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The myth of the sugar rush

Common sense and plausible mechanistic arguments suggest that carbohydrates can influence mood. But do they? And by how much?

Study under review: Sugar rush or sugar crash? A meta-analysis of carbohydrate effects on mood.

Introduction

As summer rolls around the corner, you can be sure parents are ready to stop their kids from drinking too much soda at little Billy’s pool party. As the title of the article under review suggests, acute sugar intake (or carbohydrates in isolation) has been commonly thought to lead to a ‘sugar rush’ (burst of energy) followed by a ‘sugar crash’ (fatigue from using up said burst of energy) though there is no strong scientific evidence to support this myth.

The ‘sugar rush’ notion was dispelled in the scientific community when a comprehensive 1995 meta-analysis[1] demonstrated no effect of sugar on behavior or cognitive performance in children. Nonetheless, the myth survives[2] and for almost 25 years after that meta-analysis, researchers continued to study the influence of sugar on mood. This occurred in the context of the U.S population, which uses soda, energy, and sports drinks as one of the top ten sources of energy[3] in the country. More specifically, some studies investigating the interaction between mood and simple sugars have suggested changes in cognitive performance[4] and emotional wellbeing[5].

One appealing hypothesis regarding carbohydrates (CHOs) and emotional well-being is the serotonin hypothesis. This hypothesis, whose details are depicted in Figure 1, postulates that CHO consumption leads to higher levels of the feel-good neurotransmitter[6] serotonin in the brain as a result of greater tryptophan[7] (a precursor to serotonin) availability. This is supported by reports[8] of individuals suffering from mood disorders (i.e. depression, anxiety, etc.) that tend to self-medicate with CHO-rich meals. However, one study[9] from the 1980s failed to observe increases in tryptophan or serotonin after eating, except in a protein-free meal. This tryptophan-increasing effect appears to be stopped if as little as 5%[10] of the ingested CHO drink or meal is protein.

Other studies have demonstrated conflicting results, reporting no influence[11] or even slightly detrimental[12] effects on the overall relationship between mood and CHO intake. While reviews[12] have recently covered the relationship between CHO intake and mood, studies have not been quantitatively (meta-) analyzed to determine which direction the evidence leans and to quantify how heterogeneous the literature really is. The authors of the study under review conducted a meta-analysis to fill this gap by quantitatively analyzing the short-term effects of isolated CHO intake on mood while considering potential confounding variables such as CHO dose and/or type, fasting interval, and any activity preceding mood assessment.

Carbohydrates are believed to influence behavior and mood. While some studies suggest carbohydrate intake will increase the release of the “feel-good neurotransmitter” serotonin and even boost cognitive performance, some report no influence, if not slightly detrimental effects. The study under review aims to quantify the effect and explain the conflicting results through a review and meta-analysis of results and confounding factors that may influence the putative interaction between CHO intake and mood.

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Other Articles in Issue #55 (May 2019)