THE law must be changed to make it illegal for workers to sit for more than two hours at a time says a leading health expert alarmed at rising diabetes rates.

Professor Jonathan Shaw whose 12-year "AusDiab" study on the health status of 11,000 Australians calculates that 269 adults develop diabetes every day and says drastic measure are needed.

"We need changes to occupational health and safety regulations so it is not allowed for people to sit for two hours at a time without a break," he said.

"I think everything should be on the table - taxation levers, town planning, even the layout of office spaces needs to be reconsidered to tackle the growing personal and community impact of chronic disease," he says.

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Professor Shaw from the Baker IDI Heart and Diabetes Institute has run a 12 year study of 11,000 Australians which has found young people aged 25-34 are gaining more weight than any other age group.

The study has also found people over-estimated the amount of exercise they thought they were doing by 50 per cent.

People self-reported they spent 200 minutes a day sitting but an electronic device recorded they spent an average 500 minutes a day sitting.

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The incidence of diabetes is five times higher in people who are obese and two times higher in those who are overweight.

Over the 12 year study the average gain in waist circumference among participants was 5.3 centimetres and was greater in women than in men.

"The trend for greater weight gain among people aged 25-34 is very concerning and suggests Australia still does not recognise the serious health risks associated with being overweight or obese," he said.

"The health and well-being of a whole generation of Australians is being compromised by a lifestyle rich in energy dense foods and low on physical activity," he said.

The study also found the prevalence of depression was 65 per cent higher in people with diabetes and they showed double the rate of cognitive impairment than those without diabetes.

Living in the most disadvantaged areas doubled the risk of developing diabetes, the study found.

The community needed to get serious about weight gain in the same way it got serious about smoking and gun control and water restrictions, Professor Shaw said.

Professor Shaw also advocated a tax on junk food, subsidies for healthy foods, standing desks in workplaces and stairs more accessible to encourage more physical activity.

A rule banning sitting for more than two hours might not go down well with employers but they should "look on it as a long-term investment in their employees," he said.

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Originally published as Lengthy sitting should be illegal