Average BMI for centenarians varies by population
While the concept of what constitutes a healthy weight shifts with age, research indicates that centenarians often carry a higher-than-average BMI compared to younger adults, yet they have maintained exceptional longevity. A meta-analysis in The Journal of the American Medical Directors Association found that for older adults over 65, the lowest mortality rates were observed in individuals with a BMI between 27 and 27.9. This is in contrast to the World Health Organization (WHO) standard of 18.5–24.9 for younger adults.
Centenarian studies from different countries highlight population-specific variations in average BMI, underscoring that genetics, diet, and lifestyle contribute to body composition in extreme old age. For instance, Japanese centenarians tend to have lower average BMIs than their Italian or Spanish counterparts.
Notable centenarian BMI studies
Several studies provide valuable insight into the average BMI of centenarians across different populations:
- Japanese Centenarians: A study of 467 Japanese centenarians reported a mean BMI of 19.3 kg/m², which is considered a 'normal' weight, though many were on the lower end of the spectrum and even classified as underweight.
- Portuguese Centenarians: An observational study found a mean BMI of 21.07 kg/m² among 252 Portuguese centenarians, noting that women were more likely to be underweight than men.
- Georgia Centenarians (USA): The Georgia Centenarian Study of 233 participants found an average BMI of 22.5 kg/m², with a significant number classified as underweight (31.3%) or overweight (24.9%).
The BMI paradox in older adults
The most striking aspect of this research is the so-called "obesity paradox." This phenomenon suggests that for older adults, particularly those over 65, carrying a little extra weight may be protective against mortality. While obesity is associated with a shorter lifespan and poor health in younger and middle-aged adults, this relationship appears to weaken in advanced age.
Instead of a standard inverse relationship between BMI and health, the link to mortality risk changes with age, forming a U- or J-shaped curve. At older ages, the greatest risk of death is at the extreme ends of the spectrum—the very underweight and the very obese. This suggests that for many seniors, being in the low-end 'normal' range of BMI could be more detrimental than being slightly 'overweight'.
Reasons for the apparent paradox:
- Metabolic Reserves: Slightly higher body fat may provide a metabolic reserve to draw upon during illness, which is more common in older age.
- Muscle vs. Fat: Traditional BMI doesn't differentiate between muscle mass and fat mass. A higher BMI in older adults might reflect greater muscle mass or simply a better nutritional status, both of which are protective factors.
- Reverse Causation: It is possible that individuals with lower BMI in old age lost weight due to undiagnosed illnesses or chronic health issues, thereby confounding the mortality data.
A comparison: Average centenarian BMI vs. younger adults
| Feature | Younger Adults (e.g., age 20-50) | Older Adults (65+) & Centenarians |
|---|---|---|
| Optimal BMI Range (WHO) | 18.5–24.9 (Normal) | Variable; Often higher (25–30 or higher) for better longevity |
| Mortality Risk Curve | J-shaped: risk increases with both underweight and overweight status | U-shaped: risk is higher at the extremes of underweight and obesity |
| Body Composition | Generally higher muscle mass proportion; BMI reflects fat and muscle more accurately for this age group. | Lower muscle mass (sarcopenia) is common; BMI is a less reliable indicator of health due to changes in body composition. |
| Health Implication of Overweight | Increased risk for various chronic diseases | Potential protective effect or reserve, especially against acute illness |
Limitations and complexities of BMI in the oldest old
Despite the insights from centenarian studies, several limitations and confounding factors prevent a simple conclusion about what is the average BMI of centenarians:
- Genetic Factors: Centenarians are a highly selected group with a predisposition for longevity. Their specific genetic makeup likely influences their body composition and health outcomes differently than the general population.
- Health and Weight Loss: A low BMI in extreme old age could be the result of unintentional weight loss due to illness, rather than an inherent marker of optimal health. These individuals, already in decline, may skew the data towards a lower average.
- Lack of Body Composition Data: Many studies rely solely on BMI, which is an imperfect measure. Without information on body fat percentage and muscle mass, it is difficult to determine the true health implications of BMI in this population.
- Dietary and Lifestyle Differences: Population studies can be biased by regional dietary and lifestyle patterns. For example, the lower BMI of Japanese centenarians is often linked to dietary habits such as low consumption of red meat and cholesterol.
Conclusion
The average BMI of centenarians is not a single number but rather a complex, population-dependent metric that challenges traditional weight standards for younger adults. Studies show that centenarians often fall within the 'normal' to 'overweight' categories, with a higher BMI sometimes associated with better survival outcomes due to the 'obesity paradox.' However, BMI alone is insufficient for assessing health in the very old, as factors like genetics, body composition, and pre-existing illness play significant roles. For the oldest old, a standard weight-loss regimen may not be appropriate. The focus for healthy aging should shift towards maintaining overall functionality and nutritional status rather than strictly adhering to BMI ranges designed for a younger demographic.
An authoritative link on the obesity paradox for more in-depth reading can be found at the National Institutes of Health.
