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

Abstract

A number of previous investigations have been designed to determine the effect of acute caffeine intake on the rate of fat oxidation during exercise. However, these investigations have shown contradictory results due to the differences in the exercise protocols used or the co-ingestion of caffeine with other substances. Hence, to date, there is no consensus about the effect of caffeine on fat oxidation during exercise. The purpose of this study was to conduct a systematic review followed by a meta-analysis to establish the effect of acute intake of caffeine (ranging from 2 to 7 mg/kg of body mass) on the rate of fat oxidation during exercise. A total of 19 studies published between 1978 and 2020 were included, all of which employed crossover experimental designs in which the ingestion of caffeine was compared to a placebo. Studies were selected if the exercise intensity was consistent in the caffeine and placebo trials and if these were preceded by a fasting protocol. A subsequent meta-analysis was performed using the random effects model to calculate the standardized mean difference (SMD). The meta-analysis revealed that caffeine significantly (p = 0.008) increased the fat oxidation rate (SMD = 0.73; 95% CI = 0.19 to 1.27). This increment was consistent with a significant (p = 0.04) reduction of the respiratory exchange ratio (SMD = −0.33; 95% CI = −0.65 to −0.01) and a significant (p = 0.049) increase in the oxygen uptake (SMD = 0.23; 95% CI = 0.01 to 0.44). The results also showed that there was a dose–response effect of caffeine on the fat oxidation rate, indicating that more than 3.0 mg/kg is necessary to obtain a statistically significant effect of this stimulant on fat oxidation during exercise. Additionally, the ability of caffeine to enhance fat oxidation during exercise was higher in sedentary or untrained individuals than in trained and recreational athletes. In conclusion, pre-exercise intake of a moderate dose of caffeine may effectively increase fat utilization during aerobic exercise of submaximal intensity performed after a fasting period. However, the fitness level of the participant may modulate the magnitude of the effect of caffeine on fat oxidation during exercise.

Effect of caffeine on muscle oxygen saturation during short-term all-out exercise: a double-blind randomized crossover study

“…Caffeine is sufficiently hydrophobic to pass through all biological membranes and is readily distributed throughout all tissues of the body [ 7 ]. Increased muscle oxygen saturation [ 11 ], improved function of the Na + /K + pump [ 12 ], enhanced calcium ion release from the sarcoplasmic reticulum [ 13 ] and increased fatty acid mobilization and fat oxidation [ 14 ] have been proposed as contributors for increased performance after oral caffeine intake. Interestingly, these mechanisms are mainly effective during endurance-based activities as caffeine has an amplified ergogenic effect on more oxidative fibre types [ 15 ].…”

Placebo Effect of Caffeine on Substrate Oxidation during Exercise

“…It is probable that the placebo effect of caffeine on fat oxidation during exercise is lower or even inexistent in habitual caffeine consumers, as has been recently found for muscle performance [17]. Third, a recent meta-analysis indicated that the ability of caffeine to increase fat oxidation during exercise is higher in doses ≥ 6 mg/kg when compared to~3 mg/kg [6]. Future investigations should determine if the expectancy of having received a dose of 6 mg/kg of caffeine also produces a higher effect on fat oxidation during exercise than the expectancy of receiving 3 mg/kg.…”

“…For this reason, caffeine is broadly consumed before exercise in doses from 3 to 9 mg per kg of body mass [4] (i.e., mg/kg) in the form of coffee or tea (natural sources) or in capsules/pills, pre-workout supplements, energy drinks, and caffeinated gums (artificial sources) [5]. Although less-well studied, acute caffeine intake can also produce a shifting in substrate oxidation during exercise when ingested in doses similar to the ones used for performance enhancement [6,7]. Briefly, pre-exercise ingestion of caffeine has the capacity to increase fat oxidation rate during incremental [8] and steady-state exercise [9] while it concomitantly reduces carbohydrate utilization.…”

“…Briefly, pre-exercise ingestion of caffeine has the capacity to increase fat oxidation rate during incremental [8] and steady-state exercise [9] while it concomitantly reduces carbohydrate utilization. The capacity of caffeine to shift substrate oxidation is habitually found at low-to-moderate exercise intensities, which limits its effectiveness when performing nearto-maximal intensity exercise [6,10]. Although enhanced fat utilization probably adds little to caffeine's ergogenicity in most sport situations, the utilization of caffeine prior to endurance exercise may benefit those enrolled in exercise programs for body weight loss.…”

Caffeine increases exercise intensity and energy expenditure but does not modify substrate oxidation during 1 h of self-paced cycling

“…The influence of caffeine to increase fat utilization during exercise may be an attractive effect for those individuals enrolled in exercise programs seeking body weight reduction and fat mass loss. The higher fat oxidation rate during exercise found after caffeine intake [4][5][6][7] may aid to produce a faster reduction of fat mass, as caffeine induces a higher amount of fat oxidized per exercise session [5]. However, this substance should be ingested chronically before the exercise sessions of a weight loss program to produce measurable changes in body composition as caffeine increases fat utilization in the range of 0.08 to 0.14 g/min -equivalent to 4.8-8.4 g of "extra" fat utilized per exercise session of 60 min of duration- [4][5][6][7].…”