As childhood obesity rates have reached global epidemic proportions, researchers are scrambling to figure out what carrying around extra weight when you're young means for health risks later.

There's a growing body of evidence that suggests being overweight and obese during adolescence may actually cut into life expectancy in a serious way.

In a new study out of Israel, published in the New England Journal of Medicine, researchers gathered data from 2.3 million youth and found that even a few extra pounds early on put the study participants at an increased risk of death from cardiovascular disease by early and mid-adulthood.

"The association between a higher body mass index in youth and cardiovascular death later is very robust," said lead study author Gilad Twig of the Sackler School of Medicine at Tel Aviv University, "and it didn't change in light of a very extensive sensitivity analysis."

A person's risk of cardiovascular disease and death began to increase above a BMI of 20

Twig and his colleagues were able to work from a data set involving millions of Israelis because all 17- to 19-year-olds in the country are screened for fitness before starting mandatory military service. In these army health assessments, height and weight are measured along with vision, diabetes, asthma, and other health risks.

The researchers linked this data about Israelis in their youth with cause of death information from the Ministry of Health, focusing in particular on the relationship between body mass index (a measure of body fat calculated by dividing one's weight by one's height squared) and cardiovascular death (including coronary heart disease, stroke, and sudden death from an unknown cause).

What they found was startling: Every little step a young person moved up the BMI scale toward overweight and obesity, his or her risk of death from cardiovascular issues increased. (As I've reported, BMI is an imperfect metric for individuals, but it can be useful to figure out whether children may be at an increased health risk.)

The scale they used comes from the US Centers for Disease Control and Prevention. In this chart you can see how the CDC classifies adolescent BMI according to percentiles based on age, weight, and height. A BMI that falls below the 5th percentile would classify a person as underweight. A BMI between the 5th and the 84th percentile is considered normal. The 85th to 94th percentile is overweight, and above the 95th percentile is obese.

The researchers discovered that the risk of death later in life fell along a gradient associated with adolescent BMI, with the risk increase beginning at the 50th percentile — which is currently considered within the "normal" range.

Here's what the risks looked like, specifically: For those who had a BMI below the 50th percentile, the absolute risk of cardiovascular death later in life was no higher than those at the lower end of the BMI scale (whose BMI fell within the 5th to 24th percentile, a range the researchers used as the reference group).

Adolescents with a BMI between the 50th and 74th percentile had a 32 percent increased risk of cardiovascular mortality (again, compared with the reference group). As for those with a BMI above the 75th percentile, they had a 76 percent increased risk of cardiovascular mortality compared wit the reference group.

"Overall, adolescents with a BMI below the 50th percentile had the lowest mortality rate and cardiovascular risk later in life," said Twig. In addition to percentiles, BMI can also be calculated using absolute values. They found the optimal BMI score was around 18.3 and the risk of cardiovascular disease and death started to increase above a BMI of 20.

While this is an observational study, which can't tell whether a high BMI early on caused cardiovascular death later, the fact that the researchers found mortality risk increasing along with BMI was fascinating, said Andrew Stokes, a professor who has researched obesity at Boston University. "The effects of weight on mortality appear to be graded on a continuum of risk that begins at the midpoint of the normal weight category."

Stokes noted that the current definition for child and youth BMI may be too crude, since it groups everybody — from the 5th to the 84th percentile for body fat — in the normal weight category.

Twig agrees the results raise questions about the BMI system. "I don't think a single study should lead to a change in the definition of what is the normal range of BMI," said Twig. "But I do think that defining the normal BMI range between 5th and 84th percentile is something that should be carefully considered by other studies that might look at other outcomes."

This study population was somewhat unique, and had some other limitations

The data set was massive, and the researchers did an impressive number of statistical tests to check their conclusions.

But there are some things the research can't tell us.

They only had data at two points in time: adolescent health profiles from military enrollment, and cause of death later. Although people who are overweight when they're young are likely to be overweight later in life, we don't know how the study participants' bodies changed in adulthood.

The researchers also didn't have any smoking data, and smoking often starts during the teenage years and affects body size and cardiovascular disease risk in adulthood.

There may also be something unique about this group that puts them at a higher than normal risk of death overall. As Lund University obesity researcher Paul Franks noted, the average age of cardiovascular death — less than 50 years in the study — is young. "It’s not clear why, but it’s possible that being a [soldier], there are occupational exposures that affect cardiovascular mortality," he said. [Update: Dr. Twig explained that the cohort he studied was young overall — with a maximum age of 64 — which drove down the average age of death.]

Finally, this wasn't an experiment — again, it was an observational study — so it can't tell us whether body weight caused the heart problems later, just that the two are linked.

But, Franks added: "The consistency of these data with those from other [similar studies in non-military populations] somewhat mitigates this problem." For Franks, even if the causal link between BMI and heart problem risk later hasn't been proven, "I think we can say without doubt that childhood obesity is a robust marker of premature death from specific causes."

There's still a strong basis for a link between adolescent weight and risk of disease and death later

The new Israeli research adds to a growing body of evidence that being overweight or obese in youth can hurt your heart health and increase your risk of death later.

Jennifer Lyn Baker, an epidemiologist and public health researcher at the University of Copenhagen who has led studies on adolescent BMI, said the new research checks out with findings from Northern Europe. "An advantage of this study is that it includes adolescent girls — most studies have only included boys," she said.

A recent Lancet study looked at rates of BMI from 200 countries from 1975 to 2014 and projected that if the current trends continue, by 2025, about a fifth of adults worldwide will be obese.

To curb the problem, one of the Lancet researchers, Majid Ezzati, pointed out in a statement that figuring out how to help people eat better is one step. "To avoid an epidemic of severe obesity, new policies that can slow down and stop the worldwide increase in body weight must be implemented quickly and rigorously evaluated, including smart food policies and improved health-care training."