Here's an extract from an interview with Dr John Briffa who explains it a lot better than I can:
One of the things that I love is to do things differently. In this book, the thing that captures you straight away, is on the book it says, "Lose book for good without calorie counting." That's a complete opposite of what we know and what everybody's thinking these days. Why do we not need to count calories?
John: The main reason is that it doesn't work. I know theoretically it should work. If you eat less calories than you burn, they you're going to lose weight. That's what we're told. That can work, and it can also work in the short term. It can even work quite successfully in the short term. What the studies show though, Matt, is that it doesn't work in the long term. So, if you try either eating less and or exercising more, most individuals over a period of time, a year or two, will not lose significant quantities of weight. Some of them will actually end up a bit heavier than they started. So, we also know and, you may know this from your own experience, certainly there are millions of people who have been in this position.
Forget what the studies show. They've experienced that. There are very few people who say, "Yes, I ate less and exercised more. I lost weight, and I kept it off." They're in the minority. Most people say, "Yeah, well, I'm kind of fed up. My diet went back to normal." Why is that? Usually we kind of assume that it's their failing that they didn't do it enough. They still ate too much, didn't exercise enough. They're a bit lazy. They're greedy. All of these, which can be self applied. I didn't do it correctly. I lack self control. I've got a weak will. Maybe I'm a bit inadequate or whatever. These things are common with people who fail with the calorie approach.
As I explore in the book, there's a number of different mechanisms that come into play when someone either restricts calories or exercises more, that are essentially designed to keep the body fat, to keep the body hanging onto its fat. You've kind of gone into starvation mode. The body doesn't know this is not going to end. It basically thinks, "Oh, dear. I need to protect myself." So, it changes the levels of certain hormones, like thyroid hormones. It down tunes parts of the nervous system that stimulate metabolism, so that reduces metabolism. It also has an influence on a very important hormone called leptin.
Leptin actually stimulates the metabolism and suppresses appetite. So, if levels of leptin go down as we diet, that basically suppresses the metabolism, and also makes us hungry of course. So, now we've put a big dent in the metabolism by consciously restricting calories, for example. So you might say, "Okay, I'll exercise my way out of here. I'll go to the gym more and spend more time in the gym." The problem with that is that although theoretically it works, the fundamental issue is that it doesn't burn that calories, exorcise. It really doesn't. If you've ever exercised on a piece of equipment that counts calories, you'd know this. Have you ever?
Matt: Yeah. You see the calorie numbers going up, which is great until you realize that number is equivalent to a quarter of a digestive biscuit or something like that.
John: That's exactly it. The other problem is that when people exercise more, they tend to get hungrier. So, now they're hungry. They're not actually losing weight effectively. They're subsisting on meager portions of food, because you put a dent in your metabolism. The studies that go back 60 years, there was this very famous study that was done called the Minnesota Experiment, where they took men, put them on a diet, what they call a semi-starvation diet. They did lose weight, between 20% and 26% of their weight was lost. But, their metabolism declined by 40%.
In other words, the metabolism declined to an extent far greater than you'd predict just by the fact that the body is now smaller. This is a problem, right? How are you going to make that work for you? You're not. So, instead of thinking about calories, what you might think is, that's not so important. What causes fat to get into my fat cells and stay there. It's complex, to be honest. But you know the answer to this now, because you've read a couple of my books. Basically, the nub of the issue is a hormone called insulin. Among the effects of insulin is to facilitate fat uptake in fat cells. Once it's there, it tends to keep it there. So, it's fattening
The whole transcript is here