A slight increase in weight may not expose you to serious diseases, but extreme thinness might.
In new research, a Danish study involving more than 85,000 adults found that people with a Body Mass Index (BMI) below 18.5 were almost three times more likely to suffer from illness and premature death than those who were in the mid-to-upper range of the “healthy” category.
Here are the study details that answer the question: “Which is more dangerous to health—being underweight or overweight?”
Overweight vs. Thinness
The relationship between body weight and overall health is complex. This new scientific study, which has not yet been peer-reviewed, suggests that the lowest risk of death from disease may not fall within the traditionally defined “healthy” BMI range.
Instead, the results indicate that people usually classified as “overweight” seem to have health outcomes similar to—or even better than—those with a low BMI, according to Science Alert.
Researchers found a U-shaped curve when plotting BMI against mortality rates, meaning that people at both the lowest and highest BMI levels were at greater risk of death.
Even people at the lower end of the “healthy” range faced higher risks: mortality doubled between 18.5 and 19.9. Those with a BMI between 20 and 22.4 were also 27% more likely to die early compared to the reference group.
These results are surprising, given that a BMI between 18.5 and 24.9 is usually considered ideal.
Meanwhile, being overweight did not always translate into greater risk. In the study, people with a BMI between 25 and 35 (typically categorized as “overweight” or “obese”) showed no significant increase in mortality compared to the reference group. Only those with a BMI of 40 or higher experienced a notable increase in risk—more than double (2.1 times).
Thinness Is Not Health!
The study adds a new dimension to evidence challenging the common social belief that thinness equals health. Research shows that being underweight is a health risk, especially in older age.
Having some fat reserves can help the body cope with illness. For example, patients undergoing cancer treatments such as chemotherapy often lose weight due to appetite loss and taste changes. Those with more fat reserves to begin with can draw on them, helping their bodies maintain basic functions.
On the other hand, a person with very little fat may run out of reserves quickly, limiting the body’s ability to recover.
It’s worth noting that unexplained weight loss is often a warning sign of disease; conditions such as cancer and type 1 diabetes can cause weight loss even before diagnosis. This means that a low BMI can sometimes indicate an underlying illness.
The Importance of Food
We need food to survive—without it, we die. Humanity has known this for thousands of years.
Without food, the body enters a catabolic state, breaking down tissues to obtain energy for brain function. In this process, other vital functions, such as immunity, are suspended to prioritize energy for the brain.
In this context, it’s worth noting that all Danish participants in the study underwent health exams, which are expensive and usually conducted when a problem is suspected. Researchers acknowledge that one possible explanation for their findings is that participants may have lost weight due to an undiagnosed illness—meaning the illness itself, not just the weight loss, could be driving higher mortality risk.
BMI Is Not Always a Reliable Measure
Nevertheless, the study reinforces what other research has shown: thinness is not always protective, and extra weight is not always harmful. The concept of being “fat but fit” continues to receive scientific support.
Modern medicine, which helps people manage obesity-related conditions like diabetes and heart disease, may be shifting the safest weight range higher than before.
The study suggests that a BMI between 22.5 and 30 might now represent the lowest mortality risk—at least among the Danish population studied.
But the problem is that BMI has always been a blunt tool. It does not account for important health factors such as diet, lifestyle, and fat distribution.
It can also be misleading across different racial, ethnic, or cultural groups. Body composition, fat distribution, and baseline risks vary widely in diverse populations.
In an ideal world, healthcare providers would rely on more detailed measures such as blood tests, imaging, and lifestyle data. These are costly and time-consuming but provide far more insight than the simple weight-to-height ratio.
Between Obesity and Thinness: A Nutritionist’s Perspective
Final Word
The Danish data is still preliminary. More studies and research are needed before drawing firm conclusions. But one thing is clear: extreme thinness is dangerous, and BMI alone is a weak measure of health.
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