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.
and:
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.
and:
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%.