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We have millions of years of evolution hard-wiring us to survive scarcity—compensatory survival mechanisms our body uses to defend against weight loss. So, when we start losing weight, we may unconsciously start moving less as a “behavioral adaptation” to conserve energy. There are metabolic adaptations as well. Our metabolism slows down. Every pound of weight loss may reduce our resting metabolic rate by seven calories a day. This may only translate to a few percent differences for most, but can rapidly snowball for those who achieve massive weight loss.

During one season, some of The Biggest Loser contestants famously had their metabolic rates tracked. Above and beyond the hundreds of calories less it takes to just exist 100+ pounds lighter, by the end of the season their metabolic rates slowed by an extra 500 calories a day. The mindblower was that six years later, they were retested and still had the 500-calorie-a-day handicap. So, the contestants had to cut 500 calories more than anyone else their size to maintain the same weight loss. No wonder the bulk of their weight was regained. Most remained at least 10 percent lower than their starting weight, though, and even a 7 percent drop has been shown to cut diabetes rates about in half. Still, the metabolic slowing means you have to work that much harder than everyone else just to stay in place.

Analyzing four seasons of The Biggest Loser, minute-by-minute, researchers noted that 85 percent of the focus was on exercise rather than diet, though the exercise component accounted for less than half of the weight loss. Even six years after their season ended, the contestants had been maintaining an hour of daily, vigorous exercise, yet still regained most of the weight. Why? Because they started eating more. They could have cut their exercise to just 20 minutes a day and still maintained 100 percent of their initial weight loss if they would have just been able to keep their intake to under 3,000 calories a day. That may not sound like much of a challenge, but weight loss doesn’t just slow your metabolism; it boosts your appetite.

If it were just a matter of your weight settling at the point at which your reduced calorie intake matches your reduced calorie output, it would take years for your weight loss to plateau. Instead, it often happens within six to eight months. You may know the drill: start the diet, stick to the diet, and then weight loss stalls six months later. What happened? Don’t blame your metabolism—that just plays a small part. What happened is that you likely actually stopped sticking to your diet, because your appetite went on a rampage.

Let’s break it down. If you cut 800 calories out of your daily diet— 2,600 calories a day down to 1,800— and your weight loss stalls after six months, then what happened is that at the end of the first month, you think you’re still cutting 800 calories, but you may actually only be down about 600 calories a day. By month two, you’re only down about 500; month three, 300; and by month six, you’re only eating 200 calories less than before you went on the diet.

In other words, you inadvertently suffered an exponential increase in calorie intake over those six months. Yet you may not even realize it, because by that time your body may have ramped your appetite up 600 calories. So, it still feels as if you are eating 800 calories less, but it’s actually only 200. Since an 800 calorie drop in intake may slow your metabolism and physical activity about 200 calories a day, with no difference between calories in and calories out at six months, no wonder your weight loss grinds to a complete halt.

The slow upward drift in calorie intake on a new diet is not because you got lazy. Once your appetite is boosted by 600 calories after dieting for a while, eating 200 calories less at the end is as hard as eating 800 calories less at the beginning. So, you can maintain the same disciplined level of willpower and self-control, and still end up stagnating. To prevent this from happening, you need to maintain the calorie deficit. How is that possible in the face of a ravenous appetite?

Hunger is a biological drive. Asking someone to eat smaller portions is like asking someone to take fewer breaths. You can white-knuckle it for a bit, but eventually nature wins out. That’s why I wrote How Not to Diet. There are foods that can counter the slowing of our metabolism and suppress our appetite—ways of eating to counter the behavioral adaptation and even eat more food, yet still lose weight.

Due to the metabolic slowing and increased appetite that accompanies weight loss, sustained weight loss requires a persistent calorie deficit of 300 to 500 calories a day. This can be accomplished without reducing portion sizes, simply by lowering the calorie density of meals. This can result in the rare combination of weight loss with both an increase in quality and even quantity of food consumed. I’m going to do a whole series of videos about it. The bottom line is that sustainable weight loss is not about eating less food; it’s about eating better food.

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