One ‘breakthrough’ always opens our thinking up to others. We now know that rapid fermentation of poorly absorbed low molecular weight sugars (FODMAPs) found in onions, beans, sprouts and some fruits can generate enough gas to cause pain and bloating. But this not only begs the question, what about larger carbs that constitute dietary fibre and resistant starch, but also, what about protein? The polysaccharides in fibre are conveniently ignored in dietary advice for IBS because being larger and more complex molecules and somewhat viscous, fermentation is slower. In any case, we need some fermentable carbohydrate to maintain the populations of beneficial bacteria that keep our colons healthy. Nevertheless people with IBS can find oats and ispaghula (Fybogel) gassy. Also longer chain fructans and galactans are present in many of the fruits and vegetables that also contain the smaller FODMAPs.
Fermentation of protein is a different story and it’s just breaking, In a review just published in Alimentary Pharmacology and Therapeutics, dietitian CK Yao and the team under Dr Peter Gibson from Monash University, examine the evidence. Protein is more efficiently absorbed than carbs in the diet, but nevertheless between 8 and 18g of protein may escape absorption in the small intestine and be fermented in the colon. This amount is greater in people consuming high meat diets, like body builders, those on the Atkins diet, those on acid blockers for peptic disorders and many of the inhabitants of Argentina, Australia, USA, Germany and UK. What’s more, like carbohydrate, protein absorption is likely to be affected by preparation and cooking. Are, for example, cold cooked meats or meat products more resistant to digestion? Does eating meat together with high fibre foods delay its absorption? Is meat that contains more poorly absorbed connective tissue better for us?
And is protein fermentation a problem? Well, it might be. Protein is composed of more elements than carbon, hydrogen and oxygen; it also contains significant amounts of nitrogen and sulphur. Bacterial breakdown of protein in the colon releases a range of volatile compounds, such as phenols, indoles, branched chain fatty acids and ammonia, and gases, notably hydrogen sulphide, that smells like rotten eggs. All of these products of fermentation may harm the colon, damaging the cells that line it, making it more leaky to bacteria and toxic substances and causing inflammation. High protein diets also alter the colonic microbiome, reducing beneficial bifidobacteria species and increasing potentially damaging, pro-inflammatory clostridia. Epidemiological studies show that high meat consumption and low intake of dietary fibre are both risk factors for colon cancer.
So it might seem that protein fermentation may make the gut more sensitive. Not only that but since interactions between the colonic microbiome and the gut epithelium are implicated in obesity, depression and many other low-grade, long-term illnesses, might high protein, low FODMAP diets actually increase the tendency to IBS in the longer term and even to obesity? The answer is, of course, we don’t know. It is early days and the definitive research on those questions is still a long way off. All this suggests is that the dietary management of IBS is not specific. Change one component of the diet and there may be previously unsuspected knock on effects. A paradox that may not have escaped your notice is that your dog or cat seem to thrive on a high raw meat diet without getting IBS. Perhaps it’s the way meat is cooked that may cause the problem.
There is some evidence that poorly absorbed carbohydrates may attenuate the damaging effects of protein fermentation in the colon. It is interesting that a vegan diet contains high levels of poorly absorbed protein from plant sources, but it also contains a lot of poorly absorbed carbohydrate. This might underline the importance of not making your diet carnivorous by replacing the fruit and vegetables with more meat and eggs. Most of the recipes our book, Cooking for The Sensitive Gut, due out in January, contain only modest amounts of meat and eggs and would help you maintain a nutritionally balanced diet while reducing your intake of some FODMAPs to your own personal tolerance.
I realise that many of you reading this article may be thinking – first fibre, then fat, then fruit and vegetables now meat and eggs. What on earth can I eat? I think the answer is not to be too prescriptive with your diet. Everybody with IBS should be informed about how different dietary components might affect them and should be encouraged to adopt a pragmatic approach to food restriction, reducing those foods that are rapidly fermented without overcompensating by reverting to high fat, high meat foods that will cause more problems in the long term.