The link between fibre and these diseases, all of them related to problems with our immune system, is the microbes living in our gut. This is a part of the body that has more to do with immunity than most of us realise – it’s home to millions of immune cells as well as trillions of bacteria. There’s a growing recognition that this colony of  gut bugs,  or the  microbiome as it’s called,  affects our health for better or worse depending on which bacteria are in residence,  says Professor Charles Mackay of Monash Unversity’s Department of Immunology.

“Less fibre, more processed food and the weight gain that can result from this has altered the mix of microbes in the gut,” he explains. “This can affect our immune system and may be driving the rise in allergy, asthma and autoimmune disease in industrialised countries.”

One clue to the connection between fibre and the immune system comes from rural Africa where problems like allergy and asthma are rare, diets are higher in fibre – and people tend to have a different mix of gut microbes.  A study comparing the gut bacteria of African children from Burkina Faso with those of European children from Italy, for instance, found the African children had a more diverse bunch of microbes than the Italian children.  Importantly, the Africans had more of the bacteria that digest fibre to produce short chain fatty acids (SCFAs) -  substances that help keep both the gut and the immune system healthy, says Mackay.

Besides having an anti-inflammatory effect  these SCFAs help keep  the gut lining in good shape, preventing  gut permeability or  ‘leaky gut’, a problem where the gut lining becomes weakened,  allowing things like   bacteria and waste to pass through into the bloodstream.   There’s some evidence linking a leaky gut to Type 1 diabetes, says Mackay who’s soon to take up an Australian Diabetes Council funded chair of Diabetes at the Charles Perkins Centre, The University of Sydney.

“It could be that bacteria or products of bacteria pass through the gut lining and stimulate the immune system in a way that causes it to attack the wrong target,” he says. “Although for someone to develop an autoimmune disease like Type 1 diabetes you generally have to have a genetic predisposition as well as an environmental trigger. It may be that a leaky gut is the environmental trigger,” he says.

Google ‘leaky gut’ and you’ll find claims that it’s behind a range of health problems from chronic fatigue to autism  -and  all because we’re eating grains. But research into gut permeability and autoimmune disease is still at the early stages, says Mackay.

“I think there’s some scientific basis for a possible effect of grains on the gut lining for some people but I don’t think that this means all of us – there are plenty of healthy people who eat grains,” he says.

So what should we eat to encourage the right mix of microbes we can count on to produce SCFAs? Although you’ll find advice on the internet to get your SCFAs from butter, Mackay’s advice is to eat a wide range of fibre rich plant foods.

“The levels of SCFAs produced from fibre are much greater than that derived from butter,” he says. “I don’t think there’s any substitute for the health benefits of dietary fibre.”

There’s also a scientific argument for using vinegar in dressings to add to vegetables, he adds. Vinegar’s main component is acetic acid which is also a short chain fatty acid.

Once we understand the ideal mix of microbes - how long would it take for a change of diet to improve the quality of our gut bacteria?

“We’re not sure – in humans and mice the microbiome can change very quickly and going on a bender of fast food for two weeks would change it,” Mackay says. “But we don’t know what happens then – does it go back to normal once you start eating differently?  We have ideas about which microbes are useful but we don’t understand yet what the ideal composition is.

“But when we do it has the potential to improve human health just by improving diet without spending money on developing drugs - It’s one of the most exciting developments in medical research in a long time. In the future, we’ll probably be monitoring the health of our gut microbiome and then correcting it if it’s unhealthy.”