Whatever happened to the second law of thermodynamics?

Department of Biochemistry
State University of New York Downstate Medical Center
Brooklyn, NY 11203
E-mail: rfeinman{at}downstate.edu
Department of Nuclear Medicine
Jacobi Medical Center
Bronx, NY 10461

Dear Sir:

We found the recent review by Buchholz and Schoeller (1) to be surprising in several ways. First, there was no reference to our recent review "Thermodynamics and Metabolic Advantage of Weight Loss Diets" (2), which addressed the same subject but included a more comprehensive set of references and thereby strengthened Buchholz and Schoeller's observation that high-protein, low-carbohydrate diets frequently lead to greater weight loss calorie-for-calorie than do isocaloric diets of different composition (metabolic advantage). Still more surprising was that, in opposition to these experimental observations, they insisted that "a calorie is a calorie" anyway and invoked the first law of thermodynamics. In our own review (2) we pointed out that one cannot ignore 1) the second law of thermodynamics and 2) the fact that living organisms are open systems, far from equilibrium, and therefore subject to different efficiencies depending on metabolic path.

The second law says that there is a physical parameter called "entropy," which we identify with disorder or inefficiency and, whereas energy is always conserved, entropy is not (3). In any real (irreversible) process, entropy increases and no process is perfectly efficient. The consequence is that conservation of energy (ie, the first law) is maintained by exporting high-entropy compounds (principally carbon dioxide and water) into the environment. The extent to which energy and matter are distributed among heat, chemical bonds, work, and the excreted products is determined by the specific metabolic pathway used. In our review, we presented plausible mechanisms by which dietary composition can lead to different pathways of different efficiencies.

The first law of thermodynamics never exists in the absence of the second law. Both laws are inviolate, and they must be applied correctly.

REFERENCES

1. Buchholz AC, Schoeller DA. Is a calorie a calorie? Am J Clin Nutr 2004;79(suppl):899S-906S.
2. Feinman RD, Fine EJ. Thermodynamics and metabolic advantage of weight loss diets. Metab Syndr Relat Disord 2003;1:209-19.
3. Kondepudi D, Prigogine I. Modern thermodynamics. From heat engines to dissipative structures. Chichester, United Kingdom: Wiley & Sons, 1998.