People usually gain weight because they overeat, but what makes them overeat?

A new study suggests that obese people have a different physiological response to food: they continue to salivate longer in response to a new taste than do people of normal weight.

Saliva production tends to decline in most people once they have got used to the taste of a certain food and had enough of it. The process, called habituation, is associated with a feeling of fullness, said Dale S Bond, a study author and assistant professor of research at the Miriam Hospital in Providence.

But among very obese people in the new study, all of whom were candidates for bariatric surgery, the decline in saliva in response to a stimulus — in this case, lemon juice — was slight and occurred much more gradually than among those who were of normal weight, researchers said. The study was published online in the journal Obesity Surgery.

“This is basically saying that there is a difference in how we respond to food physiologically depending on our weight status,” Bond said. “It suggests that this habituation process is impaired in people who are obese.”

“They’re not as sensitive to those feelings of fullness, and as a result, they continue to eat longer,” he added.

In the study, researchers recruited 34 obese patients who were candidates for bariatric surgery and 18 people of normal weight for comparison.

After measuring the baseline levels of saliva by placing a small amount of water on the volunteers’ tongues, they conducted 10 lemon juice tastings at regular intervals and measured the salivary changes that occurred.

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