Curly Kale: What It Actually Does in Your Body

Curly kale provides sulforaphane, a compound that activates the body's own detoxification enzymes, along with calcium your body absorbs more efficiently than from milk. How you prepare it determines how much of that reaches your cells.

Kale belongs to the Brassica family, the same group as broccoli and cabbage. What sets it apart is its concentration of glucosinolates, sulfur-containing compounds stored in the plant's cells.

When you chew, chop, or blend kale, you break those cells open. That releases an enzyme called myrosinase. Myrosinase converts glucosinolates into their active forms, most notably sulforaphane.

Sulforaphane is what researchers call a "phase II enzyme inducer." In plain language: it signals your liver and other tissues to produce more of the enzymes that help neutralize and clear potentially harmful compounds from your body.¹

Here's what makes kale stand out from other crucifers. One 2024 analysis in Food Science & Nutrition found that kale contains roughly 7 to 10 times the amount of sulforaphane found in broccoli.² That's a meaningful difference if you're choosing greens for their protective compounds.

A constraint worth noting: most of this research measures isolated compounds in lab or animal settings. Large-scale human trials directly linking kale consumption to disease prevention are still limited. The mechanism is well established. The clinical dose-response in humans is still being refined.

The way you handle kale changes what your body gets from it.

Myrosinase, the enzyme that converts glucosinolates into sulforaphane, is heat-sensitive. Boiling kale or cooking it above roughly 140°C (284°F) deactivates myrosinase, which means fewer active compounds make it to your gut.³

Steaming is gentler. Light steaming for three to five minutes preserves more myrosinase activity while still softening the leaves.

Blending raw kale keeps myrosinase fully active. The mechanical disruption actually mimics thorough chewing, breaking open more cell walls and releasing more of the enzyme. That's one reason blended greens can deliver a different nutrient profile than the same greens eaten in a salad where you might not chew every leaf completely.

Juicing is a different story. When you extract the juice and discard the pulp, you lose the fiber matrix along with some of the compounds bound to it. The glucosinolates and their breakdown products are still present in the liquid, but the fiber that slows their transit through your gut is gone.

"Kale contains higher amounts of beta-carotene, lutein, zeaxanthin, glucosinolates, and sulforaphane compared to other cruciferous vegetables including broccoli, cabbage, spinach, and Brussels sprouts." 

- Subedi et al., Food Science & Nutrition, 2024²

One cup of cooked curly kale provides roughly 5 to 6 grams of dietary fiber, a mix of soluble and insoluble types.

The soluble fiber in kale forms a gel-like substance in your digestive tract. This slows the absorption of sugars and may help moderate blood glucose response after a meal. A 2018 meta-analysis found that higher dietary fiber intake was associated with lower risk of developing type 2 diabetes.⁴

The insoluble fiber adds bulk. It supports regular bowel movements and feeds beneficial gut bacteria through fermentation in the colon.

What matters for people choosing between formats: blending preserves all of the fiber. The blender breaks fiber into smaller particles, but every gram stays in the drink. You're still getting the full fiber profile, just in a more immediately accessible form. Juicing removes most of the insoluble fiber with the pulp. That changes how your body processes the sugars and other compounds in the liquid.

This is the practical distinction between a blended green drink and a cold-pressed juice. Same starting ingredient. Different outcome in your gut.

Kale is unusually good at delivering calcium your body can use.

In a landmark 1990 study, Heaney and Weaver measured fractional calcium absorption from kale at approximately 41%, compared to about 32% from milk. The difference was statistically significant.⁵

The reason: kale is low in oxalates. Oxalic acid, found in high concentrations in spinach and Swiss chard, binds to calcium and prevents your body from absorbing it. Kale doesn't have that problem. Its calcium exists largely as soluble organic salts like calcium citrate and malate, which your gut absorbs readily.⁶

One cup of cooked kale provides roughly 177 mg of calcium. That's less than a glass of milk in absolute terms. But because of the higher absorption rate, the amount of calcium your body actually takes in is comparable.

Kale also delivers over 680 micrograms of vitamin K1 per cooked cup, more than seven times the daily recommended intake. Vitamin K1 activates osteocalcin, a protein that helps incorporate calcium into bone tissue. Without adequate vitamin K, calcium may circulate in the blood but never reach the bones where it's needed.
Data from the Nurses' Health Study found that women consuming at least 109 micrograms of vitamin K daily had a 30% lower risk of hip fractures.⁷

One note for people on blood thinners like warfarin: vitamin K affects how warfarin works. If you take warfarin, talk to your doctor before significantly increasing your kale intake. This isn't a reason to avoid kale. It's a reason to be consistent with your intake so your medication dose stays calibrated.

Most people tolerate kale well, but there are a few things worth knowing.

Digestive adjustment. Kale is high in fiber and contains raffinose, a sugar that humans can't fully digest. Your gut bacteria ferment it instead, which can produce gas. If you're not used to eating cruciferous vegetables regularly, starting with smaller amounts and increasing gradually gives your microbiome time to adjust.

Thyroid considerations. Kale contains goitrogens, compounds that can interfere with iodine uptake by the thyroid gland. A 2024 systematic review in Nutrients found that the evidence for brassica vegetables causing thyroid dysfunction in humans is weak, provided iodine intake is adequate.⁸

The documented cases of kale-related thyroid issues involved extreme consumption. One study showed a 25% decrease in radioiodine uptake after participants drank large quantities of kale juice twice daily for seven days, but their actual thyroid hormone levels remained unchanged.⁹

Cooking reduces goitrogenic activity by deactivating myrosinase. If you have a diagnosed thyroid condition, especially hypothyroidism, lightly cooking your kale and ensuring adequate iodine in your diet addresses the concern without giving up the vegetable's benefits.

For most people eating normal amounts, the goitrogen content in kale is not a clinical concern.

Kale is a strong fit for people looking to increase their intake of protective plant compounds, absorbable calcium, and fiber without relying on supplements. Women building or maintaining bone density benefit from the vitamin K and calcium combination. People managing blood sugar benefit from the fiber, particularly when kale is consumed blended rather than juiced.

If you take warfarin or another vitamin K-sensitive medication, consistency matters more than avoidance. Pick an amount of kale you'll eat regularly and stick with it so your doctor can adjust your dose accordingly.

If you have an existing thyroid condition, cook your kale and ensure you're getting enough iodine. That's a modification, not an exclusion.

And if you've tried kale raw and found it too bitter or tough, blending changes the experience entirely. The bitterness softens when combined with fruit or other ingredients, and your body still gets the full fiber and compound profile.

At Sunrise Blends, curly kale is one of the foundational greens in our blends. We use a whole-food blending process, not juicing or powdering, specifically because it preserves the fiber matrix and keeps the cell-disrupted compounds intact. The greens are frozen immediately after blending, which locks in the nutrients and enzymes that degrade with time and heat exposure.

That approach reflects what the research consistently shows: how you process kale matters as much as whether you eat it at all.

Sources cited in this article:

1. Fahey JW, Talalay P. Antioxidant functions of sulforaphane: a potent inducer of Phase II detoxication enzymes. Food and Chemical Toxicology. 1999;37(9-10):973-979.
2. Subedi S, et al. Fermenting kale enhances its functional food properties by increasing accessibility of key phytochemicals and reducing antinutritional factors. Food Science & Nutrition. 2024;12(5):3417-3430.
3. Vermeulen M, et al. Bioavailability and kinetics of sulforaphane in humans after consumption of cooked versus raw broccoli. Journal of Agricultural and Food Chemistry. 2008;56(22):10505-10509.
4. Reynolds AN, et al. Dietary fibre and whole grains in diabetes management: systematic review and meta-analyses. PLOS Medicine. 2020;17(3):e1003053.
5. Heaney RP, Weaver CM. Calcium absorption from kale. American Journal of Clinical Nutrition. 1990;51(4):656-657.
6. Weaver CM, et al. Calcium bioavailability from high oxalate vegetables: Chinese vegetables, sweet potatoes and rhubarb. Journal of Food Science. 1997;62(3):524-525.
   Feskanich D, et al. Vitamin K intake and hip fractures in women: a prospective study. American Journal of Clinical Nutrition. 1999;69(1):74-79.
7. Wichman J, et al. Do Brassica vegetables affect thyroid function? A comprehensive systematic review. Nutrients. 2024;16(8):1164.
8. Bajaj JK, et al. The role of micronutrients in thyroid dysfunction. Sudanese Journal of Paediatrics. 2020;20(1):13-19.