New data shows that the healthy BMI range isn’t as narrow as we thought, nor is it where we thought it was.
By Jyoti Kinghorn
Human beings come in a variety of shapes and sizes. It has long been observed that a healthy weight at any size correlates with a longer and healthier life. Weighing too much or too little is generally understood to cause health problems that can lead to an earlier death. But how much weight is too much weight?
One way to measure and standardize body fat is to calculate one’s body mass index (BMI). BMI (kg/m2) is calculated by dividing a person’s weight (in kilograms) by their height (in meters) squared. Currently, the World Health Organization (WHO) considers an adult BMI in the range 18.5–24.9 to be a normal and healthy BMI. They consider a BMI below 18.5 as underweight, 25.0–29.9 as overweight (or pre-obese), 30.0–34.9 as obese class I, 35–39.9 as obese class 2, and above 40.0 as obese class III. A study by the National Cancer Institute Cohort Consortium previously narrowed down the healthiest BMI to be between 22.5 and 24.9. This and similar studies have shown an association between a BMI of 25 or higher and an increased risk of all-cause mortality (death by any reason). But a recent study by Aayush Visaria and Soko Setoguchi of the Rutgers Institute for Health indicates that this assumption about healthy BMI may not be true. Individuals with overweight BMI may have the same chance of dying as those with normal BMI.
Why a new BMI study was needed
A large part of the prevailing understanding of healthy BMI and its association with all-cause mortality is based on data collected from the 1960s through the 1990s from largely non-Hispanic White populations. This sixty- to forty-year-old data does not take into account advancements in healthcare and disease management that have increased the average life span by over 10 years even as the U.S. population has gotten steadily heavier. Furthermore, the data is not representative of the racial/ethnic composition of the current U.S. population. So can the conclusions drawn from such studies be broadly applied today?
In their analysis, Visaria and Setoguchi use more contemporary and nationally representative data collected by the National Health Interview Survey (NHIS) from the years 1999 to 2018. This study included data from 554,332 adults with a mean age of 46 years old. The participants were 50 percent female, 69 percent non-Hispanic White, 12 percent non-Hispanic Black, 4.6 percent non-Hispanic Asian, 14 percent Hispanic, and 1.1 percent Native American/multiracial/other. The broad regions of residence of the participants were the Northeast (18 percent), Midwest (24 percent), South (37 percent) and West (22 percent). Comorbidities included self-reported history of cardiovascular disease, diabetes, hypertension, kidney disease, asthma, COPD, liver condition, and non-skin cancer or melanoma, among others. The median follow-up was 9 years, with maximum follow-up of 20 years. The researchers observed 75,807 deaths in this sample from all-cause mortality, and statistical models were employed to analyze the data. The researchers used the previously established healthiest BMI group (22.5–24.9) as a reference, and they compared the risk of all-cause mortality in other groups with this reference group.
Healthy BMI range may have been set too low
Contrary to previous studies, the researchers found that unadjusted risk of all-cause mortality was comparable across BMI groups in the 20–29.9 range. The risk appeared even lower in the overweight 25–27.4 BMI group compared to the reference group. The researchers narrowed the data (adjusted data) by including only never-smokers who were healthy (no self-reported history of cardiovascular disease or non-skin cancer except melanoma), and who did not die within 2 years of follow-up. As before, the risk of all-cause mortality was lower in the BMI group of 25–27.4. Using both adjusted and non-adjusted data, the risk of all-cause mortality was comparable between the reference group and the higher overweight group with a BMI range of 27.5–29.9.
Therefore, participants in the overweight non-obese groups had the same risk of all-cause mortality as those in the normal weight BMI reference group. The risk was comparable between men and women. The risk was also comparable between non-Hispanic White, non-Hispanic Black, and non-Hispanic Asian groups, though it was significantly higher in Hispanic adults with BMI ranging 25.0–29.9. In adjusted data including only healthy never-smokers, the pattern of increased risk for Hispanic adults with BMI 25.0–29.9 persisted, though the increase was no longer statistically significant.
Obese and underweight BMI still associated with shorter lives
While the overweight group did not have an increased risk of all-cause mortality, there was a big increase in the risk (21–108 percent) of obese participants with BMI of 30 and over. Other studies have shown that obesity is linked to cardio-metabolic diseases such as type 2 diabetes, high blood pressure, coronary heart disease, stroke, and predisposition to cancer, all of which are significant contributors of premature death.
There was also an increase in mortality risk for underweight individuals with BMI of less than 18.5. This matches studies that have shown a connection between an underweight BMI and increased health risks, such as certain neurological disorders, primary vascular dysregulation, and predisposition to cancer.
Old age influences BMI-related risk of mortality
There seems also to be an age factor when studying the effects of BMI. When the researchers stratified the data by age, the results indicated the risk of mortality in younger adults was reduced only up to BMI of 27.4, whereas in older adults (≥ 65) there was no significant increase in the risk even up to BMI of 34.9. These data may indicate that older adults were more resilient to adverse effects of overweight and obesity than younger adults.
No one BMI size fits all
For a measure that is so widely used to advise patients about their health and weight, BMI is not as well understood as we previously assumed. The current understanding of healthy BMI numbers appears to be based on outdated data and lacks distinction between age groups, sexes, and racial and ethnic backgrounds. However, the risk of mortality due to underweight or obesity are clear as ever. According to Visaria and Setoguchi, future studies may be helped by incorporating data such as body fat distribution and waist circumference in order to better understand what constitutes a healthy BMI.
This study was published in the peer-reviewed journal PLoS ONE.
Berrington de Gonzalez, A., Hartge, P., Cerhan, J. R., et al. (2010). Body-mass index and mortality among 1.46 million white adults. New England Journal of Medicine 363, 2211–2219. https://doi.org/10.1056/NEJMoa1000367
Golubnitschaja, O., Liskova, A., Koklesova, L., et al. (2021). Caution, “normal” BMI: health risks associated with potentially masked individual underweight—EPMA position paper 2021. EPMA Journal 12, 243–264. https://doi.org/10.1007/s13167-021-00251-4
Visaria, A., & Setoguchi, S. (2023). Body mass index and all-cause mortality in a 21st century U.S. population: A National Health Interview Survey analysis. PLoS ONE, 18(7), e0287218. https://doi.org/10.1371/journal.pone.0287218
World Health Organization. (2010, May 6). A healthy lifestyle – WHO recommendations. https://www.who.int/europe/news-room/fact-sheets/item/a-healthy-lifestyle—who-recommendations
The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.
About the Author
Jyoti Kinghorn is a science writer based out of Austin, Texas. Trained in plant molecular biology, her freelance writing career in COVID times has taken her into the realm of human biotechnology and disease. She loves writing, reading, and being outdoors with her children.