When it comes to health and fitness, and in particular to reducing the amount of fat on one’s body, the dominant story within our culture, at least from the sort of people who present themselves as experts, is that fat gain or loss is just Calories-in-vs-Calories-out so just take however many Calories you burn and eat less than that until you’re thin.

Now, obviously there is something truth to this because if you stop eating you will waste away until you die, and you will be very thin shortly before your death. (Though, interestingly, if you autopsy the corpses of people who’ve starved to death you will find tiny amounts of fat still remaining.) Of course, the problem with just not eating until you’re thin is that starvation makes you unfit for pretty much any responsibilities and it’s also bad for your health. (Among many problems, if you literally stop eating your muscles will substantially atrophy, including your heart.)

So the big question is: is there a way to eat fewer Calories than you burn while remaining a functioning adult who can do what the people you have responsibilities to need you to do, which doesn’t wreck your health?

The good news is that there are methods that accomplish this balance. The bad news is that (at least as far as I can tell) there’s no one method that works for everyone.

Since this post is about the Calories-in-vs-Calories-out mantra (from here on out, Ci-Co), I’m only going to discuss moderate Calorie restriction—oversimplifying, aiming for a deficit that results in about a half a percent of bodyweight reduction per week, for a period of 6-12 weeks, before returning to maintenance for an approximately equal length of time. (This is a version of what bodybuilders do and they’re probably the experts at losing fat because bodybuilding can be described, not entirely inaccurately, as competitive dieting.)

Now, at first glance, this isn’t too far off what the Ci-Co people seems to be saying. However, it’s very different in practice, and those differences will be illuminating, because they’re all things that the Ci-Co people get wrong.

The first big problem with trying to implement Ci-Co is: what on earth is your daily Calorie expenditure? There are highly accurate ways of measuring this which are extremely expensive with most being infeasible outside of a laboratory. Apart from that, there’s no good short term way. The best way—which is what bodybuilders do—is to carefully measure your Calorie intake and your weight over a period of time, then see what your weight does, and calculate your Calorie expenditure from your intake plus what your weight did. For example: suppose you take 3000Cal/day and over 14 days lost a pound. A pound of fat contains roughly 3600 Calories, so your actual expenditure was 3000 + (3600/14) = 3257. From there you can refine your intake to achieve what you want. (Bodybuilders also have phases where they put on muscle, which means gaining weight, so they will have to eat at a surplus to provide energy for building the extra muscle tissue.)

This looks nothing like what the Ci-Co people suggest, which usually amounts to either taking the USDA random-number of 2000 or else using an online tool which estimates your Calorie expenditure from your height, weight, and some description of how active you are. These are generally accurate to +/- 50%, which is not obviously distinguishable from useless. Using myself as an example, entering 6′ and 215 pounds with high activity, it estimated my maintenance Calories as 2900 and a weight loss target of 2450. I’ve actually been using the MacroFactor app to track approximately 100% of what I eat and weighing myself every morning when I wake up. It estimates my maintenance Calories as about 3900 Cal/day and I’m losing a little over a pound a week with a target Calories of 3200 Cal/day. On days when I eat about 2800 Calories I go to bed hungry and am very hungry the next day. If I tried to lose weight at 2400 Cal/day in a week or two I’d be constantly ravenous, unable to concentrate, barely able to do my job (I’m a programmer), and miserable to be around.

Because here’s the thing: the human body can tolerate small (consistent) Calorie deficits without worrying, but if they become too large the body freaks out and concludes that something very, very bad is going on and the top priority for the foreseeable future is getting through it. That means two things, both very bad for losing fat:

1. Spending all your waking hours trying to find enough food
2. Reducing your Calorie expenditures as much as possible to conserve what energy we do have until the bad times have past.

The second point is probably the bigger deal. What the CiCo people don’t realize is that your Calorie expenditure is nowhere near fixed. If your body thinks it’s a good idea, you can maintain on a surprisingly large number of Calories. If your body thinks it’s a good idea, you can maintain on a surprisingly small number of Calories. The former looks like having a lot of energy and feeling good. The latter looks like being tired and cold all the time.

Even worse, there is reason to believe—though this is nowhere nearly as well established—that if you make your body freak out and think it needs to survive a famine too many times, it will start to prepare for the next famine as soon as food becomes readily available again, much as people who’ve been broke a few times and also had good times tend to live like misers and save money the next time things go well. (In the the case of your body, this means gaining the fat you will need to survive the next famine, just like bears put on a ton of fat in summertime in order to get through the coming winter.)

This is why the other critical part of how bodybuilders diet is that they only do it for 6-12 weeks at a time, then take long maintenance breaks at their new weight. (The variability because they pay attention to how their body reacts and if it seems to be starting to freak out, they stop losing weight and start maintaining so it doesn’t have to adapt to the diet—there are many factors which go into how long it’s possible to diet before the body starts to freak out.) This relatively short fat-loss window ensures that the body never goes into surviving-famine mode. And the maintenance Calories are not a fixed number, either. They can easily increase for a few weeks as your body gets used to the extra food and raises your metabolism because it seems safe to do so.

When you put this all together, it’s why the Ci-Co people give the laws of thermodynamics a bad name. It may be perfectly true that losing weight is the result of one number that’s not easy to measure being lower than another number that’s impractically expensive to measure and impossible to usefully estimate, but knowing that that’s true has no practical value.

For a much more entertaining take on a closely related subject, check out Tom Naughton’s post Toilet Humor And The HOW vs. WHY Of Getting Fat.

---

This post was about the problems with Calories In vs Calories Out, but I would be remiss to point out that everything I said up above about how bodybuilders reduce fat is predicated on having a reasonably well-regulated metabolism to begin with. There are all sorts of ways for the human metabolism to become disregulated and if yours is disregulated your odds of successfully reducing fat are much lower until you figure out what’s wrong and fix it. In my own case, I’m about 99% certain that at times in my life I’ve induced insulin insensitivity in my body through excessive fructose consumption. (I can eat a pound of chocolate for lunch if I let myself and there was a period back when I was in grad school when I was drinking full-sugar Mountain Dew and eating cake mix out of the box with a spoon. That stuff has more sugar and flour in it. This is during a period when I was unemployed and depressed as well as young and dumb, and I had yet shaken off being raised during the low-fat craze of the 1980s and 1990s.) I believe some extensive low-carb eating has allowed my body to mostly reset its relationship with insulin and at this point I’m only willing to eat candy/ice cream/etc. on Christmas, Easter, and my birthday. That said, when I’m cutting (reducing fat), I find it much easier and more successful if I go back to eating low carb or even keto.

That’s me; I suspect that many people are in a similar boat because fructose is way more common in processed food than people normally realize and it’s reasonably well established that extremely high fructose consumption (much higher than anything you’d get from any reasonable intake of fresh fruit, btw) can induce non-alcoholic fatty liver disease, which seems to have a causative relationship with insulin resistance/metabolic syndrome. That said, this is not everyone who’s got excess fat. There are tons of things that can go wrong to disregulate one’s metabolism/appetite, some of them dietary, some of them endocrine, and some I don’t even begin to have an idea. The human body is unbelievably complex and there are a lot of ways it can malfunction. There’s really no substitute for trying things and seeing what works. And at least we know that it’s a good idea to get regular exercise no matter how much excess fat you’re carrying. It may not make you lean, but it will certainly make you healthier and happier than if you don’t do it. After the first few months.

Oh yeah—and I’m no expert, so please do your own research and don’t take my word for it.