What Is Normal Bmi for a Woman 5 7

February 09, 2021

3 min read

Lower death risk with normal weight transition to overweight vs. other BMI trajectories

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Disclosures: The authors report no relevant financial disclosures.

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People with normal weight early in adulthood and overweight at an older age have a lower risk for mortality than those with overweight or obesity in early adulthood or lifelong normal weight, according to study data.

Hui Zheng

"Previous studies on BMI history have focused either on older or younger populations due to data limitations," Hui Zheng, PhD, an associate professor of sociology at The Ohio State University, told Healio. "Our study models lifelong BMI trajectories and estimates trajectory-specific mortality risks. This study is the first to examine the association of lifelong BMI trajectories with mortality and the first to do this across two generations, which should uncover how BMI evolves over the whole adulthood, provide a more accurate estimate of the mortality consequence of obesity than prior studies, and yield insights on the dynamics of this relationship over time."

Adults with normal weight who transition to overweight at an older age have a lower mortality risk compared with those with overweight or obesity early in adulthood.

Defining BMI trajectories

Zheng and colleagues analyzed data from two cohorts in the Framingham Heart Study. The original cohort included men and women aged 28 to 74 years who were examined every 2 to 3 years from 1948 to 2010. The offspring cohort were the children of the original cohort and their spouses. They were examined every 3 to 4 years from 1971 to 2014. Researchers made observations from age 31 years and older. After removing participants with missing data, 4,576 adults from the original cohort and 3,753 from the offspring cohort were included in the analysis.

Participants in the original cohort were divided into seven trajectories. The class II/III obesity trajectory included adults with class I obesity at age 31 years that increased to a BMI of 40.37 kg/m² at age 60 years. The class I obesity trajectory participants started with overweight at age 31 years and increased to class I obesity. The overweight stable trajectory included those starting with a BMI of 26.27 kg/m² with overweight remaining through age 80 years. The overweight downward trajectory included adults with overweight whose BMI gradually decreased by age 80 years. The normal-weight upward trajectory included participants who progressed from normal weight of 22.41 kg/m² to overweight status at about age 55 years. The normal-weight stable group remained normal weight through age 80 years. Finally, the lower level of normal weight included adults who maintained a mean BMI of 19.47 kg/m².

Mortality risk higher with early adulthood obesity

In the original cohort, after adjusting for education and smoking behavior, the class II/III obesity trajectory had the highest mortality risk compared with the normal-weight upward group (HR = 2.18; 95% CI, 1.69-2.82). The class I obesity group had the second highest mortality risk (HR = 1.58; 95% CI, 1.36-1.82), followed by the overweight downward group (HR = 1.37; 95% CI, 1.23-1.53), the lower level of normal-weight trajectory (HR = 1.37; 95% CI, 1.2-1.56), the overweight stable group (HR = 1.34; 95% CI, 1.2-1.49) and the normal-weight stable trajectory (HR = 1.17; 95% CI, 1.06-1.3). A final model adjusting for disease index slightly weakened the associations, but these remained in the same direction for all trajectories.

The offspring cohort was divided into six trajectories similar to the original cohort. There was no overweight downward trajectory in the offspring cohort due to a small sample of people with weight loss, and the overweight stable trajectory in the original cohort was renamed overweight obesity for the offspring. Most trajectories in the offspring cohort slowly increased in weight from age 31 to 80 years, and the normal-weight upward trajectory in offspring advanced to overweight status at about age 45 years, a younger age than the original cohort.

The offspring cohort had lower mortality risks associated with obesity trajectories compared with the original cohort. After adjusting for birth cohort, sex, education and smoking behavior, the class II/III obesity trajectory had the highest mortality risk compared with the normal-weight upward group (HR = 1.8; 95% CI, 1.21-2.64), followed by the class I obesity cohort (HR = 1.43; 95% CI, 1.15-1.77), the lower level of normal-weight group (HR = 1.24; 95% CI, 0.97-1.59), the normal-weight stable cohort (HR = 1.04; 95% CI, 0.88-1.24) and the overweight obesity group (HR = 1.01; 95% CI, 0.84-1.21). The patterns remained the same in the fully adjusted model with disease index included.

"The effect of weight gain on mortality is complex," Zheng said. "It depends on timing and magnitude of weight gain and baseline weight status in early adulthood. For people with normal weight in early adulthood, moderate weight gain into overweight in later adulthood is associated with lower mortality risks compared to those who remain in the range of normal weight over the course of adulthood. But, for people with overweight or obesity status in early adulthood, weight gain is associated with excessive mortality risk."

Despite the lower mortality risks with obesity trajectories in the offspring cohort, the offspring had a higher overall mortality risk compared with the original cohort (6.4% vs. 5.4%) attributable to more participants in the offspring cohort in high-risk trajectories.

Zheng said future research should focus on understanding the mechanisms behind the lower mortality risk for the normal-weight upward trajectory. He added that a similar study should be done for younger generations once data become available.

For more information:

Hui Zheng, PhD , can be reached at zheng.64@osu.edu.

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