The Effect of Dietary Supplements on Endurance Exercise Performance and Core Temperature in Hot Environments: A Meta-analysis and Meta-regression

Abstract

Background The ergogenic effects of dietary supplements on endurance exercise performance are well-established; however, their efficacy in hot environmental conditions has not been systematically evaluated. Objectives (1) To meta-analyse studies investigating the effects of selected dietary supplements on endurance performance and core temperature responses in the heat. Supplements were included if they were deemed to: (a) have a strong evidence base for ‘directly’ improving thermoneutral endurance performance, based on current position statements, or (b) have a proposed mechanism of action that related to modifiable factors associated with thermal balance. (2) To conduct meta-regressions to evaluate the moderating effect of selected variables on endurance performance and core temperature responses in the heat following dietary supplementation. Methods A search was performed using various databases in May 2020. After screening, 25 peer-reviewed articles were identified for inclusion, across three separate meta-analyses: (1) exercise performance; (2) end core temperature; (3) submaximal core temperature. The moderating effect of several variables were assessed via sub-analysis and meta-regression. Results Overall, dietary supplementation had a trivial significant positive effect on exercise performance (Hedges’ g = 0.18, 95% CI 0.007–0.352, P = 0.042), a trivial non-significant positive effect on submaximal core temperature (Hedges’ g = 0.18, 95% CI − 0.021 to 0.379, P = 0.080) and a small non-significant positive effect on end core temperature (Hedges’ g = 0.20, 95% CI − 0.041 to 0.439, P = 0.104) in the heat. There was a non-significant effect of individual supplements on exercise performance (P = 0.973) and submaximal core temperature (P = 0.599). However, end core temperature was significantly affected by supplement type (P = 0.003), which was attributable to caffeine’s large significant positive effect (n = 8; Hedges’ g = 0.82, 95% CI 0.433–1.202, P < 0.001) and taurine’s medium significant negative effect (n = 1; Hedges’ g = − 0.96, 95% CI − 1.855 to − 0.069, P = 0.035). Conclusion Supplements such as caffeine and nitrates do not enhance endurance performance in the heat, with caffeine also increasing core temperature responses. Some amino acids might offer the greatest performance benefits in the heat. Exercising in the heat negatively affected the efficacy of many dietary supplements, indicating that further research is needed and current guidelines for performance in hot environments likely require revision.

Exertional heat stroke: nutritional considerations

“…• ↓ Intestinal permeability (Butler-Dawson et al, 2019;Gan et al, 2022;King et al, 2016) • ↓ Cytokinaemia (Bouchama et al, 2022;Ogden et al, 2021) • ↑ Leukocyte function (Bouchama et al, 2022) • ↑ Central drive and ↓ perceived effort may increase EHS risk (Carter et al, 2007) • Large doses may induce GI symptoms (Dong et al, (Roberts et al, 2021;Zuhl et al, 2015;Zuhl et al, 2014) • Higher doses may induce GI symptoms (Parker et al, 2018) • No beneficial effect at low doses (Peel et al, 2021;Roberts et al, 2021) Bovine colostrum 20 g day −1 for 14 days…”

“…The FDA has since banned the sale of supplements containing ephedrine alkaloids (FDA, 2004 ). Of note, caffeine, a widely used (and legal) stimulant among athletes and adults, may also increase EHS risk as it reduces perception of effort, fatigue and pain, and may exacerbate T c elevation during exercise in the heat (Guest et al., 2021 ; Peel et al., 2021 ). However, there is no evidence of caffeine directly contributing to EHS.…”