Eating healthy during the week and splurging on chips and burgers on the weekend may be just as damaging to a person's health as having a regular diet of junk food, a new study suggests.

The study led by Margaret Morris, the Head of Pharmacology at University of New South Wales (UNSW) in Australia, examined the impact of 'yo-yo dieting' on the gut microbiota of rats.

Human gut consists of up to 100 trillion microbial cells that influence metabolism, nutrition and immune function.

Disruption to the gut microbiota has been linked with gastrointestinal conditions such as inflammatory bowel disease and obesity.

The study was the first to compare how continuous or intermittent exposure to an unhealthy diet can impact the composition of the gut microbiota, Morris said.

"The findings indicate that intermittent exposure to junk food three days a week is sufficient to extensively shift the gut microbiota towards the pattern seen in obese rats consuming the diet continuously," Morris said.

"A reduction in the diversity of the gut's microbiota and a loss of some of the beneficial biota is clearly not a good thing for health," she said.

"While these findings are yet to be replicated in humans, those who are strict with their diet during the week may be undoing all their good work by hitting the junk food over the weekend," she said.

The researchers compared the abundance of microbiota in rats given continuous access to either a healthy diet or junk food with a group cycled between the two diets, healthy for four days and junk for three, over 16 weeks.

A range of metabolic markers, including body weight, fat mass, insulin and leptin, were also examined.

At the end of the 16 weeks, rats on the cycled diet were 18 per cent heavier than those on the healthy diet, while leptin and insulin levels in cycled rats were in between rats fed junk or healthy food.

The researchers found the microbiota of cycled rats was almost indistinguishable from rats fed a constant diet of junk, with both groups' microbiota significantly different from those in the rats fed a healthy diet.

The junk food diet also reduced the abundance of microbial species capable of metabolising flavonoids, which have been suggested to not only assist in weight loss but also exert neuro-protective functions within the brain.

Cycled rats also showed large swings in food intake, consuming 30 per cent more energy than those maintained on the healthy diet only.

When cycled rats switched back to a healthy diet, they consumed half as much food as those maintained on a healthy diet only.

"The study suggests certain gut microbiota, including Ruminococcus and Blautia, may be promising targets for future therapeutic strategies to treat metabolic disorders," Morris said.

The findings were published in the journal Molecular Nutrition and Food Research.