Too Much Protein

Adverse Effects Associated with Protein Intake above the Recommended Dietary Allowance for Adults

Background: While high-protein consumption—above the current recommended dietary allowance for adults (RDA: 45% of bodyweight in pounds or 0.8 g protein/kg body weight/day)—is increasing in popularity, there is a lack of data on its potential adverse effects.

Objective: To determine the potential disease risks due to high protein/high meat intake obtained from diet and/or nutritional supplements in humans.

Design: Review.

Subjects: Healthy adult male and female subjects.

Method: In order to identify relevant studies, the electronic databases, Medline and Google Scholar, were searched using the terms:“high protein diet,” “protein overconsumption,” “protein overuse,” and “high meat diet.” Papers not in English were excluded. Further studies were identified by citations in retrieved papers.

Results: 32 studies (21 experimental human studies and 11 reviews) were identified. The adverse effects associated with long-term high protein/high meat intake in humans were:

(a) disorders of bone and calcium homeostasis,
(b) disorders of renal function,
(c) increased cancer risk,
(d) disorders of liver function, and
(e) precipitated progression of coronary artery disease.

Conclusions: The findings of the present study suggest that there is currently no reasonable scientific basis in the literature to recommend protein consumption above the current RDA (high protein diet) for healthy adults due to its potential disease risks. Further research needs to be carried out in this area, including large randomized controlled trials.

Protein is an essential macronutrient needed by the human body for growth and maintenance. Foods rich in animal protein are meat, fish, eggs, poultry, and dairy products, while plant foods high in protein are mainly legumes, nuts, and grains.

High protein diets are promoted intensively by the nutritional supplements industry and they are considered to be “the gold standard” by many athletes for muscle development and/or body fat loss. On the other hand, several scientists claim that the overuse of protein supplements or high dietary protein intake could cause disorders to human health.

Up to 80% of breast, bowel, and prostate cancers are attributed to dietary practices, and international comparisons show positive associations with high meat diet. It should be noticed that red meat is the main dietary source of saturated fat, which has been associated with breast and colorectal cancers.

Conclusions: Despite the fact that short-term high protein diet could be necessary in several pathological conditions (malnutrition, sarcopenia, etc.), it is evident that “too much of a good thing” in diet could be useless or even harmful for healthy individuals. Many adults or even adolescents (especially athletes or body builders) self-prescribe protein supplements and overlook the risks of using them, mainly due to misguided beliefs in their performance-enhancing abilities. Individuals who follow these diets are therefore at risk. Extra protein is not used efficiently by the body and may impose a metabolic burden on the bones, kidneys, and liver.

Moreover, high-protein/high-meat diets may also be associated with increased risk for coronary heart disease due to intakes of saturated fat and cholesterol or even cancer. Guidelines for diet should adhere closely to what has been clinically proved, and by this standard there is currently no basis to recommend high protein/high meat intake above the recommended dietary allowance for healthy adults. Further investigation with large randomized controlled studies could provide more definitive evidence.