Researchers studying the effect of various food policies on changes in weight often use the simple saying that a change of 3500 kcal via diet or exercise results in a 1 lb change in weight (it is a claim we repeated in our paper in the Journal of Health Economics, which studied the efficacy of fat taxes).
I ran across this interesting post by a Mike Gibney, a public health and nutrition expert, who points out some problems with the 1 lb = 3500 kcal calculation. Here's what he has to say:
Firstly, a 1lb weight loss will not be 100% fat but will also involve the loss of some lean tissue (muscle and protein elements of adipose tissue and its metabolism). Whereas fat has an energy value of 9 kcal/g, lean tissue has a value of 4 kcal/g. The exact ratio of the loss of lean and fat in weight reduction depends largely on the level of fat in the body at the outset.
The second criticism of this rule is that it ignores time. If you shed 3,500kcal per week every week, that would differ from a deficit of 3,500 kcal per month every month. The former leads to a daily deficit of 500 kcal while the latter is just 117 kcal.
Thirdly, the 3,500 kcal rule assumes complete linearity – in other words the rule equally applies, pound after pound of weight loss. We saw above that progressive weight loss will progressively increase the % of that weight loss as lean tissue but more importantly, the 3,500kcal rule ignores a major adaptation in energy expenditure
and on this topic, he concludes:
Clearly, the continued use of the 3,500 kcal rule in predicting weight loss should cease and the recommendations of the consensus statement of the ASN and ILSI should apply: “Every permanent 10 kcal change in energy intake per day will lead to an eventual weight change of 1lb when the body reaches a new steady state. It will take nearly a year to achieve 50% and about 3 years to achieve 95%”.
Finally, I'll point out the importance of taking into account these kinds of issues when calculating the effects of fat taxes. He says that according to one study, a 20% soda tax would lead to;
a reduction of energy intake of 34-47 kcal per day for adults. Using the 3,500 kcal rule, an average weight loss of 1.60kg would be predicted for year 1 rising to 8kg in year 5 and to 16kg in year 10. However, when the dynamic mathematical model is used, the corresponding figures for years 1, 5 and 10 are, respectively, 0.97, 1.78 and 1.84 kg loss. The % of US citizens that are over-weight is predicted to fall from existing levels of 66.9% over-weight to 51.5% over-weight in 5 years time using the 3,500 kcal rate but using the dynamic mathematical model, the 5-year figure for the over-weight population in the US would be just 62.3%.