Eating a high-fat diet in your 20s and 30s heightens the risk of ill-health later on - and not just because of weight gain.
According to researchers at Qingdao University, China, fatty foods cause a reduction in, and mutation of, so-called 'good' bacteria in the gut.
Specifically, an unhealthy diet modifies microbiomes - which break down food in the stomach - and sparks a rise in inflammatory markers throughout the body.
The data, published online in the journal Gut, raises fears this could sow the seeds of metabolic disorders, such as diabetes, heart disease and stroke, over the long term.
The researchers set out to see if different levels of dietary fat alter gut bacteria in healthy young adults from China.
Dietary habits in the Asian country are moving from being low-fat, high-carb to relatively high in fat and low in carbohydrates.
The researchers divided 217 healthy 18 to 35-year-olds of normal weight into three dietary groups.
The participants then received different ratios of carbs - white rice and wheat - and fat - mainly soybean oil.
Fibre and protein intake was kept the same between all the participants.
The three end diets were either low fat, where lipids made up 20 per cent of the participants' energy intake.
Moderate fat - equal to 30 per cent of energy intake - or high fat - where lipids accounted for 40 per cent of energy intake.
Each participant stuck to their particular diet for six months.
Its impact on their gut bacteria and inflammatory markers was assessed in blood and faecal samples taken at the start and end of the experiment.
After six months, participants in all three groups lost weight, with those on the low-fat diet shedding the most.
But certain changes, with potential implications for long-term heath, were only evident in the samples from the high-fat group.