Danish Study: Being Too Thin Is Riskier Than Being Slightly Overweight
A new study from Denmark questions the conventional BMI limits and the value of the index itself. The team led by Sigrid Bjerge Gribsholt from the Steno Diabetes Center Aarhus analyzed data from nearly 86,000 adults over five years. The cohort was predominantly female (81%), with a median age of 66.4 years. The findings were presented at the annual congress of the European Association for the Study of Diabetes (EASD) in Vienna.
The results showed that people with a BMI in the overweight range (25 to 30) or even the lower obesity range (30 to 35) did not have a higher risk of mortality than people at the upper end of the normal range (22.5 to 25).
Surprisingly, not only being underweight (BMI below 18.5) but also the lower and middle normal range (18.5 to 22.5) were associated with a higher risk of mortality. Underweight individuals died 2.73 times more frequently than the reference group, while "severely obese individuals (BMI over 40) had a 2.1-fold increased risk.
Underlying Illness Often Explains Low BMI
The researchers also pointed to an effect called "reverse causation." Chronic or yet-undiagnosed illnesses such as cancer, lung diseases, or heart failure can lead to unintentional weight loss, often years before a diagnosis. This effect makes low BMI values appear riskier in statistics, although the disease is the actual cause.
To minimize this confounding factor, the team excluded people from the analysis who died in the first years of follow-up. The association still remained, although it was weakened. The study interpreted this as an indication that, in addition to illnesses, factors like low muscle mass or malnutrition also play a role in mortality risk.
Earlier large-scale analyses, such as the 2016 JAMA study on three Danish cohorts (1976–2013) or the Global BMI Mortality Collaboration (in The Lancet), had already shown an increased risk with a very low BMI and only a moderate increase with slight overweight. Both papers had already pointed out that reverse causation and smoking are key confounding factors.
What’s new about the large-scale Danish cohort study is the finer sub-division of the BMI normal range and the targeted control for "reverse causation." The lowest risk was found here in the upper normal range to a slight overweight (22.5 to 27.5), a finding that refines previous observations.
What the BMI Study Means for Your Ideal Weight
The new data suggest that a BMI below 25 doesn't need to be compulsively pursued. A BMI up to 30 may not be associated with increased mortality risk, particularly in older adults. Moreover, other factors must be taken into account, as Professor Bruun from the University of Copenhagen explains. “BMI is only part of the picture. Other important factors are how fat is distributed”, says Bruun. Fat stored deep in the abdomen around the organs, known as visceral fat, is very metabolically active and can negatively impact health.
Specifically, a person with a BMI of 35 might have no problems if the excess fat is on their hips, buttocks, and thighs. In contrast, another person with a BMI of 35 and an apple shape might be more likely to have type 2 diabetes or high blood pressure, Bruun explains. “The treatment of obesity should be personalized and consider factors like fat distribution and comorbidities when setting a target weight.”
Waist-to-Height Ratio Replaces BMI
The Danish findings confirm that BMI alone is only a crude measure. It does not differentiate between muscle and fat mass, ignores fat distribution, and excludes individual factors such as age, gender, or fitness level. An international commission recently proposed in The Lancet Diabetes & Endocrinology that obesity should be defined in the future based on a combination of BMI, fat distribution, and organ function. Professional societies recommend supplementary measurements such as waist circumference, Waist-to-Height Ratio (WHtR), or direct body fat measurements.
The Waist-to-Height Ratio is now often considered a better indicator of health risks, particularly for cardiovascular diseases. The simple rule of thumb is that your waist circumference should be less than half of your height, which corresponds to a WHtR of less than 0.5. In some countries, the prescription of weight-loss injections is already linked to this value rather than the BMI.
A slightly elevated BMI is therefore not automatically harmful. Low weight can even be riskier, especially if it is caused by a known or still-hidden disease. The BMI should therefore always be supplemented by other measurements so that your personal ideal weight is based not on a rigid number but on personalized health values.
Sources
EASD 2025 Abstract / Science Daily. 2025. Being too thin can be deadlier than being overweight
JAMA. 2016. Change in Body Mass Index Associated With Lowest Mortality in Denmark, 1976–2013
Global BMI Mortality Collaboration. The Lancet. 2016. Body-mass index and all-cause mortality
The Lancet Diabetes & Endocrinology. 2025. New definition of obesity
European Society of Cardiology. 2025. Waist-to-height ratio predicts heart failure incidence
Frontiers of Nutrition. 2025. Association of waist-to-height ratio with all-cause and obesity-related mortality in adults
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