KASTELI, Greece  Dr. Michalis Stagourakis has seen a transformation of his pediatric practice here over the past three years. The usual sniffles and stomachaches of childhood are now interspersed with far more serious conditions: diabetes, high blood pressure, high cholesterol. A changing diet, he says, has produced an epidemic of obesity and related maladies.

Small towns like this one in western Crete, considered the birthplace of the famously healthful Mediterranean diet  emphasizing olive oil, fresh produce and fish  are now overflowing with chocolate shops, pizza places, ice cream parlors, soda machines and fast-food joints.

The fact is that the Mediterranean diet, which has been associated with longer life spans and lower rates of heart disease and cancer, is in retreat in its home region. Today it is more likely to be found in the upscale restaurants of London and New York than among the young generation in places like Greece, where two-thirds of children are now overweight and the health effects are mounting, health officials say.

“This is a place where you’d see people who lived to 100, where people were all fit and trim,” Dr. Stagourakis said. “Now you see kids whose longevity is less than their parents’. That’s really scaring people.”