Imagine a system with a background/quiescent energy consumption of 1000kCal/day.
Imagine that same system can buffer up to 500kCal for up to 24 hours store excess energy in circulation.
Imagine it converts excess energy to stored energy at an efficiency of 50%.
Assume activity correlates with marginal energy consumption but also increases in the presence of excess energy.
A system such as the one described would have very different behaviors during alternate day fasting (0kCal for 24hrs, 5000kCal for 24hrs) than consuming 2500kCal daily.
The human body is more complex than the system I just described, but it is a useful model to consider for this context.
You eat less calories, your body might start consuming less calories.
Also, there are two different pathways for using glucose in the body: aerobic and anaerobic. The aerobic one produces 15x more ATP (cellular level energy) than the anaerobic one. The anaerobic one wastes more as heat. So if for some reason you're in the second one, even though you ingest the same amount of calories, the amount of energy you have usable is much less.
So yeah, calories in calories out is true, but it's not really helpful.
This is technically true but not particularly relevant.
It's quite difficult to be in only anaerobic effort, though (and I'd say pretty ill advised since that basically means stuff like all out sprinting without warmup or cooldown).
Higher intensity effort burns more calories than lower intensity (eg [1]). It's just harder to sustain.
1- https://www.webmd.com/fitness-exercise/what-to-know-heart-ra... )
Food — not lack of exercise — fuels obesity, study finds
https://www.npr.org/2025/07/24/nx-s1-5477662/diet-exercise-o...
Your body can raise or lower its temperature. It can put energy towards cell repair or cell reproduction. It can store energy as fat, or signal to burn fat, or build muscle or catabolize muscle.
It's a common story when people start to eat 20% less, continue the same lifestyle and lose exactly zero weight as the result. Their body didn't create 20% of energy out of nothing but it just started to waste less energy as body heat.
The body is more like a thermostat system. Environmental effects can convince your body to move the temperature/weight up or down, but for both weight gain and weight loss is a battle. Your body tries to maintain it's understood ideal weight. Your body if given a chance and you haven't convinced it to change the thermostat setting, will immediately do all it can to gain/loose weight. This is why crash diets never work. It takes sustained effort to convince your body that it's wrong about your initial weight.
These are excuses, in the fitness community, the response to "I can't lose weight" is "eat fewer calories", and its incredibly effective
Anything that can influence "energy out" is by your own definition "relevant".
When caloric intake is reduced, the body can decrease this type of expenditure without the person even realizing it.
I don’t understand what’s "desperate" about IF, it’s just an easy way (for some people) to lower their calories intake. It has other benefits and some caveats but it’s one way to get healthier.
Some people operate with a goal of a caloric deficit of even something as small as e.g. 200kcal. But because all these things are impossible to measure accurately, a difference of just 10% beyond a daily BMR of 2000kcal isn't just a possibility; its the norm. You run for an hour; what if that burns an extra 50kcal that your Apple Watch did not account for? You eat a slice of bread which advertises it contains 80kcal; but it actually contains 100kcal [1]? You sleep poorly, which causes some mild systemic inflammation the next day, which raises your body temperature?
[1] https://health.clevelandclinic.org/are-calorie-counts-accura...
If you had a car with a broken gas gauge you would just pump until it overflowed… same idea here.
Over a month or so if your weight is stable then you are putting in as many calories as you burn. If you’re gaining weight, you’re consuming more, and if you’re losing weight, you’re consuming less.
Adjust accordingly.
Hence: Why diets exist. That is the structure. There are good ones and bad ones.
Keto, for example, can work for weight loss not because there's anything particularly interesting about the way your body absorbs carbs versus protein and fat (there are differences, but its not the biggest reason why it can work). It can work for some people because typical protein and fat food sources are less calorically dense (by volume and weight) than carb sources. You may feel full faster; so you may naturally eat fewer calories.
Similarly: IF can work for some people because most people cannot physically eat enough to consume massive calorie counts if they time-restrict the hours they're allowed to eat. It also seems to come with some well-studied metabolic effects.
You don't have to accurately track inflows and outflows, but vibe-structuring your consumption and exertion habits based on outcomes is a privilege that, sure, some people have, but is not a panacea for every body and mind. Broadly, the people who need to make change who do this will not see the change they wanted.
Of course we do!
You’re thinking of this the wrong way. The goal isn’t “eat less” as you said, it’s “consume less calories (energy)”
On any given month if you gain weight or maintain when you want to lose, then you need to consume less energy next month. For 99% of people, that means reduce sofa, reduce sugar, reduce fat (all the energy dense stuff)
Feeling hungry? Drink massive glasses of water and eat literally all the vegetables you physically can get in. I have whole carrots for snacks most days. Cauliflower too. Cucumber is great. Frozen peas on a hot day. All of it, as much as you can eat.
Also note Americans are more obese than the rest of the world. Like gun violence, police deaths, healthcare and so much more - the rest of the world can fix it. Why not America.
For instance this is something every single person who's into body building does, because you want to be in a slight caloric surplus when bulking, and then you want to get back into a slight calorific deficit when cutting.
It turns out the body processes different calories very differently depending on a variety of factors including: baseline genetics, time of day, menstrual cycle, prior fasting, current mineral excess/debt, gut flora/fauna biome, and the composition of previously consumed food still remaining the digestive track.