A few weeks ago I completed my last Abbott World Marathon Major: the London Marathon. Over the years, the application and qualification process for these races has become more complicated, but I'll cover that at a later date. For now, I want to share an image I saw in a BBC article regarding the event:

It's based on the estimate that the average runner burns around 100Kcal per mile. I've also seen many t-shirts and stickers saying things like "WILL RUN FOR BEER" or "RUN FOR CAKE". Aside from the fact that the 100Kcal figure varies considerably depending on the pace ran, as well as the age, sex, weight and body composition, these slogans seem to suggest that one can trade exercise for food, as if our bodies were checking accounts whose balance we have to be careful to maintain. Unfortunately, the human body isn't that simple.

It doesn't matter why we do it, or even really how we do it, exercise does have a positive effect on our overall health. There is a multitude of research that indicates that exercising provides a range of health benefits: it reduces the risk of type 2 diabetes and cardiovascular disease, it has effects on blood pressure, cholesterol and mental health, and it positively impacts energy levels, sleep quality, mood, anxiety levels, and creativity. And while an increase in exercise levels alone can be enough to lose weight, most of us need the help of something else.

Typically, that something else comes in the form of modifications to a person's nutrition. The result of eating less and moving more, so the logic goes, should be weight loss. But sometimes, even that isn't as straightforward as we wish it would be. Why is that?


The relationship between exercise, nutrition and body composition is complex. Losing, maintaining or gaining weight is not as easy as balancing the number of calories ingested against the number of calories expended. That's because there's a middleman in all of us called "the metabolism".

What we call the metabolism is the process by which we take in nutrients from food and convert them to energy which we use to drive the biological processes that keep us alive. Metabolism varies from person to person and is influenced by many factors. Genetics, hormones, gender, body composition, age, and many other things.

In the media, many talk about metabolism like it is something we can control and manipulate via the use of magic potions and killer workouts. We see it on supplement packaging and articles online. We're told of tips and hacks that can “increase your metabolism" or "fix your broken metabolism”. But when it comes to metabolism, there's still debate about the merits and methods of altering it. Some even argue that we don't truly understand what a "broken" metabolism looks like.

That's why researchers around the world are working full-time to untangle the complexities of the system responsible for the function of every cell in our body. They, of all people, understand that there are many things we still don't know about the metabolism. But there are also a few things we do know. First, we know that all our energy intake comes from the food we consume. 100% of it. Second, we know that we expend energy in three main ways:

- Via Resting Metabolic Rate (RMR): The body needs a significant amount of energy to keep its vital parts and processes functioning. While lying in bed all day, the average adult would still have to expend approximately 60% to 80% of total calories to maintain critical functions.

- Via the Thermic Effect of Food (TEF): Eating, digesting, absorbing and excreting food is estimated to account for roughly 10% of the average person's daily energy expenditure.

- Via Physical Activity: This is not only purposeful exercise, like running or walking or going to a yoga class, but also the generic movement involved in things like fidgeting, transitioning from room to room, and manipulating objects with our hands.

Third, we know that while the metabolism is a smaller part of the body as a whole, it is itself a complex system. One feature of complex systems is the fact that an implemented change has effects that are not always immediate and not always obvious.

An example of this comes from the energy balance system. Those whose weight is stable--meaning, they are not continually gaining or continually losing weight--are considered to be in energy balance. But is that because the amount of calories exactly matches the amount of calories expended? Unlikely. More likely, something else is going on behind the scenes to cause that person to maintain their weight despite continual fluctuations in energy consumption, creation, storage and use. The issue is that we can't accurately say what that "something" is...


Despite the uncertainty around the mechanisms of the metabolism, most agree that it has a significant ability to adapt. For example, consider the effect of exercise on the body.

If a person increases the frequency/volume/intensity of their physical activity over a prolonged period while making zero changes to their nutritional behaviors, they will probably lose weight. That seems straightforward, right? But that person will also be likely to experience an increase in their desire to eat and drink. Performing high volumes of exercise often leads to an actual increase in dietary intake, or at least a desire to increase dietary intake. In other words: the more we exercise, the more we want to eat, and the more we do eat.

Compensation can also occur in other components of energy balance, including resting metabolic rate (RMR) and non-exercise physical activity. For example, after a run or a workout session most people actually reduce the amount of spontaneous physical activity they undertake and slow down for the rest of the day. They do things in a way that reduces the amount of movement necessary, like taking the elevator instead of the stairs, driving to the shops instead of walking, or going to bed earlier.

On top of this, there is some evidence that exercising for very long periods of time might actually slow down your metabolism in order to compensate for greatly increased physical activity. According to this theory, the view that physical activity increases total energy expenditure may only hold true until a certain (very high) threshold of activity is reached. At that threshold, total energy expenditure remains constrained despite further increases in physical activity.

Basically, your body is doing some budgeting because you still need to hold onto calories to perform basic functions and stay alive. So when you boost your physical activity a lot, it may be that your body holds back more calories to maintain these basic biological processes. In other words, your resting metabolism can slow down slightly in response to activity.

Finally, there are changes in body composition to consider. When people consistently increase their level of physical activity, they start losing body fat and often experience a degree of increase in lean muscle mass. And these changes in weight and body composition also have an effect on the energy you expend.


When it comes to manipulating body composition the only certainties are often tied to extreme behaviors. Starvation will change how a person looks. But most don't want to starve themselves, or stuff themselves. Most want subtle body composition changes at the price of a little extra effort and thought. And the best way to get that is find a physical activity you enjoy doing, do it, focus on good nutrition while listening to the signals your body sends.

There are methods, like calorie counting, that can help but ultimately the aim is simple. Eat when you're hungry, sleep when you're tired, and do what you love with people you care about.