Popeye was right. Not because of the iron — but more on that in a moment. Spinach is indeed one of the most nutrient-rich leafy greens we know. And it is also one of the most misunderstood.
The iron myth. The oxalic acid warning. The question of raw or cooked. Spinach has more stories than most other vegetables. And behind each of these stories lies something interesting — if you look closely.
The Popeye story — and the comma that changed everything
The myth that spinach is particularly rich in iron originated in the 1870s. The German chemist Erich von Wolf investigated the iron content of spinach — and made a mistake that made history. He placed the comma in the wrong place. Instead of 2.7 mg of iron per 100 grams, he mistakenly wrote 27 mg — ten times the amount.
This error went unnoticed for decades. And Popeye became a spinach icon based on this false number.
The truth: Spinach contains about 2.7 mg of iron per 100 grams of fresh leaves. That's significant — but not exceptional. Lentils contain more. Pumpkin seeds contain more. Meat contains iron in a form that the body can absorb much better.
But — and this is crucial — spinach also contains vitamin C, which significantly improves iron absorption. And it contains folate, magnesium, vitamin K, beta-carotene, lutein, zeaxanthin, kaempferol, and chlorophyll in truly impressive amounts.
Popeye was right. Just for different reasons than he thought.
What's really in spinach — the complete nutrient profile
Folate — the most underrated vitamin
Folic acid is a B vitamin that is essential for cell maturation and division processes in the body. It is indispensable for pregnant women — folic acid deficiency in early pregnancy is one of the most common causes of neural tube defects. But it is also important for everyone else: for the immune system, for blood formation, for cell regeneration.
Spinach is one of the best natural sources of folate available.
Vitamin K — the unsung bone builder
Vitamin K is much more than a clotting vitamin. Recent research shows its central role in bone health — it activates proteins that deposit calcium into the bones. Spinach contains vitamin K in exceptionally high concentrations.
Lutein and Zeaxanthin — for the eyes
Lutein and zeaxanthin are carotenoids and are important for healthy eyes. They protect the retina from UV damage and can counteract age-related macular degeneration (the deterioration of the central retina responsible for sharp vision). Spinach is one of the richest sources of both substances.
Kaempferol — the anti-inflammatory plant compound
Kaempferol has strong antioxidant properties and can prevent chronic diseases. It is the same substance we know from broccoli — and it works in spinach in a similarly concentrated form.
Quercetin — the antioxidant from onions
Yes — spinach also contains quercetin, the flavonoid (a secondary plant compound with protective effects) that we know from red onions. Quercetin is an antioxidant and can reduce infections and inflammation in the body.
Chlorophyll — more than just green pigment
Chlorophyll (the green pigment where photosynthesis takes place) is not only important for the plant. In the human body, it acts as a natural antioxidant, supports liver function, and has a detoxifying effect. Dark green spinach with an intense color contains more chlorophyll — and thus more of all these effects.
Magnesium — the energy mineral
Spinach is one of the best plant-based sources of magnesium. Magnesium is involved in over 300 enzymatic (enzyme-controlled chemical) processes in the body — muscle function, nerve function, energy metabolism. A magnesium deficiency manifests itself in muscle cramps, fatigue, and nervousness.
Nitrate — the controversial molecule
Spinach is known for its high nitrate content, which supports healthy heart function. Nitrate is converted into nitric oxide in the body, which dilates blood vessels and can lower blood pressure. This is a real health benefit — especially for athletes who use nitrate as a natural performance enhancer.
The oxalic acid question — how dangerous is it really?
Oxalic acid (an organic acid naturally found in many plants) is why some people avoid spinach. Oxalic acid binds calcium and iron — making these minerals less available to the body. It can be problematic for people prone to kidney stones.
But here are two important facts that put the discussion into perspective:
The oxalic acid content in spinach varies depending on the variety, age of the plant, and season. Spinach accumulates more and more oxalic acid in its stems during growth until June — these high concentrations decrease again in the autumn months. Spring spinach and autumn spinach are therefore less affected than summer spinach.
And: Blanching (briefly scalding with hot water) significantly reduces the oxalic acid content — the cooking water takes away most of the oxalic acid. Anyone who briefly blanches spinach and discards the water will have spinach with significantly lower oxalic acid content.
For healthy people without a tendency to kidney stones, oxalic acid is not a problem in normal consumption.
Combining it with calcium-rich foods further neutralizes oxalic acid — tomatoes with spinach, Parmesan on spinach, yogurt with it. Calcium-containing tomatoes neutralize its oxalate content.
Raw or cooked — which is better?
The answer is: it depends on what you want.
Eating raw spinach has the advantage that vitamins and minerals are fully available. When cooked, the vegetable loses some of the heat-sensitive vitamins — vitamin C and folic acid suffer from the heat.
At the same time: beta-carotene and lutein are fat-soluble (can only be absorbed by the body in combination with fat) and become more available through light heating. A splash of olive oil with steamed spinach significantly increases the absorption of these carotenoids.
The practical recommendation: baby spinach raw in salad for vitamin C and folic acid. Older spinach lightly steamed with olive oil for beta-carotene and lutein. And for oxalic acid-sensitive people: always blanch and discard the cooking water.
What foliar fertilization has to do with spinach — and why it is particularly effective here
Spinach is a very fast grower. In good conditions, it is ready for harvest in 4 to 6 weeks. This rapid growth rate means it needs a lot in a short time — a lot of chlorophyll, a lot of photosynthesis, a lot of minerals.
And this is exactly where the argument for foliar fertilization with Grünkraft Calcium comes in.
More chlorophyll through more photosynthesis. Chlorophyll is the pigment where photosynthesis takes place. More CO₂ directly in the leaf through Grünkraft Calcium means more intensive photosynthesis — and that directly means more chlorophyll. A deep, rich green is the visible proof.
More lutein, zeaxanthin, and kaempferol. These substances are formed as a protective reaction of the vital plant — as secondary plant compounds that the plant produces when it has excess energy. More photosynthesis — more energy — more secondary plant compounds.
Calcium for stable cell walls. Due to its rapid growth, spinach often has thin, sensitive cell walls. Grünkraft Calcium supplies calcium directly through the leaf — stronger cell walls mean less pest infestation, fewer fungal diseases, and longer shelf life after harvest.
Earlier and more uniform cutting. In field trials from Colombia, lettuce — a botanically similar fast-growing leafy crop like spinach — was harvested 7 days earlier than the control group. The growth rate was 16.8 percent higher. The same principle applies to spinach.
Practical: Grünkraft Calcium as a fine mist on the leaves — early in the morning when the stomata (leaf pores) are open. First application 5g per 1 liter of water. From the second application, 20-30g per 1 liter every 10-14 days. In the bed or in a pot on the balcony — the same principle.
What the soil means for spinach
Spinach is demanding when it comes to soil.
pH value: 6.5 to 7.5 — slightly acidic to neutral. If the soil is too acidic, spinach hardly absorbs iron and magnesium.
Nitrogen: Spinach is a heavy feeder (a plant that needs a lot of nutrients for its rapid growth) — it needs a lot of nitrogen for intensive chlorophyll formation. It responds immediately positively to well-supplied soils with active soil life.
Moisture: Consistent moisture is crucial. Drought stress immediately leads to flowering (bolting) — the spinach shoots up and becomes bitter. BODENKRAFT PUR Zeolith in the soil buffers dry periods.
Soil life: An active soil microbiome (the community of beneficial microorganisms in the soil) makes nitrogen, iron, and magnesium available to plants. AM+PLUS microorganisms in the watering can every 3-4 weeks give spinach the soil life it needs.
How to eat spinach properly — three important tips
Combine with vitamin C. Lemon juice over spinach salad, add tomatoes, cut in bell peppers — vitamin C significantly improves iron absorption from spinach.
Combine with fat. Olive oil, butter, nuts — fat-soluble carotenoids like lutein and beta-carotene are absorbed much better with fat.
Do not reheat. Reheated spinach contains increased nitrite levels (breakdown products of nitrate that can be problematic in larger quantities) — if you cook spinach, eat it fresh or cool it down quickly and store it in the refrigerator.
The short summary
Spinach is a real superfood — but not because of the iron that made Popeye famous. Rather, it's because of folate, vitamin K, lutein, zeaxanthin, kaempferol, quercetin, chlorophyll, and magnesium.
And the key to truly nutrient-rich spinach lies — as always — in the soil and in photosynthesis. Dark green, intensely growing spinach in mineral-rich soil with regular foliar fertilization has more of all these substances than pale, slow-growing spinach in depleted soil.
This time, the comma is in the right place.
What secondary plant compounds are and why photosynthesis determines them — in the Photosynthesis article.
Measure your spinach's Brix value yourself — in the Brix article with instructions.
All garden products in our Garden Collection.
Sources: Artgerecht.com, Spinach Ingredients and Effects | Liebherr FreshMAG, Spinach as a Superfood | Schnelleinfachgesund.de, Spinach Myth and Truth | Utopia.de, Spinach Nutrients and Iron Content | AOK Magazin, Oxalic Acid in Foods | UMID Colombia S.A.S., Field Trials Lettuce Colombia 2017-2018
