The world's diet has deteriorated substantially in the last two decades, a leading nutrition expert said on Monday, citing one of the largest studies available on international eating habits.

Poor countries in sub-Saharan Africa and Asia are seeing the fastest increases in unhealthy food consumption, while the situation has improved slightly in Western Europe and North America, said Dariush Mozaffarian, Dean of the Friedman School of Nutrition Science and Policy at Tufts University.

Between 1990 and 2010, middle and low income countries saw consumption of unhealthy foods increase dramatically, Mozaffarian said, citing information in a study he co-authored for the March edition of The Lancet Global Health journal.

The "globalization" of western diets - where a small group of food and agriculture companies have disproportionate power to decide what is produced - is partially causing the shift to unhealthy eating, Mozaffarian said.

Processed foods high in sugar, fat and starch are driving the growth of unhealthy foods.

The study reviewed 325 dietary surveys, representing almost 90 percent of the world's population, in what is thought to be the largest study yet of international eating habits.

China and India recorded some of the highest increases in unhealthy food consumption, the study said. Some countries in Latin America and Europe saw an increase in both healthy and unhealthy food consumption.

Between 1990 and 2014, roughly the same period as the study, the number of hungry people worldwide dropped by 209 million to 805 million, according to the U.N.'s Food and Agriculture Organisation (FAO).

"Most global nutrition efforts have focused on calories – getting starchy staples to people," Mozaffarian told the Thomson Reuters Foundation. "We need to focus on the quality of calories for poor countries, not just the quantity."

Old people displayed better eating habits than the young in most of the 187 countries covered in the study.

This is a worrying development, Mozaffarian said, as rates of obesity and chronic diseases like diabetes are set to increase if young people continue eating unhealthy foods.

"Young people are growing up with much worse diets than their parents or grandparents," he said.