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Yup, significant and speedy weight loss alters your metabolism to the point it may start fighting to get that weight back on. Find out why, and perhaps what you can do to avoid this effect.

"I tracked my daily calories and made sure to have a 500 calorie deficit every day," a friend told me. "At first, I lost quite a bit of weight — but then, without changing anything, I not only stopped losing, but started gaining again, until I weighed more than I did before I ever went on a diet. I think going low-calorie totally messed my metabolism up."

We've all heard this same story, in one way or another — but does (significant) weight loss really slow your metabolism down, and if so, what does that mean?

What is metabolism, and what factors impact yours?

Metabolism — which quite interestingly comes from the Greek word for "change" — is a label we use for the chemical stuff bodies do to stay alive, so things like converting food to energy and other things we need, and getting rid of waste.

Your resting or basal metabolic rate (BMR), an important concept for anyone who is actively trying to lose weight, refers to the calories it takes for your body to simply carry out all these essential functions, like breathing, keeping your heart beating, and keeping your cells in good working order. It's the calories you burn even when you're not doing anything, basically. You may be surprised to learn that somewhere between 40 and 70 percent of your daily calorie intake is spent on these basic bodily functions. 

Everything from your age to your biological sex, and from your genetic makeup to your weight and height, influence your BMR, along with certain medical conditions. But other factors, including what we eat, do impact our metabolism as well. The thermic effect of food (TEF) refers to the calories we burn digesting what we just ate, for instance. 

Does losing weight change your metabolism?

Yes, it does. As your BMI drops, your body will automatically need fewer calories to sustain itself as well, meaning that a slowed basal metabolic rate is actually a perfectly normal and expected feature of weight loss. Research has also shown, however, that the basal metabolic rate often goes through more changes than would be expected based on your new weight alone.

This kind of makes sense, too — historically, humans didn't have access to the calorie bonanza many of us do today, and losing weight was bad news and a sign of food insecurity. So this change in metabolism, callled adaptive thermogenesis, is believed to serve the purpose of preventing further weight loss and setting your body up to gain weight again. 

The whole thing goes a long way towards demonstrating why gradual weight loss with the help of minor calorie deficits is the more sustainable choice, doesn't it? When you take this approach, you're not sending your body into a starvation mode that will make it fight tooth and nail to get the weight back. 

If anyone made an effort to lose weight — lots of it — it was the Biggest Loser crew. They started out, on average, at 328 pounds. By the end of 30 weaks of blood, sweat, tears, and dieting, that average had dropped to 200 pounds. Their reduction in body fat was similarly impressive. Yet, a now famous 2016 determined that all but one gained weight again six years on, and many were very close to their starting weight. 

The answer lies in their resting metabolic rate, which suffered a long term decrease. The Biggest Loser participants' resting metabolic rates were such that they burned anywhere from just over 100 to more than a 1000 calories less by doing "nothing" — meaning their bodies expended less energy on performing their basic functions. The folks who lost the most significant amounts of weight had, you guessed it, the most drastically reduced BMR. 

It's not all bad news, though — over half of the participants "maintained at least 10 percent weight loss", the study said, and over one-fith of overweight people who lose significant amounts of weight do. Healthy lifestyle changes are also pretty likely to be maintained. 

What can you take away from this?

The only way to prevent the effects of adaptive thermogenesis from having much of an impact on you is to lose weight gradually with a healthy diet that meets your nutritional needs — so your body doesn't think it's starving (or, to be more precise, so that your body isn't starving). This is why fad and crash diets are such as bad idea and why people often regain their weight and more immediately after. (Your mind does neat things like craving food terribly to "make you binge" to help this process along, too.)

After you reach your goal weight — which should fall into the healthy weight range, and not anywhere under — you then switch to weight maintenance and stop losing weight. Yup, we know from experience that seeing the numbers on the scale go down is exhilierating and maintance is a bit of a let-down. But hey, if it's time for you to go to maintance, the point at which you neither gain nor lose, you've meet your goal. That's not just an achievement, but one that will still take some hard work to keep up. Maybe that's why they call it "maintenance". 

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