Australia's obesity epidemic is at crisis point, say researchers who can now measure how losing just six kilograms results in a major health gain.

Half the nation's adult population are well over a healthy weight, says Associate Professor Katherine Samaras, raising their risk of type 2 diabetes, heart disease and some cancers.

Obesity sparks damaging changes to immune systems, she said.                       

We've found that a modest weight loss, of about six kilograms, is enough to bring the pro-inflammatory nature of circulating immune cells back to that found in lean people, Dr Samaras said in a statement on Tuesday.

These inflammatory cells are involved in promoting coronary artery disease and other illnesses associated with obesity.

This is the first time it has been shown that modest weight reduction reverses some of the very adverse inflammatory changes we see in people with diabetes.

Dr Samaras and colleague Dr Alex Viardot, from Sydney's Garvan Institute of Medical Research, studied the immune system functioning of obese people with type 2 diabetes or prediabetes who were put on a restricted calorie diet.

Study participants were limited to between 1000 to 1600 calories a day for 24 weeks, while they also had gastric banding surgery halfway through the trial to further restrict their food intake.

The research, published in the Journal of Clinical Endocrinology Metabolism this week, found only a small amount of weight loss could reap an 80 per cent reduction in these pro-inflammatory cells as well as other positive effects for the immune system.

Excess weight disorders now affect 50 per cent of adult Australians, with obesity being the major cause of type 2 diabetes and some cancers, she said.

The situation has reached crisis point, and people must be made aware that excess fat will affect their immune systems and therefore their survival.

The study also offered new insights into why some people found it more difficult than others to lose weight.

We also showed that the activation status of immune cells found in fat predicted how much weight people would lose following a calorie restricted diet and bariatric surgery, Dr Samaras said.

Those with more activated immune cells lost less weight (and) it's the first time this has been described.