Working up a bit of a sweat can help virtually everyone's health, but it turns out that those carrying some extra kilos get more benefit.

Sports doctor Chris Hanna told the Australasian anaesthetists' conference in Auckland that among the obese, the death rate per year in those who are unfit is more than three times greater than it is for the moderately fit. He defines as moderately fit as those who do at least 150 minutes of aerobic activity such as walking, cycling and swimming each week.

But while the benefits of physical activity are not in doubt, the size of the survival advantage suggested by Dr Hanna has been questioned.

Professor Tony Blakely, an expert in mortality data at Otago University in Wellington, said New Zealand studies had not been done on the matter, but a three-fold disparity seemed implausible.

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He cited a large European study, published last year, which found that among obese people the death rate was 25 per cent higher for the inactive than the active.

"This study also found, in percentage terms, that the protective effect of physical activity was greater among obese people, compared to those of normal weight.

"So, yes, from what we know, physical activity is 'good for you', ie lowers death rates, and possibly more so, in percentage terms, if you are overweight or obese. But it does not negate the impact of BMI [body mass index] on mortality -- both obesity/overweight and physical activity matter."

Professor Blakely said a global burden-of-disease study published in the Lancet last year found that both physical activity and BMI matter. It concluded that among 60-year-olds the lowest amount of physical activity was associated with a 40 per cent higher rate of heart disease compared to the most physically active.

Auckland University heart disease researcher Professor Rod Jackson has said that the leading risk factors for cardiovascular disease are bad cholesterol, high blood pressure and smoking. Raised blood-glucose is fourth, while in fifth place and "quite a long way down" is obesity.

A Health Ministry study in 2013 found the top five risk factors for the top 10 causes of health loss in New Zealanders were, in order of significance: smoking, high BMI (weight for height), high blood pressure, high blood glucose, and physical inactivity.