* A study has just been published, which claimed to have definitively demonstrated that 'fast' carbs do not make you fat. The paper was about the glycemic index (GI), which is a flawed and imprecise measure. 'Fast' carbs are considered to be those with a high(er) GI and 'slow' carbs are seen as those with a low(er) GI.
* The study claimed to have reviewed two things:
1) Observational studies reporting associations between BMI and dietary GI.
2) Meta-analyses (pooling together) of Randomised Controlled Trials (RCTs) comparing low-GI and high-GI diets for weight loss.
(1) was accurate, (2) wasn't.
* The researchers reviewed 35 observational studies. However, none of these observational studies examined the association between BMI and GI. They were about GI and other things (e.g., cancer or heart disease) and they merely reported BMI because it would be needed as an adjustment factor in the original study. Nothing relevant to the association between BMI and GI was therefore adjusted for – energy intake, exercise, total carbohydrate intake, whether trying to lose weight or not.
* There were eight meta-analyses of RCTs to review. Five did not compare low-GI and high-GI diets for weight loss. They too were about other conditions – mostly heart disease and diabetes. The three that did examine GI and weight (loss) all found significant differences in favour of the lower GI group. By significant we mean statistically significant, not large.
* The paper had major conflicts of interest with the Grain Foods Foundation and grains/carbohydrates generally. It did not definitively show that "'fast' carbs don’t make you fat.” It might have shown how research can be (mis)used to make desired claims. It might have shown how useless GI is as a measure. It might have shown that 'fast' and 'slow' carbs are as bad as each other!
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