Alarming research shows New Zealand fast food chains have been increasing their serving sizes.

A University of Auckland study looked at almost 5500 fast food products across 12 food groups at 10 major fast-food chains between 2012 and 2016.

Across all products, researchers found a 5 per cent increase in serving size, a 6 per cent climb in energy density, a 14 per cent jump in energy per serving, and a 12 per cent increase in sodium per serving.

The serving size, energy per serving and sodium per serving had gone up in desserts and pizzas. While sodium density, energy per serving and sodium per serving had gone up in sandwiches and salads.

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But Restaurant Brands general manager of marketing Geraldine Oldham said the company, which owns KFC, Pizza Hut, Carl's Jnr and Starbucks, had not increased the size of meals and was working to make them healthier.

"We have made significant changes to the nutritional composition of our products, including reducing sodium and sugar. Since 2012 we have reduced the salt level in our burger buns by 24 per cent which meets the Heart Foundation's guide of 400mg," he said.

"In the KFC Colonel burger, the amount of sugar has gone from 11.7 grams per serve in 2011 to 7.1 grams per serve and salt has dropped from 1111 milligrams per serve down to 830 milligrams per serve."

She said their food was meant to be eaten as an occassional treat rather than every day.

Asian fast food, however, saw a significant decrease in serving size and energy per serving over the five years of the study, researchers said.

The research follows a University of Otago report that last week revealed two million New Zealanders will be obese in 20 years if trends continue. The Australian and New Zealand Journal of Public Health showed the average Body Mass Index (BMI) of Kiwis increased from 26.4 in 1997 to 28.3 in 2015.

Dr Helen Eyles, who led the Fast Food Trends in New Zealand study, said overall New Zealand's fast foods had become larger and more energy dense over the past five years.

"An important consideration from the study is the impact on young people, the highest consumers of fast foods," Eyles said.

National Nutrition Survey data found 38 per cent of 15 to 18-year-olds and 42 per cent of 18 to 30-year-olds had consumed fast food in the past month.

"Our fast food chains should make changes in line with the 'Healthy Kids Industry' pledge as part of the Government's childhood obesity plan, including measurable reductions to the serve size and overall healthiness of products," Eyles said.

She said she hoped the study would encourage the Government to develop and implement guidelines for the fast-food industry.

Researchers did not name specific outlets.