Australasian Science: Australia's authority on science since 1938

Weekend Treats Undo Healthy Weekdays

Eating well during the week only to binge on junk food over the weekend is likely to be just as bad for your gut health as a consistent diet of junk, according to a study of the gut microbiota of rats.

The human gut consists of up to 100 trillion microbial cells that influence metabolism, nutrition and immune function. Disruption of this microbiota has been linked with gastrointestinal conditions such as inflammatory bowel disease and obesity.

The new study, published in Molecular Nutrition and Food Research (, indicates that “intermittent exposure to junk food 3 days a week is sufficient to extensively shift the gut microbiota towards the pattern seen in obese rats consuming the diet continuously,” said Prof Margaret Morris of The University of NSW. “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.”

The research team compared the abundance of microbiota in rats given continuous access to either a healthy diet or junk food with a group that alternated between four healthy days and three junk food days. A range of metabolic markers, including body weight, fat mass, insulin and leptin, were also examined.

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

The researchers found that the microbiota of rats on the alternating diet was almost indistinguishable from rats fed a constant diet of junk, with the microbiota of both groups significantly different from rats fed a healthy diet.

The junk food diet also reduced the abundance of microbial species capable of metabolising flavonoids, which assist in weight loss and exert neuroprotective functions within the brain.

Rats on the alternating diet also varied greatly in their food intake, consuming 30% more energy than those maintained on the healthy diet only. When the rats switched back to a healthy diet, they consumed half as much food as those maintained on a healthy diet only.

Morris said that a greater understanding of the role of energy-rich foods and dieting on microbial changes is important given the current obesity epidemic and the prevalence of yo-yo dieting in Western countries. “The study suggests certain gut microbiota, including Ruminococcus and Blautia, may be promising targets for future therapeutic strategies to treat metabolic disorders,” she said.