Relationship between monetary delay discounting and obesity: a systematic review and meta-regression

BACKGROUND AND OBJECTIVES: Previous studies have documented that high rates of delay discounting are associated with obesity. However, studies utilizing monetary reward experiments typically report no associations, as opposed to positive associations apparent in studies utilising food-reward experiments. Our objective was to investigate the reasons behind the mixed evidence from a methodological perspective using systematic review and meta-analytic methodologies. METHODS: Seven databases (EMBASE, MEDLINE, PsycINFO, Scopus, Web of Science, Econlit and IBSS) were systematically searched. Logistic meta-regression was applied to identify the determinants of a significant association and risk of bias was assessed using a modified form of the Newcastle Ottawa cohort scale. RESULTS: A total of 59 studies were identified, among which 29 studies (49.2%) found a significant positive association and 29 (49.2%) reported no association. A higher proportion of significant and positive associations was reported in those studies utilizing 'best-practice' methods (i.e. appropriate measurement models) to estimate monetary delay discounting (15/27; 55.6%) and incentive-compatible experiments (10/16; 62.5%) than those using non-'best-practice' methods (14/34; 41.2%) and hypothetical experiments (19/43; 44.2%). All five studies utilizing both 'best-practice' methods and incentive-compatible experiments generated a positive and significant relationship. Results from a logistic meta-regression also suggested that studies employing incentive-compatible experiments (OR: 4.38, 95% CI = 1.05-18.33, p value: 0.04), 'best-practice' methods (OR: 4.40, 95% CI = 0.88-22.99, p value: 0.07), parametric methods (OR: 3.36, 95% CI = 0.83-13.57, p value: 0.04), those conducted in children/adolescent populations (OR: 3.90, 95% CI = 0.85-17.88, p value: 0.08), and those with larger sample size (OR: 1.91, 95% CI = 1.15-3.18, p value: 0.01) tended to show positive and significant associations between delay discounting and obesity. CONCLUSIONS: This review suggests that the mixed evidence to date is a result of methodological heterogeneity, and that future studies should utilise 'best practice' methods.