(FORTUNE Magazine) - Mexico's classic staple, the corn tortilla, has seen better days. Consumption has declined 25% since 1995, which happens to be around the time Mexico signed a free-trade agreement with the U.S., opening its floodgates to foreign junk food.

Ever since, consumers have tasted their way through an onslaught of sweet and salty options, arriving at an unlikely new favorite: Japanese ramen noodles imported from the U.S. Consumption has grown nearly 30% a year since 2000, up to one billion servings last year. Easy to prepare, filling, and relatively cheap at 40 cents a serving, noodle soups are now stocked in upscale grocery stores in Mexico City and tiny general stores in mountain villages.

Leading the way is the Maruchan brand owned by Toyo Suisan Kaisha (Research), a Tokyo company that has captured 81% of the Mexican instant-noodle market. It and other Japanese noodlemakers have tuned in to the Mexican penchant for chile and lime and have developed new flavors accordingly.

The noodles are especially popular among the urban and rural poor, many of whom no longer farm the corn and beans that once sustained them and instead work long hours in factories or small businesses, leaving little time for cooking. Diconsa, the government food-assistance agency, is purchasing hundreds of tons of Maruchan a year.

But the love affair with MSG-laden high-fat noodles has Mexico's public health experts on high alert. "We are seeing both malnutrition and obesity in rural areas around the country," says Teresa Gonzales de Cossio, a nutrition expert at Mexico's National Institute of Health. The increasing presence of Maruchan and other low-nutrient foods, combined with less physical activity, is a major factor in Mexico's ballooning obesity epidemic, which may earn it the laurel of most obese nation in the world in 2006.