Kale is the same species as broccoli, cabbage and Brussels sprouts, Brassica oleracea, grown for loose leaves instead of a head. Chinese kale, or gai lan, is a different group within that species. Genetic work places it apart from all the other cultivars, so studies of Chinese kale are not studies of ordinary kale.
Kale is unusually low in oxalate, the compound that locks up calcium in spinach. In an isotope study of 11 women, about 41% of the calcium in kale was absorbed, against about 32% from milk. A 150 g kale meal held 0.01 g of oxalic acid, while the same weight of spinach held 1.27 g.
Per 100 g raw, kale has about 390 µg (micrograms) of vitamin K. That is more than three times the Daily Value (the amount on US food labels). Only 4 of the 96 foods we track have more. It also ranks 3rd of 74 foods for lutein and zeaxanthin (pigments found in the eye), at 6,261 µg.
Your body takes up only part of it: when 7 volunteers ate 400 g of kale with oil, modeling put vitamin K absorption at 4.7%. Two kale scares, thallium and the thyroid, are covered below.
Health benefits of kale
The top findings from 30 studies, strongest evidence first
What the stars mean
- Strong. Several randomized trials, pooled in a meta-analysis, point the same way, or regulators (FDA or EFSA) have approved the claim.
- Moderate. Good randomized trials support it, but there are fewer of them, or their results vary.
- Limited. Small, short or single trials, or mixed results. Promising, but it needs confirmation.
Population and lab studies get no stars: they show a link or a mechanism, not proof.
How we rate evidence →- ★Lower blood sugar in small trialsA small amount of a kale-containing food lowered peak blood sugar from 176 to 163 mg/dL in 42 adults. Kale bars lowered HbA1c, average blood sugar, in 30 people with type 2 diabetes.
- ★Calcium absorbed well, more than from milkIn 11 women, about 41% of the calcium in kale was absorbed, against about 32% from milk. This was a small study of single test meals.
- ★Lower blood pressure from daily kale juiceIn 84 Korean adults with borderline readings, 300 mL of kale juice a day for 6 weeks lowered both blood pressure numbers. No placebo group was described.
- ★Higher HDL and lower LDL from kale juiceIn 32 men with high cholesterol, 150 mL of kale juice a day for 12 weeks raised HDL 27% and lowered LDL 10%. There was no placebo group.
- ★More frequent stools in constipated womenIn a small crossover trial, 4 weeks of kale increased stool frequency in constipated women compared with a control food. It helped most in people who started with smaller stools.
All 30 studies, strongest evidence first
Kale food lowered peak blood sugar from 176 to 163 mg/dL
In this double-blind, placebo-controlled crossover trial, 42 Japanese adults had the food at 7 g or 14 g. Paired with a high-carbohydrate meal, the 7 g dose gave the lower peak. Two-hour blood sugar was also lower (268 and 266 against 284 in the study’s units), and insulin did not differ between the three conditions. No adverse events were seen, and it tested a small amount of a kale-containing food in single meals, not a plate of leaves.
Kale bars lowered HbA1c and insulin resistance in diabetes
This double-blind, placebo-controlled trial gave 30 people with type 2 diabetes three bars a day for 12 weeks. Each bar held 26.25 g of freeze-dried kale, about 341 g of fresh kale a day in total. Body weight and calorie intake also fell compared with placebo, while fasting glucose and LDL (“bad”) cholesterol only trended lower. Blood sugar during the glucose tolerance test did not differ, and this was a small trial with a very large dose.
Freeze-dried kale did not beat freeze-dried peas for blood sugar
This crossover trial had 124 Saudi women with obesity take 3 g sachets of freeze-dried kale or peas three times a day. Women who took kale first lost weight, while HbA1c improved more with peas. Strong carryover between the two phases (effects lasting into the next) limits what can be concluded. The authors call for parallel-design trials.
Kale juice daily lowered blood pressure in 84 people with borderline readings
Korean adults with systolic pressure over 130 or diastolic over 85 mm Hg drank 300 mL of kale juice a day for 6 weeks. Both systolic and diastolic pressure fell, and the abstract describes no placebo group. Pressure fell regardless of their glutathione S-transferase gene variants. The effects on blood sugar and cholesterol depended on those variants, so those findings are less certain.
Kale juice raised HDL 27% and lowered LDL 10% in high-cholesterol men
This before-and-after study gave 150 mL of kale juice a day for 12 weeks to 32 men with total cholesterol above 200 mg/dL. The abstract shows no placebo group. The atherogenic index (a blood-fat risk score) fell 24.2%, and the antioxidant enzyme glutathione peroxidase rose. The results differed with smoking status.
Four weeks of kale increased stool frequency in constipated women
This pilot randomized crossover trial compared 4 weeks of kale with a control food. Kale also shifted some gut bacteria, with more Eubacterium eligens and fewer Ruminococcus gnavus. It helped most in people who started with smaller stools. A company, Q’SAI, funded the study and took part in its design, and several authors work for the testing company Metagen.
Kale and Brussels sprout capsules did not change a breast cancer marker
This randomized trial gave 78 healthy premenopausal women six capsules a day (550 mg of dried Brussels sprouts and kale each) for eight weeks. Other groups ate 40 g a day of broccoli or Brussels sprouts, or took placebo. Neither the capsules nor the added vegetable moved the urinary 2:16 hydroxyestrone ratio, a proposed and unproven marker of breast cancer risk. The capsules mix kale with Brussels sprouts, so they do not test kale alone.
Review of 123 studies: no antithyroid effect of Brassica vegetables in people
This systematic review covered lab, animal and human studies of Brassica plants and extracts, with a section on kale. The no-effect finding held especially with enough iodine. The kale evidence comes from animals: sheep and cattle fed a lot of kale on low iodine developed thyroid changes, which iodine supplements prevented. A human case of severe hypothyroidism it describes involved raw bok choy, not kale.
Women absorbed about 41% of kale calcium, against about 32% from milk
Isotope-labeled absorption test in 11 healthy women, each given an average test dose of 300 mg of calcium from kale and, for comparison, from milk. The authors credit the low oxalate content of kale, in contrast with spinach, whose calcium is poorly absorbed. It was a small study of single test meals.
Added oxalic acid did not change kale iron absorption, 10.7% against 11.5%
Two stable-isotope studies in Zurich included 16 healthy women, 13 of whom finished both. Test meals were wheat rolls with 150 g of kale, or with kale plus 1.26 g of oxalic acid. A spinach meal, which holds 1.27 g of oxalic acid naturally, gave 24% less iron absorption than kale, a gap that could be chance.
Kale with peanut butter raised toddlers’ vitamin A from beta-carotene
This randomized trial included 37 children aged 12 to 36 months. They ate 50 g of cooked, isotope-labeled kale with either 33 g of peanut butter or 16 g of lard. Blood levels of vitamin A made from the kale were higher with peanut butter. The conversion ratio was about 11 to 1 against about 13 to 1 by weight, a difference that could be chance.
Only about 5% of kale’s vitamin K reached the blood
Seven healthy volunteers ate one 400 g serving of isotope-labeled kale with 30 g of vegetable oil. The labeled vitamin K peaked in blood after 6 to 10 hours, and one volunteer showed no consistent rise. Modeling put mean bioavailability (the share absorbed) at 4.7%. It was a very small study of a single serving.
After eating kale, lutein reached the blood far more readily than beta-carotene
Seven adults ate isotope-labeled kale providing 33 µmol of lutein and 34 µmol of beta-carotene. Peak blood levels of labeled lutein, 0.38 µM, were more than five times those of beta-carotene, 0.068 µM. At the peak, 3.6% of the lutein dose was recovered against 0.7% of the beta-carotene. It tracked absorption, not health outcomes.
Kale extract drink raised macular pigment and blood lutein in macular degeneration
This pilot trial gave 20 patients with dry macular degeneration an oily kale extract or a placebo of refined rapeseed oil. For 4 weeks they drank 50 mL a day; the extract held 10 mg of lutein and 3 mg of zeaxanthin. The gains faded during a 4-week washout, though values stayed above the starting levels. It tested an extract, not leaves, and the abstract reports no vision outcomes.
100 g more crucifers daily went with about 10% lower death risk
This umbrella review (a review of reviews) covered 57 articles on 24 health outcomes. Cruciferous vegetables (the cabbage family) were also linked to lower cancer and depression risk, but 68% of the evidence was of low quality. It covers the whole group, of which kale is one member.
100 g more leafy greens daily went with about 25% lower risks
This review covered 24 meta-analyses with 29 health outcomes for green leafy vegetables and lutein. The lower risks were of death, coronary heart disease and stroke. Lutein intake was also linked to lower risk of age-related macular degeneration and cataract. These are associations for leafy greens as a group; the authors flag contamination with E. coli as a caution.
About 1.3 daily servings of greens went with slower cognitive decline
This prospective study followed 960 adults aged 58 to 99 for an average of 4.7 years. For that intake, slower decline was like being 11 years younger. Higher intakes of vitamin K, lutein, nitrate, folate, alpha-tocopherol and kaempferol were each linked to slower decline, and beta-carotene was not. The questionnaire grouped spinach, kale or collards, and lettuce, so kale is one part of the group.
People eating most greens had 14% lower type 2 diabetes risk
This meta-analysis of prospective cohorts found that four of six eligible studies reported green leafy vegetables, with 0.86 times the risk. Fruit and vegetables combined showed no clear benefit. Two of the studies defined greens as spinach, kale and lettuce, and the definitions varied.
Older women eating collards or kale monthly had 69% lower glaucoma odds
This cross-sectional study included 1,155 US women, 95 of whom had glaucoma. Eating collards or kale at least once a month was compared with less than once a month (0.31 times the odds). Carrots and canned or dried peaches also stood out. Collards and kale were counted together, the data are a snapshot, and the range is wide.
African-American women eating collards or kale often had 57% lower glaucoma odds
This cross-sectional study included 584 women, 77 of whom had glaucoma. Eating collards or kale more than once a week was compared with once a month or less (0.43 times the odds). Fruit and other vegetables were linked too. Collards and kale were counted together, and the authors call for trials.
Girls with goiter had not eaten more kale than girls without
This case-control study compared 284 schoolgirls with goiter (an enlarged thyroid) and 288 without in Qom, Iran, where goiter is common. Histories of eating soy, kale, turnip and fish did not differ between the groups. The main risks were storing iodized salt in open containers, the family’s economic position and the district. Kale intake came from a questionnaire.
Family’s raised urine thallium was traced to kale chips, then fell
This case report covers a mother and her three young children in California, whose levels fell once they stopped eating the chips. Two samples of the commercial chips held 1.98 and 2.15 mg/kg of thallium. Peak urine levels were 5.6 µg/g creatinine in the mother and 10.5 in a child (reference limits 0.4 and 0.8). It is one household and one product; the authors note dehydration and other manufacturing steps can change thallium levels, and food surveillance data are scarce.
Steamed kale bound bile acids 13% as well as cholestyramine
This test-tube digestion used a mix of human bile acids, against the cholesterol drug cholestyramine. Kale, collards and mustard greens tied at 13%, ahead of broccoli (10%) and cabbage (5%) on a dry-matter basis. Steaming improved binding compared with raw vegetables tested earlier. This is lab chemistry, not a study in people.
Common kale holds under 10 µmol of goitrin per 100 g
This review used lab analysis and modeling, not kale eaters, to study goitrin, a thyroid-blocking compound. Goitrin at 194 µmol blocked iodine uptake; collards, Brussels sprouts and some Russian kale (Brassica napus) hold enough to matter. Kale of the Brassica oleracea type, commercial broccoli and turnip tops do not. Thiocyanate expected from indole glucosinolates is well under the 40 to 69 µM already in human blood.
Kale moved thallium from root to leaf most efficiently of 25 Brassicas
In this greenhouse experiment, the Brassica varieties were dosed with thallium on purpose, and some built up as much as 8,300 µg per gram. Kale had a translocation factor (how much moves from root to leaf) of 167. In kale the metal gathered along leaf edges and veins and formed crystals at the leaf pores. It shows what kale can do on contaminated soil, not what store-bought kale contains.
On polluted land, kale and taro took up the most thallium
In this meta-analysis of 35 vegetable species on polluted land, the plant-to-soil thallium ratio was 0.133 for kale and 0.060 for taro. Uptake rose with soil thallium and was shaped by soil pH. The authors name kale among vegetables that could pose a health risk when grown on contaminated land. That says little about ordinary farm soil.
Steaming kept the most antioxidant capacity and flavonoids in kale
This lab study compared five methods for Galega kale: boiling, steaming, microwaving, pressure cooking and vacuum cooking. Every method lost some antioxidants, flavonoids, organic acids and minerals. Cooking did not cause large losses of oxalic acid, and the oxalate-to-calcium ratio stayed under 2.
Cooking cut kale’s vitamin C about 89%, beta-carotene only about 5%
Lab analysis of raw and cooked leaves of one kale variety, with losses counted on a dry-mass basis. Polyphenols fell 56% and nitrates 78%. The authors suggest eating kale raw or only lightly processed, for example blanched.
Blanching 20 minutes left 27.6% of vitamin C and 12.9% of potassium
Lab experiment on kale and spinach from Kenya, run for people with kidney failure who must limit potassium. Blanching time mattered more than water temperature. A method that cuts potassium also removes most of the vitamin C.
Kale in high-fat-fed mice lowered triglycerides, LDL and inflammation, not fat
This was a study in mice, not people. Over 12 weeks, curly green kale prevented rises in blood endotoxin and inflammation markers in the high-fat groups of 9 mice. It did not prevent fat gain or insulin resistance.
Who should be careful with kale
Most people can eat kale freely. Take a little care if one of these applies:
- Warfarin. Kale is very rich in vitamin K, which can change how this blood thinner works. You do not need to avoid it. Keep the amount steady from week to week, as the NHS advises, and check with your clinic before big changes.
- Thyroid disease or low iodine. Common kale holds little of the thyroid-blocking compound goitrin. A 2024 review of 123 studies found no antithyroid effect from Brassica vegetables in people with enough iodine. The famous case of severe hypothyroidism involved 1 to 1.5 kg a day of raw bok choy, not kale.
- Kale chips and snacks. Kale takes up the toxic metal thallium from soil better than most vegetables, and thallium has no US food standard. California officials traced one family’s raised urine thallium to kale chips holding about 2 mg/kg. In May 2026 they warned consumers off several kale snacks from one brand; we found no official testing of fresh kale.
How to choose, store and eat kale
- Raw: in a lab test, cooking cut kale’s vitamin C by about 89% but its beta-carotene by only about 5%.
- Steamed: steaming kept the most antioxidant capacity and flavonoids of five cooking methods in a lab test.
- Blanched: a 20-minute blanch left 27.6% of the vitamin C and 12.9% of the potassium, so it suits only people who must limit potassium.
Choose kale with firm, deep green leaves and no brown or yellow patches. Keep it unwashed in a plastic bag in the fridge for up to five days.
Kale is one of the more heavily treated greens. In USDA tests of 708 washed samples in 2017, nearly 95% had at least one pesticide residue. Across 2017 and 2018, 43 of 1,415 samples (about 3%) had a residue above the legal limit, against under 1% of all foods tested.
Sources
- Dietary advice for patients taking warfarin. Cambridge University Hospitals NHS Foundation Trust.
- Anticoagulant medicines: considerations. NHS.
- Violi F, Lip GY, Pignatelli P, Pastori D. Interaction between dietary vitamin K intake and anticoagulation by vitamin K antagonists: is it really true? A systematic review. Medicine (Baltimore). 2016.
- Galanty A, Grudzińska M, Paździora W, Służały P, Paśko P. Do Brassica vegetables affect thyroid function? A comprehensive systematic review. Int J Mol Sci. 2024.
- Felker P, Bunch R, Leung AM. Concentrations of thiocyanate and goitrin in human plasma, their precursor concentrations in brassica vegetables, and associated potential risk for hypothyroidism. Nutr Rev. 2016.
- Chu M, Seltzer TF. Myxedema coma induced by ingestion of raw bok choy. N Engl J Med. 2010.
- Choudhury A, Fowles J, Bartlett R, et al. Thallium exposure secondary to commercial kale chip consumption: California case highlights opportunities for improved surveillance and toxicological understanding. Int J Environ Res Public Health. 2025.
- Investigation spotlight: thallium and commercial brand of kale chips. California Department of Public Health.
- CDPH warns consumers not to eat Brad’s Plant Based products. California Department of Public Health news release NR26-021. May 19, 2026.
- Xiao X, Zhou W, Guo Z, et al. Thallium content in vegetables and derivation of threshold for safe food production in soil: a meta-analysis. Sci Total Environ. 2024.
- Sikora E, Bodziarczyk I. Composition and antioxidant activity of kale (Brassica oleracea L. var. acephala) raw and cooked. Acta Sci Pol Technol Aliment. 2012.
- Armesto J, Gómez-Limia L, Carballo J, Martínez S. Effects of different cooking methods on the antioxidant capacity and flavonoid, organic acid and mineral contents of Galega kale. Int J Food Sci Nutr. 2019.
- Mugo BM, Kiio J, Munyaka A. Effect of blanching time-temperature on potassium and vitamin retention/loss in kale and spinach. Food Sci Nutr. 2024.
- Kale. University of Nebraska–Lincoln Extension.
- Pesticide Data Program annual summary, calendar year 2017. USDA Agricultural Marketing Service.
- Pesticide Data Program annual summary, calendar year 2018. USDA Agricultural Marketing Service.
History of kale
The Greek scholar Theophrastus, writing about 2,320 years ago, named three kinds of cabbage. They were a curly-leaved one, a smooth-leaved one and a bitter wild type. The Romans later called the curly type Sabellian kale, which is regarded as an ancestor of modern kales. Genetic work points to the eastern Mediterranean as the place where the whole species was first cultivated.
The name comes from Northern Middle English cale, related to Scots kail and German Kohl. It comes ultimately from Latin caulis, meaning cabbage.
In Scotland kale was such a staple that to be “off one’s kail” means being too ill to eat. The Kailyard school of writers, including J. M. Barrie, took its name from the kale field, the kailyard.
Kale reached North America mainly by way of Russian traders. They brought Russian kale to Canada and then the United States in the 19th century. For most of the 20th century kale was mainly decorative in the US. It became a common food from the 1990s, largely because of its nutrition.
Kale is a winter comfort food in northern Europe. During the Second World War the British Dig for Victory campaign urged people to grow it.
Dutch cooks mash it with potatoes into boerenkoolstamppot. In northern Germany, groups go on a winter Kohlfahrt, or kale trip, a hike that ends with a kale dinner.
In 2011 the fast-food chain Chick-fil-A sent a letter to a Vermont artist, Bo Muller-Moore. It demanded that he stop printing T-shirts that read “Eat More Kale”. On December 11, 2014, the US Patent and Trademark Office granted him the trademark.
Sources
- Mabry ME, Turner-Hissong SD, Gallagher EY, et al. The evolutionary history of wild, domesticated, and feral Brassica oleracea (Brassicaceae). Mol Biol Evol. 2021.
- Kale. Wikipedia.
- Eat More Kale. Wikipedia.
Frequently asked questions about kale
Is kale good for you?
For most people, yes. Per 100 g, raw kale has 35 calories, 93 mg of vitamin C, 390 µg of vitamin K and 4.1 g of fiber. In trials, a small amount of kale-containing food lowered the blood sugar peak after a meal. Kale bars for 12 weeks improved HbA1c in 30 people with type 2 diabetes.
These trials are small, and no kale finding here reaches strong evidence.
Is raw or cooked kale healthier?
Neither is clearly better, and no trial here compared them in people. In lab tests, cooking cut kale’s vitamin C by about 89% but its beta-carotene by only about 5%. Steaming kept the most antioxidant capacity of five methods.
A 20-minute blanch left just 27.6% of the vitamin C. Mixing raw and lightly steamed kale is a reasonable way to get both.
Does kale contain thallium?
Kale takes up thallium from soil more readily than most vegetables. But no food standard or fresh-kale survey tells us how much is in store-bought leaves. On polluted land, a meta-analysis of 35 vegetables ranked kale among the two top accumulators.
The one documented case we found involved kale chips, not fresh leaves. A California family’s raised urine thallium fell after they stopped eating chips holding about 2 mg/kg. Thallium has no US food standard, and health officials call for more testing.
Is raw kale bad for your thyroid?
Probably not at normal amounts. Common kale holds under 10 µmol of goitrin per 100 g, far below the 194 µmol that blocked iodine uptake in testing. A review of 123 studies found no antithyroid effect from Brassica vegetables in people with enough iodine.
The case often retold in kale and thyroid articles, severe hypothyroidism after 1 to 1.5 kg of raw bok choy a day, was not kale. If you have thyroid disease, ask your clinician before eating very large raw amounts.
Is kale healthier than spinach?
They trade wins. Per 100 g raw, kale has 93.4 mg of vitamin C and 254 mg of calcium, against 28.1 mg and 99 mg in spinach. Spinach has more vitamin K (483 against 390 µg) and more lutein plus zeaxanthin (12,198 against 6,261 µg) on our pages.
Kale is the lower-oxalate green. A 150 g kale meal held 0.01 g of oxalic acid against 1.27 g for spinach, and women absorbed about 41% of kale’s calcium.
Can you eat kale if you take blood thinners?
If you take warfarin, yes, as long as the amount stays steady. Kale has about 390 µg of vitamin K per 100 g. A systematic review found mixed evidence that dietary vitamin K shifts the effect of blood thinners. An effect seemed possible only above about 150 µg a day.
The NHS advises keeping the amount of green vegetables consistent. It says the food limits usually do not apply to apixaban, dabigatran, edoxaban or rivaroxaban.
Is kale good for people with diabetes?
It may help, but the evidence is small and mixed. In a 42-person placebo-controlled trial, 7 g of kale-containing food was eaten with a high-carbohydrate meal. It lowered the blood sugar peak from 176 to 163 mg/dL.
Kale bars lowered HbA1c in 30 people with type 2 diabetes. In 124 women with obesity, HbA1c improved more with peas than with kale. Check portions and choices with your care team.
How much kale can you eat a day?
No study here sets an ideal amount. Trials used 7 to 14 g of a kale-containing food and 150 to 300 mL of juice a day. At the top end, kale bars equaled about 341 g of fresh kale a day for 12 weeks.
A cup of raw kale (21 g) has 7 calories and about 0.9 g of fiber. If you take warfarin, keep the amount steady.