Your weight affects how long you live—but it's extremely complicated

Every January, fat's in the crosshairs of health columnists, fitness magazines, and desperate Americans. This year, PopSci looks at the macronutrient beyond its most negative associations. What’s fat good for? How do we get it to go where we want it to? Where does it wander when it’s lost? This, my friends, is Fat Month.

We often think about weight loss in the short term, hoping to drop five pounds in time for vacation or 10 in time for a wedding. Of course, this kind of yo-yo dieting isn't the greatest for our health. If you're going to ask how much you should weigh, you want to think long term—what weight will keep you healthy?

This question has been at the forefront of researchers' minds for a while, and for good reason. Global obesity rates are high and steadily increasing, with 1.3 billion adults considered overweight and another 600 million in the weight range categorized as obese.

While it's true that muscle weighs more than fat, most people who are carrying around a few extra pounds are doing so in the form of adipose tissue. Unlike bone and muscle, fat cells can generate inflammation, allowing us to heal infections and protect our bodies from further damage. But having too many fat cells causes our bodies to release inflammatory proteins all the time. Many studies indicate this can increase our risk of developing cancer.

Carrying around extra fat cells also affects other physiological pathways, many of which—high blood pressure, hyperglycemia (persistently high blood glucose levels), and high cholesterol, for example—can lead to potentially fatal heart problems.

We all need some amount of body fat. So how much is too much? Using BMI (body mass index, the body mass divided by the square of the body height) and rates of overall mortality, scientists studying this question initially came to a surprising conclusion: On a population level, the relationship between BMI and mortality formed a U-shaped curve, in which the lowest point (the one where mortality was at its lowest) was actually at a BMI range considered nearly overweight (about 24.5, when 25 is overweight). Bring on all the healthy fats and carbs.

But some scientists—including Andrew Stokes, a chronic disease and global health researcher at Boston University—challenged those conclusions. Instead of a U-shaped curve, his group's follow-up studies saw the risk of mortality increase with higher BMIs. The lowest risk category, he says, is somewhere in the low-normal weight range, and risk increases pretty consistently as BMI goes up.

The earlier studies failed to account for two important factors, according to Stokes. “The normal weight category used in most of the studies combines low-risk, stable weight people with high-risk individuals who have lost weight,” he says. If a study subject spends most of their life obese and then loses weight, they might accumulate years of negative health effects. Not all of those risks disappear with weight loss. “Much of the research on obesity just uses a snapshot of weight currently," says Stokes.

Stokes equates this to the way we study smoking. You can’t simply compare nonsmokers to smokers. Nonsmokers include those who have never smoked as well as those who may have smoked for several decades and then quit.

And smoking itself is another crucial factor in these BMI studies, Stokes says. Smoking can present a huge bias in estimating risks associated with obesity, because the habit affects body weight through metabolic effects and reduced appetite. By failing to take smoking into account, your analysis may include people who have a low body weight but smoke heavily, upping their chance of an early death. Once those outliers are removed, Stokes and his colleagues argue, the relationship between excess body weight and early mortality is clear.

Of course, the range for normal BMI is pretty broad. For example, someone who is 5'4" has a normal BMI if they weigh anywhere from 108 pounds to 145 pounds. If we want to know which sliver of the range is actually best, Stokes says, researchers have a lot more work to do.

And then there's the question of whether BMI is the right metric to use at all. BMI is often criticized as a poor indicator of health. Because muscle weighs more than fat, a weightlifter could have a BMI in the overweight range, and that's to say nothing of all the other variations the human form can take. Even two folks with the same BMI and the same amount of actual body fat might face different risks due to their adipose, if one carries more in their belly and the other stores more in their hips. Studies that compare BMI to superior methods like the Dexa scan, a type of x-ray that can determine exactly how much body fat you have and where, show that the potential misclassification is not trivial. But Stokes argues that on a population level, BMI is a pretty good parameter to use. Variations in its accuracy do exist, but it's not so wildly off-base as to be useless when we make generalizations about risks throughout an entire population.

But that's the key here: we're talking about average risk. None of this is to say that someone with an ideal BMI is bound to outlive someone who is obese. There are plenty of other factors at play in determining our health, including genetics, exercise, diet, and especially stress. Body weight and the degree of fat we carry often intertwines with these other characteristics, but it's still only one factor. An individual's health can't be determined by a number on a scale, and one should always talk to a doctor when considering major lifestyle changes.

Still, while there’s no scientific consensus on what range of normal BMI equates to lowest overall mortality, Stokes hazards a guess for the lower end, perhaps 20 to 22. He also points out that many studies (on rodents and primates, anyway) show calorie restriction can increase longevity. One population he thinks we should study more are those humans who have always had a very low BMI. Ping-ponging your weight around to try to get on the lowest possible end of the healthy threshold is definitely not a good idea, but living your entire life with a BMI on the lower end—maybe even under 20—could decrease risk of diseases like cancer and diabetes. It's an important question, Stokes says, and one that not enough researchers are asking.