Why Protein Powders Like Whey and Soy May Upset Your Stomach

Protein powder can be a game changer for people striving to hit their macros, improve their performance, or build muscle. For one thing, it's so simple to include in your diet, whether you add a scoop to your smoothie, incorporate it into your pancake mix, or simply stir some into your water. But for all the benefits of protein powder, some people may experience some unsavory side effects when taking it. Whether you're ready to try protein powder for the first time and don't know what to expect, or you're already using it and wondering if it could be to blame for your bloating or breakouts, this guide can help you find the protein powder that will work best for you.

Whey protein is a protein found in dairy, and it contains all of the essential amino acids. Research shows that whey promotes muscle growth and may help reduce inflammation. However, some people can experience side effects while using whey protein powder, including acne. And for those with lactose intolerance, bloating and nausea may occur, as certain whey protein powders contain the lactose sugar that can exacerbate your symptoms. Sticking to a whey isolate will help you avoid ingesting unwanted lactose, and hopefully make eating or drinking whey protein more tolerable.

Casein protein is another protein found in dairy, and it also contains all of the essential amino acids. It's known to help reduce muscle breakdown and support muscle growth. However, because casein protein has a slow release — meaning, it takes longer to digest — some people may experience bloating, abdominal pain, or constipation after ingesting it. If someone has an intolerance to casein, they may experience more serious symptoms, including hives, rashes, wheezing, or even anaphylaxis.

Egg protein is a popular choice because of its exceptional protein quality and digestibility. According to Vivian Schemper, RDN, LDN, creator of The Healthy Plate RD, "few people have reported mild bloating and gas after consuming egg protein powder, but the reasons are not fully understood." Schemper also notes that egg protein powder may not be as filling as others on this list. In fact, one study found that it did not reduce appetite as effectively as casein or pea protein powder — something to consider if you're looking for a protein powder with some staying power.

Soy has been one of the most popular plant-based protein powders for years now. However, "[it] may cause mild digestive distress (bloating, gas, diarrhea) in individuals who are sensitive or intolerant to soy," Schemper tells POPSUGAR. She adds that one downside of soy protein powder is that it may not be as effective at making protein, possibly due to the slower digestion rate of soy or the low levels of leucine in soy protein powder, an amino acid that's critical for muscle building. Schemper also cautions that people taking thyroid medications like levothyroxine (synthroid) should talk to their doctors before trying soy protein powder, as studies have shown that soy protein supplements can interfere with the absorption of thyroid medications.

Pea protein powder is a good, plant-based alternative for those avoiding animal or soy products. "Pea protein powder is generally well-tolerated and has minimal side effects reported," Schemper explains, adding that it contains "very little fiber (unlike whole peas), so it generally doesn't cause digestive issues like gas or bloating." One word of caution, though: "Some pea protein powders can contain high sodium levels, which could be problematic for folks who need to watch their salt intake," Schemper says.