While that is a logical assumption, it’s not actually true. [1]
If you intake X calories, your body will use them to provide you energy for exercise. If you don’t exercise, you won’t always just turn it into fat - your body will find another find other uses of the calories such as producing stress hormones.
Exercise is important but will not make you lose weight alone.
No one is suggesting that you can out run an increasing calorie surplus. You can however keep your diet the same and add two hours of running to your week and lose weight.
Ask anyone who has been obese and subsequently lost weight and they'll tell you the same thing. Eating less is both a sufficient and necessary condition to losing weight. Exercise is neither sufficient nor necessary because the caloric surplus these people are on significantly exceeds the amount that you can practically burn through exercise. While you're right that exercise can help on the margin, it's just not a useful intervention that moves the needle much as far as weight loss goes.
I don't know why anyone here has the idea that I'm arguing that exercise alone is key for weight loss. I have very clearly only claimed that all else equal increasing exertion will lead to a calorie deficit.
> While you're right that exercise can help on the margin, it's just not a useful intervention that moves the needle much as far as weight loss goes.
There's more to exercise than just the thermodynamic effects of calorie expenditure. Building muscle and/or cardiovascular capacity will improve your quality of life and will complement any weight loss. You can improve your mental health by becoming more physically active, and this is well established. Those mental health gains and physical health improvements make it easier to maintain a better diet. Beyond the marginal, but significant calorie expenditures you can create a positive feedback loop.
At one stage, I started walking home from work at my fastest pace. 5 miles, 5 days a week. No other lifestyle changes. No significant weight loss.
A few years later, I started tracking calories with myfitnesspal and keeping to a limit (1600 initially, 1400 when it became easier). No exercise, but steady and impressive weight loss. I seem to have kept most of it off. I think a lot of it becomes psychological - not being afraid to feel appetite or skip meals if already satiated from earlier.
Only assuming you have a steady body weight and diet, which is a very bad assumption for most overweight and obese people.
Not to mention, the processes in your body are way more complex than all this makes them out to be. For moderate exercise, after an adaptation period of a few weeks to months, there is almost no calorie impact from the exercise itself on your total calorie expenditure: your metabolism adjusts and various internal processes are deprioritized to prioritize the exercise.
This is in fact a major component of why exercise is so healthy: it doesn't do much for weight loss, but it stops/slows down all sorts of unnecessary processes in the body that are actively harming your overall health.
> almost no calorie impact from the exercise itself on your total calorie expenditure
This is categorically false. You don’t have a magical metabolic adjustment, you simply become more efficient in performing an exercise but the calorie use never drops to effectively zero like you’re claiming. Think about this for a second, it makes no sense to think that running could ever consume zero calories, basic physics still apply.
- one, your body may crave more food because of the energy loss - the Lipostat model suggests that your body has a target weight (well, fat level, more precisley), and will adjust hunger up if calory expenditure goes up
- your internal processes use way more energy than you can consume through exercise in a modern lifestyle (2000+ calories a day just from sitting still, for many adult males), and there is a lot of room for the body to adjust those up and down to make up for the extra exercise to keep up your current weight. So if you consume 2000 Cal a day with 0 Cal from exercise, with moderate exercise you will end up conauming 1600 Cal a day from internal processes + 400 Cal a day from exercise after some time (a one time run will absolutely consume extra, if your run every day, you'll get less and less extra Cal from your runs, until you reach about net 0, assuming you're not increasing the amount of exercise constantly).
If one were getting the doctor recommended amount of exercise running for 30 minutes or some commensurate activity would not be intimidating, and it's far less likely that you would have a weight issue in the first place. Doctors recommend about 150 minutes of moderate intensity exercise per week. Imagine doing that every week, for your whole life.
That’s why there are programs like Couch to 5K which slowly work up to a nonstop 30 minute run. It doesn’t even take that much determination and it’s practically designed for obese people. I was able to complete it weeks ahead of schedule at a BMI of 30 and the 4-5/wk runs it enabled helped me lose 35 pounds in 2024 - the vast majority of the weightloss happening during the weeks that I ran.
Nitpick: a marathon is 42 kilometers (by definition). Most people would struggle to even walk that.
There's no such thing as a 5km marathon, that's a contradicition in terms. However, if one were running 150 minutes a week on a regular basis then a (42km) marathon would likely not be too intimidating a prospect. Suffice to say, most are not getting in their doctor recommended level of exercise.
Agreed that life is not that simple as a math equation, but I think baselines apply, e.g. if you are sedentary, adding 150-180 mins of moderate exercise will definitely improve your health and life.
if you are already meeting that baseline, and still have issues, then you should look at tweaking other variables in the equation, whether diet, stress, etc.
What I find worrying is the cherry picking some people do, e.g. "aha your body will get used to exercise, so I might as well not bother", then wonder why at their next visit to the doctor, they are now told to go on a steady diet of statins etc.
The only thing being claimed is that exercise doesn't significantly help with long term weight loss.
That doesn't mean exercise isn't extremely important for your health. It is, in a myriad ways. Even the mechanisms that make it not help with weight loss are some of the reasons why it is so healthy - it's taking away calories from metabolic processes that are more harmful than helpful.
You're treating the human body as a far simpler system that it actually is, leading to drawing unsupported conclusions about the efficacy of exercise as an intervention for fat loss.
I guess if I'm totally out of condition then exercising feels very inefficient. Once I'm in condition, though, it's calories in = calories and it isn't complicated, at least for endurance activites. If I pile on more activity then I need a corresponding amount of extra calories and it's simple arithmetic.
I only see that study cited by people who are looking for excuses to not exercise.
You're off the mark here - I'm in great shape and exercise more or less every day.
At the same time, I keep up with the latest science and understand the limitations of exercise as a means of sustainable weight loss for the general population.
Please stop talking about simple arithmetic - the body is not a simple machine and trying to conceptualize it as such is willfully ignorant.
It's really not. Weight, atleast if you control for muscle weight, is an extremely good predictor of health issues at almost every level. It's a huge problem for your body both in many complicated internal ways, all the way to simple mechanical ways, putting increased stress on most of your joints and bones.
If you intake X calories, your body will use them to provide you energy for exercise. If you don’t exercise, you won’t always just turn it into fat - your body will find another find other uses of the calories such as producing stress hormones.
Exercise is important but will not make you lose weight alone.
[1] https://pmc.ncbi.nlm.nih.gov/articles/PMC5556592/