Q: I’ve heard kale has a lot of health benefits. But every time I eat it, I feel sick to my stomach. What’s going on? Is there a way to fix this?
A: Kale is a rock star nutrient-dense food, containing magnesium, calcium, potassium and vitamins A, C and K, to name a few. But it’s not for everybody. Raw kale can wreak havoc on our guts. And people on certain blood thinners need to avoid eating too much kale. (More on that later.)
Let’s start with digestive issues. Kale is loaded with soluble fiber that can cause nausea and insoluble fiber that can cause diarrhea. It also belongs to a complex sugar family — called the raffinose family of oligosaccharides (RFOs) — that can cause bloating.
If you and kale don’t get on together, try this to make it easier to digest:
- Massage it: I know this might sound bizarre, but stick with me. If you’re serving a kale salad on Thanksgiving, start by discarding the stems, which are tough to eat. Slice the leaves, add them to a bowl with a dash of your favorite dressing, and use your hands to gently massage them together. This helps tenderize the fibers and makes it easier for your stomach to process.
- Cook it: Cooking kale helps ease its bitterness and softens tough fibers that can hamper digestion. You’ll still get the benefits of the fiber — after all, plant-based fibers don’t suddenly disintegrate even if you boil them (you’d have to blast them at 300 degrees Celsius or higher to do that). But cooking can compromise some of the heat-sensitive nutrients: One study showed that boiling kale decreased vitamin C levels by about 89 percent. (That won’t stop me from eating my favorite soup, caldo verde, though.)
- Blend it: It’s a myth that you “lose” all the benefits of fiber when you cook, chop or blend veggies like kale before consumption (juicing is a different story). When you make a smoothie, you’re just blending the fibers down into smaller pieces that pass more easily through your stomach — you’re not destroying the cell walls. Try blending kale with frozen banana in a smoothie or making a kale pesto.
Foods that are high in fiber are critical for our health — they lower the risk of diabetes and heart disease — so don’t drop them entirely. Instead, try exploring with foods that have different ratios of insoluble to soluble fiber to see what works best with you.
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Why kale might upset your stomach
Fiber: Fiber from vegetables is wonderful, lowering the risk of colorectal cancer, improving accidental bowel leakage and helping control blood sugars. It can be divided into soluble and insoluble forms, with many plant foods containing a mixture of both. Half a cup of cooked kale has about 0.7 grams of soluble fiber and 1.8 grams of insoluble fiber, a double-whammy.
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Humans can’t fully digest either form of fiber, and each behaves distinctly. Soluble fiber forms a viscous gel with water. That gel takes much longer for your stomach to empty out; while it sits in your stomach long after the meal is over, you can feel a heaviness, nausea and abdominal discomfort.
Another key difference is that soluble fiber is fermented by the bacteria living in your guts. This means that soluble fiber amplifies gas production through fermentation, leaving you the parting gifts of bloating and flatulence.
On the other hand, insoluble fiber tends to move waste through your system and is less rapidly fermentable by your microbiome. While beneficial for those suffering from constipation, insoluble fiber can soften stool such that, if you go to town on kale chips, diarrhea may be your prize.
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RFOs: RFOs are found in several plants — notoriously, cruciferous vegetables like Brussels sprouts and broccoli. Unlike animals with ruminant stomachs like cows — which have four compartments for this purpose — the human digestive tract wasn’t designed to absorb RFOs. That’s actually a good thing. In us, they travel relatively intact all the way to the colon, where they’re fermented and positively impact our gut’s microbiome. The downside of that, though, is they produce extra gas.
When to cut back on eating kale
Gastrointestinal symptoms aside, there are a few other situations when you may need to rethink the amount of kale you’re eating:
- On certain blood thinners: Because kale contains a high amount of vitamin K, people who take the blood thinner warfarin, which blocks vitamin K, must be careful not to exceed the limit recommended by their physician. I treated a patient a few years ago with a life-threatening bleed who was on warfarin. My team discovered he had recently started eating large kale salads every day on a health tip from his granddaughter. It was well-intentioned, but he hadn’t been aware of this important interaction.
- When breastfeeding: Ingesting very high amounts of foods known as goitrogens, including kale, can interfere with the production of thyroid hormones, potentially leading to thyroid issues in the infant. So how much is too much? One case published in the New England Journal of Medicine reported severe thyroid dysfunction in an adult after eating about 2.2 pounds of the goitrogen bok choy every day for months. As long as you maintain a varied diet when breastfeeding or otherwise, you’re probably good.
- Kale allergy: If you experience signs like wheezing or hives after eating kale, you may have a rare kale allergy and should discuss this with an allergist.
What I want my patients to know
The recommended daily fiber intake for adults is 22-34 grams. The softer, more frequent stools that come with a healthy higher-fiber diet aren’t necessarily abnormal at all. If you were an every-other-day, wipeless wonder kind of person, and now that you’ve discovered kale smoothies, you’re more of a three-times-a-day guy who is increasingly bidet-curious — as long as your bowel movements are not causing you social distress, by all means, stay the course.
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