Turn over many a nutrition bar or box of veggie burgers, and you’ll often find soy protein isolate (SPI) featured prominently on the ingredient list.
While there’s disagreement among nutritionists over whether soy is part of a healthy diet (some are concerned about its estrogenic properties but others like it as protein source for those who don’t eat meat), most agree that SPI, its super-processed offspring, should be avoided.
“A big issue with soy is that we’re eating more of it than ever before and in very processed forms like SPI,” says Middleberg Nutrition founder Stephanie Middleberg, MS, RD. So SPI may have started out as a plant, but once it gets to you, it’s far from it.
Here are four reasons nutritionists say you should probably ditch soy protein from your diet:
1. A lot of its nutrients have left the building. “Soybeans are a great quality protein because their amino acid content is similar to that in meat, and they’re a good source of fiber, minerals, and complex carbs,” says Middleberg. But to create SPI, soybeans are chemically engineered to “isolate” their protein, and this process strips out all of the other nutrients the original bean contained.
2. It contains unhealthy additives. Foodtrainers founder Lauren Slatyon, MS, RD, says that the chemical process used to isolate soy protein often leaves behind substances you don’t necessarily want to be eating, like aluminum and hexane. “Think of bathing in toxic bath oil,” Slayton says. “Even once you dry yourself off, some residue remains. Want to eat that residue?” The spray drying method used for soy can also form nitrites, compounds that can form carcinogens in the body, she explains.
3. It’s probably genetically modified. According to the USDA, over 90 percent of the soybeans grown in the U.S. are genetically modified, so most SPI comes from altered beans. “This means soy protein isolate is chemically modified, processed, and filled with pesticides,” says Middleberg.
4. It may upset your stomach. Many people have allergies or intolerances that make it hard to digest soy. But even if you’re not one of them, soy protein isolate may make your stomach rumble, says Slayton. This is because SPI has a higher concentration of trypsin inhibitors, chemicals that reduce available trypsin—an enzyme that helps digest protein—in the body.
So what to do if you’re a soy-loving vegetarian? Skip products with SPI and opt for “natural, whole protein sources like beans, lentils, nuts, and seeds, and organic, non-GMO natural sources of soy like edamame, tofu, and tempeh,” Middleberg suggests.
Slayton also suggests sticking to fermented soy sources, like miso, tempeh, and natto. “Fermentation increases the digestibility of soy, adds good bacteria, and reduces the plant estrogen content in soy foods,” she explains.
And in the end, both nutritionists agree: Like most things, soy is best enjoyed in moderation—and sticking to whole (rather than processed) foods is always a good plan. —Lisa Elaine Held