Weight loss after six months is the same for the 5:2 diet or a standard kilojoule-reduced diet, a study has found.

The popular 5:2 diet, when you eat normally for five days and heavily restrict your intake for two, does work in the short-term, a study has found.

But you can lose about the same amount of weight while on a standard diet of reducing your daily kilojoule (kJ) intake in a much less restricted way.

The mean weight loss after six months for those on the standard kJ-reduced diet was 5.5 kilos, while those on the 5:2 diet lost 5.3kg, say researchers from Austin Health and the University of Melbourne.

Researcher Margie Conley is presenting the findings to the Dietitians Association of Australia's National Conference in Melbourne.

The randomised study involved 24 obese male war veterans aged 55-75 years, who had five counselling sessions with a dietitian over the six month study.

One group went on the 5:2 diet restricting them to 2500 kJs, or 600 calories, on two non-consecutive days a week while they ate normally on the other five days.

The others, who on average had been consuming around 10,080 kJs a day, went on a diet cutting their daily intake by 2500 kJs.

Men in both groups lost body fat and girth, with the standard kJ-reduced diet group reducing their percentage body fat by an average 2.3 per cent and their waistline by 6.4cm.

The figures for those on the 5:2 group were 1.3 per cent and 8cm.

Eating just 2500 kilojoules over a day might mean having two poached eggs and some spinach for breakfast, 60g to 80g grilled chicken with a side salad of cucumber and tomato for lunch, and a small piece of fish with vegies for dinner.

"Compliance rates were similar for the two groups, but the 5:2 diet group reported being hungrier, especially early on in the study," Ms Conley said.

"Interestingly, weight loss slowed at the three-month mark for both groups, which was when the dietitian follow-up tapered out, showing support may be the key element in continuing success."