The old saying “you can never be too rich or too thin” may no longer hold true. A surprising new Danish study suggests that being underweight could actually be deadlier than being overweight for many people.

Study Findings

Researchers found that individuals with a body mass index (BMI) below 18.5 were nearly three times more likely to die within five years compared with those at the higher end of the normal range (BMI 22.5 to under 25).

BMI is a measure of weight relative to height. A BMI of 25 to 30 is classified as overweight, while anything over 30 is considered obese. Surprisingly, the study showed that those in the low-to-mid normal range (BMI 18.5 to 22.5) also faced an increased risk of death compared to those closer to 25.

The large-scale study analyzed health data from more than 85,000 Danes, with an average age of 66. Findings were presented at the annual meeting of the European Association for the Study of Diabetes in Vienna, Austria.

Risks of Underweight vs. Obesity

“Both underweight and obesity are major global health challenges,” said lead researcher Sigrid Bjerge Gribsholt of the Steno Diabetes Center at Aarhus University Hospital.

She explained that obesity can disrupt metabolism, weaken the immune system, and increase the risk of Type 2 diabetes, cardiovascular disease, and up to 15 types of cancer. By contrast, being underweight is tied to malnutrition, weaker immunity, and nutrient deficiencies.

“There are conflicting findings about the BMI range linked to lowest mortality,” Gribsholt noted. “It was once thought to be 20 to 25 but it may be shifting upward over time owing to medical advances and improvements in general health.”

Surprising Numbers

During the study’s follow-up, 7,555 participants died. Key findings included:

• Underweight (BMI <18.5): Almost 3 times higher risk of death than those in the upper-normal range.
• Severely obese (BMI 40+): More than 2 times higher risk of death.
• Low-normal (BMI 18.5–20): Twice as likely to die as the general population.
• Lower-mid normal (BMI 20–22.5): 27% more likely to die.
• Overweight (BMI 25–30): No increased risk of death, supporting the idea of being “fat but fit.”

Possible Explanations

Researchers caution that some of the elevated risk among the very thin may be tied to underlying illness, since their data came from health scans. They also suggest that carrying a bit of extra fat could be protective, especially in older adults.

Fat distribution also plays a critical role. Visceral fat—stored deep within the abdomen around the organs—is particularly harmful because it disrupts metabolic health.

"As a result, an individual who has a BMI of 35 and is apple-shaped - the excess fat is around their abdomen - may have Type 2 diabetes or high blood pressure, while another individual with the same BMI may be free of these problems because the excess fat is on their hips, buttocks and thighs.

"It is clear that the treatment of obesity should be personalized to take into account factors such as fat distribution and the presence of conditions such as Type 2 diabetes when setting a target weight."