In human thermodynamics, dietary thermodynamics is the study of the relation between thermodynamics and weight loss or weight control or balance, referring to the subject of applying the laws of thermodynamics to facilitate weight loss or weight control or to develop diet theories on how to lose weight or control weight.

Overview
It is difficult to pin down exactly who originated this subject, although it does seem to have a loose historical connection to body building, if not dieting in general; generally seeming to be a subject that arose in the 2000s.

In 2011, diet expert Zoe Harcombe explained how the laws of thermodynamics have been misapplied, in recent years, to weight loss theory, and that in addition to the first law, the second law needs to be factored into diet theory to quantify available energy and the overall energy balance of a person. [1]

First law
The central debate in diet theories, concerning thermodynamics, seems to be how to going about applying the thermodynamics to understand weight loss formulaically. The original old school method seems to be the “calorie counting” method of weight loss, often theorized to be a first law based model, such that weight loss formulaically would be defined as:

$W_L = \frac{C_{out} - C_{in}}{3,600} \,$

where WL is the amount of weight loss (in pounds), Cout is calories burned, and Cin is calories taken, such as via food substance or liquid calories. In this model, if, for instance, a typical person goes on a low calorie diet, say Cin = 800 calories per day, and goes about their normal day, thus equating Cout to the typical daily metabolic rate of say Cout = 1600 calories per day, then one would formulaically conclude that a person who adheres to this formula to the calorie would expect to loose one pound ever 4.5 days.

Second law
In recent years, some have begun to argue that the second law needs to be factored into the weight loss equation. A simple model in this direction would argue that weight loss is a isothermal-isobaric process of the state of a human molecule on going from an initial state to a final state, and that the Lewis inequality would have to hold in order for this process to occur:

A first issue in this type of analysis is the "turnover rate" factor, namely the difficult issue of differentiating between human-surface interactions and human-human interactions, the former of which, according to standard model, being attributed to activation energy lowering or rising factors, the latter of which being attributed to enthalpy and entropy change factors.

Along these lines, one may well note that "mental factors", generally deemed an entropy factor, such as one's serotonin level would play a key role in the weight loss equation. It is known, for instance, that when one is in a stable strong positional relationship (not depressed), that serotonin levels are high and that the feeling of hunger drive is low; whereas, conversely, when one in unstable positional relationships (depressed) or in fact completely unbonded with no sort of stability, that serotonin levels are low, and resultantly the hunger drive is high, whereby, resultantly food consumption tends to increase and weight tends to gain. This approach tends to explain why depression and obesity tend to be linked.

Activation energy
The general model as to how to go about quantifying atomic turnover in humans is that this factor plays a role in the activation energy of reactions between humans, in the sense that food intake and hence atomic turnover are a substrate factor, i.e. an interaction factor with the surface of the earth, which acts to facilitate human-human reactions, in the same manner as does the oxide-embedded iron surface, in the Haber process, act as a catalyst that works to lower the activation energy barrier to reaction between H2 and N2 just as does a fertile earth surface facilitate reactions between a male Mx and female Fy human molecule. There are, however, a number of issue to be worked out in overall nature of this subject applied to human-human reactions. [2]

Summary
In any event, sorting out the thermodynamics of dieting, in thermodynamic terms, i.e. whether diet and weight loss is best quantified as an either an enthalpy, entropy, activation, or free energy factor or some other attribute, is a difficult subject.

Objections
Many argue that thermodynamics has nothing to say in regards to weight loss, often tending to object to the first law method of weight loss diet formulas, arguing to the effect that all calories are not the same, or something to this effect. [3]

References
1. (a) Zoe Harcombe (obesity researcher) – ZoeHarcombe.com.
(b) Harcombe, Zoe. (2011). “Thermodynamics and Weight Loss” (V), ZoeHarcombe, Mar 9.
2. (a) Thims, Libb. (2007). Human Chemistry (Volume One). Morrisville, NC: LuLu.
(b) Thims, Libb. (2007). Human Chemistry (Volume Two). Morrisville, NC: LuLu.
3. Tunjic, Luka. (2005). Biomechanics and Weight Loss (pg. 12). Lulu.