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Potential impact on prevalence of obesity in the UK of a 20% price increase in high sugar snacks: modelling study

BMJ 2019; 366 doi: https://doi.org/10.1136/bmj.l4786 (Published 04 September 2019) Cite this as: BMJ 2019;366:l4786

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Inflated expectations of reduced obesity prevalence result from an erroneous static weight loss model

The recent modeling study by Scheelbeek et al. estimated the calorie intake changes in response to 20% price increase in sugary snacks and predicted the corresponding changes in body weight using a so-called "static model for weight loss". The authors acknowledged that the static model "might not fully reflect actual mechanisms of weight change. However, the period considered in this study is one year, and modelled weight changes during that time were expected to be relatively small. Therefore changes in energy expenditure, as described previously, are expected to have marginal impacts on our results "

Unfortunately, the authors' expectations are incorrect. As previously demonstrated in a similar modeling study investigating the potential impact of a sugar-sweetened beverage tax, the static weight loss model substantially overestimates weight losses after one year and the discrepancy exacerbates over time (https://www.ncbi.nlm.nih.gov/pubmed/21940223) when compared to the results of a validated dynamic model of weight change that correctly accounts for physiological changes in energy expenditure and energy partitioning (https://www.ncbi.nlm.nih.gov/pubmed/21872751),

More recently, dynamic models have been developed to also account for changes in appetite with weight loss (https://www.ncbi.nlm.nih.gov/pubmed/27804272) that result in predicted weight changes that are even smaller than previous dynamic models that account for only energy expenditure changes (https://www.ncbi.nlm.nih.gov/pubmed/29896621). The physiological adaptations in response to persistent interventions are expected to result in greatly diminished returns when it comes to weight loss, as has been clearly documented in obesity pharmacotherapy trials (https://www.ncbi.nlm.nih.gov/pubmed/29071809).

Ignoring the physiological changes in appetite, energy expenditure, and energy partitioning results in inflated estimates of the effectiveness of interventions and policies targeted to address obesity. We should not be misled that obesity prevalence can be easily decreased by single interventions and such interventions should not be judged to have failed if they do not live up to exaggerated promises based on erroneous assumptions.

Competing interests: No competing interests

09 September 2019
Kevin D Hall
Senior Investigator
National Institutes of Health
12A South Drive, Room 4007, Room 4007