Body Mass Tracking From Childhood to Adulthood
Predictors and Tracking of Body Mass Index from Adolescence into Adulthood
What is the problem, and what is known about it so far?
Obesity is defined as an excessive accumulation of body fat, where the body mass index is more than 30. During recent decades, the mean body mass index (BMI) (weight in kilograms divided by the square of height in meters) and the proportion of overweight and obese persons have increased worldwide. It has been recently described by health officials as one of the greatest health problems facing America, contributing to more than 300,000 deaths each year.
Studies have shown substantial tracking (people maintain their relative position within their age-sex group over time) of body weight and obesity. Only a few studies have addressed the effect of parent's body weight on the change in their children's body weight from childhood or adolescence into adulthood. Many studies show that more physical activity and higher levels of education are associated with lower BMI, and reduced physical activity plays an important role in increasing body weight. It is also reported that smoking is linked with lower BMI in men and women, and ending smoking typically tends to increase BMI at least to the level of persons who never smoked.
Why did the researchers do this particular study?
The investigators wanted to examine tracking of BMI from age 15 to 33 years, in order to examine the effect of adolescent and adult health-related behavior and parents' BMI and education on adult BMI. They also examined changes in lifestyle factors as predictors of adult overweight and obesity.
Who was studied?
In 1979 and 1981, 957 of 1086 students aged from 11-16 years (average 13 years) from six schools in Oslo, Norway filled out questionnaires. In 1999 636 of 915 students filled out follow up questionnaires. Information regarding BMI for both 1979/1981 and 1999 was available for 485 students, 235 women and 250 men, which represents 45% of those used from the main analysis from 1981. 482 of the students' parents participated in the study.
How was the study done?
The baseline questionnaire asked about smoking habits and frequency of leisure time physical activity (LTPA) defined as at least 30 minutes of physical activity that produced a sweat and shortness of breath. Each student underwent a brief health examination including measurement of height and weight and assessment of sexual maturation. Physical fitness was assessed by pulse counting while students rode ergometric bicycles. Parents completed a baseline questionnaire that included information on educational level, height, and weight. The follow-up questionnaire which was mailed to the participants asked about frequency of LTPA, smoking habits, educational level, weight and height.
What did the researchers find?
There was a strong relationship between BMI in adolescents and in adulthood for men and women combined. The most important factors explaining BMI in adults were adolescent BMI, father's BMI, smoking, and gender. These factors explained almost half the variation in adult BMI. The study also showed that the quarter of people with the highest BMI at baseline had the greatest risk of having a BMI of 30 or more as an adult. Also the half of the people with the lowest BMI at baseline had the lowest risk of having a BMI of 25 or more at follow-up. However, those who increased their physical activity level between adolescence and adulthood had a lower risk of adult overweight than those who did not. Interestingly, those who quit smoking at adolescence increased the risk of being overweight in adulthood compared to those who smoked at both time points. In addition, the risk of being overweight was reduced if both parents and participants had a high educational level.
What were the limitations of this study?
People reporting their own weight and height have a tendency to overestimate height and underestimate weight, resulting in an underestimation of BMI. However, since this was a longitudinal study, if the degree of error is the same for all heights and weights the BMI may not be affected. It was hard to draw conclusions about the effect of stopping smoking because the group of smokers was small (17).
What are the implications of the study?
This study shows that relative body weight stays the same throughout life, and that both parents' BMI and a person's own LTPA are important factors influencing adult body weight. This suggests that the foundation for adult body weight is laid already during adolescence. This implies that the importance of physical activity should be emphasized at a young age.
Summarized by Gina Trachimowicz, College of Medicine, University of Vermont
Summarized from "Predictors and Tracking of Body Mass Index from Adolescence Into Adulthood". Kvaavik, Elisabeth et. al. Archives of Pediatric & Adolescent, December 2003, Volume 157, Number 12, Pages 1212-1218.
