Here's a sobering statistic: Roughly 90 percent of people who lose a lot of weight eventually regain just about all of it.

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This stark fact underscores how far we remain from conquering the obesity epidemic spreading across the Western world, and the urgent need for us to finally be realistic about the achievable goals and benefits of weight loss.

Why is it so hard to keep the weight off? The reason is both simple and complex. Gaining a significant amount of weight — we don't know exactly how much — doesn't just puff up our fat cells; it changes our biology. Our bodies act as if that higher weight is our normal weight, defending it like a mother embracing her newborn.

First, our bodies slow our metabolism, so we must eat less to not gain weight. A study of contestants on the TV show "The Biggest Loser," for example, found that before the competition, they had an average resting metabolism rate of 2,607 calories per day. After the show, it was 2,000 calories. Six years later, even though most had returned to their pre-show weight, their resting metabolism had dropped to 1,900 calories per day.

At the same time, our bodies barrage us with hormonal signals saying we are hungry all the time, an inner voice chanting "eat, eat, eat."

Essentially, our bodies become our enemy, undermining our efforts at every turn.

Just how hard it is to keep off lost weight was illustrated in a compelling story by The New York Times in reporter Gina Kolata's yearlong project following two patients at Michigan Medicine, the academic medical center here at the University of Michigan.

That story told an all-too-common tale: two people frustrated again and again by an inability to lose weight and keep it off. They both took a serious step, undergoing bariatric surgery at Michigan Medicine. The pounds fell off and their health improved, but neither was completely satisfied with their transformation.

Researchers at Duke University found that 10 years after gastric bypass surgery, almost three-quarters of the obese patients had maintained weight loss of more than 20 percent of their presurgery weight. For a person weighing 300 pounds, that means a sustained weight loss of 60 pounds. (Less encouraging results were found for those who had a sleeve gastrectomy or adjustable gastric banding.)

This study, however, does not herald the discovery of the mythical magical bullet (or scalpel). Most patients in the study, who started with an average body mass index of 47, are still considered obese (a BMI of 30 or above). They do not have the chiseled bodies promised in magazines and strived for on reality programs such as "The Biggest Loser."

This encouraging study, then, offers more evidence of the immense challenges faced even by those who undergo the radical step of surgery to address their weight.

The good news is that groundbreaking obesity research conducted during the past few decades has established why it is so hard to sustain weight loss.

Key lessons about weight loss

Since the 1980s, when most scientists thought fat cells were inert storage vessels, we have discovered that they are engines in a vast and complicated network that interacts with the brain to control hunger, metabolic rate and other key functions.

SEE ALSO: Can an Elimination Diet Help You Lose Weight?

Unfortunately, these insights into how our bodies work have not led to reliable interventions to control them. Two lessons are clear. First, people do not regain weight because they lack willpower. Instead, powerful biological responses counter their best efforts at every turn.

Maybe the most important message comes from my Michigan Medicine colleague, Charles Burant, M.D., Ph.D., "Don't get heavy in the first place."

Easier said than done, I know. But Burant's insight suggests a radical and necessary new way of viewing obesity. Although most people know how hard it is to lose weight and keep it off, our culture still casts it as a typical health condition — a problem that can be cured through the relatively simple treatment of diet and exercise.

Obesity, instead, is better understood as a chronic condition that we bring upon ourselves. Once we let it happen, it is almost impossible to reverse. You'd think the stakes couldn't be higher.

And yet, they are. Recent research suggests that a mother's weight and eating habits before and during pregnancy can affect the genes that control her baby's weight.

This picture, however, is not entirely grim. We are also finding that a little weight loss provides a lot of health benefits. Obese people who lose even 10 or 20 percent of their body weight — like those in the Duke study — usually enjoy meaningful reductions in blood pressure, blood sugar levels and inflammation. They also report feeling much better both physically and mentally.

Although surgery is effective, our weight-loss clinic at the University of Michigan is also helping obese patients lose and keep off 10 percent of their body weight without going under the knife. That result, which can have measurable health benefits, is hard-won; it is achieved through a rigorous program of diet, exercise and counseling.

In this program, as well as in our bariatric surgery program, we are seeking clues that will enhance our ability to prevent weight gain and more reliably assure weight loss.

As we continue to explore the science of weight, we know two things for certain.

• Obese people should focus on how they feel instead of how they look.

• Realistic goals can provide lifesaving benefits.

Obesity research also tells us that we need to pay greater attention to people who are not overweight. For them, the advice is clear: Be vigilant. Watch your weight. Take action when you have gained 5 pounds. By the time it gets to 50, it may be too late.

Marschall S. Runge, M.D., Ph.D., is executive vice president for medical affairs at Michigan Medicine and dean of the University of Michigan Medical School.