New research suggests that high-protein, low-carbohydrate diets can contribute to lasting health effects in men, primarily in their testosterone levels.
A study published last week in the medical journal Nutrition and Health analyzed the results of 27 separate trials to see how low- versus high-carbohydrate diets impacted men’s health in the long term. Researchers from the University of Worcester examined a total of 309 patients, all healthy males around 27 years old, who had varying degrees of protein in their diets.
The study examined the two diets over a short-term (three week) span, as well as in the long-term, or beyond three weeks, and their impact on the subjects’ resting cortisol and testosterone levels.
Cortisol is a steroid hormone produced in the adrenal glands that helps regulate the body’s response to stress. Testosterone is a major sex hormone responsible for the development of male sexual organs, sperm production and overall muscle strength.
The University of Worcester in the United Kingdom quantified a “moderate-carbohydrate” diet as one where carbohydrates make up less than 35% of an individual's total energy intake, and a “high-carbohydrate” diet as one where carbs make up over 35% of the total energy intake. The researchers also required a 20% difference between the overall carbohydrate makeup between the two diets. Moderate and low-protein diets were stratified in the same way.
Proteins, made up of amino acids and critical in building and maintaining muscle, can be found in foods like meats, beans, legumes, dairy products and nuts; carbohydrates, essentially sugar molecules that the body converts into energy, come primarily from grains, fruits, nuts and some dairy products.
Cortisol levels were less impacted by high-protein, low-carbohydrate diets, with only a slight increase in both resting and post-exercise cortisol amounts in the short run. However, as cortisol levels returned to baseline after around three weeks on the diet, researchers posit the condition “is likely part of the adaption process to such diets, and thus may not represent a pathological state.”
Still, further research is necessary to confirm the findings on cortisol levels.
As for testosterone, researchers found that participants who consumed a moderate diet of protein with a low amount of carbohydrates had no long-term changes in their testosterone, but high-protein and low-carbohydrate diets “greatly decreased resting and post-exercise total testosterone,” which suggests “individuals consuming such diets may need to be cautious about adverse endocrine effects.”
The average decrease in testosterone levels for those on high-protein, low-carbohydrate diets was around 37%, researchers said.
“Most people eat about 17% protein, and the high protein diets which caused low testosterone were all above 35%, which is very high,” lead researcher Joseph Whittaker told the Markets Herald. “So for the average person, there is nothing to worry about, however for people on high protein diets, they should limit protein to no more than 25%.”
The study itself is subject to a number of limitations, not least of which being the limited cohort included in the research. While the homogenous trial is representative of the average male in their late 20s, the research may not be applicable to other age groups or demographics.
A second limitation was the differences in exercise regimes and type of carbohydrate intake for those on low-carbohydrate diets, which may have contributed to long term changes in testosterone levels.
“However, due to the low number of studies and risk of data dredging, these factors were not explored as possible sources of heterogeneity,” researchers wrote in part. “This issue brings up the wider debate about ‘lumping versus splitting’ in meta-analyses.”