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Dear Doctor: Does drinking coffee really increase the craving for sweets? I tend to drink a lot of coffee throughout the day, and if it's true, it explains a lot about my dietary choices.

Dear Reader: While recent studies have found a connection between caffeine and the urge for something sweet to eat, the cause and effect isn't quite that direct. It's not the coffee itself that somehow makes us long for the doughnut or Danish, but rather how caffeine affects certain chemical processes within our bodies. To get a fuller understanding of what's going on, let's take a closer look at the research -- and at caffeine.

Our daily coffee (or tea or cola or energy drink) habits give us that boost because the caffeine they contain cuts off access to the adenosine receptors in our brain. Adenosine is a chemical that helps to regulate our internal clocks. It gradually builds up throughout the day, and, as it reaches a certain level, we begin to feel sleepy. By blocking those adenosine receptors with caffeine, we get a sense of energy and alertness.

But scientists at Cornell University found that in addition to giving us a jolt, caffeine dulls our ability to perceive sweetness. When the researchers divided the study's participants into two groups, one that drank decaffeinated coffee and the other that drank regular coffee, the members in the caffeinated group were unable to accurately gauge the sweetness of a sugar solution. They rated it as markedly less sweet than the decaf group did.

Not only that, the caffeine drinkers' palates also remained dulled to the taste of sugar for at least 15 minutes after drinking their high-octane coffee. Because many people continue to reach for caffeinated beverages throughout the day, researchers suspect that the ability to taste sweetness remains suppressed.

This new insight dovetails with the group's previous research, which found that limiting a person's ability to taste sweetness actually creates a craving for it. The result is that people actively seek out sweet (and frequently high-calorie) treats.

Put the two together -- caffeine suppresses sweetness and a dulled palate leads to sweet cravings -- and your coffee habit could be laying the physiological groundwork for a serious sweet tooth.

The study also had some surprising news about how we perceive the effects of caffeine. Although neither group knew whether they were the ones drinking the decaf, almost all the study participants thought that they were the ones who got the caffeine. When they were asked to rate how alert they felt after drinking their particular beverage, both groups reported feeling equally energized.

And it turns out caffeine isn't the only thing that can influence taste. Previous studies by these same Cornell researchers found that mood plays a role. Sports fans in the throes of joy after a victory rated ice cream as markedly sweeter than did the followers of the losing team. Meanwhile, the despondent fans were measurably more sensitive to sour and bitter flavors than the happy ones.

Could the happiness boost cancel out the caffeine effect? That would make for an interesting study.

(Send your questions to askthedoctors@mednet.ucla.edu, or write: Ask the Doctors, c/o Media Relations, UCLA Health, 924 Westwood Blvd., Suite 350, Los Angeles, CA, 90095. Owing to the volume of mail, personal replies cannot be provided.)

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