AUTHOR: Dr. Jason Bush

Today, I’m writing about prebiotics and gas.  Simultaneously the source of humour and embarrassment, flatulence is a normal part of life.  You can thank (or blame) your gut microbiome for this, as they ferment and metabolise the undigested parts of your diet.  And while there’s no end to the low-brow jokes one can make about the topic, it is worth speaking seriously about gas, bloating, and flatulence.  While they may not be medically important to everyone, they do influence our behaviour and can affect our quality of life, especially when gas becomes excessive.

When we feel bloating, cramping, or other temporary sources of belly pain, we are generally describing the sensation of our digestive tract being inflated.  This change is perceived by the nerves touching the gut.  Remember that the largest network of nerves outside of the brain lies in the gut.  But not all this discomfort is directly related to gas – food and liquid can also stretch the gut, activating the same nerves.

Water drawn into the gut contributes to stretching, especially in response to high-sugar or -salt foods.  Soluble fibers, including inulin, also force water from the body into the gut, helping to explain why these products often cause bloating.  Gas, however, is the most frequent cause of gut discomfort.  While some of this gas comes from air swallowed during eating, most is produced by the fermentation of indigestible food by the gut microbiome.

Some people report that foods high in prebiotic content tend to produces excessive gas, especially those containing inulin or FOS.  Gas is produced rapidly as these short sugar chains are cut up and fermented.  The small molecular size of other similarly short prebiotics, such as GOS and XOS, also permits rapid fermentation.  This can lead to increased levels of gas and pressure in the gut.

However, not all prebiotics are quickly fermented.  Digestion resistant starch, for example, is composed of massive chains, and these molecules are processed and fermented much more slowly.  While they do release gas, they tend to do so over a longer time and produce less pressure against the gut lining.

But even people supplementing with digestion resistant starch sometimes complain of bloating and discomfort, especially when first trying the product or when increasing the dose.  This is often the case with other prebiotics.  However, this problem is generally temporary and is likely related to the microbiome adapting to increased prebiotic levels:  The bacteria that can use the prebiotic more efficiently (and produce less problematic gas as a side effect) increase in numbers over time as the amount of prebiotic in the diet increases.

Finally, while gas causing bloating and pain is a medical concern, especially for those with chronic bowel issues, the odour of the gas is more of a social problem.  Most types of gas produced by bacterial fermentation have no smell but some compounds, such as hydrogen sulfide and other sulfur-containing molecules, carry an unpleasant odour.  Remarkably, prebiotic supplementation can trigger changes in the gut microbiome and decrease the production of noxious gases.  This suggests that individuals with problematic gut microbiomes can address the issue of gas odour through prebiotic supplementation.  I have addressed how the gut microbiome varies from person to person and how this presents an intriguing opportunity for personalized medicine in another post.

In conclusion, gas is the sign of an actively feeding microbiome.  But you can help to control this activity through diet and prebiotic supplementation.  Everyone’s microbiome is different, so experiment to find the right balance.  Just remember that you’re feeding a complex ecosystem – and that the balance you seek takes time find.