BMI and mortality

Having a high BMI may not lead to an early death if you are otherwise healthy, according to a new observational study from the US.

Researchers from the Rutgers Institute for Health examined data, spanning between nine and 20 years of records, on more than half a million (554,332) Americans, dividing them into nine different BMI categories to reveal that the risk of dying from any cause was similar across a wide variety of weight ranges.

The team also found that the link between BMI and mortality may vary by age.

For older adults, there was no significant increase in mortality for any BMI between 22.5 and 34.9, which extends into BMI categories typically considered obese, and for younger adults, there was no increase in risk for any BMI between 22.5 and 27.4.

However, obesity was still clearly unhealthy, and the study, published 6 July 2023 in PLOS ONE, found that overall, there was a 21% to 108% increased risk of death linked to their weight, for adults with a BMI of 30 or over.

The patterns were largely the same in men and women and across races and ethnicities, the researchers say. The findings support the idea that BMI alone may not be a reliable indicator of overall health, the team concludes.

Lead author, Dr Aayush Visaria, explained that even though it has been well-established that elevated BMI can contribute to several cardio-metabolic conditions, the studies that have analysed the association between BMI and all-cause mortality have been inconsistent.

“Most US studies have used data from the 1960s through 1990s and have included predominantly non-Hispanic White adults,” she said.

“In our study, detailed Information on demographics, socio-behavioural factors, comorbidities, and healthcare access was also available.”

On average, participants were 46 years old, 50% female and 69% non-Hispanic White, with 35% of participants having a BMI between 25 and 30, typically defined as overweight, and 27.2% had a BMI above or equal to 30 – defined as obese.

Over the maximum follow-up period of 20 years, the researchers observed 75,807 deaths, and when additional controls for comorbidities such as diabetes and hypertension (potential colliders) were included, the risk of mortality marginally decreased among overweight BMI categories.

“These patterns became more apparent after further restricting the cohort to healthy (without non-skin cancer or melanoma or cardiovascular disease at baseline), never-smokers who did not die within the first two years of follow-up,” Dr Visaria said.

“When stratifying by age group alone, we found that the decreased mortality seen from BMI of 25.0 to 29.9 was more pronounced in older adults and that younger adults had increased mortality risk.

“Older adults also had significantly lower unadjusted risk of mortality among BMI of 30–34.9 compared to younger adults, BMI of 30–34.9; older adults: 0.85 [0.82, 0.89]; younger adults: 1.60 [1.52, 1.69]). These age-related differences persisted and remained statistically significant, after adjustment for covariates and after excluding participants who died within the first two years of follow-up.”

The BMI-mortality patterns observed in the overall population also remained largely the same in men and women, even after adjustment for covariates and restriction to healthy never-smokers.

The study pointed out that there were several plausible reasons why participants with higher BMI (25.0–34.9) may have an all-cause mortality risk much like those with a conventionally normal BMI (18.5–24.9):

  • Overweight individuals may have survival advantages in various adverse circumstances, such as critical illness, major morbidities, and severe infection that are not offset by the increased risk of chronic metabolic diseases.
  • Overweight individuals without disease may be metabolically healthy and have a more favourable body composition consisting of higher lean mass. Further, BMI alone may be insufficient in classifying high-risk adiposity–both waist circumference and weight change over time can modify BMI-mortality associations, as seen in our NHANES findings.
  • Lean individuals who develop diseases such as hypertension or diabetes may have more aggressive or treatment-resistant disease, whereas overweight or obese individuals who develop such conditions may be able to manage or even reverse disease with weight loss strategies.
  • Finally, the U.S. population has become much more diverse, with greater representation of ethnic minorities and older adults in our cohort with potentially different body compositions compared to previous cohorts.

The authors conclude that further studies incorporating weight history, body composition and morbidity outcomes are needed to fully characterize BMI-mortality associations but say that BMI in the overweight range is generally not associated with increased risk of all-cause mortality.