In a world where slimness is often equated with health, a new Danish study has flipped the narrative: being slightly overweight, or even mildly obese, may not be as deadly as once thought. In fact, in some ways it could be safer than being thin.
The new research presented at the European Association for the Study of Diabetes (EASD) conference in Vienna supports the phenomenon, often dubbed "fat but fit," and challenges the long-standing assumption that thinner is always healthier.
Food isn't just fuel; it's the foundation of survival. When the body is deprived of food, it shifts into a catabolic state. That means it starts breaking down its own tissues, muscles, fat, and even vital organs, to extract energy. The brain, being the command center, gets top priority. To keep it running, the body reroutes resources, often at the expense of other systems.
One of the first to take a hit is the immune system. It slows down or shuts off, leaving the body vulnerable to infections and disease. Repair processes stall. Hormonal balance falters. Even temperature regulation and heart function can become compromised. That's why having energy reserves can be life-saving.
In this study, researchers tracking more than 85,000 adults found that those with a Body Mass Index (BMI) in the overweight range, and even some in the lower obesity range, were no more likely to die during a five-year follow-up than those in the upper end of the "normal" range.
More startling? Individuals with BMIs in the underweight category or even the lower half of the "healthy" range had significantly higher mortality rates.
BMI is an internationally recognized standard for assessing whether individuals are at a "healthy" weight. It's calculated by dividing body mass by the square of body height, resulting in a figure expressed in units of kg/m2. A score of 18.5 to <25 kg/m2 is generally considered to be of normal, healthy weight. A BMI of <18.5 kg/m2 is categorized as underweight, whereas 25 to <30 kg/m2 is considered overweight, and over 30 kg/m2 is categorized as obesity.
Using health data, researchers examined the relationship between BMI and mortality in 85,761 individuals. The data revealed a U-shaped curve in mortality risk, with 7,555 (8%) of the participants dying during follow-up.
Underweight individuals were nearly three times more likely to die than those in the reference BMI group. Those with a BMI of 18.5–20.0 had double the risk. Even those in the 20.0–22.5 range had a 27% higher risk.
Meanwhile, overweight (25 to <30 kg/m2) and mildly obese (30.0 to <35.0 kg/m2) individuals showed no increased risk. Only those with severe obesity (BMI ≥ 40) had a significantly higher mortality rate (23%), more than double that of others.
The study's median participant age was 66.4 years, and most were undergoing health scans, which may skew the data toward individuals already facing medical issues. All of the results were adjusted for sex, comorbidity level, and education level.
These findings challenge the common belief that thinness is not always protective, and extra weight is not always harmful. In fact, being underweight, especially in older adults, can increase health risks.
Having some body fat helps the body handle illness. For example, people going through cancer treatments often lose weight due to poor appetite or taste changes. Those with more fat to begin with have reserves to support vital functions, while very thin individuals may struggle to recover.
Also, sudden weight loss can signal hidden health problems like cancer or type 1 diabetes. So, a low BMI isn't always a sign of good health; it can sometimes point to an underlying illness.
"Both underweight and obesity are major global health challenges," says Sigrid Bjerge Gribsholt, from Aarhus University Hospital, who led the research. "Obesity may disrupt the body's metabolism, weaken the immune system, and lead to diseases like type 2 diabetes, cardiovascular diseases, and up to 15 different cancers. At the same time, underweight is tied to malnutrition, weakened immunity, and nutrient deficiencies.
"There are conflicting findings about the BMI range linked to the lowest mortality. 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."
Researchers were surprised to find that having a BMI up to 35 didn't increase the risk of death, and even those with a BMI between 35 and 40 only had a slightly higher risk.
Dr. Gribsholt explained that this could be due to "reverse causation."
"Some people may lose weight because of an underlying illness," said Gribsholt. "In those cases, it is the illness, not the low weight itself, that increases the risk of death, which can make it look like having a higher BMI is protective. Since our data came from people who were having scans for health reasons, we cannot completely rule this out."
Professor Jens Meldgaard Bruun, co-author of the study, emphasizes one of the key takeaways from the findings, "BMI isn't the only indicator that an individual is carrying unhealthy levels of fat."
BMI, a simple ratio of weight to height, has long been criticized for its inability to distinguish between fat and muscle, or to account for fat distribution. Bruun stresses that where fat is stored matters more than how much there is.
"Visceral fat – fat that is very metabolically active and stored deep within the abdomen, wrapped around the organs – secretes compounds that adversely affect metabolic health," explained Brunn. "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."
This aligns with recent calls to rethink obesity diagnostics. According to one study from the Global Commission on Clinical Obesity, BMI doesn't compute the body composition. It doesn't tell us if the elevated BMI is due to excess body fat. Hence, it proposed a new framework that considers metabolic markers and fat location.
Another study by the Harvard researchers has suggested looking at BMI alone might no longer be enough. Instead, researchers found that body fat percentage and distribution are better predictors of heart disease than BMI.
Rethinking the weight debate, Brunn said, "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."
This study doesn't suggest abandoning efforts to combat obesity, instead, it urges a more personalized approach that takes into account age, muscle mass, fat distribution, and underlying health conditions.
Source: European Association for the Study of Diabetes via ScienceDaily
