It is not uncommon in youth soccer to hear parents tell the coach how important it is for their somewhat overweight child to “run around” so as to shed those excess pounds. While I agree that any sort of exercise can be good for general well-being, it is not fair to ask the coach to run your kid a bit more in the hopes of reducing a weight problem. This is particularly vexing in the case of goalkeepers, where the parental perception is that the coach, by forcing their child to stand in goal and not run around during the game, is dooming the child to a lifetime of poor fitness. There are so many things wrong with this (oversimplified) perception that it is hard to know where to start. However, I thought perhaps the best way to tackle the issue was with some simple lessons on thermodynamics and metabolism. Hopefully this will help illustrate that while exercise is certainly a good thing, it cannot be used as a tool in the battle of overweight and obesity – those problems must be tackled at a more fundamental level.
For many, the issue begins with the fact that so many doctors, pundits, politicians, nutritionists, bloggers, and journalists reduce the problem of weight gain or loss to the concept of “calories-in/calories out”. This view of the problem stems from the first law of thermodynamics, which states that energy cannot be created or destroyed, only changed from one form to another. This caloric balance view of the world is then perverted to promote caloric restriction + energy expenditure (typically through exercise) as the most straightforward and perhaps only way to lose weight. Most of us know from experience, however, that this is such an oversimplification of the situation that it is rendered unusable in the vast majority of weight loss cases. Indeed, a number of clinical studies have also illustrated the folly of this approach – it is worth looking at some of Gary Taube’s writing on the subject here and here for a nice history lesson on some of those studies.
Unfortunately, opponents of the caloric balance hypothesis are fond of saying things like “the first law of thermodynamics does not govern weight gain or loss”. What they are trying to illustrate is the significant complexity of our bodies and the multivariate nature of the obesity problem. However, that statement is just as wrong and oversimplified as the one put forth in the caloric balance hypothesis. The first law of thermodynamics must determine what goes on in our bodies, regardless of the level of complexity, but you must consider all sources of energy input and energy output to understand how it relates to weight gain or loss.
Let’s first consider the major contributors to the equation. Typical food labels will calculate % daily values based on a 2,000 calorie diet, so we will use that as a typical daily intake. Approximately 60-80% of those calories will be used by your body in normal functions at rest. This includes all normal organ functions plus the digestion of food. Put another way, the typical, healthy person should expect burn ~1600 calories per day just sitting around doing nothing (except eating). From this it is easy to see that the major “calories out” portion of the equation has nothing to do with exercise, and everything to do with the health of your basal metabolism.
In this example, what are the possible fates of the “extra” 400 calories? Well, they get used for everything you do that is not sitting and staring off into space (walking to your car, typing on the computer keyboard, chewing your food), or they can get stored as fat, or they can be used to build muscle, or they could be used to further increase your metabolic rate. That’s right, your body is actually smart enough to know, under certain circumstances, when to burn more fuel, thereby taking care of the calories-in/calories-out equation. Note that I have said nothing about how to use exercise here to consume that additional 400 calories. This is because your body is smarter than you and it doesn’t need you trying to manipulate what it was evolved to do so perfectly – maintain a steady state…we will come to that later.
Ok, of all the possible uses for the extra 400 calories, the least desirable for most of us is fat storage. If we assume that all of our non-sitting-on-our-butt activities are roughly constant from day to day, what we really want to do is decrease the “calories into fat” part of the equation, while increasing the “calories into metabolism” portion. Clearly, we are now in the realm of asking our body to “make a decision” – it needs to decide whether it will store those extra calories away for a rainy day, or whether your energy level will simply increase enough to utilize those calories.
As a chemist, I am prone to think about this in terms of chemical reactions. To anthropomorphize chemistry for a second, one can force reactions to “make decisions” by changing which chemicals you use. You can drive a reaction to the desired product by using one type of reagent (ingredient) or you can get one or more undesirable side reactions if you use a different reagent. In chemistry, you would never say “a molecule is a molecule”, yet many people invoke the “a calorie is a calorie” mantra at this point. They argue that no matter what the origin of the calorie (that is, which nutrient those calories come from), it is simply the number of those calories that dictates whether we remain slender or gain weight. This is patently ridiculous – your body works by using various nutrients to generate energy, build muscle, store fat, and do a host of other things that keeps us ticking. How on earth could we expect that it would do all of this with equal fidelity regardless of the fuel we provide? Different reagents will do different things and our body will utilize those reagents differently. In fact, Richard Feinman (not Feynman) has argued that the “a calorie is a calorie” concept actually violates the second law of thermodynamics, which is an interesting idea. However, that argument is beyond the scope of the present discussion.
Anyone who cooks knows that different reagents react differently. In the event where things go awry and you burn your dinner, you know that oil (fat), sugar, fruits, vegetables, and meat are likely to burn differently and will result in different products – burned oil gives black smoke and burned sugar gives caramel/tar, for example. This simple concept can be applied to an over-simplified view of our metabolism. If you provide your body with certain reagents, it will proceed along a certain pathway that is directed by those reagents. Let’s take dietary fat as an example. If you provide your body with the bulk of calories as fat, the most likely “reaction” is the burning of those fat molecules – your body will use them as fuel. Your body doesn’t know how to store that fat for a rainy day unless you provide two other key ingredients: sugars/carbs and insulin. In the reaction where all three components are present, the sugars can be combined with the fats to provide a “storable” form of fat called triglycerides, while the insulin then works to store those triglycerides, thereby swelling your fat cells – you get fatter.
So the apparent outcome here is that under one set of reaction conditions (eating mostly fat), your body can choose to use left-over fuel via an increased metabolic rate or activity level. Another set of conditions (fat + simple carbs/sugar + the resultant insulin spike) might lead to storage of the extra fuel as fat, and your metabolic rate might stay essentially the same. One could even imagine more extreme cases where the storage mechanisms become so dominant (again, because the fuel you are providing dictates that outcome) that the storage activities begin stealing from the ~1600 calories per day that your body normally needs for basal metabolism. That is, your metabolic rate might slow down in order to direct more calories towards fat storage. In this view, it is easy to see how someone might become overweight simply by virtue of what they eat and not how much they eat. Thus, energy is always conserved – your body is just making decisions about whether that energy is used right away or is stored for later depending on the types of calories (ingredients or reagents) being provided.
Let’s finish with a word on exercise. Exercise is good, but it has little to do with weight loss. In the above framework, we can consider exercise to be an additional part of the energy expenditure equation. First, we should note that there are plenty of calories “left” to work with to do a significant amount of exercise. A 5k run would use nearly 400 calories, and at face value would seem to take away those pesky extra calories we were trying to figure out how to use. Does this mean that we should all be running a 5k every day in order to avoid getting fat? If this were the case, one might think that our obesity epidemic would be even direr than it is currently. In fact, it just won’t work that way, because your body is pretty smart. It knows that if you use those extra 400 calories on exercise, it needs to take in even more food in order to have a little bit in reserve – you need at least a couple of calories left to brush your teeth at night, correct? So, your body makes you hungry, and you eat a little bit more. Basically, exercise has put you at a caloric deficit, but your body isn’t happy about it. If you decide to resist the hunger and not eat, you might lose a bit of weight, but your body will also rebel by decreasing your metabolic rate to recover that excess buffer of calories – you will feel tired after exercise and will feel that way for as long as you insist on operating at a caloric deficit. Note that nothing has been said here about burning stored fat for energy to make up for the deficit – that may be an option for your body, but only if you aren’t giving it fuel that it wants to store as fat in the first place. Once you store fat, it is pretty difficult to mobilize it just by running a bit more. Those of us who choose to provide our bodies with fat as a main fuel source also seem to have an easier time accessing stored fat – some have called this the state of being a “fat burning beast”. It is worth thinking a bit about that approach – it has worked for a large number of individuals, including myself.
Anyway, even this overly simplified view of your body’s inner workings is itself complicated. The take home message is that your body knows thermodynamics way better than you do, and the way to solve the problem of fat storage is not to try to manipulate the two sides of the equation (calories-in vs. calories-out), but rather to change the chemical reactions involved in the bulk of your caloric usage – your metabolism. As an organism that has on-demand access to food, providing reagents/fuel that your body wants to burn right away makes way more sense than giving it things it wants to store. Those stored calories become fat, and when your storage mechanisms are severely damaged, they may also sap your immediately available energy levels. The first law of thermodynamics certainly holds when it comes to weight gain, but you need to let your body do the math because that equation is way too complicated to solve by just going out for a run.