Abstract
BackgroundThere is evidence that caffeine increases the maximal fat oxidation rate (MFO) and aerobic capacity, which are known to be lower in the morning than in the afternoon. This paper examines the effect of caffeine intake on the diurnal variation of MFO during a graded exercise test in active men.MethodsUsing a triple-blind, placebo-controlled, crossover experimental design, 15 active caffeine-naïve men (age: 32 ± 7 years) completed a graded exercise test four times at seven-day intervals. The subjects ingested 3 mg/kg of caffeine or a placebo at 8 am in the morning and 5 pm in the afternoon (each subject completed tests under all four conditions in a random order). A graded cycling test was performed. MFO and maximum oxygen uptake (VO2max) were measured by indirect calorimetry, and the intensity of exercise that elicited MFO (Fatmax) calculated.ResultsMFO, Fatmax and VO2max were significantly higher in the afternoon than in the morning (all P < 0.05). Compared to the placebo, caffeine increased mean MFO by 10.7% (0.28 ± 0.10 vs. 0.31 ± 0.09 g/min respectively, P < 0.001) in the morning, and by a mean 29.0% (0.31 ± 0.09 vs. 0.40 ± 0.10 g/min, P < 0.001) in the afternoon. Caffeine also increased mean Fatmax by 11.1% (36.9 ± 14.4 [placebo] vs. 41.0 ± 13.1%, P = 0.005) in the morning, and by 13.1% (42.0 ± 11.6 vs. 47.5 ± 10.8%, P = 0.008) in the afternoon.ConclusionThese findings confirm the previously reported diurnal variation in the whole-body fat oxidation rate during graded exercise in active caffeine-naïve men, and indicate that the acute ingestion of 3 mg/kg of caffeine increases MFO, Fatmax and VO2max independent of the time of day.Trial registrationNCT04320446. Registered 25 March 2020 - Retrospectively registered
Highlights
Endurance performance has been traditionally understood as a multifactorial concept in which maximal oxygen uptake (VO2max), ventilatory thresholds and muscular efficiency play important roles [1]
It is known that endurance performance is poorer early in the morning and late at night compared with the afternoon [6], and that maximal fat oxidation rate (MFO) and Fatmax are higher in the afternoon compared to the morning whether in non-athlete male students [7], in untrained normalweight and obese individuals [8], or in endurance-trained athletes [9]
The present results indicate that caffeine intake increases MFO and Fatmax as well as Maximal oxygen uptake (VO2max) independent of the time of day
Summary
Endurance performance has been traditionally understood as a multifactorial concept in which maximal oxygen uptake (VO2max), ventilatory thresholds and muscular efficiency play important roles [1]. Known as the capacity to adapt fuel utilization to substrate availability, has recently been suggested an additional key factor affecting performance in endurance disciplines [2]. Given that maximal fat oxidation during a graded exercise test (MFO), and the intensity of exercise that elicits MFO (Fatmax), have been recognized as potential determinants of metabolic flexibility during exercise [3, 4], it seems plausible that both MFO and Fatmax strongly influence endurance performance. Higher fat oxidation rates (at the expense of lower carbohydrate use) at moderate exercise intensities might help spare endurance athletes’ muscle and liver glycogen stores during training and competition [5]. There is evidence that caffeine increases the maximal fat oxidation rate (MFO) and aerobic capacity, which are known to be lower in the morning than in the afternoon. This paper examines the effect of caffeine intake on the diurnal variation of MFO during a graded exercise test in active men
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