Caffeine is a natural stimulant with well-recognized metabolic benefits, however, there is a lack of studies investigating the time-of-day caffeine influence to enhance fat oxidation in women. The aim of this study was to evaluate the influence of the time of the day on the effect of caffeine on maximal rate of fat oxidation during aerobic exercise in women.

Caffeine is a natural stimulant with well-recognized sports performance benefits. Aside its performance-enhancing effect, caffeine has the potential of increasing fat utilization during aerobic exercise at submaximal intensities, lowering-down the contribution of carbohydrate as a fuel. This property of caffeine may provoke a glycogen-sparing effect in the skeletal muscle and liver for exercise situations where carbohydrate availability may be a challenge. Additionally, the capacity of caffeine to enhance fat utilization during exercise could be of interest for improving health outcomes as it may increase the rate of change in body composition in exercise programs. Maximal fat oxidation rate (MFO) during exercise is a remarkable physiological indicator associated with metabolic flexibility/body weight loss and endurance performance To date, it is unknown if caffeine increases MFO in the same proportion during morning and evening exercise trials in women. For this reason, the aim of the present study was to evaluate the influence of the time of the day on the effect of caffeine on MFO in women. We hypothesised that caffeine would increase MFO during morning and evening exercise and this effect would be of similar magnitude at both times of day.

Inclusion Criteria:

• To be non-smokers.
• To have low caffeine intake (i.e., < 50 mg of caffeine per day in the previous 2 months)
• To show no previous history of cardiopulmonary diseases or having suffered musculoskeletal injuries in the previous 6 months.
• To have a regular duration of their menstrual cycle for the previous 6 months.
• To confirm no existence of any type of menstrual disorders such as dysmenorrhea, amenorrhea, or strong symptoms associated with pre-menstrual syndrome.

Exclusion Criteria:

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