Relationship between Age and BMI – An Indicative Analysis

Body Mass Index (BMI) is a numerical value calculated from a person's weight and height (kg/m²) and is used to categorize individuals into different weight status groups such as underweight, normal weight, overweight, and obesity. Its impact on health is significant because having a BMI outside the normal range (either too low or too high) can increase the risk of various health problems. For example, a BMI in the overweight and obese categories is associated with greater risk of heart disease, diabetes, high blood pressure. Conversely, very low BMI can indicate malnutrition or underlying health conditions.

We calculated BMI values on a convenience sample of 678 residents of Johannesburg and then compared these by age. We found a statistically significant association between age and BMI, with the overall relationship approximating an inverted-U trend as shown in the figure below.

The above trend is in line with the well-documented life course pattern where BMI typically rises from adolescence through middle age, peaks around ages 50–69, and then declines in later life. This curve emerges from the interplay of biological, behavioural, and demographic factors that have been reported in previous research and are summarised below:

Metabolic and Hormonal Changes: Basal metabolic rate declines with age, while hormonal shifts (e.g., menopause in women and reduced testosterone in men) promote fat accumulation and reduce lean muscle mass, driving BMI up through midlife.

Lifestyle Accumulation: Decades of sedentary behaviour, dietary habits, and reduced physical activity compound over time, leading to gradual weight gain until retirement age.

Survival Bias and Sarcopenia: After approximate 70 years of age, average BMI drops because of survival bias. Individuals with very high BMIs have higher mortality rates, so they are underrepresented in older cohorts. In addition, age-related loss of muscle mass (lean tissue) causes weight loss, sometimes unintentionally, especially among those with comorbidities.

We further looked at just the participants aged ≤39 years for whom an unpward trend in BMI was indicated. A simple linear regression model was fitted into the data and the results show that, on average, for every additional year in age, BMI increased by approximately 0.3 units.

In conclusion, the analysis confirms the standard pattern of a rise in BMI through early and middle adulthood; which is largely reflective of metabolic changes and lifestyle patterns, and the decline in older age; which is mostly influenced by survival bias and age-related muscle loss. Understanding age-related BMI trajectory helps in public health interventions that emphasize weight management and healthy behaviors to reduce risks associated with BMI.