Thin may be in on the runway, but being supermodel svelte doesn’t help older adults reduce their risk of developing diabetes. In fact, the thinnest older adults were more often found to have diabetes than their normal-weight counterparts in a study published in the Mar. 2008 issue of Diabetes Care.

Researchers analyzed health stats collected between 1993 and 2004 in 40,000 men and 90,000 women 40 to 79 years old. All participants were diabetes-free before the study began. Over its duration, people 60 to 79 years old with a body mass index (BMI) less than 18.5—considered underweight—were 30 percent more likely to develop type 2 diabetes than those with a normal BMI of 18.5 to 24.9. As expected, diabetes rates went back up again at higher BMIs.

This underweight-diabetes link may seem counterintuitive, given the well-trodden path from obesity to diabetes. However, the finding does not necessarily suggest that being underweight causes diabetes.

Rather, in this case age is likely the biggest influence on the disease’s development, surmises lead author Toshimi Sairenchi, PhD, associate professor at the Dokkyo Medical University School of Medicine in Japan. Underweight people 40 to 59 years old did not have higher diabetes rates than their normal-weight peers, as seen in the older group, he points out.

“Aging may force the poor nutrition status in underweight people,” explains Sairenchi, “which leads to declined insulin secretion,” and therefore to diabetes.

Another influence may be genetic factors in the study’s Japanese population. “The prevalence of underweight is relatively high in Japanese older adults,” says Sairenchi, who notes that he nevertheless would expect to find similar results in the United States and Europe.

Commenting on the findings, the Mayo Clinic’s director of research, Robert Rizza, MD, says higher rates of pound-shedding illnesses among older people could also explain the curious link between underweight and diabetes. Rizza calls for additional studies to confirm the link, but says, “The bottom line is that it’s intriguing.”