Sometimes food just happens. This is a lot less cute when you are 50.

The twin scourges obesity and inactivity are raging through the world, maiming and killing like a berserker army. Each of them encourages the other: Feel free to try jogging with a hundred pound backpack, but be sure to have your last will and testament done first. Likewise, sitting still helps the fat just pile on, forcing you to be even less active, and so on and on.

Given the grim statistics for diabetes, atherosclerosis, hypertension and the secondary diseases that follow in their footsteps, you’d think the world’s governments would be waging a War on Fat. And you would definitely think it would be one of the hottest topics of science. The world is already spending billions on diets, so why can’t scientists come right out and tell us which one is the best?

Well, you see, none of them work. Or rather, pretty much all of them work, but not in the long run. Losing weight is fairly easy, but large studies show that the pounds come back on. The scientific consensus is that significant weight loss is pretty near impossible (except in the case of chronic illness, of course). It is possible to lose a little weight – like 5% or so – and keep it off. If you get serious about losing weight, however, the body eventually gets serious too. And at some point, it stops listening to you. Basically, your free will starts fading.

This is a disturbing situation, and well worth considering. We all know from experience that we have free will. We can decide to do something unpleasant and do it, and we can decide to not do something pleasant and avoid it. In each case, we clearly see ourselves as having a choice. But the law of large numbers says something else. In the long run, it is easier to ride the horse the way it is already going. In the case of the weight loss, the body will throw at us ever more frequent and intense temptations. If that is not enough, the discomfort will intrude on our lives more and more: Hunger pangs that make it impossible to sleep, chronic fatigue that makes it hard to be active and burn calories. In extreme cases, you may find that there are blank spots in your memory and empty spots in the fridge. You have no memory of having eaten, and you certainly don’t feel like you have eaten, but you have. You may even find that food is gone from your fridge while you slept. The body will defend its fat as if it were its life. And that is no coincidence.

In the wild, too much food is not the problem. For tens of thousands of years, until just recently, hunger was the real risk. A surplus of food was temporary. If you started losing weight, no matter what your weight was at the outset, the body would interpret this as a famine coming on. It still does. And it will do what it takes to defend you from starvation, even if it means tricking or outright overruling your free will.

One clue comes from people with anorexia. It has been known for a while that patients with this mental illness have a very high mortality. But a look at the causes of death shows something less obvious: A striking number of the deaths are from suicide.

Other studies show that a lot of people – especially women – sometimes want to kill themselves but cannot. Humans are basically built with a certain level of protection. When facing death, we shrink back automatically. This is a good thing, and it is in a way unfortunate that it does not always work. As I like to say, the game is rigged: You can kill yourself when you are feeling down, but you cannot become immortal when you are feeling good. But at least it turns out there is some level of defense. But some people don’t have that level of defense. And these are the same people who are able to lose weight indefinitely. In other words, as far as the brain knows, weight loss is just another type of suicide, and it will use the whole range of defenses to avoid it.

The irony of this is of course that in our time, gaining weight above the “well rounded” level is the actual suicide. The body thinks otherwise, however.

What does help, then? First of all, it is best to not have put the weight on in the first place. But the single most reliable predictor of body fat is the body fat of people around you: Friends, family and coworkers. This works even on a national level: A Japanese moving to America will gain weight over the course of just months, while an American moving to Japan will eventually lose some. But it also works on a local level. The people around you seem to somehow calibrate you to eat slightly more or less than you normally would. (I had noticed this effect on myself long before I read about it in popular science magazines, but then again I don’t always know that I am typical enough to be a useful example.)

And physical activity seems to be more important than body mass anyway. As I said, the problem is that being obese makes it hard to exercise. But you have to start at the level where you are. Remember, if you are severely obese, you are literally in the same situation as someone who carries a heavy load, so don’t try exercise that would be suitable for a slim person. That would be like them going jogging with a heavy recliner strapped to their back or something. Simply walking will be great exercise for the obese, and in the beginning you may want to limit yourself to stretches where you can find a place to sit down and rest if you get exhausted. The single biggest jump is from complete inactivity to a little exercise. Once you get used to that, your body will gradually adapt, so you can be a tiny bit more active next week again, and so on.

But trying to simply assert your willpower and stop eating (or eat just tomatoes)? Won’t work. The body comes with failsafe against that kind of crazy behavior. It keeps tabs on you and if your diet works, it will start blocking it.

Perhaps science will have a solution one day. For now, all it has is an explanation, but we can use that to adapt at least to some extent.