The influence of short-term, severe low energy availability with varying protein content on substrate metabolism, exercise performance and perceptual responses in young adults.

Low energy availability (LEA) may impact multiple biological processes, but the extent to which it affects exercise performance, and whether macronutrient composition modulates this, remains unclear. This randomized cross-over trial investigated the influence of three dietary conditions with varying energy and protein availabilities on substrate metabolism, high-intensity exercise performance and subjective responses in healthy active females (9) and males (10). Each condition lasted five days. On day one, resting metabolic rate, substrate metabolism during a 45-minute fixed-load cycling test at 65% of the peak power output achieved during an aerobic capacity test, and Wingate performance were assessed. Participants were then provided with all foods to be consumed over the next 4 days, with the following caloric and protein content: Adequate energy availability (AEA: 45 kcal·kgFFM-1·day-1; 1.5 g·kg-1 protein); low energy availability (LEA: 15 kcal·kgFFM-1·day-1, with all macronutrients reduced proportionally) and another LEA condition but with protein matched to AEA (LEA-P). They returned to the lab on the 5th day to repeat all experimental tests. Participants also completed brief, semi-structured interviews to explore their subjective responses to each condition. Results indicated that both the LEA and LEA-P diets induced a shift toward increased fat oxidation at rest, but not during exercise, compared to AEA. In contrast, Wingate performance declined only in the LEA-P trial compared to AEA, potentially due to its reduced carbohydrate content. Participants generally reported negative experiences during both calorie-restricted trials, including hunger, fatigue, weakness and frustration, with symptoms appearing more pronounced in males and during the LEA trial.