Dieting results in long term changes to hormones and muscle fibers.

[eridani]

277. When did I ever say "don't bother changing your diet'?

What I have been saying is that healthier diets and more exercise ought to be ends in themselves, not means to some pre-defined weight loss goals, because the only thing you can control is your behavior. The same behavior change has different results for different people, and THAT is what you can't control. Shifting metabolic equilibria in the direction of weight loss typically results in some weight loss, but actual results vary from not much to a great deal. The midpoint for most people who are actually fat starting out results in becoming a fat person who weighs less, but is almost certainly healthier. Given the known benefits of healthier diets and more exercise, the emphasis on weight is insane.

Twin studies clearly demonstrate that variable results are under genetic control

http://www.nejm.org/doi/full/10.1056/NEJM199005243222101

We undertook this study to determine whether there are differences in the responses of different persons to long-term overfeeding and to assess the possibility that genotypes are involved in such differences. After a two-week base-line period, 12 pairs of young adult male monozygotic twins were overfed by 4.2 MJ (1000 kcal) per day, 6 days a week, for a total of 84 days during a 100-day period. The total excess amount each man consumed was 353 MJ (84,000 kcal).

During overfeeding, individual changes in body composition and topography of fat deposition varied considerably. The mean weight gain was 8.1 kg, but the range was 4.3 to 13.3 kg. The similarity within each pair in the response to overfeeding was significant (P<0.05) with respect to body weight, percentage of fat, fat mass, and estimated subcutaneous fat, with about three times more variance among pairs than within pairs (r ≈ 0.5). After adjustment for the gains in fat mass, the within-pair similarity was particularly evident with respect to the changes in regional fat distribution and amount of abdominal visceral fat (P<0.01), with about six times as much variance among pairs as within pairs (r ≈ 0.7).

Things to note-

1. Study subjects were overfed by exactly the same number of kilocalories. At 3500 kcal per pound of fat, the bomb calorimeter theory of human metabolism says that every single one of them should have gained 24 lbs.

2. That did not happen. The pair of twins gaining the least weight gained 9.5 lbs. The pair of twins gaining the most gained 29.3 lbs. Average gain was 17.9 lbs, less than predicted from excess calorie consumption.

3. What each twin gained was pretty much directly predictable from what his twin gained.

4. Why would anyone think that weight loss is any different?