Technically, you can out-train a bad diet. If you burn more calories than you consume, you'll lose weight because, thankfully, despite how weird shit's been getting recently, the laws of thermodynamics are still a thing.

Still, it raises an interesting question: What happens when you increase physical activity levels? After all, exercise does burn calories, and those calories have to contribute somewhere. I mean, if you went for a run and expected to burn 500 calories, would your daily energy expenditure also increase by 500 calories?

Most people's response to that might be, "Yeah, obviously." Except, it's not that straightforward––so much so that this area has been a hot topic of debate.

To be clear, the contention isn't that more exercise doesn't lead to more calories burned, but rather what the actual magnitude of that increase is. In other words, does every calorie burned during exercise add to our daily energy expenditure, or does the body compensate for those additional calories elsewhere, effectively reducing how much of an increase you'll experience?

Before we answer that, we need to quickly look at what makes up our total daily energy expenditure.

What comprises total daily energy expenditure?

Daily energy expenditure is made up of several components that collectively determine how many calories you expend over the course of a day.

The largest contributor is your basal metabolic rate (BMR), sometimes referred to as resting energy expenditure, which accounts for roughly 60–70% of the calories burned each day; it's the energy your body uses to perform the day-to-day tasks that keep you alive.

Then there’s the thermic effect of food, which is the energy your body uses to digest and process what you eat.

Finally, there’s activity energy expenditure, which includes intentional exercise like lifting and running and non-exercise activity such as walking, cleaning, standing, fidgeting, and so on.

While activity energy expenditure contributes a fairly small proportion of our daily energy burn, it's the component we can actually change and modify. As such, if we took two people of the same age, sex, weight, height, etc., but one was active while the other was sedentary, the active individual would expend more energy.

The case for a constrained model of energy expenditure

Given what I just said, it would make sense then that as you do more exercise, you should also see an increase in your total daily energy expenditure. That idea, that more exercise means more calories burned, is known as the additive model.

However, research investigating highly active populations such as the Hadza hunter-gatherers of Tanzania, led by evolutionary anthropologist Herman Pontzer, has reported that despite lifestyles involving substantially more physical activity than those in industrialised societies, total daily energy expenditure was often similar once differences in body size were considered.

This led to the idea of the constrained energy expenditure model, which suggests that rather than increasing indefinitely, total daily energy expenditure is regulated within a relatively narrow physiological range.

​The theory proposes that as physical activity increases, the body compensates by reducing energy expenditure elsewhere.

For example, you might go out for a run and burn 300 calories, but then you feel sluggish and tired for the rest of the day, leading you to sit or lie down more, opt for the lift or escalator instead of the stairs, and just generally move with the urgency of my three-year-old being told to pick up her damn toys.

The end result is that sure, you burned 300 calories during the run, but then you compensated by being more sedentary the rest of the day.

​Some studies suggest this effect is more pronounced during prolonged aerobic exercise or when exercise is combined with dietary restriction. Interestingly, resistance training appears to produce less compensation.

Hold on a minute

Despite its popularity, several recent papers have challenged the constrained energy model.

​Gonzalez and colleagues argue that much of the evidence for constrained energy expenditure comes from studies comparing different people at one point in time rather than seeing what happens when those same individuals become more active. Moreover, they note that activity energy expenditure is difficult to measure accurately, and much of the research makes assumptions about energy constraint rather than measuring it directly. While the authors agree that some compensation probably occurs, they point out that the current evidence isn't strong enough to tell us how meaningful it is without better-controlled trials.

In a separate narrative review, Millward and colleagues argue the problem may be how these populations are being compared.

Pontzer suggested the Hadza didn't burn more calories than Western adults after accounting for body size, despite their high activity levels. But total energy expenditure isn't just the calories you burn during activity; it also encompasses your BMR, which is influenced by body size.

The Hadza are much smaller, so they generally burn fewer calories at rest than larger Western adults, even though they spend a large amount of their day being active.

When Millward and colleagues estimated activity expenditure separately and expressed it relative to body weight or fat-free mass, the Hadza burned substantially more through movement than the Western groups.

So what to make of all of this?

Taken together, the current evidence suggests that some degree of energy compensation almost certainly occurs when physical activity increases.

However, this doesn't mean that increasing physical activity doesn't have any impact on your energy expenditure, and it definitely doesn't mean that physical activity is pointless for fat loss or weight maintenance (leaving aside the bajillion health benefits).

As I explained in this article:

For example, a recent dose-response meta-analysis found that individuals doing more aerobic exercise saw greater weight loss, with no clear plateau up to 300 minutes per week.

While the relationship might not be exactly 1:1, it does suggest that for the average person, extra energy expenditure via exercise will have an additive effect.

Related articles:

• ​It’s Very Hard to Out-Train a Bad Diet but It’s Equally Difficult to Out-Diet a Sedentary Lifestyle​
• ​The Major Problems With Trying to ‘Out-Train’ Your Diet​
• ​How Does Improving Your Fitness Impact Your Total Daily Energy Expenditure?​
• ​Why More Exercise Doesn’t Always = More Calories Burned​

–Aa

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