More on the Thermodynamics of Body Weight

The previous post was intentionally general with no specific finger-pointing to misuse or good use of thermodynamic arguments. The misuse is so widespread, that I still prefer not to provide specific references. However, it is worth pointing to a few recent papers that have taken the issue more seriously.

First, I should note that to a high percentage of even trained scientists and doctors, "thermodynamics" means "equilibrium thermodynamics." This is all that is taught in a typical introductory course. Unfortunately, the human body can rarely be modeled accurately as being in any sort of equilibrium. At best, it may be reasonable (at least for some time scales) to model it as being in something approaching a "steady-state." As such, a complete analysis must include a study of "dynamic," "kinetic," or "nonequilibrium" effects, i.e., you must look at all inputs and outputs, and potentially the rate of change of inputs, outputs, and the state of what's inside.

One recent paper which tackles the issue head-on is by Feinman and Fine: "Nonequilibrium thermodynamics and energy efficiency in weight loss diets," Theoretical Biology and Medical Modelling 2007, 4:27. The authors specifically focus on the greater weight loss observed in low-carbohydrate diets with an emphasis on the specific "kinetics" of fat storage and dissipation arguing that simple equilibrium models fail, because real bodies, and especially real bodies that are in transition (gaining or losing weight) can be far from equilibrium requiring the consideration of dynamic effects. While they find that no experiment exists that measures all relevant variables, they are able to find evidence that dietary carbohydrate controls fatty acid storage and release via its effect on hormone levels (particularly insulin) and that this "nonequilibrium" effect can explain the greater weight loss of low-carb diets.

Another paper by Schulz and Schoeller, "A compilation of total daily energy expenditures and body weights in healthy adults," Am J Clin Nutr 1994, 60:676, reviews 22 studies which use an important (if somewhat expensive) technique for measurement of what they call "total expenditure energy" (how much energy is used over, say, a day or a week for all physical activity. The technique uses "doubly labeled water" (both the hydrogen and oxygen atoms are non-standard isotopes so that the differential kinetics of hydrogen and oxygen can be measured). The authors present data for various populations from elite athletes to normal and overweight individuals. Most notably, the total daily energy expenditure is found to vary by a factor of more than 3. To a limited extent, they were able to separate out the contributions of basal metabolic rate and physical activity, fat-free body mass and excess fat. While not specifically focused on thermodynamic equilibrium, it does add further evidence against any simple "calorie is a calorie" model of diet and weight.

And not being completely unable to resist highlighting one paper that is unwilling to draw the right conclusions from its own data, consider Brehm et al, "The role of Energy Expenditure in the Differential Weight Loss in Obese Women on Low-Fat and Low-Carbohydrate Diets," J Clin Endocr & Metab, 2005, 90:1475. Their paper reports the results of weight loss comparisons for obese women following the two diets for six months. The Low-carb group lost 50% more weight despite not being specifically calorie-restricted. (Food diaries indicated that the caloric intake was nevertheless similar between the two groups.) Not being able to measure any significant difference in resting energy expenditure or physical activity, the authors fell back on the old standby: the low-fat dieters must have cheated and underreported actual food consumption! The possibility that the results could be real apparently wasn't acceptable.