Dietary sugars and body weight: systematic review and meta-analyses of randomised controlled trials and cohort studies

The review by Te Morenga et al., (2013) sheds some light on the confused literature regarding dietary sugars and body weight. It is refreshing that the authors acknowledge that the relatively small change (~0.8 kg) in body weight attributed to alterations in sugars intake in the intervention trials is probably due to changes in energy intake and not some other characteristic unique to sugars. Their conclusion that sugars intake is a ‘determinant’ of body weight could thus be applied equally to any caloric nutrient or food.

The disparity of the studies does not allow conclusions to be drawn specifically for consumption of ‘free sugars’, which appeared to be the question posed to the authors. Since the review included the results of studies which changed consumption of sugars-containing foods, the results are confounded by changes to other caloric nutrients. The data did not allow derivation of a dose response and provide no direct support for the controversial suggestion that average population “free sugars” intake should be limited to 10% of food energy.

Some important points need to be raised with regard to what appears to be evidence of selection bias in the studies and data included in the review. Firstly, the only data used from the CARMEN Study (Saris et al., 2000) is incorrectly included in Table 2/Figure 3 (effect of reducing sugars on body weight in adults). The main result of this long term intervention should have been included in Table 3/Figure 4 (effect of increasing sugars on body weight in adults). This trial showed that increasing sugars to 30% energy for six months gave rise to a fall in body weight of 1.7 kg compared to control.

Second, the data included in Table 3/Figure 4 from the Poppitt et al. study (that was an adjunct of the CARMEN study, but involved subjects with metabolic syndrome) is the comparison between the high sugars group and the high starch group. The correct comparison should have been between the high sugars group and the controls. This shows no significant change in body weight after six months on a diet again containing 30% energy as sugars, when compared with the subject’s usual intake.

Third, the correct citation for the study by Marckmann et al., (2000) is Raben et al., (1997) [1], which reported the body weight results of this study. The present review arbitrarily cites part of the results of this study (the comparison of the high sugars group with one of the low sugars groups) while ignoring the non-significant changes in body weight seen when the high sugars group was compared to the other low sugar group.

These errors would be expected to affect the reported effect sizes and confidence intervals.

The heterogeneity of the interventions, and the admitted evidence of publication bias in the studies available for the meta-analysis, diminishes any confidence in the authors’ conclusions. Indeed, the results of studies where dietary sugars were isoenergetically exchanged for other carbohydrates or macronutrients show no effect on body weight (Fig 5). This finding, together with absence of evidence for a dose response, concur with reports by the Institute of Medicine [2], European Food Safety Authority [3], National Health and Medical Research Council [4], and the earlier joint FAO/WHO Consultation on Carbohydrates in Human Nutrition [5] that were unable to identify any level of added or total sugars intake likely to increase energy intake or obesity risk.

As the authors acknowledge, the effectiveness of the application of the results of this present analysis to the general public remain to be demonstrated.

1. Raben A., Macdonald I., Astrup A. (1997) Replacement of dietary fat by sucrose or starch: effects on 14 d ad libitum energy intake, energy expenditure and body weight in formerly obese and never-obese subjects. International journal of obesity and related metabolic disorders. Journal of the International Association for the Study of Obesity, 21, 846-59
2. Food and Nutrition Board, Institute of Medicine of the National Academies (2005) Dietary Reference Intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein, amino acids (macronutrients). The National Academies Press. Washington, DC.
3. European Food Standards Authority (2010) Scientific opinion on dietary reference values for carbohydrates and dietary fibre. EFSA Panel on Dietetic Products, Nutrition and Allergies. EFSA Journal 2010; 8(3):1462.
4. National Health and Medical Research Council (2003) Dietary Guidelines for Australian Adults. The Commonwealth of Australia, Canberra
5. FAO/WHO (1997) Carbohydrates in human nutrition (FAO Food and Nutrition Paper - 66). FAO, Rome