Why is my gas so smelly? Gender, diet and plane rides can play a role.

Q: I’ve heard your diet can affect how often you pass gas and how it smells. What else causes foul-smelling flatulence? And how much farting is normal?

A: Foul-smelling flatus is largely a function of what we eat and how it is processed by our body and microbiome. For instance, a 1971 study published in Gut measured that those on a bean-containing high-fiber diet produced gas at a rate nearly double that of those on a fiber-restricted diet — a mean of 49.4 milliliters per hour vs. 26.7 milliliters per hour, to be precise.

If you think you (or a dear friend) pass gas more frequently than everyone else, know that we do it 10 times a day on average — but up to 20 times a day would be in the range of normal. And scientists have observed that we pass gas at a similar rate whether we’re old or young.

Ninety-nine percent of intestinal gas is made up of nitrogen, oxygen, hydrogen, carbon dioxide and methane. These gases are odorless; it’s the 1% that contains sulfur, such as hydrogen sulfide, that ruins it for everyone else. When the bacteria in our colons metabolize our food, they can produce those sulfur-containing gases.

If you need a simple solution in a pinch — an upcoming work conference or a date in cozy quarters with someone special — take bismuth subsalicylate (Pepto-Bismol). Not only has it been shown to neutralize more than 95% of sulfide gases in the gut and to reduce symptoms of flatulence, but it can also prevent traveler’s diarrhea. There is some concern about salicylate toxicity if you use it regularly long term, but if you need it for a day or two to ease your social discomfort, by all means, take 524 milligrams by mouth four times a day beforehand.

What foods can cause excess gas?

Perhaps friends and foes have hinted you need a longer-term solution. In that case, there may be room for optimization in your diet.

Certain starches are not digested well by the human body and travel to the colon, where they are fermented and risk producing malodorous gas. About a third of U.S. adults cannot adequately absorb lactose, a disaccharide found in milk products.

Other culprits are foods in the raffinose family, including beans, cruciferous vegetables like broccoli and Brussels sprouts, onions, mushrooms, and even garlic. If you notice these foods triggering unpleasant flatulence, you’ll probably respond well to taking an over-the-counter medication called alpha-galactosidase (brand name Beano). It contains the enzyme humans don’t otherwise produce that can break these foods down. It may also be helpful to try a special diet, such as the low FODMAP diet with a registered dietitian for a brief period.

Do men pass more gas than women?

Flatulence discriminates against no one. The long-held stereotype that men produce more objectionable flatus than women has never been proved by science. In fact, one study found that compared with men, women have higher concentrations of hydrogen sulfide associated with fouler odor.

However, that study also noted that men produce a greater volume per passage of flatus. Because hydrogen sulfide’s ability to stimulate the nose is more dependent on its volume rather than its concentration, these factors balanced out between the sexes — the flatus of men and women was equally offensive.

Why do so many people pass gas on planes?

If you’ve been on a flight recently, you’ve probably noticed an increase in gas production — whether personally or because you’ve been seated next to a guilty party.

This behavior is attributable to the laws of elementary physics. With increasing altitude, air pressure — including intestinal air pressure — falls. According to the ideal gas law, the volume of your intestinal gas will expand. Since a small muscle called the ileocecal valve prevents that gas from traveling backward from your colon into your small intestine, it has little choice but to move forward and outward (it also probably doesn’t help that 50% of cabin air on airplanes is recirculated).

Can flatulence be a sign of a health condition?

Some may have observed that their noxious gas production skyrockets when they’re constipated and need to have a bowel movement. Think about it: When you’re constipated, your stool is just sitting there in your colon getting fermented into oblivion. The closer that stool is to the exit hatch, the more quickly the resulting gas will escape, much to the horror of your co-workers at lunch. The urge to go is also associated with contractions of the gut, which accelerates gas forward.

Besides constipation, if you have a disease that affects how well your small bowel absorbs nutrients — such as inflammatory bowel disease, pancreatic insufficiency or celiac disease — you’ll also have increased gas because more of that partially digested food will land in your colon for your microbiome to feast upon.

So while we work to normalize this human bodily process, don’t let embarrassment keep you from bringing up flatulence-related concerns with your provider, who can help ensure your symptoms are not a sign of something more serious.

What I want my patients to know

There are other purported treatments out there for intestinal gas like probiotics — which small studies have shown may help with the discomfort, though not odor per se, of flatulence mainly in the setting of irritable bowel syndrome. Researchers have also studied briefs made from carbon fiber fabric that help absorb bad smells.

But most of the time when someone comes to me concerned about what they think is the worst flatulence in the world, when they actually spend a day counting how often it happens — we discover it’s no more frequent than anyone else’s.