What Science Says About Kale
1. Composition, and why it varies
Kale's figures move with cultivar, season and soil, which is why single numbers mislead. A 2024 study measured field-grown 'Darkibor' kale at six farms and reported 228–350 µg of phylloquinone (vitamin K), 8.09–10.65 mg of lutein and zeaxanthin combined, and 113–147 mg of vitamin C per 100 g of fresh leaf; the authors note the sample was too small to attribute the spread to any one growing condition. [1] A 2015 review places dark-green leaves such as kale among the richest dietary sources of both carotenoids and finds their levels shifting with maturity, climate and farming practice. [2]
2. Lutein and zeaxanthin
Lutein and zeaxanthin are the main components of macular pigment, concentrated in the central retina. A meta-analysis of 20 randomised trials in 938 patients with age-related macular degeneration and 826 healthy subjects found that supplementing them raised macular pigment optical density in both groups. [3] The lutein in kale is readily absorbed: when seven adults ate 400 g of isotopically labelled kale with 30 g of oil, 3.6% of the lutein dose was recovered in plasma at peak, against 0.7% of the beta-carotene dose. [4]
3. Cholesterol — a trial of kale juice
In a 2008 study, 32 men with cholesterol above 200 mg/dL drank 150 ml of kale juice a day for twelve weeks. HDL cholesterol rose by 27% and the HDL-to-LDL ratio by 52%, while LDL cholesterol fell by 10% and the atherogenic index by 24.2%; glutathione peroxidase activity and serum selenium also increased, and body weight and waist size did not change. [5] It is one of very few trials of the juice rather than the leaf, but a single-arm before-and-after study with no control or placebo group — a direction worth studying rather than a settled finding.
4. Blood glucose after a meal
In a randomised, double-blind, placebo-controlled crossover study, 42 Japanese adults ate a kale-based food, 7 g or 14 g, with a high-carbohydrate meal. Peak blood glucose was 163 and 162 mg/dL against 176 mg/dL on placebo, and the two-hour area under the curve was lower as well; insulin levels did not differ. [6] This was a single-meal test with a dried kale food, not with juice.
5. Glucosinolates, and the enzyme that unlocks them
The predominant glucosinolates in kale are sinigrin, glucoiberin and glucoraphanin, with glucobrassicin as the main indole glucosinolate. They are not bioactive as they sit in the leaf: the plant's own enzyme myrosinase converts them into isothiocyanates and related breakdown products when the tissue is damaged by cutting or chewing. [7]
That conversion decides how much reaches the bloodstream. In one human study isothiocyanates were about six times more bioavailable than their parent glucosinolates. [8] In another, twelve men excreted 32.3% of the isothiocyanate equivalents from fresh broccoli but only 10.2% from steamed broccoli, because heat inactivates myrosinase. [9] How the leaf is handled therefore matters as much as what the leaf contains.
6. Calcium, and the oxalate that usually blocks it
Kale's calcium is absorbed unusually well: in 11 women, fractional absorption from a 300 mg test load averaged 40.9% from kale against 32.1% from milk. [10] The reason is how little oxalate kale carries. A 300 g portion contained 0.1 mmol of oxalate against 6.6 mmol in the same weight of spinach, and magnesium absorption from the kale meal was higher too, 36.5% against 26.7%. [11]
7. Is kale a superfood?
A 2019 review of kale's botany, nutrients and biological activity set out to test the word and concluded that it remains unclear why kale should be declared superior to other cruciferous vegetables, and asked openly whether the reputation rests on evidence or on other factors. [7] Kale's case is breadth — carotenoids, glucosinolates, vitamin C, vitamin K and well-absorbed calcium in one leaf — rather than any single remarkable compound.
What has not been studied
Only one published trial has tested kale juice, and it had no control group; nearly everything else in the literature comes from kale leaves, dried kale foods or isolated compounds. Leaf composition does not carry over predictably into a pressed juice: phylloquinone and the carotenoids are fat-soluble and held in the chloroplast membranes of the leaf, myrosinase is inactivated by heat, and pressing separates most of the fibre from the liquid. The sections above describe the plant.
Vitamin K and anticoagulants
Kale leaves are among the richer food sources of vitamin K, which matters for anyone taking warfarin or another vitamin K antagonist. In 43 outpatients starting warfarin, week-to-week change in dietary vitamin K intake correlated inversely with INR variability, and a weekly change of 714 µg was associated with a one-unit change in weekly INR. [12] In a randomised, double-blind trial in 70 patients with unstable control, 150 µg of vitamin K a day reduced the standard deviation of INR and increased time in the target range; the same paper reports that unstable patients tend to have lower habitual vitamin K intake than stable ones. [13] A 2014 Cochrane review of that evidence found only two small trials and judged the data insufficient to show an overall benefit from supplementation. [14] What the evidence supports is consistency rather than avoidance: anyone on these medicines should speak to the prescribing doctor before changing their diet substantially.
References
- Kim BF, Lupolt SN, Santo RE, et al. Nutrients and non-essential metals in darkibor kale grown at urban and rural farms: a pilot study. PLoS One. 2024;19(4):e0296840. doi:10.1371/journal.pone.0296840
- Walsh RP, Bartlett H, Eperjesi F. Variation in carotenoid content of kale and other vegetables: a review of pre- and post-harvest effects. J Agric Food Chem. 2015;63(44):9677–9682. doi:10.1021/acs.jafc.5b03691
- Ma L, Liu R, Du JH, Liu T, Wu SS, Liu XH. Lutein, zeaxanthin and meso-zeaxanthin supplementation associated with macular pigment optical density. Nutrients. 2016;8(7):426. doi:10.3390/nu8070426
- Novotny JA, Kurilich AC, Britz SJ, Clevidence BA. Plasma appearance of labeled beta-carotene, lutein, and retinol in humans after consumption of isotopically labeled kale. J Lipid Res. 2005;46(9):1896–1903. doi:10.1194/jlr.M400504-JLR200
- Kim SY, Yoon S, Kwon SM, Park KS, Lee-Kim YC. Kale juice improves coronary artery disease risk factors in hypercholesterolemic men. Biomed Environ Sci. 2008;21(2):91–97. doi:10.1016/S0895-3988(08)60012-4
- Kondo S, Suzuki A, Kurokawa M, Hasumi K. Intake of kale suppresses postprandial increases in plasma glucose: a randomized, double-blind, placebo-controlled, crossover study. Biomed Rep. 2016;5(5):553–558. doi:10.3892/br.2016.767
- Šamec D, Urlić B, Salopek-Sondi B. Kale (Brassica oleracea var. acephala) as a superfood: review of the scientific evidence behind the statement. Crit Rev Food Sci Nutr. 2019;59(15):2411–2422. doi:10.1080/10408398.2018.1454400
- Shapiro TA, Fahey JW, Wade KL, Stephenson KK, Talalay P. Chemoprotective glucosinolates and isothiocyanates of broccoli sprouts: metabolism and excretion in humans. Cancer Epidemiol Biomarkers Prev. 2001;10(5):501–508. PMID 11352861
- Conaway CC, Getahun SM, Liebes LL, et al. Disposition of glucosinolates and sulforaphane in humans after ingestion of steamed and fresh broccoli. Nutr Cancer. 2000;38(2):168–178. doi:10.1207/S15327914NC382_5
- Heaney RP, Weaver CM. Calcium absorption from kale. Am J Clin Nutr. 1990;51(4):656–657. doi:10.1093/ajcn/51.4.656
- Bohn T, Davidsson L, Walczyk T, Hurrell RF. Fractional magnesium absorption is significantly lower in human subjects from a meal served with an oxalate-rich vegetable, spinach, as compared with a meal served with kale, a vegetable with a low oxalate content. Br J Nutr. 2004;91(4):601–606. doi:10.1079/BJN20031081
- Couris R, Tataronis G, McCloskey W, et al. Dietary vitamin K variability affects International Normalized Ratio (INR) coagulation indices. Int J Vitam Nutr Res. 2006;76(2):65–74. doi:10.1024/0300-9831.76.2.65
- Sconce E, Avery P, Wynne H, Kamali F. Vitamin K supplementation can improve stability of anticoagulation for patients with unexplained variability in response to warfarin. Blood. 2007;109(6):2419–2423. doi:10.1182/blood-2006-09-049262
- Mahtani KR, Heneghan CJ, Nunan D, Roberts NW. Vitamin K for improved anticoagulation control in patients receiving warfarin. Cochrane Database Syst Rev. 2014;CD009917. doi:10.1002/14651858.CD009917.pub2
