When arguing against a specific scientific claim it is always desirable to be able to say that the claim violates an established law of science. Creationists attempt this with their argument that evolution violates the second law of thermodynamics (it doesn’t). The temptation is that such arguments are short and pithy, they seem conclusive, and they avoid the need to wade through a dense and complex set of scientific evidence and theories.

In the “diet wars” the first law of thermodynamics has been thrown around a lot. Up to now I have been aware of two camps defending their position with thermodynamic arguments. The first (and the one that I find most compelling) is the calorie in vs calorie out camp, that argues that the laws of thermodynamics apply to people too. This means that weight management must be a function of calories in (the total calories consumed by a person) – calories out (the total caloric expenditure, including metabolic processes, waste heat, exercise, and others). Thermodynamics must be obeyed and so if one wishes to lose weight they must burn more calories than they consume.

The second camp are the defenders of special weight-loss diets who claim that the type of calories one consumes significantly affects weight loss. They reject the “calorie is a calorie” mantra, and instead preach about the evils of carbs, or fats, or glycemic index. They argue that all calories are not equal because some calories are more efficient than others – they require less energy to metabolize. If you want to lose weight you want to consume inefficient calories (i.e. – more of the energy from these calorie sources is wasted as heat, or they require greater overall metabolic activity, so less is available for muscles and other uses). Therefore, they argue, thermodynamics (when efficiency is considered) favors manipulating macronutrients (protein, carbohydrates, and fats) for weight loss.

While I agree that this is a legitimate thermodynamic argument, what has not been demonstrated (either from a basic science perspective, or in weight loss research) is that efficiency has a significant effect. The bulk of weight loss studies show that total caloric intake does correlate pretty well with weight loss, at least in the short term.

I was surprised to learn that now there is a third camp, using thermodynamic arguments to claim, essentially, that dieting does not work.

Sandy Szwarc who writes the Junkfood Science blog (although billed as a “skeptical” blog she has earned a mixed reputation and seems to deny any link between diet, weight, and health) recently wrote an entry entitled The First Law of Thermodynamics in real life.  To jump to her conclusion, she writes:

The pop belief that people can simply eat less and exercise more and control their weight defies the first Law of Thermodynamics. 

To arrive at this conclusion she makes a rather bizarre and incoherent argument. Actually, reading this blog entry I had the feeling I was reading two separate articles.  The substantial middle of the entry is a mostly reasonable argument about the limitations of diet and exercise for weight loss. However, this is sandwiched in between arguments involving the first law of thermodynamics that are nothing but irrelevant straw men and non sequiturs.

It is as if she had a reasonable, but dense, argument to make about the complexities of weight control, but decided to wrap it up in a thermodynamics argument to give it more pith and punch. All she managed to do was hopelessly confuse her readers and distract from her real points.

The First Law of Thermodynamics

Szwarc writes:

As is often the case when science is dummied down into soundbytes, it becomes wrong. Such is the case in the distortion of the Law of Thermodynamics which has been simplified into the popular wisdom: “Calories in = calories out.” This simplistic adage has become something “everyone knows” to be true. It’s behind widely held beliefs that managing our weight is simply a matter of balancing calories and exercise. While that’s been used to sell a lot of calorie-reduced diets and calorie-burning exercise programs for weight loss; sadly, it’s also been used to support beliefs that fat people “most certainly must be lying” about their diets and activity levels, because otherwise their failure to lose weight would seem to “defy the Law of Thermodynamics.”

While it might seem inconceivable, this simplified maxim is little more than superstition and urban legend. To realize this fact requires us to first go back to physics class and fill in the missing half of the first Law of Thermodynamics.

The first Law of Thermodynamics, or energy balance, basically states that in a closed system, energy can neither be created nor destroyed, only transformed or transferred.

There are a number of straw men in her argument.  The first is that weight loss is all about calories and exercise, and that this is based upon thermodynamics. She is distorting this position, however. The thermodynamic argument recognizes that there are many sources of “calories out” and this is not equated with exercise. Overall physical activity can be increased without specifically exercising. When writing about this I and others point out that most calories are consumed by our basic metabolic rate. One way to increase calories out is to increase metabolism.

She confuses practical advice about what works with thermodynamic arguments. The thermodynamics are absolutely clear – matter and energy cannot vanish or be poofed into existence; the balance sheet must balance.

From a practical point of view, exercise does increase calories out, and it increases basic metabolic rate, and muscle tissue burns off more calories than fat, and fit people are likely to burn more calories just going about their day. Further, while we recognize that calories out can be increased by simply increasing metabolism, there is no safe and healthful way to do this directly. Stimulants work short term, but are not safe and cause rebound weight gain.

Her argument about a closed system vs an open system is irrelevant – another straw man.  The calories in vs calories out argument is about calories going into and out of the human system – it treats it, by definition, as an open system.

To further muddy things she writes:

Balance in an open system, like the human body, is when all energy going into the system equals all energy leaving the system plus the storage of energy within the system. But energy in any thermodynamic system includes kinetic energy, potential energy, internal energy, and flow energy, as well as heat and work processes. 

The first sentence is true – all energy must ultimately equal out or thermodynamics is violated. Her second sentence is also true – all forms of energy must be considered. But she makes it seem as if the second sentence refutes the first, which it doesn’t.

Her comment that the thermodynamic argument is used to accuse fat people of lying about their food intake is a non sequitur. It allows for the fact that there may be metabolic differences among people.  Also, it is possible that overweight people do not lie about their caloric intake, they just grossly underestimate it.

The Biology of Weight Control

Szwarc then goes on to discuss various studies concerning the biology of weight control. In this section I actually agree with most of her arguments, which essentially add up to the fact that dieting does not work. To quickly summarize her points – most people who lose weight by dieting will gain it back, it requires a huge effort of will to significantly alter one’s “natural” weight, and it is even difficult to gain weight if one is naturally skinny.

There are various reasons for this, but the major one is that our bodies evolved in a calorie-limited environment. Surviving lean times was a priority, and so when we reduce our caloric intake our bodies interpret that as starvation and reduce the basal metabolic rate to conserve energy. When we overeat our bodies interpret that as a time of plenty and takes advantage of the extra calories by increasing metabolic expenditures. This has the net effect of resisting any significant change in body weight.

While I have significant disagreements with some of the ultimate conclusions Szwarc draws from these facts, I agree with others. I agree that these well-established facts of biology make it very difficult for most people to lose weight, and that dieting almost always fails for these reasons.

If she had simply written an article summarizing this research in order to make this point, she would have had an excellent article, and one important to the public discourse on dieting.

But (even the thermodynamic nonsense aside) she also overreaches in her conclusions, mainly through cherry picking her data. She writes:

In the 1980s, Dr. Leibel had advertised to find people who’d maintained 100 pound weight losses for at least a year and a half. A colleague at the lab, Dr. Bruce Schneider, said that “he got six people and all of them were wacked.” Successful long-term losers are “monomaniacal and completely obsessed with their weight.” They’d made weight control their life, becoming extremely upset if they didn’t jog a certain number of miles a day, counting calories and constantly fantasizing about food, exhibiting every sign of dysfunctional eating behavior.

While it makes sense that obsessive-compulsive exercising is one way to keep weight off, it is premature to conclude that only the “wacked” can maintain weight loss. For example, the National Weight Control Registry tracks people who have lost an average of 66 pounds and kept it off for more than 5 years. While 90% do exercise every day, most of them get their exercise by walking. Most also made sensible changes to their eating habits.

She then turns to the notion of a set point, writing:

Dr. Leibel and colleagues at Rockefeller University later showed that when someone gains only about 10% of weight over their natural set point, their metabolisms increase by at least 16% over and above the expected increase for their size, as the body works hard to balance energy to maintain its natural size. 

Again, there is a great deal of evidence to support the conclusion that our metabolism adjusts in response to weight gain or weight loss – but this does not mean there is an unavoidable set point of “natural” weight. This cannot be the entire picture. For example, over the last 20 years average weights in this country (and the West in general) have been steadily increasing. Just watch the animated map of the US showing obesity trends.

Weight and set point cannot be only about genetics, that cannot explain the undeniable trend toward increased weight in America. It is difficult to pin down what other factors are playing a role – and everyone likes to blame their favorite boogy man. It is likely a combination of increased sedentariness and increased portion size. There may be other factors as well, including metabolic effects or changes in food choices. This is a separate and complex question. But genetics alone is insufficient as an explanation.


Szwarc’s thermodynamics argument is worthless and misleading – an unnecessary distraction from her real points, which are themselves a mixed bag. I agree with her that diet and exercise alone are not sufficient to explain weight gain or loss, as we also need to consider major changes in metabolism. I also agree that will power is not sufficient for most people to make long term changes to their weight. It is simply too difficult to maintain. I agree that the evidence shows that dieting often does not work. And I agree that genetics has a huge influence on our weight and body type – not everyone can and should be skinny.

However, I disagree with her ultimate conclusion that any conscious attempt at weight control is hopeless (unless you are a fanatic) and we are slaves to our genetic set points. Recent history belies this conclusion.

Rather – I would conclude that weight control is difficult, but not impossible. In addition, I believe that the popular weight loss industry is making the problem worse by distracting the public from those strategies for which there is at least some evidence of efficacy and focusing on minute, short term, and probably insignificant effects from manipulating macronutritent proportions.

The evidence suggests that long term weight loss is possible through lifestyle changes. Increasing daily activity and regular exercise is helpful. People have difficulty estimating what they eat, so keeping a diary is also very helpful.

Further, if will power is not the answer because it is too difficult to maintain, then it would seem that the answer is to make lifestyle changes as easy as possible so they can be sustained. From a public health point of view, healthful choices need to be made easier. The real lessons from the studies that Szwarc cites is that long term weight control requires sustainable strategies, not quick fixes and not magical diets.



  • Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.

Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also has produced two courses with The Great Courses, and published a book on critical thinking - also called The Skeptics Guide to the Universe.