"Gluten-free" may be the latest diet fad, but new research casts some doubt on its presumed health benefits.
In a large study of US health professionals, scientists found that those with the least gluten in their diets actually had a slightly higher risk of developing type 2 diabetes over a few decades.
The findings do not prove that a low-gluten diet somehow contributes to diabetes. But the study raises questions about the long-term benefits of avoiding gluten, which many people assume to be a healthy move.
Some people - namely, those with the digestive disorder celiac disease - do have to shun gluten, said lead researcher Geng Zong.
But there is little research on whether other people stand to gain from going gluten-free, said Zong, a research fellow in nutrition at Harvard T H Chan School of Public Health, Boston.
That's a big evidence gap, according to Zong - given the popularity and expense of gluten-free foods.
Gluten is a protein found in grains such as wheat, rye and barley. Gluten-free diets are a must for people with celiac disease - an autoimmune disorder in which gluten-containing foods cause the immune system to attack the small intestine.
But gluten-free, or at least gluten-light, diets have caught on as a way for anyone to lose weight and improve their health.
One recent study found that the number of Americans who say they've gone gluten-free tripled between 2009 and 2014.
The new findings are based on nearly 200,000 US health professionals whose health and lifestyle habits were followed over three decades.
The low-gluten fad did not exist when the study period began, in the 1980s, Zong pointed out. But participants' gluten intake naturally varied, based on how often they ate foods like bread, cereal and pasta.
Over 30 years, just under 16,000 study participants developed type 2 diabetes - a disease in which blood sugar levels are persistently too high. Obesity is one of the major risk factors.