An article has just been published by Whittaker et al called “Low-carbohydrate diets and men's cortisol and testosterone: Systematic review and meta-analysis” (Ref 1). That’s a good title. You know straight away what the paper is about and that it has used best available evidence. The rest of the paper followed suit. It was well laid out and thorough.
The background to the paper noted that individual studies had presented conflicting evidence about low-carb diets and endocrine (hormonal) impacts and thus the researchers wanted to explore this further. Testosterone is the primary male sex hormone; vital for reproduction. Cortisol is the body’s main stress hormone.
The introduction provided a good summary of the interplay between testosterone and cortisol. Testosterone and cortisol are biochemically opposed in many ways. The administration of exogenous cortisol lowers testosterone. Exercise tends to increase cortisol and decrease total testosterone. Low testosterone is associated with an increased risk of chronic disease, including type 2 diabetes (Ref 2) and cardiovascular disease (Ref 3). High cortisol is associated with an increased risk of chronic disease with higher levels being associated with an increased risk of cardiovascular disease mortality (Ref 4).
A systematic review (look at all evidence) and meta-analysis (pool it together to see the overall effect) was undertaken. This review searched for intervention studies involving healthy adult males. A low-carb diet was defined as below or equal to 35% of calories in the form of carbohydrate. That’s not as low as the Noakes Windt cut-off of 26% (Ref 5), but it’s better than the usual 45% definition (and a novel inclusion criterion ensured a clear differentiation between carb intakes – see below).