FORTALEZA, Brazil — Children’s squeals rang through the muggy morning air as a woman pushed a gleaming white cart along pitted, trash-strewn streets. She was making deliveries to some of the poorest households in this seaside city, bringing pudding, cookies and other packaged foods to the customers on her sales route.

Celene da Silva, 29, is one of thousands of door-to-door vendors for Nestlé, helping the world’s largest packaged food conglomerate expand its reach into a quarter-million households in Brazil’s farthest-flung corners.

As she dropped off variety packs of Chandelle pudding, Kit-Kats and Mucilon infant cereal, there was something striking about her customers: Many were visibly overweight, even small children.

She gestured to a home along her route and shook her head, recalling how its patriarch, a morbidly obese man, died the previous week. “He ate a piece of cake and died in his sleep,” she said.

Mrs. da Silva, who herself weighs more than 200 pounds, recently discovered that she had high blood pressure, a condition she acknowledges is probably tied to her weakness for fried chicken and the Coca-Cola she drinks with every meal, breakfast included.

Nestlé’s direct-sales army in Brazil is part of a broader transformation of the food system that is delivering Western-style processed food and sugary drinks to the most isolated pockets of Latin America, Africa and Asia. As their growth slows in the wealthiest countries, multinational food companies like Nestlé, PepsiCo and General Mills have been aggressively expanding their presence in developing nations, unleashing a marketing juggernaut that is upending traditional diets from Brazil to Ghana to India.

A New York Times examination of corporate records, epidemiological studies and government reports — as well as interviews with scores of nutritionists and health experts around the world — reveals a sea change in the way food is produced, distributed and advertised across much of the globe. The shift, many public health experts say, is contributing to a new epidemic of diabetes and heart disease, chronic illnesses that are fed by soaring rates of obesity in places that struggled with hunger and malnutrition just a generation ago.

The new reality is captured by a single, stark fact: Across the world, more people are now obese than underweight. At the same time, scientists say, the growing availability of high-calorie, nutrient-poor foods is generating a new type of malnutrition, one in which a growing number of people are both overweight and undernourished.