Effect of reducing total fat intake on body weight: systematic review and meta-analysis of randomised controlled trials and cohort studies

Dear Dhastagir, Johannes, Uffe and Stephen,

Thank you all for your interesting discussion on our research!

Dhastagir – thank you, and yes, this about our culture of food, isn’t it?

Johannes – perhaps I should stress that we are not talking about weight reducing diets here, our paper is about how we eat permanently. If we ensure that how we eat day to day, year to year is a bit lower in fat and that the remaining fat is modified (so, we cut down on saturated and trans fats) permanently we know that this will reduce our weight and protect us from cardiovascular events. And no, we have no competing interests in this.

If advice to eat a lower fat diet is so bad, as some letters suggest, one would expect the results of the RCTs to show considerable weight gain on the lower fat diets as well as deterioration of lipids and blood pressure. This was not the case – maybe we should ask, "why?". To dismiss the result as irrelevant because it does not agree with our preconceived notions serves no purpose in solving the problem of obesity. Our meta-analysis provides important information based on "real" long term clinical trials. Maybe as scientists we should think about its implications for population nutritional recommendations. Evidently, people who switch to a lower fat diet make changes to their diet that help control body weight and other CVD risk factors. Maybe it is unfair to assume that those switching to lower fat diets increase their energy intake by increasing consumption of high GI foods loaded with free sugars. Of course it is important to know what is best to replace the fat, but our paper cannot answer this question.

Uffe – we did not include the study by Cornier as it aimed to reduce the weight of 21 heavy women (and was also too short, running for only 16 weeks). We are not reporting on how well weight loss trials work, or which work better – we are looking at long term eating patterns (which was why we excluded all studies that aimed to reduce weight in any of the participants).

Stephen – yes, the studies varied in exactly what they replaced the fat calories with. Generally there did seem to be an increase in carbohydrates across the studies, but it was usually not clear whether this was due to increases in refined or less refined carbohydrates, and the effect on protein did vary. But despite this the weight loss was remarkably consistent, and was greater when the reduction in fat was greater.

We agree with you all that the main dietary message for everyday eating for people and populations is not going to be “cut down on fat”. Of course not. However, there is VERY good evidence that it should be part of the message, and as Stephen suggests maybe the issue is what we begin to replace the saturated and trans fats we cut down on with.

Best wishes to all of you for the festive season, Lee and Murray (on behalf of all the authors)