Sunday, June 30, 2019

Acute carbohydrate consumption on mood: Some studies only reported results from questionnaire subscales that yielded significant differences while omitting those that did not

Fizzing out: no effect of acute carbohydrate consumption on mood. Michael D. Kendig, Margaret J. Morris. Neuroscience and Biobehavioral Reviews (2019) PII:S0149-7634(19)30362-8.


Policy interventions increasingly target sugar-sweetened beverage (SSB) consumptionto discourage excess sugar intakegivenevidence for detrimental effects on body weight, metabolic function, anddental health. Yet it is also interesting to consider other impacts ofSSBs, given ongoing debate around their effects on mood and alertness.

The systematic review and meta-analysis by Mantantzis and colleagues (2019)makes atimely contributionto this topic. The study collates results from 31 experiments (1259 participants, 176 effect sizes) onthe acute effects of carbohydrate-containing drinks on moodmeasures.The authors note that mood is often a secondary outcome about which few direct predictions are made.

In these experiments, participants are asked to consume a beverage containing carbohydrates beforere ceiving questionnaires asking how they feel, often alongside cognitive tests. The carbohydrate is typically a sugar such as glucose, sucrose, fructose, or some mix of these. In the mood questionnaires usedmost commonly,the Profile of Mood States (POMS) and Bond-Lader VAS (BL-VAS),participants ratehow strongly their current affective state aligns with a range of positive and negative emotions, either on 5-point (POMS) or visual analogue (BL-VAS) scales. Mantantzis et al.(2019)generate effect sizes for nine affective domains, which are then combined to form a composite score foroverall mood. The authors also compared studies that assessed mood 0-30, 31-60 or over 60 minutes after consuming carbohydrates, with a view to identifying underlying mechanisms. For example, Mantantzis et al.(2019) note that past research has shown that carbohydrate consumption increases levels of the serotonin precursor tryptophan, particularly after an hour post-consumption. Thus, stronger positive mood effects should be seen after an hour if tryptophan underlies this relationship.

Strikingly, there was no evidence for positive effects of carbohydrate consumption on mood, and few significant differences between carbohydrate and placebooverall. Carbohydrates significantly reduced alertness relative to placebo when assessed at 31-60 minutes, and significantly increased fatigue when assessed 0-30 minutes after consumption. No effects of carbohydrates were found at any interval onthe other seven domains. Contrary to authors’ hypotheses, heterogeneity between studies was low, despite substantial variability in carbohydrate dose and type.The only instance of high heterogeneity was on measures of fatigue assessed61+ min post-consumption. Follow-up analyses of moderator variables revealeda trend for carbohydrate to reduce fatigue in studies involving physically demanding tasks.On the composite score of ‘overall mood’, there was a marginally significant difference favouring carbohydrate over placebo after 60 minutes. However, the authors found trends towards publication bias at this interval, noting that some studies only reported results from questionnaire subscales that yielded significant differences while omitting those that did not, skewing the overall effect size. This resul tshould therefore be interpreted with caution.

Additionally, authors were unable to obtain effect sizes for ~40% of eligible articles(20/51), underscoring the need for improved data sharing practices. This new evidence that carbohydrate intake does not enhance mood, at least over the short-term,is consistent with an earlier meta-analysis (Wolraich et al., 1995) which found no evidence for effects of sugar on behaviou rin children.

By contrast, there is evidence for improved cognition following acute glucose intake,though these effects are sensitive to procedural variables such as fasting duration and glucose dose (reviewed by Riby, 2004). Moreover, extensive evidence from rodent studies indicatesthat diets high in sugar can impair aspects of learning and memory (Beilharz et al., 2016), and a recent study in people reported positive associations between self-reported sugar intake and depressive symptoms (Kn├╝ppelet al., 2017).

What might explain the discrepancy between the null results found by Mantantzis and colleagues and popular notions of the ‘sugar high’ or ‘sugar rush’? Several differences between controlled laboratory studiesand real-world scenarios are interesting to consider. One key factor is blinding: Participants were unaware of whether their drink contained carbohydrate or placebo (typically non-nutritive sweetener) in 25 of the 31 studies assessed by Mantantzis et al. (2019). Similarly, the meta-analysis by Wolraich et al. (1995) on sugar and activity in children included double-blind experiments. The use of blinding precludes expectancy effects, where in preconceived notions about the effects of sugar increase the tendency to see such effects.Yet one recent study found that even expecting sugar increased tension (Giles et al., 2018), suggesting that such expectancy effects may not always be positive. Second, in real-world settings SSBsare often consumed in environments or occasions that are rewarding,such as social gatherings and work breaks. Repeated consumption in these settings may producepositive associations between sugary drinks and positive mood–a centre piece of advertising strategies. In research studies the potential for contextual variables to influence mood is usually controlled or eliminated. Whereas study participants consume these drinks in isolation and often after a fast, day-to-day consumption of sugary drinks is often accompanied by solid foods, such as fast food meals, which may drive –or wash out –mood effects.

As SSBs typically contain caffeine, people may misattribute mood benefits to the carbohydrate rather than caffeine content. One factor that appears to warrant further investigation is palatability, which we believe is among the primary driversof sugary drink consumption.


We believe this contribution to research on diet and mood will inform public debate on sugar consumption. It adds important new evidence suggesting that sugar consumption has nopositive effects on mood, at least under the conditions used in these experiments, and appears to increase fatigue and lower alertness. This shifts the balance of evidence for sugar intake further towards harm and away from any beneficial effects.

Beilharz, J. E., Maniam, J., & Morris, M. J. (2016). Short-term exposure to a diet high in fat and sugar, or liquid sugar, selectively impairs hippocampal-dependent memory, with differential impacts on inflammation. Behav BrRes, 306, 1-7.

Frank, G. K., Oberndorfer, T. A., Simmons, A. N., Paulus, M. P., Fudge, J. L., Yang, T. T., & Kaye, W. H. (2008). Sucrose activates human taste pathways differently from artificial sweetener. Neuroimage, 39(4), 1559-1569.

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