IF AUSTRALIANS continue to outstrip the weight gains of previous generations, today's young adults could knock as much as two years off their life.

Under that alarming scenario, women's average longevity would fall to 82, and men's would fall to 78. But the situation could well turn out even worse for men - because the projections are based on the most recent national statistics, while recent NSW figures for boys demonstrate more dramatic weight increases.

D'Arcy Holman, who calculated the lifespan losses, warned the effect of the obesity epidemic was underestimated because health gains in other areas were masking deaths caused by excess kilos. In the US, he said, scientists had estimated obesity had increased deaths by 8 per cent over 20 years, but these were hidden by reduced deaths from smoking, high cholesterol and high blood pressure.

"Inevitably the same competing trends have been at play in Australia," he wrote in the analysis for the Western Australian Public Health Advocacy Institute.

Many commentators have suggested weight will shorten the lives of today's children relative to earlier generations, but the analysis compiled by Professor Holman, a professor of public health at the University of Western Australia, marks the first systematic assessment of those claims. His calculations compared overweight and obesity rates of children in 1995 to those in 1985, and assumed there would be a similar proportional increase when those children reached adulthood. If the projections play out, the situation will be even worse than if Australia catches up to present US obesity rates, he said.

And because the figures are averages across the whole community, they also understate the effect on individuals who are obese - who lose three to four years of life compared with people in the healthy weight range.

Professor Holman's projections were cited yesterday in a discussion paper released by the Federal Government's preventative health taskforce, which suggested a range of health targets to be achieved by 2020, including halting and reversing the rise in obesity and reducing the number of daily smokers to 9 per cent or less of the population.