Practical Issues in Evidence-Based Use of Performance Supplements: Supplement Interactions, Repeated Use and Individual Responses

Abstract

Current sports nutrition guidelines recommend that athletes only take supplements following an evidence-based analysis of their value in supporting training outcomes or competition performance in their specific event. While there is sound evidence to support the use of a few performance supplements under specific scenarios (creatine, beta-alanine, bicarbonate, caffeine, nitrate/beetroot juice and, perhaps, phosphate), there is a lack of information around several issues needed to guide the practical use of these products in competitive sport. First, there is limited knowledge around the strategy of combining the intake of several products in events in which performance benefits are seen with each product in isolation. The range in findings from studies involving combined use of different combinations of two supplements makes it difficult to derive a general conclusion, with both the limitations of individual studies and the type of sporting event to which the supplements are applied influencing the potential for additive, neutral or counteractive outcomes. The repeated use of the same supplement in sports involving two or more events within a 24-h period is of additional interest, but has received even less attention. Finally, the potential for individual athletes to respond differently, in direction and magnitude, to the use of a supplement seems real, but is hard to distinguish from normal day to day variability in performance. Strategies that can be used in research or practice to identify whether individual differences are robust include repeat trials, and the collection of data on physiological or genetic mechanisms underpinning outcomes.

Fig. 1

Strategies often used in the presentation and interpretation of results of supplements studies: Identification of individual responses to the intervention, and the comparison of results to the close outcomes often seen in real life sports competition. Both of these devices can lead to a misunderstanding of the real results due to the failure to appreciate the day to day variability in performance of the participants in the study or real-life athletes

Fig. 2

Effects of large doses of caffeine (9 mg/kg) taken 60 min prior to running (a) or cycling (b) at an intensity equivalent to 85% of maximal aerobic capacity (VO₂max) on time to exhaustion in a group of seven highly trained runners. Note that one subject (open square) is different to the other six subjects (filled diamond) in showing a consistently small response to caffeine. Adapted from Graham and Spriet [79], with permission