This article was written by Viome, the most comprehensive gut microbiome test on the market.

You probably feel pretty good about yourself every time you load up your smoothies, salads, and grain bowls with spinach. (Us too.) After all, it’s a superfood, so it’s gotta be healthy. But the thing about spinach—and many other “healthy” plant-based foods—is they contain a little thing called oxalates, which can cause kidney stones or keep you from absorbing certain nutrients.

What even are oxalates?

Oxalates are naturally occurring salts produced by the body and most plants. Though we don’t know for sure, many think they’re what give plants their bitter taste, probably to protect them from predators like, well, us. Amounts vary, but they’re found in pretty much every plant-based food, from rhubarb and beets to nuts and dark, leafy greens like spinach. Your body can also convert vitamin C into oxalates, which means fruits like berries and kiwis, which contain both oxalates and vitamin C, can be double trouble.

Here’s the thing: Oxalates in and of themselves aren’t bad. For most people, oxalates from food join up with minerals like calcium and iron and exit your body when you go to the bathroom—or they get broken down by bacteria in the gut (more on that later).

When oxalates become a problem

If you’d actually heard the word oxalate before today, it was probably in the context of kidney stones. That’s because calcium oxalate stones are the most common type—and they’re just what they sound like: calcium + our good friend oxalate. The two should be able to hang out in the kidneys together, but when there’s too much oxalate or too little liquid in your urine, they combine to create crystals, which can stick together to become kidney stones (ouch!).

Another problem with oxalates is that when they latch onto minerals, they can keep you from reaping the benefits of those nutrients. For instance, even though spinach is high in calcium, its oxalates prevent some of the calcium from being absorbed (though it’s worth noting that if you were to drink a glass of milk with your spinach, you’d still get all the calcium from the milk).

The good news is some oxalates are broken down by bacteria in the gut. The bad? Not everyone has the right bacteria for the job. Perhaps the most well-researched one is Oxalobacter formigenes. Though some members of Lactobacillus and Bifidobacterium species may play a role as well, O. formigenes is known to metabolize oxalate, and studies suggest it can help reduce the risk of kidney stones. But as with most bacteria, antibiotics can kill off these good guys, potentially leaving your gut defenseless.

Viome, the most comprehensive gut microbiome test on the market, analyzed tens of thousands of people and discovered that about 30 percent of them lacked the microbes necessary to metabolize oxalates properly. In other words, “healthy” foods like spinach may not actually be healthy for them.

So… I should stop eating spinach?

Not necessarily. Many foods that contain oxalates are good for you and packed with nutrients. But it’s true some people are more sensitive to oxalates. For example, if you have a digestive issue like inflammatory bowel disease or have had gastric bypass surgery, oxalates are more likely to build up in your kidneys. Viome has also found that a low-oxalate diet may help people with digestive issues.

The fact is everyone is different. If you really want to know what your body can handle, consider Viome. Viome is the only gut microbiome test able to analyze your gut at the molecular level to see if your microbes actually metabolize the foods you’re eating. This is because Viome uses metatranscriptomic sequencing technology—a technical way of saying it has the power to see not only your microbes but also what they are metabolizing, producing, and using.

After Viome finds out what’s going on in your gut, its expertly trained AI machine crunches the numbers and gives you precise, individualized results. In other words, you’ll find out whether spinach is actually healthy for you.

Viome’s mission is to put your health back into your own hands by providing personalized dietary recommendations that empower you and restore your body to its innate purpose.

For a super-limited time, Greatist readers can get $100 off a Viome kit when they use code GREATIST12 at checkout.*

This is part of Greatist’s series with Viome and was reviewed by the Viome experts listed below. Stay tuned to find out what else Viome can show you about your gut and learn more about Viome here.

*Greatist receives an affiliate fee on purchases made. Offer valid through October 31, 2018, 11:59 p.m. PT.