The surprising foods that are messing with your gut
By Chloe Lambert (New Scientist 15/9/18)
We're finally starting to understand which foods are causing tummy troubles for so many, and the culprits challenge everything we thought we knew about healthy eating.
The simplest way to feed a healthy gut is to eat a varied diet, especially fruit and vegetables
SOUTH Beach, paleo, vegan, juice cleanse… and FODMAPs. Short for fermentable oligosaccharides, disaccharides, monosaccharides and polyols, the name FODMAPs certainly doesn’t have instant appeal, but a diet focused on avoiding these substances is catching on with the public and the medical profession alike. The low-FODMAP diet is based not on celebrities’ waistlines or detox bunkum, but on the premise that a healthy gut leads to a happy life. So popular is it proving that there are now claims the diet could alleviate everything from indigestion to chronic fatigue.
Over the past few years, we have become much more clued up about the extensive influence of the gut in health and disease, and the impact our lifestyle choices can have on what some researchers like to call our “second brain”. Gluten, a protein found in grains such as wheat, barley and rye, has taken much of the blame, with a growing number of people claiming that they have some sort of gluten intolerance. Global sales of gluten-free food rose 12.6 percent in 2016, and specialist supermarket aisles now heave with gluten-free products, even though the idea that people can be gluten-sensitive even if they don’t have the autoimmune disorder coeliac disease has been largely debunked.
Now the gut health tide is turning once again, and it appears that gut problems linked to certain foods like bread might be real for many. What’s more, the secret to dealing with these problems could fly in the face of established healthy eating advice.
The most common cause of gut problems is irritable bowel syndrome (IBS), a catch-all term for a poorly understood constellation of symptoms including bloating, diarrhoea and constipation. It affects millions, although it doesn’t appear to be on the rise, with the incidence in the West put at between 5 and 15 per cent of the population. If, however, it feels like you can’t go to a dinner party without someone talking about their gut problems, that might be down to a shift in awareness. “People are noticing symptoms more and reporting them,” says Peter Gibson at Monash University in Melbourne. “Sixty years ago we had no criteria to diagnose IBS, and people with gut symptoms just put up with it.”
“Too much fibre, fruit and veg could be causing a rise in gut complaints”
IBS symptoms overlap with those of coeliac disease. For coeliacs, consuming gluten causes symptoms such as diarrhoea, constipation, cramps and fatigue, and triggers a faulty immune reaction which damages the lining of the gut. Although unrelated to IBS in origin, coeliac disease has been attracting attention because it is proving to be more common than previously thought, affecting 1 per cent of people. However, many who test negative for it – showing no signs of making antibodies in response to gluten, or of gut damage – still complain that wheat products make them feel unwell. This has been labelled non-coeliac gluten sensitivity, although the condition remains controversial.
Some argue that gluten is poorly digested by many people, and that its ubiquity in modern, processed food is at odds with the diet that the human gut evolved to deal with. “Wheat is a friend that has outstayed its welcome for some of us,” says David Sanders at the University of Sheffield, UK. Scepticism over whether gluten can cause symptoms in non-coeliacs has been fuelled by the fact that cutting it out has become a fashionable lifestyle choice. Studies show that many non-coeliacs are choosing to avoid gluten because they believe doing so is healthy, despite little evidence for its benefits.
In one study of 22,000 adults in the US, the proportion eating a gluten-free diet more than triples from 0.5 percent in 2009 to 1.7 percent in 2013, although the proportion who had coeliac disease stayed roughly constant. Also in 2013, 30 per cent of adults in the US reported trying to cut down on gluten.
“The gluten-free craze is a marketeer’s dream and the perfect worried-well scenario,” says Kevin Whelan at King’s College London. “We are seeing people avoiding it in the belief this will help them lose weight or improve their overall health. But beyond coeliac disease, gluten’s role in optimising health is nowhere near what people think it is.”
Even if gluten is not itself the culprit, we are now understanding why so many people believe they are sensitive to it. A growing body of evidence over the past few years points the finger at FODMAPs. These carbohydrates are present in numerous foods, including wheat, which contains some called fructans. “If you go on a gluten-free diet you reduce probably 50 per cent of your FODMAPs,” says Gibson. Rather than having some sort of sensitivity to gluten, it seems that for some people with IBS, FODMAPs could be the trigger.
After researchers noticed that people with IBS felt more tired when they ate lots of FODMAPs and less so on a low-FODMAP diet, there has been hope it might help people with chronic fatigue syndrome, who also have a high incidence of IBS. But Gibson says the evidence so far is weak and the diet’s effect on fatigue only happens among people with IBS.
Although it is appealing to think that we might all benefit from fewer FODMAPs in our life, “if you don’t have bowel problems there is absolutely no rationale for being on a low-FODMAP diet”, says Gibson. “This is not a diet for good health.” One drawback to the diet is that it cuts out foods that are important sources of vitamins, fibre and nutrients such as calcium. Recently, there have also been concerns that it depresses levels of good bacteria in the gut, and the long-term consequences of the diet remain unclear.
Many people trying to avoid FODMAPs struggle to work out which foods to eliminate. “It has helped an awful lot of people,” says Peter Whorwell, a gastroenterologist at the University of Manchester, UK. “But for one patient it will be tomatoes, the other lettuce, the other none of the above.”
Intriguingly, a third of people with IBS who try the diet reap no benefits at all. For them, some researchers are now looking at a more controversial alternative.
When it comes to gut health, one of the most common pieces of advice has been to eat plenty of fibre, such as wholegrain bread and fibrous vegetables, which help keep the gut working properly and can also reduce the risk of cancer and cardiovascular disease. But it now seems that fibre may also be part of the problem. Some types of fibre release gas when fermented in the bowel, causing irritation in sensitive individuals.
Whorwell was one of the first to demonstrate that reducing fibre could be beneficial in IBS, with a study finding that wheat bran made 55 per cent of people with IBS feel worse and only 10 percent feel better. The idea has been gaining ground, although it has lately been overshadowed by the fuss over FODMAPs. Whorwell often advises his patients to try switching from wholegrain to white bread. “Fibre is just as important as FODMAPs in aggravating irritable bowel,” he says.
Whorwell even speculates that a push for healthier diets – high in fibre and fruit and vegetables – may be contributing to a rise in gut complaints. “Thirty years ago, nobody had heard of five a day,” he says. “It amazes me how many vegetables people are eating these days.”
The trouble is that advising patients to eliminate both FODMAPs and fibre flies in the face of good nutrition. In future, IBS treatment will probably involve supplements containing targeted combinations of beneficial, slow-fermented fibres, says Whelan, who is studying fibre’s effects on the gut.
And although gastroenterologists remain divided over the issue of gluten, some believe it may yet prove to have adverse effects beyond coeliac disease. “The research evidence is nowhere near catching up with the marketing machine,” says Whelan. “But I think non-coeliac gluten sensitivity exists. Some people, when given a gluten challenge, do get symptoms, though identifying who they are and how they differ from people actually responding to fructans is difficult.”
The good news for those with gut problems is that the diversity of symptoms and causes are being investigated like never before. For those who don’t, however, it might be time to step off the bandwagon, says Sanders. “If you don’t have any symptoms, you don’t need to be on any of these diets.”
What to eat for a healthy gut
Probiotics – foods prepared with live “good” bacteria, such as yogurt, or supplements – have been shown to help with conditions like irritable bowel syndrome; they may also be beneficial for people with weakened immune systems. But their long-term effects on overall health are not yet known.
Fermented foods and drinks such as kimchi, sauerkraut, kombucha and kefir are gaining in popularity. Their effects on health again aren’t yet clear, but consuming them won’t do any harm and might improve the microbial diversity in your gut.
For people in good general health, prebiotics, which encourage the growth of good bacteria, are the way to go, says Monika Fleshner at the University of Colorado, Boulder. “It’s difficult to introduce a bacteria that will stay there and colonise,” says Fleshner. “If you provide existing bacteria in the gut with prebiotic nutrients that will help their growth, it seems a more natural way to support your gut ecology than trying to ingest a live bacteria and hoping it sticks.”
The simplest way to feed a healthy gut is to eat a diversity of fruits and vegetables. But for prebiotics, good sources include green bananas; vegetables in the sunflower family such as artichokes, radicchio, lettuce, chicory, tarragon and salsify; and those related to lilies such as leeks, asparagus, onions, garlic, shallots and chives.
No junk please
Gastroenterologist and science writer Giulia Enders recommends cooking carbohydrates such as potatoes and pasta and later eating them cold or reheating them. The period of cooling makes starch more resistant to human digestion, meaning it stays intact to feed microbes in the large intestine.
Tim Spector at King’s College London has shown that intermittent fasting may encourage the growth of helpful anti-inflammatory bacteria. He also recommends avoiding junk food, because additives such as emulsifiers seem to kill good bacteria. In a bit of DIY research, Spector found that when his son went on a fast-food diet for 10 days, his gut microbes reduced by 40 per cent. Don’t try that at home.
