Researchers point an improvement in fatty acid mobilisation and blockage of receptors as possible explanations, which may have implications for athletes using caffeine as an ergogenic aid during morning training or competition.
“The recommendation to exercise on an empty stomach in the morning to increase fat oxidation is commonplace,” says lead study author of this research, Francisco José Amaro-Gahete.
“However, this recommendation may be lacking a scientific basis, as it is unknown whether this increase is due to exercising in the morning or due to going without food for a longer period of time."
Afternoon exercise
To add further weight to the benefits of timing caffeine ingestion, the team also found its effects to be more pronounced if the exercise was performed in the afternoon rather than the morning.
Here, the team point to higher core body temperature resulting in increased energy metabolism, and improved muscle compliance as possible explanations behind the boost in caffeine’s benefits.
Studies cited also highlight the exercise-induced catecholamine peak as higher in the afternoon than in the morning promoting an increase in lipolysis in skeletal muscle and adipose tissue. This may explain the higher fat oxidation rates observed in the afternoon.
The team from the university of Granada and Rey Juan Carlos University in Madris enrolled 15 men (mean age, 32) in the trial, where they were asked to complete a cycling test on four separate occasions every seven days.
Subjects ingested three milligrams per kilogram (mg/kg) of caffeine or a placebo at 8am and 5pm (each subject completed the tests in all four conditions in a random order).
For each exercise test conditions, the hours elapsed since last meal, physical exercise, or consumption of stimulant substances) were standardised, and fat oxidation during exercise was calculated accordingly.
A diurnal variation?
Findings revealed Maximal Fat Oxidation Rate (MFO), the intensity of exercise that elicited MFO (Fatmax) and maximum oxygen uptake (VO2max) were significantly higher in the afternoon than in the morning.
Compared to the placebo, caffeine also increased mean MFO by 10.7% in the morning, and by a mean 29.0% in the afternoon.
Additional results found caffeine also increased mean Fatmax by 11.1% in the morning, and by 13.1% in the afternoon.
“The existence of a diurnal variation in fat oxidation during exercise was confirmed, the values being higher in the afternoon than in the morning for equal hours of fasting,” adds Amaro.
The combination of acute caffeine intake and exercise at moderate intensity in the afternoon seems to be the best scenario for individuals seeking to increase the amount of fat utilized during continuous aerobic exercise,” the study concludes.
“Whether higher doses of caffeine induce greater effects on whole-body fat oxidation during graded exercise tests and further improves endurance performance remains to be investigated.”
Source: Journal of the International Society of Sports Nutrition
Published online: DOI: 10.1186/s12970-020-00400-6
“Caffeine increases maximal fat oxidation during a graded exercise test: is there a diurnal variation?”
Authors: Mauricio Ramírez-Maldonado et al.