Kale: Nutrients, What Research Shows and Safety

Kale (Brassica oleracea var. sabellica) is a leafy vegetable in the cabbage family (Brassicaceae). The leaves are eaten, raw or cooked. People eat it for its vitamins, fiber and plant pigments. This page keeps kale as a food apart from supplements, powders and extracts, and apart from hype. It is information, not a prescription.

Read this first

• Warfarin (a blood thinner): kale is very high in vitamin K, which affects how warfarin works. The advice from MedlinePlus is to eat a steady amount of vitamin K foods from week to week and to talk to your doctor before changing your diet. [2] Do not start or stop eating kale in large amounts without telling your clinic.
• Thyroid: kale contains goitrogens (compounds that can interfere with the thyroid). The Linus Pauling Institute says these are not of clinical importance unless eaten in large amounts or when iodine is also low. [3]
• Allergy to any food is possible. For trouble breathing or swelling of the face or throat, call 911. We did not find a source we could read on kale allergy specifically.
• Evidence in one line: kale is a nutritious vegetable. Studies of leafy greens as a group link them to lower heart disease risk, but those are observational; trials of kale itself are few and small.

What is in it (USDA values)

Source: USDA FoodData Central, "Kale, raw" (SR Legacy, ID 168421). [1] The per-cup column is our arithmetic from the USDA values and the USDA portion of 1 cup = 21 g. % Daily Value uses the FDA 2016 Daily Values; values vary with variety, ripeness and storage.

USDA also lists lutein plus zeaxanthin (6,261 µg per 100 g) and beta-carotene (2,873 µg per 100 g), plant pigments of the carotenoid family. [1] Kale is also named by the Linus Pauling Institute as a source of glucosinolates (about 67 mg per cup of chopped raw kale), sulfur-containing compounds that are turned into isothiocyanates when the leaf is chopped or chewed. [4] Most of the evidence on these compounds comes from the lab and from animals; the human studies below do not show that they prevent disease.

What research says

Eaten as food versus taken as a supplement or extract. The diabetes trial used a freeze-dried kale bar and the hormone trial used capsules. Both are processed products at amounts well beyond a typical serving, so their results do not transfer to eating kale in meals. We did not find a source we could read on kale powders or juices, so we make no claim about them.

How much is a normal serving

• USDA lists 1 cup of raw kale as 21 g. [1] That is a household portion in a nutrient database, not an advised amount.
• The NHS counts 80 g of fresh, canned or frozen vegetables as one portion of its 5 A DAY (reviewed June 2026), and the WHO recommends at least 400 g of fruits and vegetables a day for people older than 10 (fact sheet, January 2026). [11][12] These are public health figures, not advice for you.
• A laboratory comparison of eight vegetables (it did not include kale) found frozen produce comparable to fresh for vitamins overall, with beta-carotene lower in some items. [13] We did not find kale-specific data on cooking losses, so we make no claim.
• USDA values above are for raw kale. Cooked kale is a different food in the database.

Safety, allergy and medicines

• Warfarin and vitamin K: one raw cup supplies about two thirds of the Daily Value for vitamin K (USDA). [1] MedlinePlus says vitamin K foods can change how warfarin works, advises consistent amounts week to week, advises against eating large amounts of leafy green vegetables, and says to talk to your doctor before changing diet. [2] Kale is not named on that page; it is a leafy green. The aim is steadiness and telling your anticoagulation clinic, not necessarily avoiding it. Other blood thinners work differently; ask your pharmacist. See foods and medicine interactions.
• Thyroid (goitrogens): the Linus Pauling Institute lists kale among cruciferous vegetables that contain goitrogens and says they are not of clinical importance unless eaten in large amounts or when iodine deficiency is also present. [3] It gives no amounts. People with a thyroid condition or on thyroid medicine should ask their clinician. See thyroid conditions and iodine.
• Kidney stones (oxalate): the NIDDK avoid-list for calcium oxalate stones names spinach, rhubarb, nuts, peanuts and wheat bran, not kale (page last reviewed May 2017). [15] A small study measured very little oxalic acid in kale. [8] Anyone with a history of stones should still ask a clinician what to eat.
• Large amounts: the trials above gave processed kale. Whether regularly eating very large amounts of kale is harmless was not covered by sources we read. Proper evidence not found.
• Allergy: proper evidence on kale allergy not found in the sources we read. Stop eating it and seek care after any reaction; for breathing trouble or swelling of the face or throat call 911.
• Pesticide residues: we could not read a source (EWG and USDA Pesticide Data Program pages were not reachable), so we make no statement on kale residues. Washing is covered below.
• Pregnancy: proper evidence on kale in pregnancy not found in the sources we read; ask your clinician or midwife.

Choosing, storing and food safety

• Choose leaves that are not bruised or damaged; for bagged greens, pick packs that are refrigerated or on ice. [14]
• Rinse all produce under running water, including bagged greens not labeled pre-washed. Do not use soap, detergent or produce wash. Washing reduces bacteria but does not remove them all. [14]
• Refrigerate pre-cut or packaged produce, at 40°F or below. [14]
• Leafy greens were linked to E. coli contamination in one review; see leafy greens. [10]

Related pages

• Cruciferous vegetables
• Leafy greens
• Foods and medicine interactions: vitamin K, grapefruit and others
• Sulforaphane and glucosinolates in cruciferous vegetables
• Thyroid conditions and iodine, biotin and herbal products
• Fresh, frozen and canned produce