Sugar substitutes make people feel they can indulge in a sweet treat or a soft drink without the calories. But research suggests that consuming low-calorie sweeteners at the same time as carbohydrates prevents the body from using blood sugar effectively, increasing the risk of health problems.

Dana Small at Yale University in New Haven, Connecticut, and her colleagues asked 60 healthy people to consume 7 beverages of a single type over 2 weeks. The beverages contained either sugar, the low-calorie sweetener sucralose, a non-sweetening carbohydrate or both sucralose and the carbohydrate.

The researchers tested the participants’ brain responses to sweet flavours and assessed their insulin sensitivity, a measure of how well the body regulates blood sugar. Low insulin sensitivity can precede diabetes.

People who’d consumed the sucralose–carbohydrate combination had lower insulin sensitivity after the trial than before. In addition, their brains showed less activity in several regions that usually respond to sweet taste by switching on and helping to regulate metabolism. Participants who drank beverages containing sugar, sucralose alone or carbohydrate alone experienced none of these effects.