Why, Exactly, Do Our Bodies Fight Us on Weight Loss?

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Photo: Alexandra Grablewski

A recent story in the New York Times likely had a lot of people thinking they would never, ever reach their goal weight. The paper reported on a study that followed contestants from season eight of The Biggest Loser and found that, six years later, all but one had regained much of the weight they’d lost, and their resting metabolisms were even slower than after the 2009 finale. Plus, their levels of a hunger-controlling hormone — which were practically zero after the show — had only rebounded to about half the previous level.

In short, the contestants burned fewer calories and felt less full than before they ever set foot on the ranch. Regardless of how you might feel about the controversial show, any logical person would wonder, What the hell? Wouldn’t our bodies want us to be at a lower, healthier weight?

Unsurprisingly, it’s complicated. Here’s a look at the whys and hows of weight loss and weight re-gain.

Our caloric thermostat

To understand why our bodies can’t just be happy for us after we slim down, think of a thermostat. If it’s getting hotter or colder in your house than the temperature you’ve programmed in, the HVAC system kicks into gear to regulate things.

What we eat and what we burn off are usually coupled, says Michael Rosenbaum, an obesity researcher and professor of pediatrics and medicine at Columbia University Medical Center. “If you exercise more and burn more calories, you’re probably going to eat more. If you eat less, you’re probably going to burn fewer calories to compensate for that,” he says.

This system is great for maintaining your usual weight, but pretty terrible if you’re trying to kiss some of those pounds good-bye. It’s obviously possible to burn more calories than you eat, but your body will fight you as you slim down — and when you try to maintain your new weight — via a process known as metabolic compensation.

And that’s on top of natural post-weight-loss changes in your resting metabolism, or the amount of calories your body needs while lying down. (Resting metabolic rate, or RMR, is mainly determined by genetics and muscle mass.) To continue the thermostat analogy, a 4,000-square-foot house costs more to heat and cool than a one-bedroom apartment does. In the same way, larger bodies need more calories to operate than smaller ones. So when a larger-bodied person becomes smaller, they require fewer calories than they used to just as a matter of course.

We evolved to hang on to fat

Now is when you factor in the reality that your body wants you to get back to your starting weight. Metabolic compensation is basically a genetic survival mechanism where a thinner body burns fewer calories during activity than it did before. Being able to store extra calories as fat would have increased chances of survival during times of famine, and also improved women’s ability to reproduce, Dr. Rosenbaum says.

“So you would predict that the human genome is very enriched with genes that favor the storage of extra calories as fat, and very poorly endowed with genes that favor the preservation of leanness, even though we would love to have those genes right now,” he says. “We evolved for a different world.”

All of this means that, for example, a 200-pound woman who lost 50 pounds will burn fewer calories than a woman who weighs 150 pounds without dieting. Another woman who loses the same amount will have a different metabolic adaptation. And both of the dieters will likely be hungrier, too: Not only does metabolism slow in response to weight loss, but levels of hormones that regulate hunger also shift.

Our drive to eat changes, too

The two main hunger hormones are leptin and ghrelin, though they work differently. Leptin, known as the satiety hormone, is secreted by fat cells and helps tell the brain when you’re full. As fat cells shrink, they produce less leptin and your brain doesn’t get the message that you’re satisfied. The hunger hormone, ghrelin, comes from the stomach and tells the brain that it’s time to refuel. In the recent Biggest Loser study, participants’ leptin levels plummeted by the end of the show and only halfway recovered six years later. Other research has shown that ghrelin increases after weight loss and doesn’t return to baseline, at least after a one-year follow-up.

“Those hormones have a huge impact on weight regain and weight maintenance, because it’s not just what your metabolism is doing, it’s also your drive to eat,” says Holly Lofton, assistant professor of medicine and the director of the medical-weight-management program at NYU Langone Medical Center. Studies have shown that these metabolic and hormonal changes persist at six years even in people who were much less overweight than contestants on The Biggest Loser and lost as little as 10 percent of their weight, Dr. Rosenbaum says.

And he adds that there are changes in your brain, too: Food has a greater reward value after you’ve lost weight. “The neural circuitry that’s involved in food restraint becomes less active, you will have to eat more to reach satiation, and you will be less aware of how much you’ve actually eaten.”

You might be thinking that since Biggest Loser contestants lose weight so fast — up to 240-pounds-in-seven-months fast — that’s why their bodies freak out. But Dr. Lofton says that the speed of weight loss doesn’t matter. It’s the amount of weight dropped that does, and whether you lost muscle (which is more metabolically active) at the same rate as fat. Dr. Rosenbaum says it’s not clear. W. Scott Butsch, an instructor of medicine at Harvard Medical School and a physician at the Weight Center at Massachusetts General Hospital, agrees. He says that while it may be true that greater weight loss leads to greater metabolic changes, existing studies haven’t compared changes between groups of people losing weight at different speeds.

Some research indicates that people who lost 5 percent of their body weight have no metabolic penalty and people who drop 10 percent have theirs slow by 20 to 200 calories per day — a far cry from the 200 to 800 described by the Times, though the people in the story lost between 23 and 56 percent of their weight.

Why some people naturally weigh less

So why does a certain weight seem to be easier to maintain for one person than another? The concept of a weight set point is just that — a concept, says Dr. Lofton. But the idea is that the body wants to be a certain weight, and as we lose or gain weight, our hormones and metabolism will shift to get back to that number. Everyone’s set point is different (thanks to things like height) and it can change over time, thanks to things like genetics, previous weight-loss attempts, aging, and hormonal shifts like pregnancy and menopause.

Dr. Butsch explains it as your brain understanding that there’s a degree of fat mass that the body needs as well as how many calories it requires to function. This raises an interesting question: Why would someone’s thermostat be set at a weight that’s higher than what the body needs to survive? He says that people who are overweight sometimes have a degree of leptin resistance, which means their brain isn’t getting good intel on their satiety levels, and they keep eating and storing more fat than needed.

In Dr. Lofton’s experience, and, again, she’s a doctor at a weight-management clinic, set points tend to get higher over time, not lower. Dr. Rosenbaum agrees: “The overwhelming body of evidence is that you can drive this set point up, but not down, at least in animal studies.” Though taking anti-obesity medications can lower people’s set points, as can weight-loss surgery, says Dr. Butsch.

And since the body sees weight loss as an illness, it only makes things harder for people who’ve lost weight before, says Dr. Lofton. “It’s as if our body’s remembering the previous weight loss attempts and what we did — usually cutting calories, increasing exercise — and changing  that hormonal response so that we see fewer weight-loss results as time goes on using the same mechanisms.”

Women in particular might have a harder time keeping weight off, because we have more fat mass thanks to our breasts and hips, and fat stores tend to increase after pregnancy and menopause. That’s why body-fat standards are higher for women than for men. Sadly, the more fat cells you have, the slower your metabolism is, Dr. Lofton says. “Men tend to lose weight faster and more consistently than women. That’s my observation.”

People do keep it off, though. “About 50 percent of the people that I see who’ve lost weight with conventional methods need to be on some type of medication to help them maintain their weight, because they struggle with hunger, they struggle with these hormones, or because their metabolic rate has gotten to a point where they just can’t sustain it all the time,” she says. That means that medically supervised weight loss does work for about half of her patients.

How you can fight back against your physiology

All of this talk about slowed metabolism and increased hunger might make people think long-term weight loss is a lost cause, but that’s just not true. There are things you can do to offset some of the adaptations that your body makes to get you back to your set point.

First and foremost, Dr. Lofton recommends having protein at every meal and snack. Not only does it help you feel full, but it’s the building block for muscles. You want to lose as little muscle as possible while slimming down because it burns more calories at rest than fat does.

As for exercise, she recommends starting out with cardio until you’ve shed about half of the weight you want to lose. Strength training is great for building muscle but it can seemingly backfire in the beginning. “If we increase resistance training and muscle mass and we haven’t done cardio to burn fat, we’re going to see a weight increase initially — which is very frustrating to patients and can lead them to give up on whatever plan they’re doing.” After you hit that halfway mark, she suggests ramping up strength training to prepare for maintenance.

Dr. Butsch agrees. “That’s why we tell people after they lose a lot of weight initially to focus on resistance training, because you’ve probably lost a lot of muscle mass.”

And in general, don’t eat “diet” foods that are low in fat to save calories. Just like protein, you need fat to feel full. Low-fat foods are often higher in sugar or fake sugar than their standard counterparts to compensate for the taste, and they can trick your brain to make you even more hungry. In fact, processed, packaged foods are also easier for your body to digest, which doesn’t help with fullness (or calorie burn), so most dietitians recommend eating as much whole food as possible. Yes, this means cooking and meal-prepping and even buying fruits and nuts to snack on as opposed to energy bars (though in a pinch, bars are better than going hungry).

Losing weight and keeping it off is not a pipe dream, but being realistic about how our bodies react — and what we can do in response — is helpful for everyone.

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