Is it all right to have a few extra pounds or not?

The latest study in a years-long back and forth over the question says not.

Body mass index (BMI) is a measure of body fat based on height and weight. Shutterstock

This research, covering more than 30 million people around the world, suggests that people in the normal range of weight — neither overweight nor underweight — live longer than people who are even just a little overweight.

“It's definitely not good for health to be overweight or obese,” says Dagfinn Aune of Imperial College London, who led the research.

Aune was among the many researchers critical of two Centers for Disease Control and Prevention studies published in 2005 and in 2013 in the Journal of the American Medical Association that found overweight people actually had a 6 percent lower risk of death than people who weighed less.

The studies used body mass index — a measure of height versus weight — to show those with a BMI of 25 to 30 lived longer and had lower risks of dying from heart disease and cancer than those in the "optimal" index range of 20 to 24.9.

Like many others, Aune didn’t believe it.

“Unfortunately, sometimes bad science is published and gets a lot of media attention — misleading the public,” Aune said in an email to NBC News.

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So he and colleagues went through data from around the world covering 30.3 million people and 3.74 million deaths.

They only included people who had never smoked, because smokers often weigh less than non-smokers. But smokers have a much higher risk of dying from a long list of conditions.

They also weeded out anyone who might have a long-term health condition, again with the idea that these people might weigh less and thus skew the calculation.

“In the analysis of all participants the lowest mortality was observed in those with a BMI of around 25,” they wrote in their report, published in the British Medical Journal’s online publication The BMJ.

“However, the lowest mortality was observed in the BMI range of 23-24 among never-smokers,” they added.

And for people with no health problems, those with a BMI of 22-23 were the least likely to die early, and when they looked at people with 20 to 25 years worth of records, the people with BMIs of 20 to 22 lived the longest.

The World Health Organization defines “normal” BMI as 18.5-24.9. BMI is calculated by dividing weight in kilograms by the square of your height in meters.

That means someone who is 5 feet 5 inches tall and weighs 149 pounds has a BMI of 24, which is considered healthy. Add a pound and the same person has a BMI of 25 and is considered overweight. At 170 pounds this person has a BMI of 40 and is considered obese. Someone 6 feet tall and 180 pounds would have a BMI of 24.4 and would be obese at 230 pounds.

The National Institutes of Health has a BMI calculator online here.

Even when they took into account geographical location, age, education, alcohol, physical activity, height, dietary pattern, and intake of fat, fruit, and vegetables, the pattern held.

It’s an important question as the whole world gets larger.

A survey of 188 countries shows that nearly 30 percent of the global population, or 2.1 billion people, are either overweight or obese. Not a single country has lowered its obesity rate since 1980.

When people are too heavy, they have a much higher risk of heart disease, cancer, stroke, arthritis, diabetes and a range of other ills.

Half of all U.S. adults have diabetes or blood sugar so high they're almost diabetic. More than two-thirds of Americans are either overweight or obese.

No one study is ever the last word. Scientists and doctors like to share their findings in real time so they can gradually build a consensus on what makes sense and what doesn't.

The CDC and WHO both advise keeping weight in the accepted healthy range, meaning under a BMI of 25.