A study has found that many Australian pre-schoolers may be missing out on the vital nutrients they need for good health.

A study in the Medical Journal of Australia finds that many children are missing out on Omega 3 and fibre, while consuming too much saturated fat.

Professor Maria Makrides and her colleagues from the Women and Children's Health Research Institute in Adelaide studied 300 children.

She says the findings suggest Australian children are following a diet that could lead to long-term damage to their health.

Fourteen per cent of the children studied were found to be obese.

Meanwhile, British scientists say an increasingly overweight world population is threatening global food security and the environment.

They have for the first time calculated the total weight of humanity at 287 million tonnes, with the average adult weighing 62 kilograms.

The researchers say if it the US average was replicated worldwide it would be like adding an extra 1 billion people to the global population.