If you love your mashed potatoes, better have a small stack instead of a huge one. According to one new theory, potatoes and other carbohydrates eaten in large quantities by a sedentary population may be a major contributor to America's alarming rate of diabetes.

Researchers at Harvard School of Public Health say the problem is starch. When the starch hits your mouth, its molecules are turned into sugar and head for your bloodstream. Your body is getting pure glucose.

The flood of blood sugar sets off a chain reaction. Insulin pours out of the pancreas. Triglycerides shoot up. HDL, the good cholesterol, drops dramatically. It's the perfect setup for heart disease and diabetes, say the doctors.

It's not just potatoes that create this effect. The problem is the same with white bread, bagels, and white rice.

Potatoes are supposed to be one of the great foods. They contain calcium, niacin, iron, and vitamin C. You don't have to give them up, but they should be eaten in moderation, especially by people who don't get much exercise.