It’s likely that the type of protein you’re choosing could be contributing to your workout farts—and to the pretty pungent odor of them, too. That’s because some popular high-protein foods (like meat and eggs) are rich in sulfur, which can turn into stinky gasses when it ferments in the gut. Or, you may be eating another ingredient along with your protein source that your body has trouble fully digesting, leading to extra fermentation and especially rancid rips. Here are some common ones to keep in mind.
Lactose
A lot of people turn to dairy products—whether we’re talking whey or casein powders or whole foods like cottage cheese—as a convenient way to take in more protein. But they’re also some of the more common causes of room-clearing farts, thanks to the sugar lactose.
That’s why a lot of people equate drinking protein shakes with intestinal gas, Dr. Applegate says: “Some whey protein contains lactose, and that can be difficult to digest for someone who is intolerant.” If that’s you, the milk sugar can sit in your gut where bacteria feast on it, causing excess, smelly gas and bloating. And depending on the brand and how it’s made, some whey protein powders may end up with more lactose than others.
A lot of people have trouble digesting and absorbing lactose; some estimates suggest up to 68% of the world’s population. And while lactose intolerance can cause lots of digestive issues in some people, others may not even realize they have a problem absorbing it—until, say, they start slugging back more whey shakes and notice they’re bloated, farty, and gassy more often than before, sports nutritionist Marie Spano, MS, RD, CSCS, tells SELF.
Sugar alcohols
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Sugar alcohols—chemicals that are added to foods to make them taste sweet without adding as many calories as actual sugar or leading to blood sugar spikes—are a common ingredient in a lot of protein bars, Spano says. The problem is that our bodies aren’t able to fully digest and absorb them, Cummings explains: They end up sitting around in your gut, feeding bacteria and producing more gas and bloating (and potentially even triggering diarrhea).
“Maltitol, sorbitol, mannitol, and xylitol are sugar alcohols that can increase gas and bloating, and some people are more sensitive to it than others,” Spano says. Even relatively small amounts—like just one protein bar—can cause problems for some people, she says.
Fiber
A lot of people turn to fiber-rich grains and legumes as good sources of plant-based protein, but they come with a potentially farty downside: Too much fiber can cause gas, Cummings says. This can be especially true if you’re sensitive to FODMAPs, a type of carbohydrate that tends to linger undigested in the gut, where it ferments and leads to gas and bloating. Beans and lentils, common in some vegan protein bars, often contain these highly fermentable carbs.
Some protein bars also contain added fiber—often listed as inulin or chicory root in the ingredient list—that can cause stomach issues for some people. This is actually considered a prebiotic, which means it ferments in the gut and feeds healthy gut bacteria. But too much in one sitting can be a problem. “The bacteria in our gut munch on it, and once they break it down, the byproduct is gas,” Spano says. Some protein bars contain nearly half the recommended amount of daily fiber in just one serving, so it can be easy to eat too much without even realizing it. (A lot also contain both added fiber and sugar alcohols, making for a particularly putrid one-two punch.)
