The direct effects of drinking beverages with added sugar or other caloric sweeteners on bodyweight are difficult to discern. A meta-analysis of randomised controlled trials, published in the journal Obesity Reviews, calls for further robust research into the effect of reducing intake of such beverages by overweight individuals.
The relationship of calorific beverages with obesity has been investigated in animals and humans, however original studies as well as reviews and meta-analyses thereof report conflicting results, with some reporting no effect at all. To disentangle the evidence, a group of researchers from the University of Alabama, USA, performed a meta-analysis of randomised controlled trials (RCT), following a defined set of criteria. In a meta-analysis, results from different studies on a specific topic are collected and jointly analysed in order to reach a general conclusion based on the accumulated scientific data. RCTs are considered the best source of evidence, because these studies can control for both known and unknown influencing factors.
A nutritively sweetened beverage (NSB) was defined by the authors as something one drinks, which has a nutritive sweetener (one that provides energy) added. The review did not include studies on ‘diet or lite drinks’ (which only contain non-nutritive sweeteners), 100% fruit juice, unsweetened milk, meal replacements or alcohol. Human studies had to last at least 3 weeks, in which subjects were randomly assigned to either drink or not drink NSBs and had their body fat measured before and after the intervention. Studies involving subjects in a stressed condition e.g. ill, or pregnant, were excluded. Also excluded were those that did not explicitly answer the research question, which was the case for three studies which previous analyses included erroneously (in the authors opinion); a full explanation is provided in the review article.
Both published and unpublished literature was screened and a total of 12 studies met the inclusion criteria. As these varied in their approach they were analysed categorically. Meta-analysis of four studies that added NSBs to a persons’ diet showed dose-dependent increases in weight. The relatively short study duration (up to 12 weeks), small sample sizes, and the potential for consistent confounding factors were highlighted as limitations that required cautious interpretation of the findings.
Meta-analysis of those studies that attempted to reduce consumption, relating to the effectiveness of education, overall did not find an effect on body mass index (BMI; body weight in kg divided by the square of height in m). However, a further analysis using measures of BMI at baseline (only 3 studies) suggested that interventions to reduce consumption may be beneficial particularly for individuals who are overweight, potentially leading to significantly more weight lost or less weight gained compared to overweight control subjects.
The current evidence base does not focus directly on whether long-term consumption of NSBs influences BMI. Other factors that may play a part, for example, include liquid versus solid energy forms and the presence of sweetness. Alternative beverages, not included in the definition of NSBs, such as coffee, tea, sports drinks, fruit juices or alcohol, may have different implications on body weight.
Conclusions must be made based on the totality of the evidence. Experimental evidence and short term trials show additional energy consumed in beverages is not compensated for, and leads to weight gain. In light of the obesity problem facing policy makers, the paper concludes the need for future research to focus on filling the gaps. There is a need for well-powered and adequately controlled randomised trials to test specifically the efficacy (the output versus the input; as opposed to the effectiveness) of programmes that reduce NSB intake, especially among overweight individuals. This could be further supported by mechanistic studies.
For further information, see
Mattes RD, Shikany JM, Kaiser KA and Allison DB. (2011). Nutritively sweetened beverage consumption and body weight: a systematic review and meta-analysis of randomized experiments. Obesity Reviews 12(5):346-365, doi:10.1111/j.1467-789X.2010.00755.x