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.

"Thermodynamics" of Weight Gain and Loss

A common belief among the proponents of calorie restriction and exercise as a means of weight loss is that "thermodynamics" (or more particularly, conservation of energy) says it must be so. The basic premise is that:

Energy-in - Energy-out = Energy-stored-as-fat

Therefore, if you want to lose weight (make Energy-stored-as-fat negative), then Energy-out must exceed Energy-in. This is then interpreted to mean that you must either eat less or exercise more to change this balance. People then go into elaborate calculations as to how many Calories you burn with various sorts of exercise and how many Calories are in different diets.

A few authors have recognized that this same equation can be interpreted a little differently. They argue that no one can actually regulate their energy intake precisely enough to have a constant weight over any length of time. Therefore Energy-stored-as-fat must be regulated by the endocrine system and be more or less fixed. Then, the thing that has to change when you eat more or less is the Energy-out; i.e., the body adjusts its energy expenditure (without a specific conscious exercise program) to burn available excess energy, and conversely, if you get an unusually large amount of exercise (run a marathon, for example), then you will naturally eat more to replace the energy used.

I believe the real picture is a good deal more complicated than any of these simple descriptions. As a physicist and thermodynamicist, I look at a "control volume" (a human body in this case), and consider all of the flows of mass and energy in and out. Any net differences must be stored internally. Inputs include the food and drink consumed and the air breathed in (most of the time, direct energy inputs [mechanical or thermal] can probably be ignored as small). Outputs include waste excreted, air breathed out (with a different composition!) and any mechanical work performed. It is important also to recognize the energy associated with differences in the internal potential energy associated with the any chemical changes that may occur between matter taken in and matter excreted. These represent a form of possible difference between energy-in and energy-out.

So, consider someone who is eating, say, an "extra" 1000 Calories a day. He could store it as a quarter pound of fat and gain a couple of pounds per week. He could get restless and move around a lot more, expending at least some of the excess as mechanical energy. His peripheral capillaries could dilate so that he loses more energy thermally through his skin. He could simply excrete some of it as undigested. I have always suspected, for example, that there is an upper limit to the rate at which food can be processed in the body, and if you exceed that limit by binge-eating or feasting on a particular day, then most of the excess is simply excreted rather than stored.

People often report that even large increases in the amount of exercise they get does not result in weight loss. Some portion of this may be a consequence of muscle development ("good" weight gain). But to a large extent, it is very hard not to just eat more in response to the body's repair and recovery processes after exercise.

Similarly, if you try to lose weight by starving yourself, your body tends to try to conserve energy to compensate (and you may be chronically suffering as your body tells you to eat more!). People who literally go on starvation diets (either intentionally or as a result of disaster) often report a permanent reduction in their "metabolism" to the point where they eat much less after the period of starvation (or quickly gain back any weight lost).

All of this means, that if you really want to change body weight (either up or down), you really need to be tinkering with the endocrine system to stimulate release or storage of fat. That's rather complicated, and I don't understand it very well myself. It's a bigger topic than I want to tackle today, anyway. Moderate amounts of exercise are probably beneficial. But probably the simplest thing that most people can do is to control carbohydrate consumption, particularly simple sugars and starches (including those found in "whole" grains and most fruits). These tend to set off a chain of hormonal events including a large spike in insulin production, triglyceride production and ultimately fat storage. If you keep your insulin and triglyceride levels low, you tend to take fat out of storage instead.

Anyway, the purpose of this post is not to promote any particular approach to weight management, but merely to argue that if you're going to invoke "thermodynamics" in discussing it, make sure you are properly accounting for all the energy inputs, outputs, and transformations that are occurring, and be very careful that you are drawing the right conclusions.