With the U.S. confronting an obesity epidemic that shows no signs of abating, some doctors and scientists are divided on just how dangerous extra weight is.

The debate is a critical one when you consider that about two-thirds of U.S. adults are classified as either overweight or obese, meaning they have a body-mass index (calculated as weight in kilograms divided by height in meters squared) of 25 or higher.

While most everyone agrees weight loss is a good goal for those who are heavy, some researchers now believe that fitness is more important than fatness when it comes to long-term health and longevity.

They cite studies showing that people in the overweight category—or those with a BMI from 25 to 29.9—actually have better survival rates in some situations than their normal-weight counterparts, a finding known as the “obesity paradox.”

Others say the risks related to obesity are being underestimated. They say most of the studies supporting the obesity paradox are flawed in that they don’t take into account people’s weight history.

Carl J. Lavie, the medical director for cardiac rehabilitation and director of exercise laboratories at the John Ochsner Heart and Vascular Institute in New Orleans, says overweight people can be healthy, so fitness should be the goal. Andrew Stokes, assistant professor in the department of global health and Center for Global Health and Development at Boston University’s School of Public Health, disagrees, saying the risks associated with extra weight are being underestimated.

YES: Being Fit Is More Important Than Being an Ideal Weight

By Carl J. Lavie

First and foremost, I don’t promote overweightness or obesity or suggest that normal-size people gain weight. That said, fat isn’t always the demon it is made out to be when it comes to a person’s long-term health and mortality.

In a perfect world, everyone would be lean, fit and metabolically healthy, maintaining balanced blood-sugar levels, healthy lipid profiles and good blood pressure throughout life. However, this is far from the case in the U.S., where nearly three out of four adults are classified as either overweight or obese.

The good news for those who may be struggling to lose weight and keep it off is this: As I explored in my book “The Obesity Paradox,” there can be surprising benefits to carrying around a few extra pounds. In fact, being out of physical shape and having low overall fitness is actually a far greater danger to health than fat, especially in people who are only slightly overweight.

A major 2013 study published in the Journal of the American Medical Association and led by Katherine Flegal of the Centers for Disease Control and Prevention found that although obesity was associated with much higher risk of death, it was all due to the higher mortality in moderate and severely obese people, or those with body-mass indexes (BMIs) greater than 35 and 40, respectively.

The mildly obese, or those with BMIs of 30 to 34.9, actually had a 5% lower mortality than did the normal BMI subjects, which wasn’t quite statistically significant. More important, the overweight people, or those with BMIs between 25 and 29.9, had the lowest mortality and a significant 6% lower mortality than the normal BMI subjects, according to the researchers, who analyzed 97 studies involving nearly 2.9 million adults.

Although the paper was controversial and harshly criticized by some, many obesity experts around the world applauded the results because it supported their data and observations. Furthermore, it got the conversation moving in a different direction—one that must account for the quality of one’s fitness and metabolism regardless of weight and fatness.

Many overweight or mildly obese individuals can have normal blood glucose levels, lipid profiles and blood pressure, and if these people are physically active and have a good level of fitness, substantial evidence indicates that they can be extremely healthy—more so than lean individuals who are unfit or metabolically unhealthy. So while preventing more severe obesity is indeed a necessary goal, the vital takeaway from the latest scientific literature is: Fitness is more important than fatness for long-term health.

I have emphasized in many of my medical papers that being heavier increases the risk of coronary heart disease, as well as other cardiovascular diseases such as hypertension, heart failure and atrial fibrillation. However, as numerous reports show, the overweight and even the mildly obese have a better short- and medium-term prognosis and survival compared with normal BMI subjects with these same diseases, a phenomenon called the obesity paradox and possibly better termed an overweight paradox.

When people gain weight, the majority of weight is in fat pounds, but some muscle weight increases, as well, more so if one is exercising. By contrast, a thin person who is physically inactive and physically weak, with poor fitness and low muscle mass and muscular strength, is actually “sicker” than the heavier person who is fitter. And very thin people (BMI in the upper teens and low 20s and especially less than 18.5, which is considered “underweight”), generally have the worst prognosis and survival for many conditions, partly because they have fewer fat reserves to fall back on when major illness takes a toll on their bodies.

Some critics of the obesity paradox take issue with the fact that many obesity studies don’t differentiate between people who have always been normal weight and people who used to be overweight or obese but lost weight (which could be bad if their weight loss was due to illness). They also suggest that the beverage industry may have influenced results by funding some research. The truth is, the studies supporting the obesity paradox are large, clean and robust and have been published in prestigious journals, something over which the beverage industry has no control.

There’s no doubt that preventing obesity at the extreme end is important. But the situation for those who are overweight or mildly obese isn’t “doomsday,” as they can be very healthy despite the extra pounds. For these people, which represent a large number of adults in the U.S., weight loss is a good goal, but preventing weight gain and maintaining optimal fitness is a more important goal.

Dr. Lavie is the medical director for cardiac rehabilitation and director of exercise laboratories at the John Ochsner Heart and Vascular Institute in New Orleans. Email him at reports@wsj.com.

NO: Research Supporting the Idea Suffers From a Significant Flaw

By Andrew Stokes

It is almost hard to imagine that in the early 1960s, at the peak of the smoking epidemic, about half of American men and a third of American women smoked cigarettes. Although lung-cancer rates started climbing as early as the 1920s and studies showing a link to smoking began to appear soon thereafter, appreciation of the harms of smoking remained remarkably low for many decades.

Not until the landmark Surgeon General’s report of 1964 declared that cigarette smoking causes lung cancer did public perception start turning. To fight faltering sales, the tobacco industry launched an aggressive campaign to sow doubt about the harms of smoking that would last for many years. This campaign, which enlisted respected physicians and researchers, sought to portray the science on the health effects of smoking as unresolved.

We find ourselves in a similar situation today even if the culprits may not be as obvious as before. The proportion of the U.S. population that is overweight (defined as having a body-mass index of 25 to 29.9) or obese (BMI of 30 or greater) has increased rapidly in recent decades, and excess weight is a well-established risk factor for diabetes, cardiovascular disease and cancer. Yet, as was the case historically with smoking, the effects of carrying extra weight on the risk of dying apparently remain an open controversy.

Some research in recent years has suggested that being in the overweight range may actually lower a person’s risk of dying prematurely. Some in the food and drink industry have embraced the finding, funding researchers whose work supports the idea that extra weight may be good for you. Other skeptics, both inside and outside of academia, have jumped on board, claiming that the hazards associated with excess weight are overblown.

Why does it remain unclear after so much research whether being overweight is helpful or harmful for longevity? There is one simple but surprisingly overlooked problem with previous studies examining the effects of being overweight and obese: They fail to account for weight history.

Looking at weight at a single point in time, like almost all obesity studies do, misses the fact that low weight can be a sign that something is wrong—that is, sometimes people are slim because they have a serious illness such as heart disease or cancer. To epidemiologists, this bias is known as confounding by illness or reverse causality.

Using information on weight history, it is possible to address this seemingly intractable source of bias. Weight history makes it possible to distinguish people who were slim throughout their lives from those who were formerly overweight or obese but lost weight. Again, the importance of this distinction can be seen in a simple analogy to smoking.

If you study the health effects of smoking by comparing smokers to nonsmokers, you may very well conclude that smoking isn’t all that bad for you. However, the reason isn’t that smoking is inconsequential but that the comparison is biased. Nonsmokers include both low-risk people who never smoked and high-risk people who used to smoke but quit—many of them after developing an illness. To deal with this bias, epidemiologists factor in smoking history to divide current nonsmokers into former smokers and never smokers.

In similar fashion, any study that fails to distinguish never-overweight/obese people from the former overweight/obese will arrive at biased results. This doesn’t mean that all people who maintained normal weight throughout their lives are healthy. Likewise, not all people who were formerly overweight/obese and lost weight are at high risk of dying. Some of them lost weight through healthy lifestyle change, and there is strong evidence that this form of weight loss reduces risk of cardiovascular disease, though science has shown that intentional weight loss is difficult to achieve and even more difficult to sustain.

But my research with Samuel Preston of the University of Pennsylvania suggests that taken as a whole, people who were formerly overweight/obese and are now of normal weight are a high-risk group because of illness-induced weight loss. Conflating this group with the never-overweight/obese category—as almost all prior studies do—obscures the real health risks associated with excess weight.

Flawed research has helped to create an appearance of much greater uncertainty than actually exists around the science on overweight and obesity. With about two-thirds of Americans currently classified as overweight or obese, it is vital that we don’t become complacent and that we do everything in our capacity, at individual, community and national levels, to aggressively tackle this epidemic in the U.S.

Dr. Stokes is an assistant professor in the department of global health and Center for Global Health and Development at Boston University’s School of Public Health. He can be reached at reports@wsj.com.