They love your guts: microbes and what your diet does to them

Studies have uncovered links between particular foods and a healthy diversity in our intestinal ‘populations’

Diversity is the spice of life. And it seems the same goes for your gut bugs – as we get on in years, at least. That’s the finding of the Eldermet study, which tracked 178 older people who were living in the community or in long-term care.

The study, which was led by Prof Paul O'Toole at University College Cork and the Alimentary Pharmabiotic Centre, found that people in care who had less diverse diets tended to have less diverse populations of bacteria in their gut, and this was also linked with poorer health.

The study, which was published in the journal Nature last year, was an eye- opener for co-author Fergus Shanahan, professor of medicine at UCC, who never expected to find such a strong correlation.

“It seems that for the elderly it’s not enough to have calories and nutrients, you have got to have diversity in the diet too,” he says. “And the prediction now is that there will be new foods for promoting this diversity in the microbiome [the ecological community of micro-organisms that share our body space], and that we may in the future be able to measure the gut bacteria as biomarkers of health in the elderly.”

The study draws attention to the goings-on deep in your gut, which is packed with trillions of bacterial cells – hundreds of different species – that far outnumber the cells in your own body.

And even though you can’t see those bacterial cells, what they are and what they are doing matters. If you have a lack of diversity, if the “wrong sort” is hanging around in there, or if there’s some unruly microbial behaviour, it could spell trouble.

In recent years studies have linked the gut microbiota with the risk of gut inflammation, colon cancer and obesity, and they might even have implications for mental health.

However, while we might be somewhat at the mercy of our gut bugs, it seems they in turn are at the mercy of what we feed them through our diet.

Antibiotics or illness can cause havoc, and gut microbes tend to be more malleable in children and the elderly, but overall diet is the main determinant of which microbes are in the gut and which are behaving, explains Shanahan, who directs the Science Foundation Ireland-funded Alimentary Pharmabiotic Centre in Cork.

“Over a lifetime, diet is the most important thing that influences your microbiome,” says Shanahan, who earlier this month scooped the award of SFI researcher of the year.

“The microbiota in health in adult life is relatively stable – although perhaps not as stable as we once thought – and with new molecular techniques we are finding that environmental and lifestyle factors, especially diet, are influencing the composition of the microbiota. Also, the bacteria are making and producing things, and depending on what you feed them, they will behave differently.”

So what should we be feeding them? It’s a quickly evolving area of study, but so far it seems that the old advice to eat fibre and a diversity of foods holds true.

Fabulous fibre
Recent advances in genetic technology mean that we can now get a better handle on the bacteria in the gut by looking at their DNA – although typically the experiments are done in what comes out the other end, where faecal bacteria can give a good sense of the populations in the bowel.

That has led to an explosion of studies into how diet is linked to gut bugs, and it's still a relatively new area, explains Dr Paul Cotter, but so far the findings are galvanising the long-held view that fibre in the diet is a good thing.

Specific types of carbohydrate – including inulin, which is found in vegetables such as chicory and Jerusalem artichoke – can act as “prebiotics” to encourage the growth of bacteria in the large intestine. And Cotter reckons the list of prebiotics is set to grow.

"Now we are starting to identify new types of probiotics that we weren't aware of maybe five to 10 years ago," he says. "And maybe the next type of 'good diet' and prebiotics will be the ones that enhance these bacteria."

The bigger picture
However, Cotter cautions against targeting the gut microbiota alone as a panacea that can solve health problems.

“My general attitude is that any intervention needs to be part of a larger solution,” he says. “So modulating the gut microbiota won’t allow you to eat junk food every day.”

It's a view shared by APC researcher Prof Catherine Stanton, whose research at Teagasc is looking at how probiotics can chop up fats in the diet to increase levels of omega-3 fatty acids for the host.

“I’d always recommend a balanced approach to diet – you can’t pick one component and say you have to eat lots of that because that is good for you,” she says. “You have to eat a balanced diet with a variety of different nutrients and a good mixture of fruits and vegetables – they provide fibre but they also provide other components such as polyphenols, which research is starting to show can also positively impact on microbiota and gut health.”

The gut bugs of lean and obese people tend to differ, and this has sparked interest in the role microbiota could play in obesity or its prevention. Dr Paul Cotter of the Alimentary Pharmabiotic Centre has been finding that whey protein seems to modulate gut microbiota in a protective way. "We fed mice a high-fat diet and then introduced milk proteins into the diet – either whey protein or casein," says Cotter. "The animals that were given whey protein gained considerably less weight than the ones that got casein. With whey the mice didn't develop what you would think of as a 'lean' microbiota, but the gut microbes did change and we think they were compensating for the high-fat diet."

What about humans? In an as-yet-unpublished study that worked with elite athletes and mere mortals with low and high body mass indices, APC researchers found that exercise and whey consumption were linked with a more diverse gut microbiota, which is generally considered healthy, according to Cotter.