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When one of pediatrician Jim Sears' kids asks for a soda or other sweet treat, he sometimes asks them a question: "Is this a good time to be suppressing your immune system?"
Sears, who is co-host of The Doctors TV show and a contributor to the popular AskDrSears parenting website, is a firm believer in a widespread idea: A big dose of sugar can immediately suppress your immune system and make you more vulnerable to colds, flu and other infections.

So, at a time when people are especially keen to protect themselves from H1N1, or swine flu, it's worth asking: Is it true? Can a few spoonfuls of sugar really help make you sick?

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Denver nutrition therapist Kate Pfeiffer has no doubt. "Limiting sugar should be the first line of defense against infectious disease," she says. She wrote a column for Examiner.com titled: "Worried about the Swine Flu? Avoid Sugar!" In it she cites a 1973 study, published in the American Journal of Clinical Nutrition. The same study is cited at AskDrSears.com and on many alternative-medicine sites.

In the study, researchers at Loma Linda University gave volunteers 100 grams of sugar (20 teaspoons, roughly the amount in a liter of soda). The researchers then drew blood from the volunteers and mixed in some bacteria. They found that infection-fighting white blood cells from people who had just gorged on sugar gobbled up many fewer bacteria than those who had just fasted or eaten an unsweetened starch.

But that's not evidence that would convince most doctors, says Aaron Glatt, an infectious-disease specialist who is president and chief executive officer of New Island Hospital in Bethpage, N.Y. Glatt says he has heard of the sugar theory, but as far as he knows, there are no studies showing people who eat a lot of sugar actually get more cases of colds and flu.