Secrets of Metabolism
If you’re trying to manage your weight, you’ll need to mind your metabolism.
Metabolism, specifically resting metabolism, is the body’s engine. It’s the energy you burn just to keep your heart beating, your lungs breathing, and your other organs running.
Unless you’re an elite athlete, resting metabolism accounts for 60% to 75% of all the calories you burn each day, and it varies a lot from person to person.
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People who have a naturally high metabolic rate can eat more, without gaining weight, than people who burn calories at a slower pace.
Sounds great, right? You’ve got this awesome internal combustion engine that burns hundreds of calories a day without you having to do a single sit-up.
Now for the bad news: It’s hard to boost your resting metabolism much beyond its natural set point, though it is possible to slow it down.
Here’s what science has shown can put a dent in your ability to lose weight and keep if off.
Sleep
Researchers have known that short sleep leads to weight gain. People who don’t get at least 6 hours of sleep at night are prone to overeating, and they usually crave starchy, sugary foods.
It turns out that overeating isn’t the only issue when you’re cheating sleep; not getting enough shut-eye also slows metabolism.
Researchers at the University of Pennsylvania recently brought 36 healthy adults into their sleep lab. Over 5 days, half the group was only allowed to sleep 4 hours a night; the other half got to sleep up to 10 hours at a time.
Even though the sleep-restricted group was active and awake for more hours of the day, their resting metabolisms slowed by about 50-60 calories a day, says senior study author Namni Goel, PhD. Goel studies sleep medicine at the University of Pennsylvania School of Medicine.
It’s not a huge amount, but “that can add up across multiple nights of sleep restriction,” she says.
Even more concerning, Goel says, is that metabolism sputters just as appetite goes up. Her sleep-restricted volunteers were eating about 500 more calories each day, so the total calorie imbalance just from not getting enough sleep was substantial -- around 550 calories a day, enough to lead to about a pound of weight gain each week.
Protein
Kevin Hall, PhD, a senior investigator at the National Institutes of Health, has been doing experiments to test the idea that all calories are equal, no matter where they come from. With fats and carbohydrates, that seems to be true. But Hall says protein seems to be a different story.
“It looks like there might be some benefit to increasing the amount of protein that you eat,” he says, though more research is needed before he can say for sure. And he’s not sure why protein might boost metabolism, but he has some theories.
The body spends more energy trying to digest and absorb proteins than it does fats and carbohydrates.
“It also might increase metabolic rate, but very slightly. We’re not talking about huge increases here,” Hall says.
Diets that switch the body’s calorie primary fuel source from sugar or carbohydrates to fats also may help with your metabolism. Diets that do this are called ketogenic diets. They tend to be higher in fats or proteins and lower in carbohydrates.
In a recent study, Hall had 17 overweight or obese men follow two different low-calorie diets. The first was a diet that was higher in carbohydrates and lower in fats. The second was a ketogenic diet that was lower in carbohydrates and higher in fats. Protein was kept the same between the two diets. The men lost weight on both diets, but their metabolisms were slightly higher on the ketogenic diet.
The take-home message for people who are looking to lose weight is that if you’re going to cut calories, don’t cut your protein intake.
Eating more protein may help keep your resting metabolism high, which can help you both lose weight and keep it off after.
Weight Loss
When we lose weight, our bodies fight hard to regain it.
Eric Ravussin, PhD, director of the Nutrition Obesity Research Center at the Pennington Biomedical Research Center, likens this resistance to weight loss to what happens when you pull on a spring.
“The more you pull your weight away from your natural settling point, the more your body is going to resist,” he says. One way the body resists weight loss is to slow down its resting metabolism. The more rapid and extreme the weight loss, the more metabolism appears to slow. A recent series of studies has shown exactly how dramatic the metabolic slowdown after weight loss can be.
Hall spent 6 years following contestants from season eight of "The Biggest Loser" reality show. The show takes obese people and pairs them with trainers who push them through extreme exercise -- up to 4 1/2 hours each day -- and strict diets so they quickly lose weight.
At the end of the competition, which lasts for 7 months, some people had lost as much as half their starting weight. The trouble is that their metabolisms slowed even as the pounds flew off.
By the end of the show, when they were at their lowest weight, their resting metabolisms had dropped by more than 600 calories a day, on average.
Researchers had expected some slowing in their daily calorie burn, but the metabolic plunge was even more than scientists had predicted. And contrary to what experts had expected, their metabolisms never adjusted after their extreme weight loss. In some cases, they slowed even more.
Thirteen of the 14 contestants regained some of the weight they lost. Four contestants are heavier now than before they joined the show. Some have said their junk food cravings are still there, though their capacity to burn them off isn’t.
“We took a look at this extreme case of very huge lifestyle changes, huge amounts of weight loss because we wanted to see how strongly the body responds when you intervene to such a large degree. The answer is pretty darn strongly,” Hall says.
Hall thinks that hormones -- particularly the hormone leptin, which banishes hunger -- may play a role.
In a different study, "Biggest Loser" contestants had 80% less leptin at the end of their weight loss than a similar group of people who’d lost weight after bariatric surgery.
Scientists are currently testing whether giving leptin injections after weight loss might preserve metabolism and prevent weight regain.
Until there’s a drug to prevent weight regain, the take-home message here, says Ravussin, is that slow and steady is a better way to lose weight if you want to have a better chance of keeping it of
Even better, Hall says, is to try to change the way you think of weight loss. Instead of going on a diet -- dramatically cutting calories and killing yourself at the gym -- to get to a certain weight, he says it’s better to focus on adopting habits you’ll be able to stick with over the long run.
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By Brenda Goodman, MA
