Based on new analysis the number of New Zealanders who are obese will nearly double from 1.1 million in 2015 to 2 million in twenty years’ time  – with major health impacts if public health action is not taken.

The review of New Zealand’s BMI (body mass index) trends by University of Otago researchers was published today in the Australian and New Zealand Journal of Public Health.

“High BMI has now overtaken tobacco as the greatest contributor to health loss in New Zealand, which emphasises the public health importance of these findings,” says researcher Dr Ross Wilson

Wilson and Professor Haxby Abbott from Otago’s Centre for Musculoskeletal Outcomes Research, say BMI and obesity rates are continuing to increase in New Zealand and their expectations were they would continue to do so for the foreseeable future.

The pair said recent decades had seen a significant rise in BMI around the world but New Zealand is amongst the world’s most obese developed countries with prevalence rates of obese people (a BMI greater than 30kg/m2) trebling between 1977 and 2011-13.

Based on observed trends this increase is expected to continue with our average BMI increasing from an overweight 26.4 in 1997 to 28.3 in 2015 and by the early 2030s the average Kiwi is expected to be obese with the average BMI exceeding 30.

The results also show increasing ethnic and socioeconomic disparities with the BMI of Māori and Pacific people  – and those living in socially deprived neighbourhoods – being higher again. By 2038, the average BMI among Pacific people is projected to exceed that of the general population by 7.1-8.1kg/m2.

Wilson and Abbott are calling for public health policy measures to address the situation.

The Body Mass Index is a measure of body fat based on height and weight that applies to adult men and women. Between 18.5kg/m2 and 25kg/m2is considered the healthy weight range, anything at 30kg/m2 or above is considered obese, Dr Wilson explains.

Wilson and Abbott considered potential influences on obesity from the data of 76,294 adult New Zealanders, including the effects of an aging population and cohort effects reflecting the increased exposure of younger New Zealanders to a high-obesity environment during critical phases of childhood development, to see whether these would explain the trends in population BMI. They did not.

Instead, the researchers say the recent increases in population BMI can be attributed to “period effects”, or changes in the physical, socio-cultural or economic environments which may affect BMI by shaping behaviours such as food choice and levels of physical activity.

“These results suggest that, in the New Zealand context, the forces behind the obesity epidemic have largely been contemporaneous (period) influences on BMI, such as greater availability and consumption of high-energy, low-nutrient foods and lower levels of physical activity across all cohorts, rather than cohort-specific factors,” the researchers state in their paper.

“Altering or mitigating these environmental influences is therefore crucial to slow or reverse projected increases in population obesity.”

The findings emphasise the need for effective public health measures to address the causes of the obesity epidemic, the pair say.

“These projections imply that, unless addressed by improved public health policies, ongoing population BMI increases are likely to increase the premature mortality, population health loss, healthcare system costs and workplace productivity losses associated with the obesity epidemic.”

Tobacco control may be a useful parallel for considering the importance of comprehensive reforms across a range of policy areas (eg, taxation, advertising restrictions, product regulation), in halting the spread of a public health epidemic, the researchers state.

“A comprehensive obesity reduction strategy might include, among other things, improving the relative affordability of healthy foods (eg, through taxation, subsidies), restrictions on marketing of unhealthy foods and promotion of active modes of travel such as walking and cycling.”

High BMI is the greatest contributing risk factor to health loss in New Zealand. Healthcare costs associated with treating overweight and obesity-related conditions in New Zealand were estimated to be NZ$624 million in 2006, representing 4.4 per cent of all healthcare spending. Given ongoing increases in obesity prevalence over the past decade, current obesity-related healthcare costs are likely to be substantially higher than this, the researchers say.

This research was supported, in part, by the Health Research Council of New Zealand.

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