A lot of the nutrition researchers continue to argue that the body needs to obey the laws of thermodynamics, that is energy in minus energy out has to equal weight gained or weight lost. Using that definition, the way to lose weight is to create a negative caloric balance (by cutting food or increasing energy expenditure) such that the weight disappears.
That viewpoint neglects the fact that the body is not a closed system, and the internal cellular metabolism depends on many aspects, one of which is thought to be the macronutrient composition of the food that goes in. For example, protein is known to have a thermogenic affect, which increases metabolism slightly.
I read a study tonight which was trying to compare diets, but managed to make another point (one they probably didn’t realize). The study followed 811 people over the course of two years, and watched what happened as each of them dieted using diets slightly different in composition. The target calorie intake was targeted at 750 kCal below baseline.
If you subscribe to the theory that a 750 kCal deficit will ultimately lead to weight loss, then it is rather easy to compute the theoretical weight reduction over the course of two years. 750 kCals a day is 273,750 kCal a year. Since there are 3500 kCal in a pound, that means over the course of a year each person should have lost around 78 lbs.
What were the results? Peak weight loss was achieved at 6 months, and averaged around 6 kg (15 lbs). By the end of two years, the average weight loss was only 4 kg (around 10 lbs). So, according to the conventional wisdom, these people should have lost 78 lbs on average, but only lost around 10 lbs.
The most obvious answer is to simply eat less. This advice is logically satisfying as energy balance is clearly tipped in a negative direction, at least temporarily, and serves as a conveniently simple treatment sound-bite for a 15 min doctor’s visit. However, evidence that meaningful weight loss is sustained for more than a few years following a low-calorie diet is hard to find. The reason for this lack of sustainability is thought to include both environmental influences and a complex regulatory system in the body, which is geared towards limiting weight loss during hypocaloric states and restoring baseline body weight, even when that baseline weight is considered socially and medically undesirable.
You can read the whole study here if you want