Kale
Kale (Brassica oleracea var. sabellica; in Hebrew, “קייל”) is a leafy vegetable from the cruciferous family — the same family that includes broccoli, cauliflower and cabbage. Botanically, it is a cabbage that does not form a head. It is one of the most nutrient-dense vegetables relative to its calories, and its cold hardiness allows it to be grown through most of the year.
100 grams of raw kale provide only about 49 calories, but also about 390-700 mcg of vitamin K (hundreds of percent of the daily intake), about 93-120 mg of vitamin C (more than an orange), about 500 mcg retinol equivalents of vitamin A and about 150 mg of calcium.
The advantage that sets it apart from spinach: oxalates
This is the most important point for understanding where kale fits in, and it is almost always missing from comparisons.
Spinach is considered a “plant source of calcium” — but that is misleading. Spinach is very rich in oxalates, acids that bind calcium in the gut to form an insoluble complex. The result: calcium absorption from spinach is only about 5%.
Kale, by contrast, has low oxalate content. Calcium absorption from it is estimated at about 50% — higher availability even than that of milk (about 32%). In other words, although spinach contains more calcium on paper, kale actually delivers far more available calcium.
This also explains why the common warnings about oxalates and kidney stones apply to spinach and chard — but less so to kale.
Research-based health benefits
An exceptional amount of vitamin K: A single serving provides hundreds of percent of the daily intake. Vitamin K1 is essential for blood clotting and for activating osteocalcin — the protein that fixes calcium in bone tissue. The combination of vitamin K with available calcium makes kale one of the best plant foods for bone health.
Sulforaphane and glucosinolates: Like all crucifers, kale contains glucosinolates. When the plant tissue is damaged — by cutting or chewing — an enzyme called myrosinase converts them into isothiocyanates, chief among them sulforaphane. Sulforaphane is one of the most studied plant compounds in the context of activating the Nrf2 pathway, which switches on phase 2 detoxification enzymes. An important practical point: myrosinase is heat-sensitive and is deactivated by prolonged cooking. Brief steaming (3-4 minutes) preserves it; long boiling destroys it almost completely.
Lutein and zeaxanthin: Carotenoids that accumulate in the macula of the retina and filter harmful blue light. Kale is one of the densest sources of them, and studies link their intake to a reduced risk of age-related macular degeneration (AMD).
Vitamin C: Raw kale contains more vitamin C than an orange per unit of weight. The vitamin is sensitive to heat and water — another reason to prefer it raw or briefly cooked.
Fiber and nutrient density: About 4 grams of fiber per 100 grams, at only 49 calories. This is one of the best ratios of nutrients to calories in the entire diet.
Polyphenols: Kaempferol and quercetin, which are linked to anti-inflammatory activity.
Downsides and health considerations
Interaction with anticoagulants — the most significant consideration: The high vitamin K content poses a real problem for people taking warfarin (Coumadin), whose action is based precisely on inhibiting vitamin K. The rule is not “avoid” but “keep intake consistent” — a sudden change in amount, in either direction, is what destabilizes INR control. Coordinate with your physician. Note: modern anticoagulants (DOACs such as apixaban and rivaroxaban) are not affected by vitamin K.
Goitrogens and the thyroid gland: Glucosinolates in crucifers may compete with iodine for uptake into the thyroid. In practice, the risk is relevant only with extreme intake (a large amount of raw kale every day, for example in smoothies) combined with iodine deficiency. For anyone who gets enough iodine and eats kale in reasonable amounts, there is no concern. Cooking significantly reduces the goitrogenic activity.
Insoluble fiber and digestion: A large amount of raw kale can cause gas and bloating, especially in people with a sensitive digestive system. Light steaming softens the fibers and helps a great deal.
Thallium and metals: Kale, as a cruciferous vegetable, efficiently absorbs minerals from the soil — and so may also accumulate heavy metals in contaminated soils. Reports about this are mostly exaggerated and not based on substantial research, but varying your sources is a good rule anyway.
Pesticides: Kale consistently appears on lists of the vegetables with the highest pesticide residues. Thorough washing or choosing organic is especially relevant here.
Nutrition facts per 100 grams (raw)
| Nutrient | Amount |
|---|---|
| Calories | 49 |
| Carbohydrates | 8.8 g |
| Dietary fiber | 3.6 g |
| Protein | 4.3 g |
| Vitamin K | 390-700 mcg |
| Vitamin C | 93-120 mg |
| Vitamin A (retinol equivalents) | 500 mcg |
| Calcium | 150 mg (~50% absorption) |
| Lutein + zeaxanthin | 8,200 mcg |
Practical preparation
Massaging the kale — the key trick for salad: Raw kale leaves are tough and fibrous. Massaging the leaves by hand with a little olive oil and salt for 2-3 minutes mechanically breaks down the cell walls, softens the texture significantly and reduces the bitterness. It is the difference between a kale salad that is hard to eat and a successful one.
Remove the central stem: It is tough and bitter. Fold the leaf lengthwise and cut along the stem.
Cook briefly, not long: Steaming for 3-4 minutes preserves the myrosinase enzyme (needed to form sulforaphane) and most of the vitamin C. Prolonged boiling leaches water-soluble vitamins into the water and deactivates the enzyme.
Combine with fat: Vitamins A and K, as well as lutein, are fat-soluble. A kale salad without oil, tahini or avocado wastes a good part of its value.
Kale chips: Bake briefly at 150°C (300°F) with a little oil. High temperatures burn the thin leaves and damage the nutrients.
Summary
Kale justifies a good part of its reputation — not because of vague “energy” but because of a measurable combination: an exceptional amount of vitamin K, calcium with about 50% absorption availability (higher than milk’s), lutein for the eyes, and sulforaphane. Its main advantage over spinach is precisely what it does not contain — oxalates. Two real caveats: coordinating with a doctor for people taking warfarin, and understanding that long cooking cancels out a good part of the advantage.
Frequently asked questions
Question: Kale or spinach — which is better? Answer: It depends on the goal, but for calcium kale is clearly better. Spinach is rich in oxalates that bind calcium, and absorption from it is only about 5%; from low-oxalate kale, absorption is about 50%. Spinach is richer in iron and folate.
Question: Can I eat kale if I take Coumadin? Answer: Yes, but you must keep your intake consistent and coordinate with your doctor. The problem is not vitamin K itself but sudden changes in its amount, which destabilize INR control. People taking modern anticoagulants (DOACs) are not affected.
Question: Does kale harm the thyroid gland? Answer: In reasonable amounts — no. Its glucosinolates may compete with iodine, but this is relevant only with extreme intake of raw kale combined with iodine deficiency. Cooking significantly reduces the effect.
Question: Is raw or cooked kale better? Answer: Brief steaming (3-4 minutes) is the best balance — it preserves the myrosinase enzyme and most of the vitamin C, and softens the fibers for easier digestion. Long cooking harms both; fully raw is harder to digest.
Question: How do you make a kale salad less tough? Answer: Massage the leaves by hand with a little olive oil and salt for 2-3 minutes. The massage breaks down the cell walls, significantly softens the texture and reduces bitterness. It is also worth removing the tough central stem.
Question: How much vitamin C is in kale? Answer: About 93-120 mg per 100 grams — more than in an orange. The vitamin is sensitive to heat and water, so eating it raw or cooking it briefly preserves it much better than boiling.