Has increased soda consumption driven the increase in weight gain and obesity?

One might think that the link between drinking sugared beverages and gaining weight is beyond the need for confirmation, but that if one wanted such confirmation, the evidence would be abundant and robust. Surprisingly, this is not the case.

Of course, if you just add soda to your diet and change nothing else – exercise no more than you did before – you will gain weight. But soda has not been demonstrated to be worse for weight gain than any other form of calories, whether from milk, meat, fruits or chocolate cake. The evidence that soda calories are somehow different from other calories is weak, and has not been accepted by the experts charged by the government with advising the public on diet and health.

In terms of a direct relationship between soda consumption and weight gain or loss, systematic reviews, which evaluate the weight of evidence from numerous studies and rank the results according to the most rigorous experimentation, show widespread disagreement about what the evidence says:

“Not conclusive” (Bachmann et al. 2006, no declared industry funding)
“Equivocal” (Pereira et al. 2006, no declared industry funding)
“Strong” (Malik et al. 2006, no declared industry funding)
“Probable” (World Cancer Research Fund, 2007, no declared industry funding)
“Strong” (Vartanian et al. 2007, no declared industry funding)
“Near zero” (Forshee et al. 2007, industry funded)
“Limited evidence” (Gibson S, 2008, independent consultant, industry funded)
“Open to debate” (Wolff et al. 2008, no declared industry funding)
“Moderate epidemiologic evidence” (USDA dietary guidelines, 2010)
“Difficult to discern” (Mattes et al. 2010, partly funded by National Institutes of Health; authors declared previous industry funding)

The systematic reviews that did not find strong evidence all cited methodological problems with individual studies claiming a positive association between soda and weight gain (for example some studies didn’t control for physical activity, so if soda drinkers are also more lethargic, the correlated obesity may be due to lack of activity).

One of the reviews that found “strong” evidence (Malik et al., 2006) reported several studies as showing a positive correlation, writes Gibson, when only some of the findings within these studies were, in fact, positive. Two of the 30 studies in this review were also confounded by the inclusion of diet drinks.

Gibson, who looks at the quality of the other systematic reviews in addition to the individual studies, also notes that the World Cancer Research Fund review was based on just six studies (the other reviews ranged from 12 to over 80 studies) and that the finding was bumped up in the final editing from “limited-suggestive” evidence for a link to “probable.”

Malik et al. also took aim at Forshee et al. for “analytic errors in their meta-analysis;” Foreshee et al. made corrections but said they didn’t affect the substantive conclusions.

It’s also worth noting a recent and related major systematic review of the evidence on sugar and health (Ruxton et al. Is Sugar Consumption Detrimental to Health? A Review of the Evidence 1995–2006, Critical Reviews in Food Science and Nutrition, 50:1–19, 2010) found: 

“In the case of obesity, the evidence did not reveal a positive relationship with sugar; indeed some studies showed an inverse relationship.” As for sugar sweetened beverages, “there were suggestions of a greater risk of weight gain when subjects drank considerable amounts of SSB.”

This review was funded by the Sugar Bureau  and the authors note that “a condition of which was that Sugar Bureau representatives or associates played no role in the selection or methodological assessment of papers, nor in the interpretation or writing up of the results.”

This, however, raises an important concern, frequently articulated in scientific research where there are commercial interests: has industry funding contaminated the data on soda and weight gain? Some researchers believe it has.

 

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