Saturday, October 06, 2007

Thermodynamics and weight loss

The following is a post about thermodynamics on the Michael R. Eades blog. It makes for a great read and helps explain why a calorie in vs calorie out is not really the case.

Probably no laws of physics have been so over invoked and less understood than the laws of thermodynamics. Everyone it seems is using the laws of thermodynamics to justify every position imaginable in the field of weight loss. Journalists often throw out the laws of thermodynamics to prove or disprove dietary regimens they’re writing about. Authors of various blogs and other online sites rabbit on about how the laws of thermodynamics are aligned with their pet theories. And even worse, research scientists - who really should know better - more often than not misquote the laws of thermodynamics, especially when talking about the possibility of a dietary metabolic advantage. ‘It can’t be valid,’ they sniff, ‘it violates the laws of thermodynamics.’
So, I figured is was time to delve into these mysterious laws so that readers of this blog at least can know thermodynamic nonsense when they see it.
When you get a grasp of the laws of thermodynamics it becomes pretty easy to see how they can be confusing not only to the great unwashed masses but even to scientists who have never really taken the time to study them. Thermodynamics are seemingly simple at first glance, but the more you dig into them, the more complex they become. To see what I mean, take a look at the
syllabus for the thermodynamics course at MIT and skim through a few of the lectures.
Before we jump into these laws, I want to show you why scientists typically heap scorn on anyone who claims to have somehow violated the laws of thermodynamics.
The author of a
book of thermodynamics that I have writes the following:
No violation of any law of thermodynamics is known to have occurred in over 200 years of research in this area.
Most physicists consider the Second Law of Thermodynamics the most universal ‘governor’ of natural activity that has ever been revealed by scientific study.
Sir Arthur Eddington wrote in 1915
If someone points out to you that your pet theory of the universe is in disagreement with Maxwell’s equations — then so much the worse for Maxwell’s equations. If it is found to be contradicted by observation — well, these experimentalists do bungle things sometimes. But if your theory is found to be against the second law of thermodynamics I can give you no hope; there is nothing for it but to collapse in deepest humiliation.
And Ivan Bazarov wrote the following in a thermodynamics text from 1964:
The second law of thermodynamics is, without a doubt, one of the most perfect laws in physics. Any reproducible violation of it, however small, would bring the discoverer great riches as well as a trip to Stockholm. The world’s energy problems would be solved at one stroke. It is not possible to find any other law (except, perhaps, for super selection rules such as charge conservation) for which a proposed violation would bring more skepticism than this one. Not even Maxwell’s laws of electricity or Newton’s law of gravitation are so sacrosanct, for each has measurable corrections coming from quantum effects or general relativity. The law has caught the attention of poets and philosophers and has been called the greatest scientific achievement of the nineteenth century.
Now that you somewhat understand the strong feelings of those in the know about thermodynamics, you can see why they would disparage anyone purporting to break or repeal these laws. And it helps to understand the vituperation heaped on Robert Atkins who wrote one of the most hubristic and outright ignorant statements imaginable showing a total lack of understanding of the laws of thermodynamics when he said:
When I make this claim, that you can lose more weight on a higher number of calories, I seem to be breaking the law—one of the hallowed laws of thermodynamics. Many powers-that-be get terribly provoked when I repeal their laws. But the calorie theory is a false law that is meant to be broken, and ketosis/lipolysis is the instrument for breaking it.
As reported in Gary Taubes
Good Calories, Bad Calories, this comment and others like it may have lead John Yudkin to say of Atkins’ book that its “chief consequence [may have been] to antagonize the medical and nutritional establishment.”
But, since Atkins wasn’t really a physicist, it’s easy to see how he could have become confused.
There are four laws of thermodynamics, but we’re going to concern ourselves in this post only with the first and second laws. The other two laws - the zeroth law and the fourth law involve temperature, are highly theoretical, and aren’t really relevant to the discussion at hand.
The first law of thermodynamics is the conservation of energy law and states that energy can neither be created nor destroyed. Another way of stating this law is to say that the energy of a system plus surroundings is constant in time. This first law is where the mistaken idea that ‘a calorie is a calorie’ that misguided people always want to parrot comes from. And on the surface it seems to make sense. If energy can’t be created or destroyed why wouldn’t a calorie always be a calorie? That’s where the second law comes in.
The second law of thermodynamics says that the entropy of the universe increases during any spontaneous process. What this means is that it is impossible for a system to turn a given amount of energy into an equivalent amount of work. It is this second law that is really the ‘a calorie is a calorie’ law, and, in fact, the second law shows, in terms of weight loss at least, that a calorie isn’t necessarily a calorie.
These two laws of thermodynamics can be summed up cleverly. The first law says you can’t get something for nothing, and the second law tells you that you can’t break even.
Since it’s the second law that applies to living, breathing animals, and since it is the one most often confused in the calorie issue, let’s look at it a little more closely. The second law is the law driving chemical reactions, and since we’re nothing but a bunch of walking chemical reactions it is the one that applies most to us.
The second law is a dissipation law in that it says that in any reaction that is irreversible (most of the chemical reactions that give us life) there is a loss or dissipation of energy in that reaction. If substance A converts to substance B via a chemical reaction in the body, then substance B has a lower energy than substance A. In other words energy is lost to the universe in that reaction. There is no reaction that doesn’t end up without a loss of some energy to the universe. This loss of energy is called entropy.
The second law can kind of be summed with this equation:
calories in = calories out + entropy
If we substitute numbers in the above equation it could look like this:
100 calories in = 70 calories out + entropy
If we solve this equation for entropy, we can see that entropy is 30 calories. Or, in this case, 30 calories of energy are lost.
The larger the number for entropy, the more inefficient the system is, i.e., more energy lost from the system forever.
For example, when you drive a car only about 10-12 percent of the energy contained in the gasoline actually is converted to the work of propelling the car - the rest is lost to heat (entropy). This irretrievable loss is the reason a perpetual motion machine can never be built although many have tried. No matter how efficiently such a machine might be designed it will ultimately run down because of these little energy (entropy) leaks here and there. (I’ve used entropy as if it is synonymous with energy when in technical terms it really isn’t, but it’s easier to think of it that way.)
How does this apply to weight loss?
Each of the many chemical reactions in the body end up dissipating energy. We get our energy in the form of calories from the food we eat. This energy gets consumed in all the countless chemical reactions that go on all the time. Just like an automobile, we are not all that efficient. We don’t convert calories to energy on a one to one basis because of the loss of energy to the universe described by the second law.
This is all basic stuff, but it gets interesting when we start to look at how the different macronutrients (fat, protein and carbohydrate) affect the process.
As I’ve
discussed in this blog frequently, we need to maintain our blood sugar in a fairly narrow range. We need blood sugar to supply energy to certain cells that can’t use it in any other form (the red blood cells, some brain cells and others). We can get plenty of sugar into our blood and have no trouble keeping our blood sugar up if we eat carbohydrates. The carbohydrate-containing foods get broken down into their sugar molecules that are then absorbed from the intestines directly into the blood. In our high carb world our problem isn’t too little sugar but too much. But in the early years of our existence on the planet it wasn’t like this. We didn’t have access to the bounty of easily absorbed carbs that we do today, yet we still had the need for sugar in our blood. As a consequence we evolved mechanisms to convert other nutrients - primarily protein - into sugar.
If we have a diet containing plenty of carbohydrate, the carbohydrate goes into the blood as sugar. There are very few chemical reactions along the way, and there is a loss of energy to the universe with each of these reactions. But, since there aren’t many conversions, there isn’t a lot of energy loss.
If we have no carbohydrates (or few) in the diet, however, it’s a different story. In order to maintain the necessary sugar level in the blood the body is forced to make sugar out of protein, which isn’t a simple operation. Look in any basic biochemistry textbook and you can see all the reactions required to convert protein to sugar, and each one of these reactions consumes energy just to take place but loses energy to the universe in the process as well. It’s much less efficient for the body to convert protein to sugar than it is to simply take the sugar as it comes in already formed.
The second law of thermodynamics virtually mandates that there be a larger loss of energy when one has to convert protein to sugar instead of merely using the sugar as it comes in. Since there are 4 kcal of energy in a gram of sugar and 4 kcal of energy in a gram of protein, it should be apparent that less of the 4 kcal in a gram of sugar will be dissipated than will be the 4 kcal in a gram of protein if this gram of protein has to first be converted to sugar.
And, consequently, one would think that a diet low in carbohydrate and higher in protein and fat (both of which have to be converted to sugar) would bring about a greater weight loss than a diet of the same number of calories but with higher levels of carbohydrate. In fact, the second law of thermodynamics predicts this very phenomenon. But despite this rather obvious notion that complies perfectly with the second law, many ignorant people continue to cling to the idea that ‘a calorie is a calorie’ despite that idea flying in the face of the second law. I suppose these people discount the second law. If so, then they should spend their time putting together a perpetual motion machine, which, if they could, would garner them a lot more fame than their inane posturing on the inevitability of the second law might do.
A classic example of how the second law works is in the difference between regular and premium gasoline. Both regular and premium have the same exact number of calories per gallon, but premium burns more efficiently. In other words, the calories contained in the premium gas get ‘wasted’ at a lower percentage in propelling the car along the road than do the calories in the regular. A high-performance automobile designed to squeeze the most out of a gallon of gas will get better mileage on premium than on regular gasoline, yet the calories in are exactly the same.
In the human body this inefficiency can be measured as an increase in metabolic rate and an increase in body heat being produced under laboratory conditions. One would assume that since the second law is inviolable and always in operation that people eating a diet low in carbohydrates and high in protein would produce more heat than those consuming the same number of calories but composed of a much higher percentage of carbohydrates. And that is exactly what is found.
In a paper (full text
here) published in the Journal of the American College of Nutrition researchers examined this effect in ten healthy young women who consumed either a high-protein, low-carbohydrate or a lower-protein, higher-carb diet of the same number of calories. The researchers used these women as their own controls, providing them with the first diet followed by measurements in the lab, then 54 days later with the second diet and lab evaluation.
Precise measurement of heat and metabolic rate showed that when the women followed the high-protein, low-carb diet they produced almost twice as much heat as they did when consuming the higher carb diet of the same calories. In the higher-carb diet the entropy was smaller than in the higher-protein diet, which would be expected from the second law.
As the authors of the paper put it:
These data demonstrate that meal-induced thermogenesis at 2.5 hours post-meal averages about twofold higher on a HP, low fat diet versus a HC, low-fat diet. Generally, postprandial thermogenesis has been associated with the protein content of a meal, and our data confirm this relationship. However, the difference in the energy cost of HP versus HC diets, particularly in the context of weight loss promotion, has not been addressed by healthcare professionals. Increased diet-induced thermogenesis, in association with the preservation of REE [resting energy expediture], may contribute to the reported weight loss success of diets high in protein with moderate levels of carbohydrate and lends credence to the observation that weight loss on HP diets is predominately body fat, not body water.
Bear all this in mind the next time you tell someone that it is possible to lose more weight on a greater number of calories as long as those calories are low-carb calories, and that someone pooh poohs you with the old ‘That can’t be possible. It violates the laws of thermodynamics. A calorie is after all a calorie.’ Ask them precisely which laws of thermodynamics it violates and ask them to tell you how. Then sit back and watch the fun.

1 comment:

Anonymous said...

We should have taken nutritious, fibrous and low calorie foods, that is the Thermodynamics low.