How BMI Can Miss Unhealthy Fat in the Liver

One of the important complications of obesity is NAFLD – non-alcoholic fatty liver disease. Quite simply, this is a condition where unhealthy fat accumulates in the liver. It leads to inflammation and fibrosis, progressing to cirrhosis, liver cancer, or liver failure in severe cases. Often, it goes undetected until these more severe problems emerge. Because we define obesity as the accumulation of excess or abnormal fat that can impair health, NAFLD is clearly a manifestation of obesity. But new research reminds us that relying on BMI is not really adequate as a signal for this condition. In fact, BMI screening alone can miss the problem of elevated body fat and unhealthy liver fat.

In this new study, nearly half (46 percent) of individuals with a normal BMI but elevated body fat also had undiagnosed NAFLD.

Normal Weight Obesity

In essence, this was a study of normal weight obesity. Arch Mainous and colleagues studied a sample of individuals from 2017-2018 NHANES data. These were 1,352 persons with a BMI less than 25 – in the range of a healthy BMI. But more than ten percent of them (173) had excess body fat. The weighted prevalence was about 18 percent.

Among these persons with a normal BMI and elevated body fat, 46 percent had undiagnosed NAFLD – unhealthy liver fat. But for those without elevated body fat, the prevalence of undiagnosed NAFLD was much lower at 25 percent. The authors of this research sum it up:

A large proportion of patients with healthy BMI have NAFLD. Individuals with elevated body fat percentage at healthy BMI are more likely to have undetected NAFLD. These results bring into question the clinical usefulness of BMI in guiding screening for NAFLD.

A Reminder: BMI Does Not Define Obesity

So the bottom line here is one more reminder that BMI does not define obesity very well. Rather, it is abnormal or excess fat tissue that lies at the heart of this disease.

Click here for this study by Manous et al.

Transmitted Light Brightfield Microscopy of Toad Liver, image by ZEISS Microscopy, licensed under CC BY-NC-ND 2.0

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May 18, 2022