Effects of different caffeine doses on fat oxidation and cardiovascular response during exercise at FATmax in overweight/obese female college students.

PURPOSE: This study examined the acute effects of different caffeine doses on fat oxidation and cardiovascular responses at rest and during exercise at the intensity of maximal fat oxidation (FATmax) in overweight/obese sedentary female college students.

METHODS

In a randomized trial, eleven participants (age: 20.2 ± 2.3 years; BMI: 26.36 ± 1.78 kg/m²) completed four conditions: placebo (cellulose) or caffeine at 3, 5, or 9 mg/kg. Each session comprised 60 min of seated rest followed by 40 min of treadmill walking at FATmax. Substrate oxidation (via indirect calorimetry), blood pressure, and the fingertip perfusion index (PI) were measured at specific time points: at rest (0, 30, and 60 min after capsule ingestion) and immediately post-exercise (100 min). Additionally, gas exchange and heart rate were recorded continuously throughout the entire 100-min session. Data were analyzed using SPSS 26.0 with two-way repeated-measures ANOVA (or Friedman test for non-normal data) and Bonferroni-adjusted post-hoc comparisons. Significance was set at p < 0.05.

RESULTS

Caffeine did not alter the resting heart rate or substrate oxidation. During exercise, caffeine at 5 and 9  mg/kg significantly increased the heart rate and blood pressure (p < 0.05), while caffeine at 3 mg/kg elicited no such cardiovascular effects. The PI decreased across all caffeine groups (p < 0.05). Caffeine at both 3 and 5 mg/kg enhanced fat oxidation compared to placebo and the 9 mg/kg dose (p < 0.05). Carbohydrate oxidation was lower with 5 mg/kg caffeine than with placebo (p < 0.05), and both the 3 and 5 mg/kg doses showed reduced carbohydrate oxidation relative to the 9 mg/kg dose (p < 0.05).

CONCLUSION

Acute caffeine intake at 3 and 5 mg/kg enhanced fat oxidation during FATmax exercise in sedentary overweight/obese females, whereas 9 mg/kg provided no additional metabolic benefit. However, the 5 mg/kg dose was associated with increased cardiovascular strain, which was not observed with the 3 mg/kg. Therefore, a dose of 3 mg/kg appears to offer an optimal balance between stimulating fat oxidation and maintaining cardiovascular safety in this population during acute exercise.