Author(s): ELHUSSEINI, R., FARES, E.J., Institution: AMERICAN UNIVERSITY OF BEIRUT, Country: LEBANON, Abstract-ID: 190

INTRODUCTION:
Caffeine is one of the most widely consumed bioactive compounds globally, primarily ingested through coffee, tea, and caffeinated beverages. Its ergogenic potential is well established, yet the literature remains inconsistent regarding its effects on substrate utilization, particularly at low doses and during low-intensity exercise. While high caffeine doses (~5 mg/kg) are commonly studied, recent evidence suggests that much lower doses may elicit meaningful physiological effects. Importantly, few studies have examined ecologically valid doses equivalent to everyday coffee consumption or explored sex-specific metabolic responses. This secondary analysis investigates whether a low caffeine dose (135 mg), equivalent to a double espresso, enhances fat oxidation during low-intensity endurance exercise.
METHODS:
We conducted a sex-stratified secondary analysis of a randomized, single-blind, crossover study involving 29 healthy adults (15 men, 14 women). Participants consumed 500 ml of either plain water, water with 135 mg anhydrous caffeine, or yerba mate following a 12-hour fast. After a 40-minute absorption period, participants completed graded cycling exercise from 20W to 80W in 20W increments. Respiratory quotient (RQ), energy expenditure, heart rate, and delta efficiency were measured using indirect calorimetry. Given the similar caffeine and polyphenol content and near-identical metabolic responses of the yerba mate treatments, these were combined for analysis. Changes in RQ from rest to each workload (ΔRQ) were calculated to quantify shifts in substrate utilization.
RESULTS:
In men, caffeine produced the lowest RQ across all workloads and the smallest increase from rest to 80W (ΔRQ = 0.099), compared with water (ΔRQ = 0.114) and yerba mate (ΔRQ = 0.112). Post-hoc analyses revealed significantly lower RQ values for caffeine at 60W and 80W (p < 0.05), indicating enhanced fat oxidation and attenuation of the shift toward carbohydrate utilization. In women, no consistent or statistically significant differences in RQ or ΔRQ were observed between interventions.
CONCLUSION:
This analysis demonstrates that the significant drink × intensity × sex interaction reported in the original study was specifically driven by caffeine’s metabolic effects in men. Consistent with emerging evidence that low caffeine doses can exert meaningful ergogenic effects, our findings extend this concept to substrate utilization during low-to-moderate intensity exercise using an ecologically valid dose equivalent to a double espresso. A 135 mg caffeine intake significantly enhanced fat oxidation in men but not in women, underscoring the importance of sex-specific analyses and supporting the metabolic relevance of low-dose, real-world caffeine consumption during everyday physical activity.

References
Grgic J. Nutrition. 2022;97.
Zaki SB et al. Current Developments in Nutrition. 2025 (accepted).