A long-term mass study of Israeli teens shows that even moderate levels of overweight could be hazardous to cardiac health later on in life.
The study by Prof. Jeremy Kark and Dr. Hagai Levine from the Hebrew University-Hadassah Braun School of Public Health and Community Medicine shows that teens with a BMI generally regarded as healthy could see increased risk of heart attacks and other cardiovascular problems when they get older.
The study was based on a national database of 2.3 million Israeli 17-year-olds whose height and weight were measured between 1967 and 2010 and recorded by the Israeli army in preparation for their enlistment. The results of the study were published Thursday in the New England Journal of Medicine.
Long criticized by many nutritionists as inaccurate and unrelated to modern lifestyles, the Body Mass Index (BMI) has nevertheless been the main indicator used by doctors and government health officials to determine whether an individual is of “normal” weight, overweight, or obese.
BMI is a person’s weight in kilograms divided by the square of height in meters, and there are thousands of BMI calculators online. Generally, a BMI that is under 25 is considered normal weight, while anything above that number is considered overweight or obese.
For kids and teens under 20 years of age, the BMI needs to be adjusted for frame as well as gender, and there are special “growth charts” that list healthy weights for the various ages and weights. Anything up to the 85th percentile on that chart – which corresponds to an adult BMI of 25 – is considered normal weight.
The researchers conducted the study in light of the significant increase in childhood obesity over the past several years. A report released earlier this month by the Central Bureau of Statistics showed that 21 percent of first-graders in 2014 were overweight, which means Israel is catching up to the US, where about one in four of that cohort are overweight according to current BMI measures.
But those measures of what constitutes a healthy weight may be way off, based on the Hebrew U study. Following the health records of the several million study subjects over the years (based on their records in Israel’s National Health Insurance system), the study included a total of 42,297,007 person-years of follow-up. The researchers discovered that there was increased risk of cardiovascular death in the group that was considered within the “accepted normal” range of BMI – even in subjects who were above the 20th percentile, corresponding to a BMI of 20. For those in the 50th to 74th percentiles – still considered within the normal weight range on the youth chart – the risk for heart problems was as much as 50% higher, according to the findings.
According to the study, what constitutes “healthy” in 17-year-olds is defined significantly more strictly than on the BMI scale. In order to “fit” into the 20th percentile, a 5 foot 10 inch (178 centimeter) 17-year-old male would have to weigh no more than 139 pounds (63 kilograms). According to BMI charts, a 17-year-old of that height could weigh as much as 160 pounds (72.5 kilograms) and still be considered normal weight.
The study has its advantages and limitations, said the researchers, chief among the latter being that, in the absence of specific data on the lifestyles the subjects lived, it was difficult to attribute the increased cardiovascular risk specifically to overweight in the teen years.
“We were unable to account for important cardiovascular lifestyle risk factors that may confound the BMI association, although adjustment for smoking had no effect in other studies,” the researchers said.
Another limitation was that, as the data came from IDF recruitment records, “the study sample was highly representative of the Israeli adolescent male Jewish population, but it was less representative of Israeli women, and our findings need to be confirmed in a racially and ethnically diverse population.”
But there’s no doubt that the study’s conclusions need to be taken seriously, the researchers said.
“The large size of our study, which incorporated more than 42 million person-years of follow-up, provided adequate statistical power to assess the associations within the currently accepted normal range of BMI values. The classification of BMI according to the accepted normal range (i.e., the 5th to 84th percentiles and a BMI ranging from 18.5 to 25.0) may underestimate the risk associated with being overweight in adolescence. Our findings appear to provide a link between the secular trends in adolescent overweight and coronary mortality during the past decades. In contrast to the steep decline in the rate of death from cardiovascular causes among older age groups, cardiovascular mortality among young adults has not decreased or the decline has slowed in several developed countries.”
“Our findings appear to provide a link between the trends in adolescent overweight during the past decades and coronary mortality in midlife,” said Prof. Kark, the paper’s senior author. “The continuing increase in adolescent BMI, and the rising prevalence of overweight and obesity among adolescents, may account for a substantial and growing future burden of cardiovascular disease, particularly coronary heart disease.”