A suggested tax on sugar-sweetened beverages (SSBs) may reduce obesity in 220 000 adults in South Africa, research by the University of the Witwatersrand has found.

"By instituting a 20% tax... it is predicted to reduce energy intake by about 36 kilojoules per day, resulting in a 3.8% reduction in obesity in men and a 2.4% reduction in obesity in women," researcher Mercy Manyema said in a statement on Wednesday.

This meant there would be 220 000 fewer obese adults in South Africa.

Manyema said it was government's responsibility to protect the health of its population.

"One way of doing so is through nudging people to make healthier and more sustainable choices. An SSBs tax has the potential to do this in addressing obesity-related diseases," said Manyema.

The results of the research were published in an open-access journal "Plos One" on Tuesday.

It found taxing SSBs could affect obesity in the country, particularly among young adults.

The academics said South Africans had become more obese over the past 30 years, and South Africa was presently considered the most obese country in sub-Saharan Africa.

Over half of South Africa's adults were overweight or obese – 42% of women and 13% of men.

"While SSBs alone may not be the only reason for an increase in body fat, these fizzy drinks do not contain any essential nutrients, have a high sugar content and a strong link to weight gain," said the paper's senior author, Prof Karen Hofman.

"Drinking just one SSB a day increases the likelihood of being overweight by 27% for adults and 55% for children."

The paper follows a recommendation by Health Minister Aaron Motsoaledi on the need to regulate foods high in sugar to address obesity and its related diseases.

The Democratic Nursing Organisation of South Africa said it supported the proposed tax.

"Sugar-sweetened beverages have been swelling the country's health care costs for too long," spokesperson Sibongiseni Delihlazo said.

He said the tax would result in more revenue for the state and additional funds to buy medication and build clinics and hospitals.

He said the union believed business models should take into consideration social responsibility and customers' well-being.

"After all, selling beverages that will ultimately retard and kill customers is counter-productive to the sustainability of the very same companies."