“People think carbs are the enemy, protein is your friend,” said Eleanor Dwyer, a research analyst with the firm, and “that any health concerns are overblown.”
Experts note, however, that there is only so much protein the body can use. “The body only digests and absorbs a certain amount of protein at every meal,” about 20 to 40 grams, said Jim White, a registered dietitian and exercise physiologist who spoke on behalf of the Academy of Nutrition and Dietetics. “People think that if they fill up with protein, it will be a magic bullet, whether for weight loss or to get in better shape and build muscle — but that’s not proving to be true.”
“You can eat 300 grams of protein a day, but that doesn’t mean you’ll put on more muscle than someone who takes in 120 grams a day,” Mr. White said. Meanwhile, “you’re robbing yourself of other macronutrients that the body needs, like whole grains, fats, and fruits and vegetables.”
Short-term studies suggest that high protein, low carbohydrate diets may promote weight loss and help to preserve lean muscle, and that eating protein helps satisfy hunger. But a recent small trial found that older women who lost weight on a high protein diet did not experience one of the important benefits that usually follow weight loss, an improvement in insulin sensitivity, which reduces the risk of developing Type 2 diabetes. Large population studies also suggest an association between habitual high protein intake and a heightened risk of diabetes.
Doctors also have concerns about the long-term effects of maintaining a high protein diet. Studies show that protein-rich diets do not preserve muscle mass over the long term, and doctors have long cautioned that a high-protein diet can lead to kidney damage in those who harbor silent kidney disease by putting extra strain on the kidneys. Up to one in three Americans are at risk for kidney impairment because of high blood pressure or diabetes, according to the National Kidney Foundation.
Furthermore, some researchers worry that the muscle building properties that consumers seek in protein may be a double-edged sword, perhaps even leading to an increased risk of cancer.
“One of the benefits and concerns about high protein intake, especially animal protein, is that it tends to make cells multiply faster,” said Dr. Walter Willett, chairman of the department of nutrition at the Harvard T. H. Chan School of Public Health. “That’s good in early life, when you’re a growing child. But in later life, this is one of the fundamental processes that increase the risk of cancer.”