What Science Says About Kale
1. Lutein and zeaxanthin
Raw kale contains 6,261 µg of lutein and zeaxanthin per 100 g in USDA data. [1] A 2015 review places dark-green leafy vegetables such as kale and spinach among the richest dietary sources of both carotenoids, and notes that their levels in kale shift with maturity, climate and farming practice. [2]
Lutein and zeaxanthin are the main components of macular pigment, concentrated in the central retina; a meta-analysis of 20 randomised trials found that supplementing them raised macular pigment optical density. [3] Kale's lutein is also readily absorbed: when seven adults ate 400 g of isotopically labelled kale, 3.6% of its lutein was recovered in the blood at peak, compared with 0.7% of its beta-carotene. [4] No EU health claim has been authorised for lutein — EFSA judged the evidence on lutein and normal vision insufficient — so we describe the compound, not a promise. [5]
2. Cholesterol — the kale juice trial
In a 2008 study, 32 men with total 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%; body weight and waist size did not change. Glutathione peroxidase activity and serum selenium also increased, and the response depended on smoking status. [6]
It is a rare study of the juice itself, but a small one without a control group, so it shows a direction worth studying rather than proof.
3. Blood glucose after a meal
In a randomised, double-blind, placebo-controlled crossover study, 42 Japanese adults ate a kale-containing food (7 g or 14 g) with a high-carbohydrate meal. Peak blood glucose was 163 and 162 mg/dL compared with 176 mg/dL on placebo, and the two-hour area under the curve was also lower; insulin levels did not differ. [7] This was a single-meal test with a kale-based food, not kale juice.
4. Glucosinolates — the cabbage family's signature
The predominant glucosinolates in kale are sinigrin, glucoiberin and glucoraphanin, with glucobrassicin as the main indole glucosinolate. They are not bioactive until the plant enzyme myrosinase converts them into breakdown products, including isothiocyanates, when the leaf is cut or chewed. [8]
That conversion matters in the body too. In a human study, isothiocyanates were about six times more bioavailable than their parent glucosinolates. [9] In another, people excreted 32.3% of the isothiocyanates from fresh broccoli but only 10.2% from steamed broccoli, because heat inactivates myrosinase. [10] How much of this conversion happens in a bottled juice has not been measured.
5. Calcium with little oxalate
Kale's calcium is well absorbed: in 11 women, fractional absorption averaged 40.9% from kale and 32.1% from milk. [11] The reason is kale's low oxalate content — a 300 g portion contained 0.1 mmol of oxalate against 6.6 mmol in the same amount of spinach, and magnesium absorption from the kale meal was higher, at 36.5% versus 26.7%. [12]
6. Is kale a superfood?
A 2019 review of kale's botany, nutrients and biological activity concluded that it remains unclear why kale is declared superior to other cruciferous vegetables. [8] Its strength is breadth — carotenoids, glucosinolates, vitamins and minerals in one leaf — rather than a single miracle compound.
What has not been studied
The only published trial of kale juice is the small uncontrolled study above; most of what we know comes from kale leaves, powders and isolated compounds. The amounts of lutein, vitamin K, calcium or glucosinolates in the finished juice have not been measured, and juicing removes most of the fibre of the whole leaf. The findings above describe the plant, not a promise about the juice.
Vitamin K and blood thinners
Kale leaves are among the richest food sources of vitamin K. People taking warfarin or similar anticoagulants need to keep their vitamin K intake consistent, because sudden changes can alter the effect of the medicine — speak to your doctor before drinking kale juice regularly. [13]
References
- US Department of Agriculture, Agricultural Research Service. FoodData Central: Kale, raw (SR Legacy, FDC ID 168421). 2019.
- 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
- EFSA Panel on Dietetic Products, Nutrition and Allergies (NDA). Scientific Opinion on the substantiation of health claims related to lutein and maintenance of normal vision (ID 1603, 1604, further assessment). EFSA J. 2012;10(6):2716. doi:10.2903/j.efsa.2012.2716
- 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
- National Institutes of Health, Office of Dietary Supplements. Vitamin K — Fact Sheet for Health Professionals.
