Tuesday, October 28, 2014

Gary Taubes and the First Law of Thermodynamics

It has been my observation that many people don't understand the application of the First Law of Thermodynamics (FLT) especially how it applies to the discussion of diet and nutrition, and the biological system that is a human being, or any other animal.

Gary Taubes is a science journalist who has stirred controversy in his writings about the FLT. We hope to explain what is going on here.

The FLT states that the change of energy in a system is the difference between energy going in and energy going out. This seems rather obvious. If we say that a person has gained weight, the FLT says that she has taken in more energy than she expended.

Let's label the statement "person has gained weight" as "A", and the statement "person took in more energy than was expended" as "B". The FLT says only that A is equivalent to B. They are the same. A means B, and B means A. The FLT allows us to substitute one for the other, because they are equivalent.

Here's an example of how the FLT applies, wherein Gary understands, but Don does not:

Gary:  "I got fatter over the holidays"
Don:  "Well, you ate more calories than you burned."
Gary:  "Duh! I just said that!"

Now let's look at how to misapply the FLT. If I say A, and you say that's because B, we get the following:

Gary: "I have gained weight."
Don: "That's because you took in more calories than you expended."

Aha! Don is now espousing the usual energy-balance hypothesis of weight gain. But the FLT says that we can substitute in the above. The same argument stated differently (yet equivalently) becomes:

Gary: "I have gained weight."
Don: "That's because you have gained weight."

This above illustrates why any invocation of the FLT in this context is a tautology; it's just a restatement of the proposition. It doesn't explain anything -- it asserts no causation. The FLT does not say A causes B, or B causes A, it simply says that these two statements are equivalent: A means B. When one asserts such a causation, now that is no longer the FLT, it is a hypothesis. Correlation (in this case, exact correlation) is not necessarily causation!

It may well be true that overeating causes weight gain, but there is another hypothesis available to us, what Gary Taubes has called "the alternative hypothesis": weight gain causes overeating. This reverses the causality of the standard hypothesis, and we have not violated the FLT either way (in fact, it may be the case that one, both, or neither hypothesis is true; that is why we need more rigorous studies.)

One of Taubes' often repeated examples of the alternative hypothesis is explaining growth in children. Children as they grow take in more calories than they burn and gain weight, but it's not caused by overeating. It's caused by growth hormones pushing raw materials into the body to construct tissues, which causes great hunger and provokes compensatory eating in order to maintain baseline metabolism. Growth drives hunger and eating, it's not the other way around.

Now, since we've seen the energy balance hypothesis many times explained with an analogy to money in the bank, let's consider the following. Suppose a very financially naive person has an investment that pays her $100 dollars a month in interest. She can do arithmetic, however, and budgets very carefully, spending that $100 dollars a month, so the account balance regularly floats back to zero. Now suppose that luckily, interest rates go up. Now she receives $105 dollars per month, but still budgets for $100. Slowly her bank account fattens up. At her yearly review, she asks her financial advisor why she has gained wealth. The advisor says, "Well, that's because [$60] more went into your account than came out!"  Exasperated, she says, "Duh! I just said that!" and fires him. A different advisor counsels, "All you have to do is spend more money, and the balance will go down to zero again." He doesn't get the job either, but finally a third explains how financial interest works and the question is answered satisfactorily.

If we want to find an explanation for weight gain, we must look elsewhere than the First Law of Thermodynamics for an answer.

1 comment:

PhilT said...

The equation can be reshuffled in various ways :

Food you must eat = Food stored + Food used


Food your body can use for energy = Food you ate - food your body stored

and as you say these are all equivalances, not assingments like:

Let Food stored = Food eaten - Food used.

As well as "calories" conservation of mass means these can be grams of fat as an alternative to calories.