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Having a higher BMI for a long period of time may also increase mortality risk. Alexey Kuzma/Stocksy
  • A social demographer investigated sources of bias in the use of body mass index (BMI) to determine risk of mortality.
  • He found that factoring in weight history and body shape influences the relationship between BMI and mortality.
  • These biases from body shape and weight history may account for the “obesity paradox” where previous research has shown a survival benefit from overweight and lower levels of obesity in particular groups.
  • Taking these biases into account, he noted that a higher BMI correlates linearly with higher mortality rates, showing that the risk of being overweight and having obesity has previously been underestimated.

Mortality risk estimates for being overweight or having obesity vary. However, some studies show that some individuals with overweight have a similar mortality risk to those with a healthy BMI.

Other research even suggests that obesity may protect against mortality in older patients or those with certain chronic conditions in what is known as the ‘obesity paradox’. However, other studies suggest that a higher BMI increases mortality risk.

Results may vary between studies due to differences in study design, participants’ weight history, and how lifestyle factors such as smoking and physical activity are considered.

Attending to different factors that influence the likelihood of overweight and obesity across time could help researchers better understand how obesity affects mortality risk.

Recently, Ryan Masters, a social demographer and associate professor of sociology at the University of Colorado, Boulder of the University of Colorado Boulder, investigated how different factors, including body type and weight history, affect obesity-related mortality rates.

He found that mortality risk increases linearly as BMI increases and that those with ‘healthy’ weight have the lowest mortality risk, countering some previous research findings.

The study was published in Population Studies.

For the study, Prof. Masters analyzed data from the National Health and Nutrition Examination Surveys (NHANES) 1988–94 and 1999–2006. Altogether, the data contained 17,784 cases of obesity and 4,468 deaths. Data included:

  • participants’ BMI
  • measures of body shape such as waist and thigh circumference
  • self-reports of past weight at multiple time points
  • biomarkers for cardiometabolic disease such as blood sugar levels

He split participants into groups according to their BMI:

  • A BMI of under 18.5 was considered underweight
  • 18.5–25: healthy weight
  • 25–30: overweight
  • 30–35: class 1 obese
  • 35+: class 2 obese

“In addition to BMI, a waist circumference, which indicates more metabolically active visceral fat, is important to consider,” Dr. Christopher Still, board certified and fellowship-trained specialist in obesity and weight management with Geisinger, who was not involved in the study, told Medical News Today.

“Waist circumference often has more implications for one’s health than subcutaneous fat that accumulates around the buttocks and hips. In general, men with a waist circumference greater than 40 inches and women with a waist circumference greater than 35 inches connote more visceral fat that is often more prone to prediabetes, diabetes, sleep apnea, and fatty liver disease.”
— Dr. Christopher Still

After analyzing the data, Prof. Masters found that over 20% of the sample in the healthy BMI range were either overweight or had obesity 10 years prior to the survey and that their health was significantly worse than those who had maintained a healthy BMI over the same period.

He also found that participants who had gained weight in recent years had significantly better health profiles than those who maintained higher weights for longer.

He further found that indicators of body fat and shape varied across BMI samples and were linked to indicators of poor health, cardiometabolic disease, and mortality risk.

After adjusting for weight history and body fat and shape variables, he found that overweight and obese BMI levels strongly increase mortality risk in both younger and older adults.

Dr. Lana Castellucci, chair of the World Thrombosis Day Steering Committee, and assistant professor of medicine at the University of Ottawa, who was not involved in the study, told MNT:

“This study emphasizes the limitations of BMI reporting in surveys. BMI is captured at a single point in time when someone completes a survey, and it does not consider how long people are at their current BMI. This leads to different types of bias, overestimation or underestimation, of the relationship with obesity and mortality.”

“For example, if a person who recently gains or loses weight fills out the survey, the BMI recorded is at the time of the survey, and the weight changes may not be accurately considered in effects on health and diseases,” she noted.

Ramy H Bishay, endocrinologist and a Juniper spokesperson, who was also not involved in the study, pointed out the drawbacks of relying on BMI.

“The utility of BMI in its ability to predict mortality has been in question for over 15 years, and it’s still regarded as a crude measure of obesity by most experts. [B]MI has many pitfalls, [especially for] lean, muscular individuals, older populations, and those with significant disproportionate fat distribution, the BMI is likely to be less useful,” he told MNT.

Dr. Angela Fitch, co-founder and chief medical officer ofknownwell, and president of the Obesity Medicine Association, who was not involved in the study, touched on the issue of racial and cultural differences when using BMI.

“Anytime you are making assumptions using BMI data it is hard as we know that BMI is an indicator of excess adiposity (body fat) but not an absolute predictor of metabolic health. Also, the original BMI cut-offs were primarily in Caucasian people so applying BMI cutoffs to a population of people of cultural diversity is challenging,” she told MNT.

“The limitations of these findings is that though authors show that BMI is not an ideal measure of increased health risk, no simple alternative to measure health is proposed,” Dr. Mir Ali, bariatric surgeon and medical director of MemorialCare Surgical Weight Loss Center at Orange Coast Medical Center in Fountain Valley, California, who was not involved in the study, told MNT.

“Therefore, BMI is still a useful tool. What the study does show that BMI over 25 does increase a person’s risk for developing obesity related diseases,” he said.

“The findings of this study are limited by the fact that the study only examined U.S. adults and the data used was from the NHANES survey, which does not have a representative sample of the general population,” Suzanne Manzi, pain management physician at Performance Pain & Sports Medicine, who was also not involved in the study, told MNT.

“Additionally, the study did not consider any other potential confounding factors, such as lifestyle or environmental factors, which may also influence the BMI-mortality association. Finally, the study relied on self-reported BMI, which may be subject to recall bias and inaccuracies,” she added.

Whilst accepting the importance of body shape and the inability of BMI to capture differences, Masters points out that any replacement measure would still be subject to the survival bias of those with recent weight gain and the negative survival bias of recent weight loss.

Dr. Bishay acknowledged that BMI still has its place in predicting disease risk but that a more accurate assessment will need follow-ups over time.

“Still, BMI has its role in predicting disease—cancer, type 2 diabetes, nonalcoholic fatty liver disease, hypertension and stroke risk. What this study however is shedding to light, is that a crude estimation of BMI on mortality risk is not static, and one needs to consider the trajectory of BMI changes over time and the reasons underpinning those changes,” he said.

“This analysis by Masters is likely to generate plenty of discussions and heated debate in defense of BMI as a predictor of mortality, which is becoming increasingly crude and obsolete in the face of more accepted measures such as waist circumference, waist-to-hip-ratio, and the Edmonton Obesity Staging System,” he concluded.

The author cautions that taking into account the length of time someone has had excess weight considerably adjusts the proportion of deaths due to being overweight and having obesity from negligible to 16.5%.