High-protein, low-carbohydrate diets can significantly decrease testosterone levels according to a meta-analysis published in the journal Nutrition and Health.

In an analysis that included data from 27 studies, researchers found that moderate-protein (<35%), low-carbohydrate diets had no consistent effect on resting total testosterone. However, high-protein (≥35%), low-carbohydrate diets greatly decreased resting testosterone (standardised mean difference = −1.08, p<0.01; ∼5.23 nmol/L). According to the authors this represents a 37% decrease in normal testosterone levels.

Under normal circumstances the body regulates protein intake through appetite. However, more extreme diets and also bodybuilding can result in unbalanced intakes that involve low carbohydrates and high protein.

Lead researcher Joseph Whittaker says, ‘Most people eat about 17% protein, and the high-protein diets which caused low testosterone were all above 35%, which is very high. 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 researchers also looked at cortisol levels and the various diets and found no consistent correlations.