The comfortable association of leanness with good health is misleading for around 20% of people whose body mass index (BMI) falls into the normal-weight range, according to research coming out of Germany.
In a paper published in the journal Cell Metabolism, scientists Norbert Stefan, Fritz Schick and Hans-Ulrich Haring, of University Hospital in Tubingen, report that metabolically unhealthy lean people have a 300% greater chance of dying compared to healthy people in the same BMI bracket.
This is in stark contrast to the small proportion of obese people who are, despite their high BMI, metabolically healthy. Stefan and colleagues report risk of death across all-cause mortality classifications for this group is only 25% higher than that of healthy lean people.
To make their findings, the scientists analysed 981 volunteers across all BMI divisions. All were first checked against the standard checklist for diagnosing metabolic syndrome.
Those with results that exceeded at least two of the accepted safe maximum readings were deemed to be clinically unhealthy. Participants in this category included 18% of people within the normal BMI range – a figure, say the scientists, which accords broadly with the results of larger studies.
After this base-level classification, Schick and his colleagues drilled deeper, using functional magnetic resonance imaging and spectroscopy, along with blood tests, to determine fat mass and distribution around the body, fat deposits on the liver, insulin sensitivity and blood vessel thickness.
As expected, among obese participants, abdominal fat levels and nonalcoholic fatty liver disease were strong predictors of metabolic syndrome. Among lean people, however, the strongest predictor of poor metabolic health turned out to be skinny lower legs.
Unusually thin legs, the researchers cautiously conclude, may indicate a gene-derived difficulty with storing fat in the lower limbs, and that this is linked to increased risk of cardiovascular poor health.
The scientists noted similarities between the phenotype, or body-shape, of the unhealthy normal BMI cohort and people with certain rare disorders, such as lipodystrophy, in which the body is unable to sustain adequate fat reserves.
The condition has many variants, but is often experienced by people with HIV.
The findings, say the researchers, provide evidence for the existence of a “lipodystrophy-like phenotype in the general population”.
The research adds to growing evidence that existing weight descriptions – under- and overweight, normal, and several gradings of obese – are too simple and obscure a picture that is more complex.
Earlier this year, for instance, the American Journal of Gastroenterology reported on new findings concerning lean people who had non-alcoholic fatty liver disease (NAFLD), the most common type of chronic liver malfunction.
The research showed clear metabolic differences between normal-weight and obese people with the condition. An accompanying editorial highlighted the need to classify “‘lean’ NAFLD as a unique phenotype with specific genetic associations deserving of further investigation.”
Andrew Masterson is a former editor of Cosmos.
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