Eating a high-fat diet is bad for the bugs that live within each of us and help our health, scientists say.

And separate research has just revealed that these bacterial communities in our gut – called the microbiome – have been evolving to adapt to Kiwis' love of fruit and vegetables.

The human microbiome has become a frontier in science; researchers now know that it contributes to everything from immunity and obesity to mental health.

A first-of-its-kind study just published by a team of scientists found a diet high in fat was linked to unfavourable changes in the type and numbers of gut bacteria, along with a rise in inflammatory triggers in the body.

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The researchers – among them the University of Otago's Professor Jim Mann - warned these effects may sow the seeds for the development of metabolic disorders, such as diabetes, and heart disease and stroke over the longer term.

Previous experimental research suggests that a high-fat diet created an imbalance in gut bacteria and made the gut "leaky" - while observational studies suggest that diet is strongly linked to the volume and type of bacteria in the gut.

The researchers wanted to see if different levels of dietary fat might alter gut bacteria in healthy young adults from China, where dietary habits were moving from a traditional low-fat, high-carbohydrate diet, to one relatively high in fat and low in carbs.

The team split a group of healthy young adults of normal weight into three dietary groups by altering the ratio of carbs – white rice and wheat flour – and fat – mostly soybean oil – but otherwise, keeping fibre and protein content the same.

The three diets were: low fat, equivalent to 20 per cent of energy intake; moderate fat, equal to 30 per cent of energy intake; and high fat, accounting for 40 per cent of energy intake.

Each participant stuck to their particular diet for six months, and its impact on their gut bacteria and inflammatory triggers was assessed at the start and end of the six months.

After the trial period, participants in all three groups had lost weight, with those on the low fat diet losing the most.

But certain changes, with potential implications for long term heath, were evident in the samples from the high-fat diet group.

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Although there were no major changes in the overall volume of gut bacteria among the three groups, the numbers of beneficial bacteria that produce short chain fatty acids, including butyrate, had increased in the low fat diet group.

Butryate was a key energy source for bowel cells and has anti-inflammatory properties.

By contrast, the numbers of these beneficial bacteria had fallen in the high-fat diet group, while numbers of "unhelpful" bacteria that have been found in the guts of people with type 2 diabetes, for example, had increased.

Certain bacteria, such as Blautia species, which were associated with lower cholesterol levels, were abundant among those on the low-fat diet.

Bacteroides species, which were associated with higher cholesterol levels, were more common among those on the high fat diet.

What's more, the higher fat diet was associated with significant and potentially detrimental changes in long chain fatty acid metabolism, resulting in higher levels of chemicals that are thought to trigger inflammation.

The opposite was true for the low fat diet.

"Compared with a lower fat diet, long-term consumption of a higher fat diet appears to be undesirable… for young healthy adults whose diet is in transition from the traditionally consumed lower fat, higher carbohydrate diet to one characterised by an appreciably higher fat content," the researchers concluded.

But their findings might also have implications for other countries - including New Zealand.

"These findings might also have relevance in developed countries in which fat intake is already high."

MORE FIBRE, MORE BUGS

Meanwhile, another study co-authored by New Zealand scientists has discovered the first gut bacterium that specialises in breaking down a hard-to-digest substance found in plants.

Remarkably, their findings suggest that the human gut microbiome is evolving to accommodate Kiwis' consumption of fibre-rich foods.

"M. pectinilyticus is a dedicated microorganism for breaking down pectin, a dietary fibre that makes up 40 per cent of the plant cell wall in common fruits and vegetables such as kiwifruit and tomato," explained Plant and Food Research scientist Dr Caroline Kim.

"The process wasn't well-understood until now because few pectin-degrading bacteria exist and none as specialised as M. pectinilyticus."

This had left a large gap in our knowledge of how this abundant and important component of human diet was used inside our bodies.

Kim and colleagues analysed samples and dietary intakes of 44 healthy people in New Zealand over 10 weeks.

They found that the presence of M. pectinilyticus positively correlated to the participants' pectin consumption - the more fibre one eats, the more likely that this beneficial microorganism is present.

"The high degree of specialisation shows that the typically abundant pectin consumption in the human diet may have placed evolutionary pressure on our gut microbiome to make room for specialist bacteria with dedicated niche and function for pectin degradation," Kim said.

"Since M. pectinilyticus only utilises pectin and no other types of carbohydrates, this organism will provide valuable insights into how gut microbes interact with plant pectin and ultimately begin the process of plant digestion in the human colon."

FIVE WAYS TO LOVE YOUR BUGS

• Eat plenty of high-fibre foods such as legumes, beans, peas, oats, bananas, berries, asparagus, and leeks.

• Eat a diversity of healthy foods. A wider range of foods nurtures bacterial diversity.

• Avoid fatty foods and artificial sweeteners.

• Probiotics, prebiotics and fermented foods can have limited benefits, a better, healthier diet is key.

• Improve your lifestyle: more sleep, more activity, lower stress helps your microbiome as well.