Low-carbohydrate diets and men's cortisol and testosterone: Systematic review and meta-analysis

Abstract

Background: Low-carbohydrate diets may have endocrine effects, although individual studies are conflicting. Therefore, a review was conducted on the effects of low- versus high-carbohydrate diets on men's testosterone and cortisol. Methods: The review was registered on PROSPERO (CRD42021255957). The inclusion criteria were: intervention study, healthy adult males, and low-carbohydrate diet: ≤35% carbohydrate. Eight databases were searched from conception to May 2021. Cochrane's risk of bias tool was used for quality assessment. Random-effects, meta-analyses using standardized mean differences and 95% confidence intervals, were performed with Review Manager. Subgroup analyses were conducted for diet duration, protein intake, and exercise duration. Results: Twenty-seven studies were included, with a total of 309 participants. Short-term (<3 weeks), low- versus high-carbohydrate diets moderately increased resting cortisol (0.41 [0.16, 0.66], p < 0.01). Whereas, long-term (≥3 weeks), low-carbohydrate diets had no consistent effect on resting cortisol. Low- versus high-carbohydrate diets resulted in much higher post-exercise cortisol, after long-duration exercise (≥20 min): 0 h (0.78 [0.47, 1.1], p < 0.01), 1 h (0.81 [0.31, 1.31], p < 0.01), and 2 h (0.82 [0.33, 1.3], p < 0.01). Moderate-protein (<35%), low-carbohydrate diets had no consistent effect on resting total testosterone, however high-protein (≥35%), low-carbohydrate diets greatly decreased resting (−1.08 [−1.67, −0.48], p < 0.01) and post-exercise total testosterone (−1.01 [−2, −0.01] p = 0.05). Conclusions: Resting and post-exercise cortisol increase during the first 3 weeks of a low-carbohydrate diet. Afterwards, resting cortisol appears to return to baseline, whilst post-exercise cortisol remains elevated. High-protein diets cause a large decrease in resting total testosterone (∼5.23 nmol/L).

High-protein diets and testosterone

“… Whittaker and Harris’ (2022) meta-analysis found that whilst low-carbohydrate diets > 3.4 g/kg/day protein decreased TT, low-carbohydrate diets < 3.4 g/kg/day protein had no consistent effect on TT. A re-examination of the relevant studies within the meta-analysis, and the wider literature, indicates that diets > 3.4 g/kg/day protein decrease TT, but diets < 3.4 g/kg/day do not; and that this effect is driven by protein intake, rather than carbohydrate or fat.…”

“…The meta-analysis in question found that diets > 3.4 g/kg/day protein decreased TT, a finding that was based on three studies, which are listed at the top of Table 2 ( Whittaker and Harris, 2022 ). Protein intakes > 3.4 g/kg/day are above the general population's, commonly used diets, and the vast majority of athletes ( Table 1 ).…”

“…High-protein diets have a number of benefits including increased satiety, weight loss and greater preservation of lean mass during caloric deficits ( Leidy et al, 2015 ). However, a recently published meta-analysis found that low-carbohydrate, high-protein diets (> 3.4 g/kg of bodyweight/day) (g/kg/day) caused a large decrease in men's total testosterone (TT) (∼5.23 nmol/L) ( Whittaker and Harris, 2022 ). This finding has ignited considerable discussion online and in the media ( Altmetric, 2022 ); although the accuracy of such coverage has varied greatly, arguably leading to more confusion than clarity.…”

Global Scientific Trends on Healthy Eating from 2002 to 2021: A Bibliometric and Visualized Analysis

“…Although the recommended approaches such as low-carbohydrate high-protein diet, ketogenic diet, and intermittent fasting were effective for reducing weight to get started [ 64 , 65 , 66 ], for the long term, the most effective method is still not identified because different dieting models have promoted similar weight loss [ 67 ]. Additionally, all extreme diets against obesity are hard to sustain and caution is required due to safety issues [ 68 , 69 ]. Meanwhile, overweight/obesity among college students [ 70 ] is usually overestimated as a result of following the fashion of thinness, slim food recipes, or regimens that are frequently promoted on social media [ 71 , 72 ].…”

Obesity, type 2 diabetes, and testosterone in ageing men

“…response to high protein low carbohydrate diets [75]. However, there is marked heterogeneity among high protein diets with variation in the types of protein, carbohydrate, and fat, as well as the presence of fibre.…”