(Amy Cavenaile/The Washington Post; iStock)

You lose weight, and then you gain it all back. That grim reality is a mainstay of our obese society. But a new study of contestants from the reality TV show “The Biggest Loser” suggests that, for them, that re-gain was all but pre-ordained. Contestants were severely obese and exercised and dieted competitively for 30 weeks. The unusual circumstances (most of us don’t lose weight with the whole world watching) led to unusual weight loss — over 100 pounds on average. But the process slowed their metabolism in such a way that it left them having to eat much, much less — 500 calories a day less — than other people just to maintain that weight loss. It’s not surprising that, after six years, all but one regained significant amounts of weight, and four ended up heavier than before the competition. But what does that mean for the rest of us who want to lose weight? Is weight loss always futile?

It isn’t. Although the study was a damning indictment of the show, it doesn’t apply to those of us trying to lose weight with less extreme measures. There’s a substantial body of research on whether, and how much, your metabolism slows after weight loss, and the “Biggest Loser” study is a definite outlier. No other study shows such a large slowdown over such a long period of time.

If you’re trying to lose weight, the message from that body of research is that all is not lost. Most people who manage to lose weight aren’t looking at a lifetime of 500 fewer daily calories. Metabolism may slow somewhat, but not that much, and probably not for that long.

The question isn’t whether you burn fewer calories after you lose weight, because you absolutely do. There’s less of you to move around, to pump blood through, and to keep at 98.6 degrees. Smaller people burn fewer calories than larger people. The question is whether your metabolism goes into a mode where it tries to be thrifty, to conserve calories and do all those basic functions using less energy. If it does, it means you have to eat less than someone who is the same weight but didn’t have to drop pounds to get there. Is there a metabolic penalty, in other words, for getting lighter?

Much of the evidence shows that there is some, but the amount varies. And some of the evidence shows none.

In the none category, there’s a study of women who lost 5 percent of their body weight (with a high-protein diet and exercise) with no metabolic penalty. And two studies of people who underwent bariatric surgery, all of whom lost significant weight, showed no disproportionate decrease in metabolism. Sometimes, there’s a temporary metabolic penalty that disappears after a while. A 2012 meta-analysis that looked at data from nearly 3000 subjects from 71 different studies found no penalty.

Among research that does find a metabolic penalty, the numbers run much lower than the 500 calories that “Biggest Loser” contestants experienced. Several studies find penalties in the 20-100 calories/day range, and several more are in the 100-200 range.  The ones that track the penalty over time often find that the penalty decreases, at least a little, after a year.

What’s a dieter supposed to do? “There are many variables,” and calculating the expected decrease in metabolism is tricky, says Eric Doucet, professor at the University of Ottawa and co-author of several papers on the subject. “There are different formulas,” he explains, and some studies base their estimates on body weight, some on fat-free mass, and some use complicated calculations involving things like how much your liver weighs. Nevertheless, he says, most of the scientists working on the subject — “six or seven labs around the world” — are all finding that a slowed metabolism is an expected consequence of weight loss.

Just how much, and for just how long is a question we don’t have all the evidence to answer precisely, according to Doucet. And there’s so much variability among individuals that an average wouldn’t be helpful in predicting what one particular person could expect. The range in most of the research is between nothing and a couple hundred calories a day, and many studies show that number decreasing after about a year. It’s a bummer, certainly, but it’s not the death knell for weight loss.

The most telling indication that the high metabolic price “Biggest Loser” contestants paid isn’t a dieter’s destiny is a study that followed 13 pairs of subjects, matched for gender, weight and age. In each pair, one was a “Biggest Loser” contestant and the other a bariatric surgery patient. At the seven-month mark, the contestants showed an average penalty of 419 calories (and they weren’t followed after that point). After six months, the surgery patients showed a 201-calorie penalty, but after a year it disappeared.

If you lose weight, it’s reasonable to expect to pay a metabolic penalty. That’s the bad news. But the good news is that it won’t be huge and it may be temporary. And the best good news is that you lost weight.