Benefits
Supports healthy HDL and LDL cholesterol levels
In a 12-week study, 32 men with high cholesterol drank 150 mL of kale juice a day. HDL cholesterol rose about 27 percent and the LDL cholesterol level fell about 10 percent, with the HDL-to-LDL ratio and the atherogenic index improving. There was no placebo group and everyone took the juice, so the size of the effect is uncertain.
Blood pressure readings in the kale-juice studies
In a 6-week study, 84 adults with higher-than-ideal blood pressure drank 300 mL of kale juice a day. Both systolic and diastolic readings fell across all of the genetic subgroups tested. Changes in blood lipids and blood sugar varied with the participants' glutathione S-transferase genes. This was a single-arm study with no placebo group.
Antioxidant enzyme activity and oxidative stress markers
In the kale-juice studies, red-cell catalase activity rose and a measure of DNA damage fell after 6 weeks, while plasma vitamin C and beta-carotene rose in some genetic subgroups. In the men's cholesterol study, glutathione peroxidase activity and blood selenium rose, though a fat-oxidation marker did not change. All were single-arm, before-and-after measurements.
A source of the antioxidant carotenoids lutein and zeaxanthin
Raw kale supplies about 6,260 micrograms of lutein plus zeaxanthin per 100 g, among the higher amounts in common vegetables, along with about 93 mg of vitamin C. Lutein and zeaxanthin are the carotenoids that concentrate in the macula of the eye. This describes kale's nutrient content; it is not a claim that kale treats or prevents any eye condition.
LDL cholesterol in a freeze-dried kale trial
In a 12-week placebo-controlled trial, 30 adults with type 2 diabetes ate freeze-dried kale bars supplying about 79 g of kale powder a day. LDL cholesterol and fasting glucose showed favorable trends, while HbA1c, insulin resistance, body weight and calorie intake fell compared with placebo. The trial was small and needs to be repeated in a larger group.
Mechanism of action
Carotenoid and vitamin antioxidants
Kale is rich in the carotenoids lutein, zeaxanthin and beta-carotene and in vitamin C, which can neutralize reactive oxygen species. In the kale-juice studies, the activity of the body's own antioxidant enzymes, catalase and glutathione peroxidase, rose after several weeks of daily intake.
Glucosinolates and sulforaphane
Like other cruciferous vegetables, kale contains glucosinolates including glucoraphanin. When the leaf is chopped or chewed, the plant enzyme myrosinase converts glucoraphanin to sulforaphane, which switches on phase II detoxication and antioxidant genes in laboratory work. Sulforaphane is covered in more detail on our separate broccoli extract page.
Soluble fiber and bile acids
Kale provides soluble fiber, which can bind bile acids in the gut so the liver uses cholesterol to make more of them. This is a proposed route for the lipid changes seen in the whole-leaf studies and has not been isolated as the cause in people.
Clinical trials
Single-arm, 12-week study of 150 mL/day kale juice, measuring blood lipids and antioxidant markers before and after, with no placebo group. (Kim et al. 2008, Biomed Environ Sci)
32 men with hypercholesterolemia (total cholesterol over 200 mg/dL), recruited from university faculty and staff in Seoul, Korea.
After 12 weeks, HDL cholesterol rose 27 percent and the HDL-to-LDL ratio 52 percent, while LDL cholesterol fell 10 percent and the atherogenic index fell 24.2 percent. Glutathione peroxidase activity and blood selenium rose, but the fat-oxidation marker malondialdehyde did not change. With no placebo arm and everyone taking the juice, the trial cannot separate the juice from other changes over time.
Single-arm, 6-week study of 300 mL/day kale juice in people with mildly raised blood pressure, analyzed by glutathione S-transferase M1 and T1 genotype, with no placebo group. (Han et al. 2015, Nutr Res Pract)
84 subclinical hypertensive adults in South Korea with systolic blood pressure over 130 mmHg or diastolic over 85 mmHg.
Systolic and diastolic blood pressure fell significantly in all genotype groups after 6 weeks. HDL cholesterol rose and LDL fell only in those with the GSTT1-present genotype, and blood glucose fell only in some genotype groups, so lipid and glucose effects varied by genetics. There was no placebo arm.
Analysis of antioxidant vitamins, DNA damage and oxidative stress markers in the same 6-week, 300 mL/day kale-juice cohort, by GSTM1 and T1 genotype, with no placebo group. (Lee et al. 2018, Nutr Res Pract)
84 subclinical hypertensive adults in South Korea (the same cohort reported for blood pressure and lipids).
A measure of DNA damage fell and red-cell catalase activity rose in all genotypes after 6 weeks. Plasma vitamin C rose in some genotypes, and beta-carotene, glutathione peroxidase activity and nitric oxide rose only in one genotype group. Because this reports the same single-arm cohort as the blood pressure paper, it is one study, not an independent replication.
12-week randomized, double-blind, placebo-controlled trial of freeze-dried kale bars providing about 79 g/day of kale powder (roughly 340 g fresh kale), versus placebo bars. (Jeppesen et al. 2024, Nutrients)
30 adults with type 2 diabetes in Denmark (kale bar or placebo bar).
HbA1c, insulin resistance, body weight and calorie intake fell significantly in the kale group versus placebo, and fasting glucose and LDL cholesterol showed favorable trends. Total body fat mass and the glucose area under the curve on an oral glucose tolerance test did not differ. The authors called the trial small and asked for a larger study.