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The link between body weight and health is more complicated than often assumed. The research, which is yet to be peer-reviewed, suggests the lowest risk of death may not sit neatly in the traditional "healthy" body mass index (BMI) range. In conversation with Rachel Woods, Senior Lecturer in Physiology at the University of Lincoln, we explore why these findings matter and how they challenge long-standing assumptions about thinness, fatness, and health. UNDERWEIGHT CARRIES THE HIGHEST RISK "The link between body weight and health is more complicated than often assumed," Woods explains. The Danish study, presented at the Annual Meeting of the European Association for the Study of Diabetes, revealed a U-shaped curve between BMI and mortality. Those with BMIs below 18.5 faced nearly three times the risk of premature death, while even those at the lower end of the 'healthy' range (18.5-19.9) saw double the risk. At the other end of the scale, "carrying extra weight did not always translate into greater risk," Woods notes. People with BMIs between 25 and 35 - typically labelled overweight or obese - showed no significant increase in mortality. Only those with a BMI of 40 or above saw their risk more than double. FAT RESERVES CAN BE PROTECTIVE "Having some fat reserves can help the body cope with illness," Woods writes. For example, patients undergoing chemotherapy often lose weight due to appetite loss. "Those with more fat reserves at the start can draw on them, helping their bodies continue essential functions." LOW BMI CAN SIGNAL UNDERLYING ILLNESS "Unintentional weight loss is often a warning sign of illness," Woods emphasises. Diseases such as cancer or type 1 diabetes can lead to weight loss before diagnosis, meaning that low BMI may reflect hidden health problems. "The trouble is, BMI has always been a blunt tool," Woods says. It doesn't take into account diet, lifestyle, fat distribution, or ethnic differences. Developed nearly 200 years ago from a small European sample, BMI still overlooks diversity in body composition. RETHINKING THE HEALTHY BMI RANGE According to Woods, "modern medical advances, which help people manage obesity-related conditions such as diabetes and heart disease, could be shifting the safest weight range higher than before." The study suggests that BMIs between 22.5 and 30 may now carry the lowest mortality risk. The headline message is clear: "Being very thin is dangerous, and carrying some extra weight may not shorten life. BMI alone is a fragile measure of health."