Registered nutritionist Adrienne Benjamin sheds light on why smelly wind occurs, which foods to avoid, and when it’s important to consult a GP, in recognition of IBS Awareness Month.
Adrienne explains that most intestinal gas is actually odourless. The gases we commonly pass—including nitrogen, hydrogen, carbon dioxide, methane, and oxygen—do not have a smell. The offensive odour comes from trace gases produced by gut bacteria during the fermentation of undigested foods, particularly sulphur-containing compounds such as hydrogen sulphide.
Contrary to popular belief, vegetables aren’t always to blame for smelly wind. While some vegetables do have higher sulphur levels, animal products like meat, dairy, and eggs contain more sulphur and are often more responsible for foul-smelling gas. The smell depends not on the quantity of gas produced but on the type of fermentation and the specific bacteria involved.
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Increasing fibre intake—especially fermentable fibres found in legumes and certain fruits and vegetables—feeds gut bacteria. Since this fibre isn’t digested in the small intestine, it reaches the colon where microbes ferment it, producing gas. Suddenly boosting fibre can lead to increased bloating and stinkier wind within 24 to 72 hours as the gut microbiome adapts. Gradually increasing fibre allows for easier adjustment and greater tolerance.
Each person’s gut microbiome is unique, influencing their reaction to fibre. Regular high-fibre consumers generally develop a diverse bacterial community that ferments fibre more efficiently, causing less discomfort.
Sulphur-rich foods such as eggs, garlic, onions, cruciferous vegetables (broccoli, cabbage), and red meat increase sulphur gas production by bacteria, resulting in the notorious “rotten egg” smell. These effects typically take 6 to 24 hours to manifest as food moves into the large intestine. In individuals with faster digestion or sensitivities, symptoms can appear sooner.
People with lactose intolerance may experience symptoms within 30 minutes to two hours of consuming dairy, as lactose isn’t properly digested due to low lactase enzyme levels. Undigested lactose reaches the colon, where bacteria ferment it, producing gas, bloating, and sometimes diarrhea. A hydrogen breath test can diagnose lactose malabsorption by measuring hydrogen in the breath after lactose intake. An elimination diet may also be used to confirm sensitivities.
Unexpected culprits include certain sweeteners, particularly sugar alcohols like sorbitol, xylitol, and maltitol, which aren’t absorbed well in the small intestine and ferment in the colon, increasing gas. Some artificial sweeteners can alter gut bacteria, worsening symptoms for those with sensitive digestive systems or IBS.
Foods high in fermentable carbohydrates, known as FODMAPs, can cause gas production lasting 24 to 48 hours depending on digestion speed. Additionally, large or fatty meals slow digestion and prolong fermentation, meaning symptoms may reflect food eaten the previous day rather than recent meals.
Adrienne emphasizes that passing gas is normal—gut bacteria produce 2 to 4 litres of gas daily, and most people pass wind 10 to 20 times per day. This may increase with age due to reduced digestive function and muscle control. However, medical advice should be sought if gas is accompanied by persistent abdominal pain, unexplained weight loss, changed bowel habits, blood in stool, severe bloating, or worsening symptoms.
If gas becomes frequent or disruptive, consult a GP to rule out conditions like lactose intolerance, coeliac disease, or IBS. Dietary adjustments, mindful eating, reducing processed foods, managing stress, and supporting gut health can help alleviate symptoms.