Here’s why you can’t exercise off your food, and why you shouldn’t even try.
Here’s one myth that unfortunately I, and a lot of you, grew up believing:
When you eat something that you feel doesn’t ‘fit’ into your diet, you can just go to the gym and ‘work it off.’
I’m sad to say that I spent the majority of my 20s and part of my early 30s with this mindset, slogging away for hours in the gym and on running trails trying to right my perceived dietary ‘wrongs.’ Ugh. I want those hours of my life back.
How many times have you worked out, then had an increased sense of permissibility when it came to eating? As in, ‘I just did a spin class, so I can drink 3 margaritas!’
Exercise has health benefits, but on its own, it hasn’t been associated with weight loss. That doesn’t mean you should stop exercising, but rather, you should think about activity in a different way.
Exercise isn’t meant to be a punishment for eating, and it certainly shouldn’t be. Eating anything, whether you consider it to be outside of your normal intake or not, isn’t grounds for punishment.
Food is food. Let’s not turn our choice to eat a piece of cake into a moral failing that needs redemption, either via hours of exercise or, negative self-talk and starvation.
Giving yourself ‘permission’ to eat something because you’ve exercised, is also problematic.
Mostly in the way of, you don’t need permission to eat. Ever.
But for all of you who are trying to exercise off weight without changing your diet, this piece in Vox is the perfect explainer why this is probably a bad idea.
Your body doesn’t work like that. And now, we have even more research to back that up.
Remember that post I wrote a few weeks back about the study showing that metabolism stays stable between the ages of 20 and 60? (Read the post here)
Well, we seem to be on a metabolism research roll, because not even a month later we now have another great paper in Current Biology that examines how exercise affects our basal energy expenditure (BEE – which is essentially the calories we expend breathing, thinking, digesting…just living.)
As in, when we exercise, what happens to our metabolism and the calories we burn?
This latest research sheds some light on this question. And it turns out, our bodies may be compensating for those calories.
In short, just because you burn calories exercising, doesn’t mean that your body’s total energy expenditure increases as much as you think it does. I know this sounds weird, because how does that even work – you burn calories while you work out, so they should naturally count in our total daily calories burned.
Welp. Probably not as much as we thought it did.
Let’s start from the beginning.
What was the study looking at?
The participants were 692 men and 1062 women, aged 19-96 years. Researchers controlled for sex, age, and body composition.
The researchers wanted to test several energy management models for humans – simply put, how humans adapt (or not) in terms of energy expenditure, when activity levels rise.
They had three models to explore:
- The additive model, in which energy burned through activity is added to total energy expenditure on top of basal energy needs. It assumes that AEE (activity energy expediture) and BEE (again, basal energy expenditure) are independent. In other words, when we exercise, those calories are simply added on to the amount of calories our bodies expend just being alive.
- The performance model, in which energy burned through activity actually raises basal energy expenditure. In other words, when we exercise, that activity causes an uptick in the calories we expend just being alive.
- The compensation model, in which energy burned through activity decreases basal energy expenditure. In other words, when we exercise, our bodies compensate for those calories burned by decreasing the calories we expend just being alive.
You can see these models in the graphic below, from the study.
You can also see in the graphic that BEE in animals accounts for far less of total energy expenditure than in humans. Interesting.
What did the study find?
The study found that the compensation model was how most of us adapt to increased activity energy expenditure.
To put it into layperson’s terms, when you work out for 4 hours trying to burn off that cake you ate, your body lowers the amount of energy it expends to keep itself alive, by an average of 28%, to compensate for the increased burn from activity.
In the words of the researchers, “In humans, energy compensation averages 28%, i.e., only 72% of the extra calories we spend on additional activity translates into extra calories burned that day.”
Here’s an amazing graphic that pretty much explains it all:
So that 400 calories you burned doesn’t translate into 400 calories of extra food. Even if your device’s calorie numbers were actually correct – which they probably aren’t – you’re probably burning around 72 calories for every 100.
It’s all about metabolism and how our bodies are always working towards homeostasis, in order to keep us alive. The body loves to keep things the same. Compensatory mechanisms like this one are one way the body achieves that.
The study showed that there was no difference between men and women in rates of compensation. The rate of compensation also did not vary by age.
But here’s something interesting that researchers found: obese people seem to have a larger rate of compensation – up to 49%. It seems as though as fat mass increases, so does this compensatory mechanism.
We don’t know, however, if this is something that certain individuals genetically have (a ‘thrifty’ phenotype), or if it happens because of gained fat mass.
Study authors had this to say:
It appears then that individuals with greater fat levels are predisposed to increased adiposity either because they are stronger energy compensators or because they become stronger compensators as they get fatter.
If the former, then two people can be equally active, yet one puts on fat mass while the other stays lean. If the latter, then such a positive feedback loop may imply that using exercise as a strategy to escape high adiposity becomes less and less effective.
I was curious about where do the supposed energy savings come from with this compensation. Do we stop breathing as much? Do we fidget less?
I asked John Speakman, one of the study investigators, who told me this:
First the person gets more efficient at doing the same stuff, or it starts to shut down some things. The biggest savings come from stopping doing things – ie reducing your maintenance costs by doing less. One way to think about it is like cleaning the house. We do that all the time and it takes us time and effort to do it. If I made you go for a run every day you might have less time and energy available to clean the house – so that maintenance activity would get neglected.
Question is of course by cutting back on these things is there a downside? In the above scenario for example you would get a more messy house. However, since we don’t know exactly what is being cut back at the moment we can’t say. Obviously exercise seems to bring benefits overall so if cutting back these things has negative effects then they seem to be offset by the benefits from exercise itself.
Every time I post something about how exercise isn’t the greatest way to lose weight, someone invariably counters my point by asking me why we rarely see overweight athletes.
Speakman gave me his two cents on this:
I guess it depends on what sports you choose and what you refer to as athletes. There are lots of overweight shot putt throwers and weightlifters. So I guess you mean those doing running swimming and other aerobic sports. The study we did actually specifically screened out athletes so we did not address how much compensation they show.
You also need to consider that most athletes are on precise diets, and again, we still do burn calories by exercise.
Looks like we still have a lot of work to do, but studies like this one get us closer to a greater understanding of how our bodies work.
When you consider compensation via reduction in energy used and by eating more, along with permissiveness, you can understand why we’ve been overestimating the role that exercise plays in weight loss.
Speakman puts it all into perspective like this:
In lean people 72% of calories burned on activity translate into energy burned at the end of the day and even in those with obesity it is 51%. One way to think about it is overall activity costs less than we thought it did – but it still costs energy to do it.
So yes, exercise does play a role in weight loss. But it also has a ton of benefits beyond that (and again, exercising just for the purpose of weight loss can turn activity into a punishment).
My recommendation? Move your body in a way you enjoy, and try not to focus on calories.