Kale vs. Spinach:
Which Green Is Actually Better?

Spinach has had an excellent PR run. Popeye, countless green smoothie recipes, and a reputation as the default healthy green have given it an almost untouchable position in the public imagination. So let's actually look at the numbers — because when you compare kale and spinach side by side with real nutritional data, the picture is more nuanced than most people expect.

The short answer: both are exceptional foods, and eating either regularly is a win. But on balance — and especially when you factor in nutrient bioavailability, not just raw content — kale comes out ahead. Here's why.

The Baseline: Calories and Macros

Per 100 grams (raw), kale and spinach are nutritionally similar at the macro level:

Kale: ~49 calories, 4.3g protein, 8.8g carbohydrates, 1.5g fiber, 0.9g fat
Spinach: ~23 calories, 2.9g protein, 3.6g carbohydrates, 2.2g fiber, 0.4g fat

Spinach is lower in calories and higher in fiber relative to weight, which gives it an edge in satiety-per-calorie metrics. Kale has more protein per 100g. Both are negligible sources of fat, though kale contains slightly more omega-3 fatty acids (ALA) than spinach. Neither will move the needle meaningfully on your macro targets — but that's not why you're eating leafy greens anyway.

Vitamins: Kale Wins Decisively

This is where the comparison gets interesting. USDA nutritional data (FoodData Central) shows the following per 100g raw:

Vitamin K: Kale: 705µg (882% DV) vs. Spinach: 483µg (604% DV). Both are extraordinary sources — kale wins.

Vitamin C: Kale: 120mg (133% DV) vs. Spinach: 28mg (31% DV). This is a significant gap. Kale has more than four times the vitamin C of spinach. Vitamin C is water-soluble, heat-sensitive, and one of the most commonly deficient vitamins in Western diets. This round goes firmly to kale.

Vitamin A (beta-carotene): Kale: 241µg RAE (27% DV) vs. Spinach: 469µg RAE (52% DV). Spinach wins here — it contains significantly more beta-carotene. However, beta-carotene absorption is enhanced by dietary fat, and both foods are typically consumed with fat-containing dressings or oils, so practical absorption may be similar.

Vitamin E: Spinach: 2mg (13% DV) vs. Kale: 1.5mg (10% DV). Spinach edges ahead slightly.

Folate (B9): Spinach: 194µg (49% DV) vs. Kale: 141µg (35% DV). Spinach wins on folate — important for anyone who's pregnant or managing homocysteine levels.

Minerals: The Oxalate Factor

On paper, spinach appears to be a mineral powerhouse. Per 100g: 99mg calcium, 2.7mg iron, 79mg magnesium — impressive numbers. Kale: 150mg calcium, 1.5mg iron, 47mg magnesium.

But here's the critical nuance that most comparisons miss: spinach is extremely high in oxalic acid (oxalates), an antinutrient that binds to calcium and iron in the digestive tract and dramatically reduces their absorption. Research published in the American Journal of Clinical Nutrition found that calcium absorption from spinach is only about 5% — compared to approximately 40% from kale. That means spinach's superior calcium content on a nutrition label translates to significantly less calcium actually reaching your bloodstream.

The same principle applies to iron. Spinach's iron is largely bound by oxalates and phytates, reducing its bioavailability. Kale, as a cruciferous vegetable, has a much lower oxalate content, making its iron more accessible — especially when paired with vitamin C (which kale conveniently provides in abundance).

This is one of the most important reasons why kale is nutritionally superior in practice, not just in theory. What a food label says and what your body actually absorbs can be dramatically different.

Antioxidants and Phytochemicals

Both greens are rich in antioxidants, but their profiles differ in meaningful ways.

Spinach is high in lutein and zeaxanthin — carotenoid antioxidants concentrated in the eye's macula that protect against age-related macular degeneration and cataracts. A 2019 review in Nutrients highlighted spinach as one of the best dietary sources of these specific compounds. If eye health is your priority, spinach deserves credit here.

Kale, on the other hand, is a cruciferous vegetable — which means it contains glucosinolates that convert to isothiocyanates (including sulforaphane) upon chewing or processing. Spinach, as an Amaranthaceae plant, does not contain glucosinolates at all. The cancer-preventive research around sulforaphane — pioneered at Johns Hopkins since the 1990s — applies to kale but not to spinach. Kale also contains higher concentrations of quercetin and kaempferol, flavonoids with well-documented anti-inflammatory and anti-cancer properties in peer-reviewed literature.

On total antioxidant capacity (ORAC scores, though these have limitations), kale and spinach are comparable per gram. But kale's unique glucosinolate content gives it a phytochemical advantage that raw antioxidant measurements don't capture.

The Verdict

If you had to pick one green and eat it exclusively for the rest of your life, the data makes a strong case for kale:

Vitamin C: Kale wins (4x more). Bioavailable calcium: Kale wins (due to low oxalates). Bioavailable iron: Kale wins (same reason). Vitamin K: Kale wins. Anti-cancer phytochemicals (glucosinolates/sulforaphane): Kale wins — spinach doesn't have them at all.

Spinach wins on folate, vitamin A (raw content), and lutein/zeaxanthin. These are meaningful advantages, especially for specific populations.

The honest conclusion: eat both. They're not the same food, and their different phytochemical profiles mean they offer different benefits. A diverse diet is always better than a single-food approach.

But if you're going to concentrate one green into a daily supplement — freeze-drying it to lock in maximum nutrition and taking it every morning — kale is the right choice. The breadth of its nutrient profile, its bioavailability advantages, and its unique glucosinolate content make it the most defensible single-food investment you can make. Which is, not coincidentally, exactly why OnlyKale exists.