May 14, 2024

expert reaction to conference abstract about BMI cut-off for over 40s

A conference abstract presented at the European Congress on Obesity looks at BMI thresholds for obesity in over 40s. 

Dr Katarina Kos, Senior Lecturer in Diabetes and Obesity, University of Exeter, said:

“Whilst awareness of unhealthy fat accumulation leading to obesity is important, BMI does neither capture muscle content nor the location of fat deposition.  One way to measure unhealthy fat/abdominal fat is waist circumference which gives a good indication of heart disease risk though clinically less used.  Some individuals may want some personalisation and fine tuning of their body composition related risk if at the threshold between being overweight and obese, and this study addresses the age factor.  The authors indicate that as we grow older this fine tuning may be more important, the bigger question is in what makes us as the public more inclined to act upon our surplus weight.  Are we more likely to change if we are being labelled with obesity at a lower BMI?  And this other than being a statistic as a person with obesity.  The number of people with obesity worldwide increases drastically with the proposed BMI threshold. Or are more of us simply going to feel stigmatised?”

Dr Adam Collins, Associate Professor of Nutrition, University of Surrey, said:

“Interesting idea and something that harks back to the flaws in BMI as a proxy for excess adiposity.  Given that BMI is simply weight adjusted for body size (i.e. height).  And assumes that higher weight is directly proportional to higher body fat.

“This is a good sample size with robust measures of adiposity (DXA).  But as the authors state, this also needs to be looked at across different ethnicities– e.g. South Asians, who may have a propensity for higher visceral fat at a given BMI.

“The problem with BMI has always been the grey area between 25 and 30.  Or the “overweight” category.  When comparing with actual measures of adiposity (i.e. body fat), some individuals will be misclassified due to low body fat and higher muscle mass, whilst others can have as high or higher levels of body fat than those of greater BMI.

“Hence the sensitivity (how consistently it can detect true adiposity) and specificity (to only detect those with true adiposity) of BMI is problematic <30.

“The notion that this sensitivity and specificity is dependent on age makes sense, and translates as more of a potential issue in older people below BMI 27 rather than below BMI 30.  Clinically, this is important in terms of a threshold for health concern.

“However, it is important to note that total adiposity (i.e. % body fat) is not, in itself, a direct marker of health – there is a strong association between adiposity and health, but you can still be at risk with low total adiposity (e.g. visceral fat /ectopic fat) compared to those with high total adiposity but low visceral/ectopic fat.  It would be more appropriate to relate to central (abdominal) or visceral adiposity; in this regard, waist circumference could be a useful additional marker.  More specifically, visceral and ectopic fat (fat where it shouldn’t be) are better markers of adiposity risk.  For example, liver fat or intramuscular fat.  Closely linked to insulin resistance, diabetes risk and cardiovascular disease.  Admittedly this would only be measurable via MRI imaging.

“Of course, triangulating this with other markers of risk, e.g. Blood Pressure and blood parameters, would be helpful to fully establish risk.  But triaging individuals for this assessment based on these lower BMI thresholds would be clinically very useful.”

Abstract title: ‘New BMI Cut-Off Points for Obesity in Middle-Aged and Older Adults in Nutrition Settings in Italy’ by Marwan El Ghoch et al.

This was presented at the European Congress on Obesity and was under embargo until 23:01 UK time on Tuesday 14 May 2024.

There is no full paper.

Declared interests

Dr Katarina Kos: “I have no conflict of interest.”

Dr Adam Collins: “No conflict of interest with this.”