Sugar-sweetened beverages, weight gain and nutritional epidemiological study design.

In this issue of the British Journal of Nutrition, Libuda et al. (1) describe their findings from the Dortmund Nutritional and Anthropometric Longitudinally Designed (DONALD) study on the relationship between sugar-sweetened beverages as well as fruit juice intake and changes in relative body weight over a 5-year period in 119 boys and 116 girls of average age 12 years at study commencement. The diet data, based on four to six 3 d weighed records, were more extensive and more carefully collected than in many studies. In boys, there was a direct association of the average age-standardised BMI with baseline fruit juice intake, not confirmed for percentage body fat or any of the data on changes in intake. This lack of association occurred in a metabolic environment of strongly increasing daily energy intake (mean 8·9MJ increasing to 11·1MJ over 5 years), probably corresponding to increased energy needs, although this supposition could not be directly verified as physical activity was not assessed. The boys’ increase in beverage consumption was disproportionately regular soft drinks, though 100% fruit juices also increased proportionate to increased energy intake. We agree with the authors that pubertal development may have obscured any potential effect of soft drink and other energetic beverage consumption on body-weight status in boys. We expect an effect of soft drinks, all high-fructose energetic beverages, or conceivably all energetic beverages as the boys pass out of adolescence, becoming less active. The DONALD study may eventually have data to address this point. In contrast, in girls there was an increase in age-standardised BMI and a nearly significant corresponding relationship with percentage body fat with concurrent increase in soft drinks and fruit juice (daily energy intake essentially stable, mean 7·6MJ increasing only to 8·1MJ over 5 years). Of interest, the girls’ increase in these beverages was almost entirely 100% fruit juice. The authors note the higher social acceptance of fruit juices. Fruit juices contain nutrients besides sugar, but like soft drinks they are characterised by a high fructose content. This finding of ‘change-on-change’ (change in body fatness regressed on change in fruit juice) has the strong epidemiological design feature of a quasi-experimental design. While it lacks the randomisation of a true experiment, this design has in common with a clinical trial that the girls changed their fruit juice intake and the investigators then followed the body fatness outcome variable. The authors look at the correlation of energy consumption from beverages and the rest of energy consumption to test for energy compensation, on the assumption that a zero correlation means no energy compensation, while a negative correlation means that energy compensation took place. They found no significant cross-sectional correlation, for example, no indication of energy compensation in the individual or pooled cross-sectional surveys. However, again with the power of the quasi-experimental design, the change in the rest of energy intake was significantly inversely related to increase in energy from beverages in both boys and girls, which the authors interpreted to mean partial energy compensation. We caution that the coefficient 20·28MJ per MJ of beverages may be attenuated, given that diet has substantial withinperson variability, even when measured with 3 d weighed food records. Furthermore, the reduction in energy intake from the rest of food may not be perfectly linearly related to the increase in energy from beverages, which is assumed in the model. Still, 20·28 is substantially less than 21, which would indicate full energy compensation, and we tentatively accept that the DONALD study contributes to the evidence that the energy in soft drinks and fruit juices is only partially compensated for. Although not the central point, nutritional researchers should take note of the statistical method used. The authors summarised their data using the powerful technique of repeated-measures regression, which, in a single statistical procedure, simultaneously compares cross-sectional and longitudinal aspects of prospective data, while properly accounting for within-person correlations. In particular the ‘change-onchange’ analysis is a weighted average of pair-wise differences across the four to six examinations, and so averages out random variation that would be present in any one pair, for example the comparison of the 5-year follow-up with baseline. This technique is complex to use, but has tremendous potential for sorting through the complicated data that is obtained in long-term prospective studies and in shortor long-term feeding studies. Some limitations of the present study include the following. (1) Participants chose the first day for the dietary records, which could result in some systematic bias such as social desirability bias. (2) The sample size is small by epidemiological standards, and measurement error is most problematic in youth. (3) Confounding by other dietary factors, especially beverages, was not completely addressed. The analysis of the rest of energy focuses on energy compensation, but does not address the possibility that the beverages studied are a stand-in for other specific aspects of diet. (4) The authors’ use of the term ‘energetic beverages’ is too comprehensive and could be misleading, in that they omitted several important energy-containing drinks, including dairy products, hot tea, coffee and alcohol. (5) The present study population has a low prevalence of overweight and obesity, so it may not be the most suitable for testing the British Journal of Nutrition (2008), 99, 1169–1170 doi: 10.1017/S0007114507868498 q The Authors 2007