Does Caffeine Increase Fat Metabolism? A Systematic Review and Meta-Analysis

Abstract

Whether caffeine (CAF) increases fat metabolism remains debatable. Using systematic review coupled with meta-analysis, our aim was to determine effects of CAF on fat metabolism and the relevant factors moderating this effect. Electronic databases PubMed, SPORTDiscus, and Web of Science were searched using the following string: CAF AND (fat OR lipid) AND (metabolism OR oxidation). A meta-analytic approach aggregated data from 94 studies examining CAF’s effect on fat metabolism assessed by different biomarkers. The overall effect size (ES) was 0.39 (95% confidence interval [CI] [0.30, 0.47], p < .001), indicating a small effect of CAF to increase fat metabolism; however, ES was significantly higher (p < .001) based on blood biomarkers (e.g., free fatty acids, glycerol) (ES = 0.55, 95% CI [0.43, 0.67]) versus expired gas analysis (respiratory exchange ratio, calculated fat oxidation) (ES = 0.26, 95% CI [0.16, 0.37]), although both were greater than zero. Fat metabolism increased to a greater extent (p = .02) during rest (ES = 0.51, 95% CI [0.41, 0.62]) versus exercise (ES = 0.35, 95% CI [0.26, 0.44]) across all studies, although ES was not different for studies reporting both conditions (ES = 0.49 and 0.44, respectively). There were no subgroup differences based on participants’ fitness level, sex, or CAF dosage. CAF ingestion increases fat metabolism but is more consistent with blood biomarkers versus whole-body gas exchange measures. CAF has a small effect during rest across all studies, although similar to exercise when compared within the same study. CAF dosage did not moderate this effect.

Effect of Acute Caffeine Intake on Fat Oxidation Rate during Fed-State Exercise: A Systematic Review and Meta-Analysis

“…This could be because untrained individuals have a greater window of improvement to achieve performance, whereas trained subjects may have a greater degree of adaptation and lower margin for improvement through nutritional interventions, limiting the potential of caffeine to enhance fat oxidation. However, research can also be found reflecting an ergogenic effect of caffeine of similar magnitude and an effect on fat oxidation derived from caffeine intake in highly trained athletes and less-trained individuals [ 5 , 16 ]. Therefore, more research is needed to find out whether the effect of caffeine on the rate of fat oxidation is indeed dependent on the level of training or if this is just an artefact of the number of studies that used either type of participant.…”

“…In this latter meta-analysis [ 16 ], the effect of combining caffeine with fasting periods shorter and longer than 3 h before exercise was analysed and the report confirmed that caffeine enhances fat metabolism, independently of the fasting period used before exercise. However, this meta-analysis [ 16 ] included studies with very different natures, with a mix of studies carried out at rest and during exercise and with very diverse methods to assess fat metabolism (gas exchange, blood metrics of triglycerides, FFA, and glycerol, among others). Additionally, an increase in fat metabolism induced by caffeine is not per se linked with higher fat oxidation within the working muscle, as the potential higher release of FFA induced by caffeine (i.e., lipolytic effect) must be accompanied by a higher reliance on fat in the mitochondria of the working muscle.…”

“…A recent meta-analysis examined the effect of caffeine on the rate of fat oxidation in interventions of fixed exercise intensity in a fasted state (>5 h since the last meal), which reported a positive effect of caffeine on fat oxidation rates during fasted exercise [ 15 ]. Another meta-analysis has been recently published claiming that caffeine significantly improves fat metabolism at rest and during exercise [ 16 ]. In this latter meta-analysis [ 16 ], the effect of combining caffeine with fasting periods shorter and longer than 3 h before exercise was analysed and the report confirmed that caffeine enhances fat metabolism, independently of the fasting period used before exercise.…”

“…Another meta-analysis has been recently published claiming that caffeine significantly improves fat metabolism at rest and during exercise [ 16 ]. In this latter meta-analysis [ 16 ], the effect of combining caffeine with fasting periods shorter and longer than 3 h before exercise was analysed and the report confirmed that caffeine enhances fat metabolism, independently of the fasting period used before exercise. However, this meta-analysis [ 16 ] included studies with very different natures, with a mix of studies carried out at rest and during exercise and with very diverse methods to assess fat metabolism (gas exchange, blood metrics of triglycerides, FFA, and glycerol, among others).…”

Ergogenic Effects of the Combination of Caffeine and New Zealand Blackcurrant Supplements: A Double-Blind Singe-Case Experimental Study

“…According to a meta-analysis by Conger et al [35], caffeine significantly promoted fat metabolism that was operationalized by RER (ES = 0.19). Compared to the BL phase, the RER values of both subjects decreased in the CAF (0.83) and ALL (0.84-0.85) phases and thus, could indicate increased fat metabolism.…”