Archive for the ‘thermodynamics’ Category

 

One of my favorite legal terms, collateral estoppel, refers to procedures to prevent re-litigation of issues that have already been settled in court. From the same root as stopper, that is, cork, it prevents harassment and wasting of the court’s time. The context is the recent flap over a poster presented by Kevin Hall which has started re-trying the case of whether all diets have the same metabolic efficiency, a question which, in my view, has been adjudicated several times. I put it this way because frequently I have made an analogy between evidence-based-medicine (EBM) and evidence as presented in a court of law. My main point has been that, in the legal system, there are rules of evidence and there is a judge who decides on admissibility. You can’t just say, as in EBM, that your stuff constitutes evidence.  My conclusion is usually that EBM is one of the self-congratulatory procedures that allows people to say anything that they want without having to defend their position. EBM represents one of the many corruptions of research procedure now under attack by critics (perpetrators ?) as in the recent editorial by Richard Horton, editor of The Lancet. One thing that I  criticize medical nutrition for is its inability to be estopped from funding and endlessly re-investigating whether saturated fat causes heart disease, whether high protein diets hurt your kidneys, and whether a calorie is a calorie. It seems that the issue is more or less settled — there are dozens of examples of variable energy expenditure in the literature. It would be reasonable to move on by investigating the factors that control energy balance, to provide information on the mechanisms that predict great variability and, most important, the mechanisms that make it so small in biological systems — most of the time, a calorie is a calorie, at least roughly. Funding and performing ever more expensive experiments to decide whether you can lose more or less weight on one diet or another, as if we had never done a test before, is not helpful.

Several bloggers discussed Hall’s study which claims that either a calorie is a calorie or it is not depending on whether, as described by Mike Eades, you look at the poster itself or at a video of Kevin Hall explaining what it is about. Mike’s blog is excellent but beyond the sense of déja-vu, the whole thing reminded me of the old joke about the Polish mafia. They make you an offer that you can’t understand.  So, because this is how I got into this business, I will try to explain how I see the problem of energy balance and why we might want this trial estopped.

I have taught nutrition and metabolism for many years but I got into nutrition research because the laws of thermodynamics were, and still are, invoked frequently in the discussion. Like most chemists, I wouldn’t claim to be a real expert but I like the subject and I teach the subject at some level. I could at least see that nobody in nutrition knew what they were talking about. I tried to show that the application of thermodynamics, if done correctly, more or less predicts that different diets will have different efficiencies (from the standpoint of storage, that is, weight gained per calorie consumed).

But you don’t really need thermodynamics to see this. Prof. Wendy Pogozelski at SUNY Geneseo pointed out that if you think about oxidative metabolic uncouplers, that is all you need to know. “Coupling,” in energy metabolism, refers to the sequence of reactions by which the energy from the oxidation of food is converted to ATP, that is, into useful biologic energy. The problem in energy metabolism is that the fuel, as in many “combustion engines,” is processed by oxidation — you put in oxygen and get out CO2 and water . The output, on the other hand  is a phosphorylation reaction — generation of ATP from ADP, its low energy form. The problem is how to couple these two different kins of reactions. It turns out that the mitochondrial membrane couples the two processes (together called oxidative phosphorylation). A “high energy” state is established across the membrane by oxidation and this energy is used to make ATP. Uncouplers are small molecules or proteins that disengage the oxidation of substrate (food) from ATP synthesis allowing energy to be wasted or channeled into other mechanisms, generation of reactive oxygen species, for example.

BLOG_car_analogy_May_16The car analogy of metabolic inhibitors. Figure from my lectures. Energy is generated in the TCA cycle and electron transport chain (ETC). The clutch plays the role of the membrane proton gradient, transmitting energy to the wheels which produce forward motion (phosphorylation of ADP). Uncouplers allow oxidation to continue — the TCA cycle is “racing” but to no effect. Other inhibitors (called oxidative phosphorylation inhibitors) include oligomycin which blocks the ATP synthase, analogous to a block under the wheels: no phosphorylation, no utilization of the gradient; no utilization, no gradient formation; no gradient, no oxidation. The engine “stalls.”

In teaching metabolism, I usually use the analogy of an automobile where the clutch connects the engine to the drive train . The German word for clutch is Kupplung and when you put a car in neutral your car is uncoupled, can process many calories of gasoline ‘in,’ but has zero efficiency, so that none of the ‘out’ does the useful work of turning the wheels. Biological systems can be uncoupled by external compounds — the classic is 2, 4-dinitrophenol which, if you are familiar with mitochondrial metabolism, is a proton ionophore, that is, destroys the proton gradient that couples oxidation to ADP-phophorylation.  There are natural uncouplers, the uncoupling proteins, of which there are five, named UCP-1 through UCP-5. Considered a family because of the homology to UCP-1, a known uncoupler, it has turned out that at least two others clearly have uncoupling activity. The take-home message is that whatever the calories in, the useful calories out (for fat storage or whatever) depends on the presence of added or naturally occurring uncouplers as well.

This is one of many examples of the mechanisms whereby metabolic calories-out per calorie-in could be variable.  The implication is that when somebody reports metabolic advantage (or disadvantage), there is no reason to disbelieve it. Conversely, this is one of the mechanisms that can reduce variability.

In fact, homeostatic mechanisms  are usually observed. You don’t have to have a metabolic chamber to know that your intake is variable day-to-day but your weight may be quite stable. The explanation is not in the physics which, again,  predicts variation, but rather in the biological system which is always connected in feedback so as to resist change. However strong the homeostasis (maintenance of steady-state), conversely, everybody has the experience of being in a situation where it doesn’t happen. “I don’t understand. I went on this cruise and I really pigged out on lobster and steak but I didn’t gain any weight.”  (It is not excluded, but nobody ever says that about the pancake breakfast). In other words, biochemistry and daily experience tells us that black swans are to be expected and, given that the system is set up for variability, the real question is why there are so many white swans.

So it is physically predicted that a calorie is not a calorie. When it has been demonstrated, in animal models where there is control of the food intake, or in humans, where there are frequently big differences that cannot reasonably be accounted for by the error in food records, there is no reason to doubt the effect. And, of course, a black swan is an individual. Kevin Hall’s study, as in much of the medical literature, reported group statistics and we don’t know if there were a few winners in with the group. The work has not been reviewed or published but, either way, I think it is likely to waste the court’s time.

 

…the association has to be strong and the causality has to be plausible and consistent. And you have to have some reason to make the observation; you can’t look at everything.  And experimentally, observation may be all that you have — almost all of astronomy is observational.  Of course, the great deconstructions of crazy nutritional science — several from Mike Eades blog and Tom Naughton’s hysterically funny-but-true course in how to be a scientist —  are still right on but, strictly speaking, it is the faulty logic of the studies and the whacko observations that is the problem, not simply that they are observational.  It is the strength and reliability of the association that tells you whether causality is implied.  Reducing carbohydrates lowers triglycerides.  There is a causal link.  You have to be capable of the state of mind of the low-fat politburo not to see this (for example, Circulation, May 24, 2011; 123(20): 2292 – 2333).

It is frequently said that observational studies are only good for generating hypotheses but it is really the other way around.  All studies are generated by hypotheses.  As Einstein put it: your theory determines what you measure.  I ran my post on the red meat story passed April Smith  and her reaction was “why red meat? Why not pancakes” which is exactly right.  Any number of things can be observed. Once you pick, you have a hypothesis.

Where did the first law of thermodynamics come from?

Thermodynamics is an interesting case.  The history of the second law involves a complicated interplay of observation and theory.  The idea that there was an absolute limit to how efficient you could make a machine and by extension that all real processes were necessarily inefficient largely comes from the brain power of Carnot. He saw that you could not extract as work all of the heat you put into a machine. Clausius encapsulated it into the idea of the entropy as in my Youtube video.

©2004 Robin A. Feinman

 The origins of the first law, the conservation of energy, are a little stranger.  It turns out that it was described more than twenty years after the second law and it has been attributed to several people, for a while, to the German physicist von Helmholtz.  These days, credit is given to a somewhat eccentric German physician named Robert Julius Mayer. Although trained as a doctor, Mayer did not like to deal with patients and was instead more interested in physics and religion which he seemed to think were the same thing.  He took a job as a shipboard physician on an expedition to the South Seas since that would give him time to work on his main interests.  It was in Jakarta where, while treating an epidemic with the practice then of blood letting, that he noticed that the venous blood of the sailors was much brighter than when they were in colder climates as if “I had struck an artery.” He attributed this to a reduced need for the sailors to use oxygen for heat and from this observation, he somehow leapt to the grand principle of conservation of energy, that the total amount of heat and work and any other forms of energy does not change but can only be interconverted. It is still unknown what kind of connections in his brain led him to this conclusion.  The period (1848) corresponds to the point at which science separated from philosophy. Mayer seems to have had one foot in each world and described things in the following incomprehensible way:

  • If two bodies find themselves in a given difference, then they could remain  in a state of rest after the annihilation of [that] difference if the  forces that were communicated to them as a result of the leveling of  the difference could cease to exist; but if they are assumed to be indestructible,  then the still persisting forces, as causes of changes in relationship,  will again reestablish the original present difference.

(I have not looked for it but one can only imagine what the original German was like). Warmth Disperses and Time Passes. The History of Heat, Von Baeyer’s popular book on thermodynamics, describes the ups and downs of Mayer’s life, including the death of three of his children which, in combination with rejection of his ideas, led to hospitalization but ultimate recognition and knighthood.  Surely this was a great observational study although, as von Baeyer put it, it did require “the jumbled flashes of insight in that sweltering ship’s cabin on the other side of the world.”

It is also true that association does imply causation but, again, the association has to have some impact and the proposed causality has to make sense.  In some way, purely observational experiments are rare.  As Pasteur pointed out, even serendipity is favored by preparation.  Most observational experiments must be a reflection of some hypothesis. Otherwise you’re wasting tax-payer’s money; a kiss of death on a grant application is to imply that “it would be good to look at.…”  You always have to have something in mind.  The great observational studies like the Framingham Study are bad because they have no null hypothesis. When the Framingham study first showed that there was no association between dietary total and saturated fat or dietary cholesterol, the hypothesis was quickly defended. The investigators were so tied to a preconceived hypothesis, that there was hardly any point in making the observations.

In fact, a negative result is always stronger than one showing consistency; consistent sunrises will go by the wayside if the sun fails to come up once.  It is the lack of an association between the decrease in fat consumption during the epidemic of obesity and diabetes that is so striking.  The figure above shows that the  increase in carbohydrate consumption is consistent with the causal role of dietary carbohydrate in creating anabolic hormonal effects and with the poor satiating effects of carbohydrates — almost all of the increase of calories during the epidemic of obesity and diabetes has been due to carbohydrates.  However, this observation is not as strong as the lack of an identifiable association of obesity and diabetes with fat consumption.  It is the 14 % decrease in the absolute amount of saturated fat for men that is the problem.  If the decrease in fat were associated with decrease in obesity, diabetes and cardiovascular disease, there is little doubt that the USDA would be quick to identify causality.  In fact, whereas you can find the occasional low-fat trial that succeeds, if the diet-heart hypothesis were as described, they should not fail. There should not be a single Women’s Health Initiative, there should not be a single Framingham study, not one.

Sometimes more association would be better.  Take intention-to-treat. Please. In this strange statistical idea, if you assign a person to a particular intervention, diet or drug, then you must include the outcome data (weight loss, change in blood pressure) for that person even if the do not comply with the protocol (go off the diet, stop taking the pills).  Why would anybody propose such a thing, never mind actually insist on it as some medical journals or granting agencies do?  When you actually ask people who support ITT, you don’t get coherent answers.  They say that if you just look at per protocol data (only from people who stayed in the experiment), then by excluding the drop-outs, you would introduce bias but when you ask them to explain that you get something along the lines of Darwin and the peas growing on the wrong side of the pod. The basic idea, if there is one, is that the reason that people gave up on their diet or stopped taking the pills was because of an inherent feature of the intervention: made them sick, drowsy or something like that.  While this is one possible hypothesis and should be tested, there are millions of others — the doctor was subtly discouraging about the diet, or the participants were like some of my relatives who can’t remember where they put their pills, or the diet book was written in Russian, or the diet book was not written in Russian etc. I will discuss ITT in a future post but for the issue at hand:  if you do a per-protocol you will observe what happens to people when stay on their diet and you will have an association between the content of the diet and performance.  With an ITT analysis, you will be able to observe what happens when people are told to follow a diet and you will have an association between assignment to a diet and performance.  Both are observational experiments with an association between variables but they have different likelihood of providing a sense of causality.


The big news in the low carb world is that Consumer Reports has published, for the first time, faint praise for the Atkins diet. However, the vision one might have of CR employees testing running shoes on treadmills doesn’t really apply here. They did not put anybody on a diet, even for a day. They didn’t have to. They have the standards from the government. Conform to the USDA Guidelines and CR will give you thumbs up. It probably doesn’t matter since, these days, most people buy a food processor by checking out the reviews on the internet — there are now many reviews online of what it’s like to actually be on a low-carbohydrate diet, so rather than follow CR’s imaginings of what it’s like, you can check out what users say — Jimmy Moore, Tom Naughton and Laura Dolson together get about 1.5 million posts per month with many tests and best buy recommendations. What caught my eye, though, is the ubiquitous Dean Ornish; the ratio of words written about the Ornish diet to the number of people who actually use it is probably closing in on a googol (as it was originally spelled). The article says: “to lose weight, you have to burn up more calories than you take in, no matter what kind of diet you’re on. ‘The first law of thermodynamics still applies,’ says Dean Ornish, M.D.

That’s how I got into this field. My colleague Gene Fine, and I published our first papers in nutrition on the subject of metabolic advantage and thermodynamics and we gave ourselves credit for reducing the number of people invoking laws of thermodynamics. “Metabolic advantage” refers to the idea that you can lose more weight, calorie-for-calorie on a particular diet, usually a low-carbohydrate diet. (The term was used in a paper by Browning to mean the benefits in lipid metabolism of a low-carbohydrate diet, but in nutrition you can re-define anything you want and you don’t have to cite anybody else’s work if you don’t want to). The idea of metabolic advantage stands in opposition to the idea that “a calorie is a calorie” which is, of course, the backbone of establishment nutrition and all our woe. As in the CR article, whenever the data show that a low-carbohydrate diet is more effective for weight loss, somebody always jumps in to say that it would violate the laws of thermodynamics. Those of us who have studied or use thermodynamics recognize that it is a rather difficult subject — somebody called it the physics of partial differential equations — and we’re amazed at how many experts have popped up in the nutrition field.

Finding the right diet doesn’t require knowing much thermodynamics but it is an interesting subject and so I’ll try to explain what it is about and how it’s used in biochemistry. The physics of heat, work and energy, thermodynamics was developed in the nineteenth century in the context of the industrial revolution — how efficiently you could make a steam engine operate was a big deal.  Described by Prigogine as the first revolutionary science, it has some interesting twists and intellectual connections. The key revolutionary concept is embodied tin the second law which describes the efficiency of physical processes.  It has broad philosophical meaning.  The primary concept, the entropy, is also used in communication and  the content of messages in information theory.  The entropy of a message is, in one context, how much a message has been garbled in transmission.  The history of thermodynamics also has some very strange characters, besides me and Gene, so I will try to describe them too.

First, we can settle the question of metabolic advantage, or more precisely, energy inefficiency. The question is whether all of the calories in food are available for weight gain or loss (or exercise) regardless of the composition of the diet. Right off, metabolic advantage is an inherent property of higher protein diets and low carbohydrate diets. In the first case, the thermic effect of feeding (TEF) is a measure of how many of the calories in food are wasted in the process of digestion, absorption, low-level chemical transformation, etc. TEF (old name: specific dynamic action) is well known and well studied. Nobody disputes that the TEF can be substantial for protein, typically 20 % of calories. It is much less for carbohydrate and still less for fat. So, substituting any protein for either of the other macronutrients will lead to energy inefficiency (the calories will be wasted as heat). A second unambiguous point is that in the case of low-carbohydrate diets, in order to maintain blood glucose, the process of gluconeogenesis is required. You learn in biochemistry courses that it requires a good deal of energy to convert protein (the major source for gluconeogenesis) into glucose.

So, right off, metabolic advantage or energy inefficiency is known and measurable. Critics of carb restriction as a strategy admit that it occurs but say that it is too small in a practical sense to be worth considering when you are trying to lose weight. These are usually the same people who tell you that the best way to lose weight is through accumulation of small changes in daily weight loss by reducing 100 kcal a day or something like that. In any case, there is a big difference between things that are not practical or have only small effects and things that are theoretically impossible. If metabolic advantage were really impossible theoretically, that would be it. We could stop looking for the best diet and only calories would count. Since we know energy inefficiency is possible and measurable, shouldn’t we be trying to maximize it.  But what is the story on thermodynamics? What is it? Why do people think that metabolic advantage violates thermodynamics? What is their mistake? More specifically, doesn’t the first law of thermodynamics say that calories are conserved? Well, there is more than one law of thermodynamics and even the first law has to be applied correctly. Let me explain. (Note in passing that the dietary calorie is a physical kilocalorie (kcal; 1000 calories).

There are four laws of thermodynamics. Two are technical. The zeroth law says, in essence, that if two bodies have the same temperature as a third, they have the same temperature as each other. This sounds obvious but, in fact, it is an observational law — it always turns out that way. The law is necessary to make sure everything else is for real. If anybody ever finds an experimental case where it is not true, the whole business will come crashing down. The third law describes what happens at the special condition known as the absolute zero of temperature. In essence, the zeroth and third laws, allow everything else to be calculated and practical thermodynamics like bioenergetics pretty much assumes it in the background.

The second law is what thermodynamics is really about — it was actually formulated before the first law — but since the first law is usually invoked in nutrition, let’s consider this first. The first law is the conservation of energy law. Here’s how it works: thermodynamics considers systems and surroundings. The thing that you are interested in — living system, a single cell, a machine, whatever, is called the system — everything outside is the surroundings or environment. The first law says that any energy lost by the system must be gained by the environment and any energy taken up by the system must have come from the environment. Its application to chemical systems, which is what applies to nutrition, is that we can attribute to chemical systems, a so-called internal energy, usually written with symbol U (so as not to confuse it with the electrical potential, E). In thermodynamics, you usually look at changes, and the first law says that you can calculate ΔU, the change in U of a system, by adding up the changes in heat added to the system and work done by the system (you can see the roots of thermo in heat machines: we add heat and get work). In chemical systems, the energy can also change due to chemical reactions. Still, if you add up all the changes in the system plus the environment, all the heat, work and chemical changes, the energy is neither created nor destroyed. It is conserved.

Now, why doesn’t the first law apply to nutrition the way Ornish thinks it does? To understand this, you have to know what is done in chemical thermodynamics and bioenergetics, (thermo applied to living systems). If you want to. In nutrition, you can make up your own stuff. But, if you want to do what is done in chemical thermodynamics, you focus on the system itself, not the system plus the environment. So, from the standpoint of chemical thermodynamics, the calories in food represent the heat generated by complete oxidation of food in a calorimeter.

In a calorimeter, the food is placed in a small container with oxygen under pressure and ignited. The heat generated is determined from the increase in temperature of the water bath. (Before the food measurement, we determine the heat capacity of the water bath, that is, how much heat it takes to raise the temperature). The heat is how we define the calories in the food. The box around the sample in the figure shows that we are measuring the heat produced by the system, not the system plus the environment, that is, not applying the first law. If you applied the first law, the calories associated with the food would be zero, because any heat lost in combustion of the food would show up in the water bath of the calorimeter. The calories per gram of carbohydrate would be 0 instead of 4, the calories per gram of fat would be 0 not 9, etc. So, in studying reactions in chemical thermodynamics, energy is not conserved, it is dissipated. When systems dissipate energy, the change is indicated with a minus sign, so for oxidation of food, generally: ΔU < 0. So, no, the first law does not apply. That’s one of the reasons that “a calorie is not a calorie.”
There is an additional point that we assumed in passing. In chemical thermodynamics, the energy goes with the reaction, not with the food. It is not like particle physics where we give the mass of a particle in electron-volts, a measure of energy, because of E=mc2. What this means, practically, is that the 4 kcal per gram of carbohydrate is for the reaction of complete oxidation. Do anything else, make DNA, make protein and all bets are off.
The bottom line is that, contrary to what is usually said, thermodynamics does not predict energy balance and we should not be surprised when one diet is more or less efficient than another. In fact, the question to be answered is why energy balance is ever found. “A calories is a calorie” is frequently what is observed (although there is always a question as to how we make the measurement). The answer is that insofar as there is energy balance, it is a question of the unique behavior of living systems, not physical laws. Two similar subjects of similar age and genetic make-up may, under the right conditions, respond to different diets so that most of what they do is oxidize food and the contributions of DNA or protein synthesis, growth, etc. may be similar and may cancel out so that the major contribution to energy exchange is the heat of combustion.
But thermodynamics is really not about the first law which, while its history is a little odd, it is not revolutionary. Intellectually, the first law is related to conservation of matter. Thermodynamics is about the second law. The second law says that there is a physical parameter, called the entropy, almost always written S, and the change in entropy, ΔS, in any real process, always increases. In ideal, theoretical processes, ΔS may be zero, but it never goes down. In other words, looking at the universe, (any system and its surroundings), energy is conserved but entropy increases. The first law is a conservation law but the second law is a dissipation law. We identify the entropy with the organization, order or information in a system. Systems proceed naturally to the most probable state. In one of the best popular introductions to the subjects, von Baeyer’s Warmth Disperses and Time Passes, entropy is described in terms of the evolution of the organization of his teenage daughter’s room.  To finish up on calorimeters, though, there is Lavoisier’s whole animal calorimeter.

One of Lavoisier’s great contributions was to show that combustion was due to a combination with oxygen rather than the release of a substance, then known as the phlogiston. Lavoisier had the insight that in an animal, the combination of oxygen with food to produce carbon dioxide was the same kind of process. The whole animal calorimeter was a clever way to show this. The animal is placed in the basket compartment f. The inner jacket, b, is packed with ice. The outer jacket, a, is also packed with ice to keep the inner jacket, cold. The heat generated by the animal melts the ice in the inner jacket which is collected in container, Fig 8. Lavoisier showed that the amount of carbon dioxide formed was proportional to the heat generated as it would be if an animal were carrying out the same chemical reactions that occur, for example, in burning of charcoal. “La vie est donc une combustion.” His collaborator in this experiment was the famous mathematician Laplace and people sometimes wonder how he got a serious mathematician like Laplace to work on what is, well, nutrition. It seems likely that it was because Laplace owed him a lot of money.