A new study by researchers at Oxford Population Health and in China has shown that entering adulthood with a healthy body weight is associated with a substantially lower risk of premature death from cardiovascular disease, cancer and respiratory disease. The study of approximately half a million Chinese adults is published in Science Bulletin.
Most evidence linking body mass index (BMI) to mortality comes from measurements taken in middle or later adulthood, when illness-related weight loss can affect the results (so-called reverse causality). In this study, researchers looked at the associations between BMI at 25 years old and the overall risk of death, as well as death from specific diseases.
They used data from the China Kadoorie Biobank (CKB) study in which participants from 10 rural and urban regions of China were recruited at around 52 years of age and then followed for an average of 12 years. They used participants’ self-reported weight at 25 years old and their measured height at the start of the CKB study to calculate BMI in early adulthood.
Key findings:
- Higher BMI at age 25 (BMI25) was associated with a higher risk of death overall and from specific causes, including cardiovascular disease, cancer and respiratory disease, with risk increasing steadily with BMI25;
- People who were obese during their early adulthood (with a BMI25 of 28 kg/m2) had an 85% higher risk of premature death and death from cardiovascular disease compared with those whose BMI25 was between 18.5 and 20 kg/m²;
- Respiratory disease mortality showed a particularly strong association with early adulthood obesity and was linked to a more than doubling of risk;
- Higher BMI25 was associated with increased cancer mortality overall, but with a lower risk of breast cancer mortality among women;
- Higher BMI25 was also strongly associated with higher levels of blood glucose, but not blood pressure (after taking into account BMI in later life).
The positive associations between BMI in early adulthood and mortality were shown to be independent of body weight later in life, indicating that excess weight in early adulthood has lasting effects that are not fully reversed by subsequent weight change.
Many previous studies have reported a J-shaped relationship between BMI and risk of premature death, suggesting that both low and high BMI may be harmful. However, such studies typically relied on BMI measured later in life, when chronic disease can lead to weight loss. By focusing on BMI in early adulthood, this study found little evidence of increased mortality risk linked to lower BMI levels.
Huaidong Du, Associate Professor and Nutritional & Cardiometabolic Epidemiologist at Oxford Population Health, said ‘Our results suggest that the harmful effects of obesity begin early and can persist across the life course. While reducing obesity at any age is beneficial, maintaining a healthy weight from early adulthood appears to offer the greatest long-term protection.
‘Although obesity was relatively uncommon in early adulthood in this cohort of the Chinese population, the strength of the associations has important implications as obesity rates among adolescents and young adults continue to rise worldwide.’
‘Future studies on blood biomarkers may provide new insights into the mechanisms linking BMI in early adulthood with diabetes, breast cancer and mortality.’