In theory, you could exercise more and lose weight. After all, a calorie deficit is a calorie deficit. So, whether you create that deficit by reducing your food intake or increasing your physical activity, you should end up with the same result, right? In theory, yes. But what might make sense in theory doesn’t always pan out in the real world.
Here are some issues with relying on exercise as your primary mode of creating a calorie deficit.
1. How do you know how many calories you’re actually burning?
If you want to lose weight, you need to create a caloric deficit. Calculating this isn’t too difficult. There are hundreds of calculators on the internet designed to do just that. But once you have your fat loss calorie target, you need to eat this amount consistently over time. So, if you’re aiming for a 500-calorie deficit each day, you need to be sure you’re reducing how much you eat each day by some amount close to 500 calories. In the case of exercise, you need to be sure you’re burning 500 calories each day.
And this brings us to the first problem: How do you know how many calories you’re burning each day? The only viable way for the average person to track their caloric burn is by using a smartwatch. Unfortunately, smartwatches are terrible at accurately reporting energy expenditure.
A study published in 2022 looked at the accuracy of three different smartwatches for heart rate and energy expenditure across various activities such as sitting, walking, running, cycling, and resistance training. 1
While the watches were reasonably accurate for heart rate, the energy expenditure data showed errors for each device across all activities from ~15% to ~50%.
For example, the Apple Watch 6 had a mean absolute percentage error (MAPE) of 14.9% ± 9.8% during running, the lowest error among the three devices tested in the study. Said another way, the average error was 14.9%, with a standard deviation of 9.8% (a measure of variability). In practical terms, if you go for a run and your Apple Watch informs you that you burned 500 calories, you might have actually burned as few as 475 calories or as many as 624 calories, considering errors within one standard deviation. If we extend this to two standard deviations, the errors get larger: you might have only burned ~330 calories, or you might have burned ~670 calories.
Let’s look at another example. The Fitbit Sense had the highest mean absolute percentage error of 45.1% ± 22.5% during walking. So if your Fitbit reports you burned 500 kcal walking, the actual calories burned could be as low as 387 kcal or as high as 838 kcal (considering errors within one standard deviation).
It’s also worth remembering these errors could be even larger if you’re doing multiple activities across the week. To help illustrate this, let’s say you lift weights 3 times a week, go for a run 2 times a week, and also walk 30 minutes each day, which is a very reasonable training schedule. Now, let’s assume your Apple Watch tells you that you burn about 2000 kcals each week. In reality, you might have burned ~1400 kcals or ~2600 kcals (assuming errors within two standard deviations, which includes most of the typical variability in the measurements––about 95% of the possible values).
Additionally, you don’t know where your device will fall within that error range. Your watch might be off by only a little bit or by a lot. So, it’s almost impossible to create a consistent calorie deficit if the method you use to determine energy expenditure is woefully inaccurate.
2. The amount of exercise required to create a substantial deficit isn’t realistic for most people
Oh, you don’t believe me? Cool – here’s exhibit A: In one study, researchers took a bunch of twins and made them exercise on stationary bikes twice per day to create a 1000 kcal deficit. Their food intake was kept constant to ensure they were eating at maintenance. 2
By the end of the study, the average weight loss was 5 kg. But here’s the kicker: To achieve this weight loss through exercise alone, they had to exercise over 100 minutes or 1.6 hours per day. That wasn’t a typo. Unless you’re an athlete whose literal job is to work out, good luck trying to sustain this level of exercise every day until you achieve your weight loss goal.
But even if we ignore the last two points, there’s another factor that can greatly impact the number of calories you burn.
3. Energy compensation
Most people think if they do more exercise, they’ll burn more calories. This might be true at lower activity levels, but as activity levels increase, energy expenditure doesn’t increase linearly since the body will adjust other components of your metabolism to ‘constrain’ total energy expenditure.
The latest research on this topic suggests this compensation averages ~30%; however, it can be as high as 45%. 3
You might burn 500 kcals during your exercise session but once we account for energy compensation, the actual increase in energy expenditure would only be 350 kcals (on average).
But if you continue eating the same amount as you are without accounting for energy compensation, it’s easy to see how, in the best case, your progress will be slower than it should be.
The elephant in the room: food intake
If you’re not paying attention to how much you’re eating, it’s very easy to overeat without realising, especially since hunger can increase at higher levels of physical activity.
For example, the image below shows the actual weight loss (green dots) versus the predicted weight loss (yellow dots) in groups who increased their weekly exercise by ~650 kcals (8KKW) and ~1500 kcals (20KKW) over 24 weeks. 4
Some participants maintained or even gained weight despite increasing their exercise. The researchers explained the lack of weight loss in these individuals was due to an increase in caloric intake and hunger, increased cravings for sweets, and compensatory health beliefs (e.g., “I exercised, so I deserve to eat more”). Interestingly, a secondary analysis of this study found the 20KKW group was also experiencing energy compensation––they saw a bigger drop in non-exercise activity thermogenesis, which ties in with the previous point. 5
Hopefully, you can see just how difficult it is to out-train your diet
You can’t quantify how many calories you’re burning in the first place, so you’ve lost before you even started––how much of a deficit are you actually in? And unless your name’s David Goggins, good luck trying to create and sustain a deficit through exercise alone that has you losing weight consistently each week.
But even if you can accomplish the above, you can’t control or account for the energy compensation that’s likely to occur, which will further reduce the number of calories you’re expending and, by extension, the size of the deficit you’re in.
Finally, if you’re not paying attention to how much you’re eating, you may end up overeating, resulting in a smaller deficit or no deficit at all.
I mean, fuck, I feel tired just reading that.
This is why I always say, “Eat for fat loss.” It’s way easier to cut a few hundred calories from your diet than it is to burn an equivalent amount through exercise. Focusing on your nutrition also means you’re only focused on one thing—track your food intake consistently and accurately and watch how your body weight changes. If you’re losing slower than expected or faster than you’d like, adjust your intake accordingly.
Using exercise as your primary mode of creating the deficit is like walking to another country when you could catch a flight. Sure, you might be able to do it, but it’s going to fucking suck, and it’s definitely not the most efficient way to achieve the goal. Use your diet to create the deficit and train to retain muscle and strength and, you know, for the gazillion other health benefits exercise provides.
Gettit? Good. Now, please excuse me. I have a flight to catch.
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