Spinach's real strengths are folate, vitamin K1, and lutein, not iron. Its iron is harder to absorb than the iron in meat to begin with, and the oxalate in the leaves also binds iron and calcium. Part of that oxalate dissolves in water, so blanching the leaves and pouring the water away removes a share of it; the chapter on oxalate, kidney stones, and blanching is devoted to this.
Spinach is a dark leafy green you can eat raw, blanched, stir-fried, or in soup. Its volume is deceptive: a big handful of raw spinach collapses into a small clump in the pan, because it is over 90% water.
In practice · Raw or blanched?
Most people can eat spinach either raw or cooked. The differences come down to three things.
Portion: spinach is over 90% water, so a bowl of cooked spinach is really a big pile of leaves. Counted as cooked greens, you take in much more folate and vitamin K1 than it looks.
Oxalate: blanching and pouring the water away removes part of the soluble oxalate. If you stir-fry it, or make soup and drink the broth, most of the oxalate stays on your plate. If you have had calcium oxalate kidney stones, blanching first is the safer choice.
Vitamins that dissolve in water and dislike heat: folate and vitamin C also dissolve in water and are sensitive to heat, so the longer you blanch, the more of them goes down the drain with the oxalate. Keep blanching short: lift the leaves out as soon as they wilt.
Chapter 2
Rich in folate and vitamin K1
Spinach is extremely low in calories but very high in nutrient density. For very few calories, it brings folate, vitamin K1, lutein, potassium, magnesium, beta-carotene, and vitamin C all at once.
Folate is its signature. The word comes from the Latin folium, "leaf", because folate was first isolated from spinach leaves. Folate is a vitamin the body needs to make DNA and for cells to divide, and it matters especially before conception and in early pregnancy (see Folate).
Mechanism · Beyond folate: how its nitrate works
Beyond folate, spinach has a few more highlights: vitamin K1 (Vitamin K1 (Phylloquinone); dark leafy greens are among its main food sources, and it runs clotting and bone), potassium (Potassium & Sodium), magnesium (magnesium), and lutein and zeaxanthin (Lutein + Zeaxanthin, two pigments concentrated in the macula at the back of the eye).
One more is often overlooked: spinach is a rich source of dietary nitrate. In the body, nitrate is first converted to nitrite and then to nitric oxide (), which relaxes blood vessels. In a of randomized trials, nitrate supplementation lowered blood pressure modestly; most of those trials used beetroot juice or nitrate supplements, though, not spinach itself.
Note that this is the nitrate that occurs naturally in vegetables, a separate matter from the nitrite added to processed meat.
Chapter 3
Why its iron is hard to absorb
The iron on the label and the iron your body can take are not the same number. Spinach's iron is non-heme iron — the plant kind, not bound in heme — and it is absorbed several times less efficiently than the heme iron in meat to begin with. The oxalate in the leaves also binds part of the iron and calcium into insoluble compounds that pass straight out. So "eat spinach to load up on iron" works far less well than the content number suggests.
Vitamin C in the same meal (lemon juice, bell pepper, tomato) helps that meal's non-heme iron absorb a bit better. Spinach's calcium is locked up by oxalate too and is poorly absorbed, so calcium has to come from other foods.
Mechanism · Two doorways for iron into the gut wall
Iron has two doorways into the gut wall (Hunt 2003). Heme iron comes from red meat and other animal foods; it uses its own path and is barely bothered by oxalate, phytate, or tea polyphenols. Non-heme iron comes from plants, and it has to be converted to the ferrous (divalent) form before a transporter in the gut wall called DMT1 will carry it in. The door is narrow, and a lot is waiting in line.
Spinach uses the second door, and it brings its own lock: oxalate combines with iron and calcium into insoluble compounds and drags absorption down further. So the content number looks good, but what reaches your body takes another discount.
Two pairings change this narrow door. They do not turn spinach into red meat:
Vitamin C (lemon juice, bell pepper, tomato) reduces ferric iron to ferrous iron and holds onto it so it does not precipitate, which lets that meal's non-heme iron absorb a bit betterDo not count on spinach for calcium: oxalate locks spinach's own calcium inside the gut, so very little is absorbed, and "spinach for calcium" is essentially a myth. Turn it around, though, and calcium from other foods in the same meal grabs the oxalate in the gut first and reduces how much oxalate is absorbed; that is the subject of the chapter on oxalate, kidney stones, and blanching These boosts and blocks are clear in single-meal studies but usually much smaller in a varied everyday diet, and whether your body is short of iron often matters more to how much you absorb than what you ate.
Spinach is low in protein and fat, so it cannot be a meal on its own. Its place on the plate is as a side and a source of folate and vitamin K1, alongside a staple food and some protein.
Chapter 4
Oxalate, kidney stones and blanching
The most common kidney stone is the calcium oxalate stone, and spinach is among the highest-oxalate foods. Part of that oxalate dissolves in water, so a quick boil with the water poured away markedly reduces the soluble oxalate in spinach. That changes the amount of oxalate; it does not turn spinach into a different vegetable.
If you have never had a kidney stone, eating spinach normally is nothing to worry about. If you have had calcium oxalate stones, you can cut back on the highest-oxalate foods such as spinach, and eat them in the same meal as calcium-containing foods: calcium grabs oxalate in the gut, and the two leave together in the stool (see Kidney Stones).
A CLOSER LOOK
The discarded water carries oxalate away
The flow concerns only the soluble fraction; it does not remove all oxalate, and eating the broth keeps that fraction with the meal.
1The soluble part enters water
Oxalate has soluble and insoluble fractions; the soluble part can move into the cooking water.
2Water must leave to carry it away
Discarding that water removes the oxalate that has dissolved out of the vegetable.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
Mechanism · What blanching and mealtime calcium each do
Oxalate comes in soluble and insoluble forms. The soluble part moves into water, so a quick boil with the water poured away markedly lowers the soluble oxalate in high-oxalate vegetables (Chai and Liebman 2005). If you stir-fry or steam, or eat the dish with its broth, most of the soluble oxalate stays in the food.
Blanching does not make spinach "less nutritious." It removes part of the portion that binds iron, binds calcium, and, once it reaches the urine, looks for calcium to crystallize with. The drag on iron and calcium absorption eases a little along with it.
The counterintuitive half is calcium. To prevent calcium oxalate stones, do not avoid calcium — eat enough of it: enough calcium in the meal grabs oxalate in the gut so the two leave together, which reduces how much oxalate is absorbed and how much reaches the urine (see Kidney Stones). So spinach with tofu does not cause stones; it is exactly the pairing that stops oxalate in the gut. "Do not count on spinach for calcium" and "have calcium with the meal" are two sides of one fact: calcium in the gut acts as a decoy for oxalate, not as a calcium source that spinach itself can supply.
If you take warfarin, the point is not to skip leafy greens but to keep your vitamin K1 intake steady rather than swinging up and down; follow your doctor's advice. This is general information; for personal medical questions, ask a doctor.
"Popeye downs a can of spinach and his muscles bulge" is how generations have pictured spinach, and it is where "spinach is an iron powerhouse" comes from. It does not survive a closer look: spinach's iron is non-heme iron, poorly absorbed to begin with, and oxalate ties up part of it, so judging by iron content alone overstates how much iron it gives you.
What spinach is really strong in is folate, vitamin K1, lutein, potassium, magnesium, and nitrate. Treat it as a low-calorie, nutrient-dense leafy green and it lives up to the name; treat it as an iron miracle and you will be disappointed. For efficient iron, the heme iron in red meat and similar foods does the work (see Iron).
Myth · Is the decimal-point story true?
There is also a well-known decimal-point story about spinach: that its iron content was once copied down ten times too high, and that is how Popeye's image was born. The story itself has no solid evidence behind it and is most likely an urban legend.
But it stumbles onto a correct conclusion. By weight, spinach's iron content is not low for a vegetable; the problem was never the amount but the absorption. The door for non-heme iron is narrow, and oxalate holds iron back at that door. So spinach was never an efficient iron food. For iron, heme sources such as red meat are more direct; people who eat no meat need plant iron, such as beans, paired with vitamin C (see Iron).
Not turning any single food into a legend, and seeing clearly where it is really strong, is more useful than remembering any one number.
References · 11
U.S. Department of Agriculture, Agricultural Research Service. (2019). FoodData Central: Spinach, raw and cooked. Very low calorie; rich in folate, vitamin K1, lutein, potassium, magnesium; iron is non-heme and bound by oxalate. fdc.nal.usda.gov
National Institutes of Health, Office of Dietary Supplements. (2021). Vitamin K — Fact Sheet for Health Professionals. Fact sheet (updated March 29, 2021; Wayback snapshot 19 September 2026): free phylloquinone is about 80% absorbed, but absorption from food is much lower - the body absorbs only 4% to 17% as much from spinach as from a tablet; Table 2: natto, 3 ounces, 850 mcg (as MK-7); egg, hard boiled, 1 large, 4 mcg (as MK-4) (fact sheet). ods.od.nih.gov/factsheets/VitaminK-HealthProfessional
Siervo, M., Lara, J., Ogbonmwan, I., & Mathers, J. C. (2013). Inorganic nitrate and beetroot juice supplementation reduces blood pressure: a systematic review and meta-analysis. The Journal of Nutrition, 143(6), 818-826. Dietary nitrate modestly lowered blood pressure (effect strongest from beetroot/leafy-green nitrate). 10.3945/jn.112.170233
EFSA Panel on Contaminants in the Food Chain. (2008). Nitrate in vegetables — scientific opinion. EFSA Journal, 6(6), 689. Leafy vegetables (including spinach) are major dietary nitrate sources; dietary nitrate from vegetables is part of the nitrate-nitrite-nitric-oxide pathway. 10.2903/j.efsa.2008.689
National Institutes of Health, Office of Dietary Supplements. (2024). Iron — Fact Sheet for Health Professionals. Fact sheet (updated September 4, 2025; Wayback snapshot 21 September 2026): RDAs 8 mg/day for men and for women 51+, 18 mg women 19-50, 27 mg pregnancy; UL 45 mg/day from age 14; bioavailability about 14%-18% from mixed diets with meat, seafood and vitamin C and 5%-12% from vegetarian diets; serum ferritin below 30 mcg/L suggests iron deficiency and below 10 mcg/L IDA, but inflammation can raise ferritin; supplemental iron of 45 mg/day or more may cause nausea and constipation; people with hereditary hemochromatosis are at risk of iron overload (fact sheet). Heme vs nonheme: heme iron (lean meat and seafood are the richest sources) has higher bioavailability than nonheme iron, and other dietary components affect it less; calcium might reduce the bioavailability of both forms; heme iron is about 10%-15% of total iron intake in western populations. The sheet gives no separate heme and nonheme absorption percentages (fact sheet, Wayback 2026 snapshot). ods.od.nih.gov/factsheets/Iron-HealthProfessional
Hunt, J. R. (2003). Bioavailability of iron, zinc, and other trace minerals from vegetarian diets. The American Journal of Clinical Nutrition, 78(3 Suppl), 633S-639S. Heme iron is absorbed far more efficiently than non-heme iron and is less affected by dietary inhibitors. The abstract gives NO absorption percentages and does not itself state the heme vs non-heme comparison above: it says iron and zinc absorption is lower from vegetarian than non-vegetarian diets, vegetarians have lower iron stores, adverse effects have not been shown with varied vegetarian diets in developed countries, and routine iron supplementation has no demonstrated benefit. The heme 15-35% / non-heme 2-20% figures sometimes attached to this record are not in it (abstract, PMID 12936958). 10.1093/ajcn/78.3.633S
Chai, W., & Liebman, M. (2005). Effect of different cooking methods on vegetable oxalate content. Journal of Agricultural and Food Chemistry, 53(8), 3027-3030. Boiling markedly reduces soluble oxalate in high-oxalate vegetables such as spinach. 10.1021/jf048128d