February 1, 2011;164:223-229; Hematol. Oncol. Clin. North Am, June, 2012;26(3):527–42).

Possible Benefit from High Protein Diet after Age 65?
The study mentioned above found that a high-protein diet for people over 65 was associated with a longer life span. However, it is not very likely that eating protein helped them live longer. It is more likely that older people who are healthy eat more food and more protein and that being healthy results in a longer life.

Protein and Muscle Strength in People over 65
With aging everyone loses muscles, even those who continue to exercise. A recent study followed 1,007 men, average age 67.4, for seven years. Those with a high protein intake maintained muscle strength better than those with low protein intake (J Am Geriatr Soc, Epub Feb, 2014). Another study found that men and women over 65 who ate more protein had larger muscles (J Nutr Health Aging, 2014;18(2):171-7). The 2,726 men and women were followed for four years. Those in the high protein group had less loss of muscle size assessed by dual-energy X-ray absorptiometry. However, the high protein group did not have better strength or speed; they had the same six-meter walking speed and step length as the low-protein group.

It is likely that a high-protein diet does not help older people live longer or to have larger and stronger muscles. It is more likely that the people who take in more protein are healthier to begin with and therefore eat more food which contains more protein. Illness causes loss of muscle size and strength, and people in poor health eat less food and thus take in less protein.

What Does This Mean for You?
I recommend that you reduce your intake of animal protein (meat, dairy and eggs) and increase plant sources of protein in your diet, including beans, whole grains, nuts and other seeds. We do not know whether protein helps older people to live longer and have larger and stronger muscles.

Checked 2/1/17

March 9th, 2014
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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