Obesity in Adolescents May Carry Same Risk for Premature Death as Smoking

Laurie Barclay, MD

February 26, 2009

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February 25, 2009 — Obesity in adolescents may confer the same risk for premature death in adulthood as smoking more than 10 cigarettes a day, and even overweight has a risk comparable with that for less heavy smoking, according to the results of a record linkage study reported in the February 25 Online First issue of the BMJ.

"While smoking is already a major problem in developing countries, obesity is becoming one in all regions of the world except the very poorest," write Martin Neovius, from Karolinska Institute and Karolinska University Hospital in Stockholm, Sweden, and colleagues. "Despite that, the combined effects on mortality associated with these two risk factors and their interaction in late adolescence are not known. We hypothesised that overweight and smoking in late adolescence increase the risk of mortality and that their effects are synergistic."

The study cohort consisted of 45,920 Swedish men (mean age, 18.7 ± 0.5 years) who were observed for 38 years. Records from the Swedish military service conscription register, cause of death register, and census data were linked.

At mandatory military conscription tests from 1969 to 1970, body mass index (BMI) and smoking status were recorded. Underweight was defined as BMI less than 18.5 kg/m2, normal weight as BMI between 18.5 and 24.9 kg/m2, overweight as BMI between 25 and 29.9 kg/m2, and obesity as BMI of 30 kg/m2 or more. Smoking status was based on self-report as nonsmoking, light smoking (1 - 10 cigarettes per day), or heavy smoking (> 10 cigarettes per day). Cox proportional hazard ratios (HRs) for all-cause mortality were adjusted for muscle strength, socioeconomic status, and age.

During 1.7 million person-years of follow-up, there were 2897 deaths. Mortality risk was increased in overweight men (HR, 1.33, 95% confidence interval [CI] 1.15 - 1.53; incidence rate [IR], 23/10,000 person-years; 95% CI, 20 - 26) and in obese men (HR, 2.14; 95% CI, 1.61 - 2.85; IR, 38/10,000 person-years; 95% CI, 27 - 48) vs normal weight men (IR, 17/10,000 person-years; 95% CI, 16 - 18).

Separate analyses of smokers and nonsmokers yielded similar relative estimates. Although underweight men were not at increased risk (HR, 0.97; 95% CI, 0.86 - 1.08; IR, 18/10,000 person-years; 95% CI, 16 - 19), extremely underweight men (BMI < 17 kg/m2) had increased mortality rates (HR, 1.33; 95% CI, 1.07 - 1.64; IR, 24/10,000 person-years; 95% CI, 19 - 29).

There was no significant relative excess risk from interaction between BMI and smoking status in any stratum, and all estimates of interaction were small, except for the combination of obesity and heavy smoking (relative excess risk, 1.5; 95% CI, −0.7 to 3.7).

Mortality risk was increased in light smokers (HR, 1.54; 95% CI, 1.41 - 1.70; IR, 15/10,000 person-years; 95% CI, 14 - 16) and in heavy smokers (HR, 2.11; 95% CI, 1.92 - 2.31; IR, 26/10,000 person-years; 95% CI, 24 - 27) vs nonsmokers (IR, 14/10,000 person-years; 95% CI, 13 - 15).

Limitations of this study include lack of data on women, use of BMI as a marker for fatness, inability to exclude effects of residual confounding caused by unmeasured confounders, and the possibility that the risk for mortality might also be related to changes in weight and smoking during follow-up.

"Regardless of smoking status, overweight and obesity in late adolescence increases the risk of adult mortality," the study authors write. "Obesity and overweight were as hazardous as heavy and light smoking, respectively, but there was no interaction between BMI and smoking status. The global obesity epidemic and smoking among adolescents remain important targets for intensified public health initiatives."

Dr. Neovius was funded by Arbetsmarknadens Forsakrings-och Aktiebolag and another author by the Swedish Heart-Lung Foundation and the Swedish Research Council. The other study authors have disclosed no relevant financial relationships.

BMJ. Published online February 25, 2009.

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