Spinach

Spinach (Spinacia oleracea)

Spinach (in Hebrew, “תרד”) is one of the most nutrient-dense vegetables there is: 100 grams provide four times the daily requirement of vitamin K, half of the vitamin A and almost half of the folate — in just 23 calories.

And yet, the two things spinach is most famous for are actually its weakest points: its iron is barely absorbed, and its calcium is almost unavailable. Both are the fault of the same compound — oxalates.

That doesn’t make spinach a bad vegetable. It just means it’s worth eating it for what it is truly good at.

Nutrition facts per 100 grams

Spinach’s real strength lies in the first three highlighted rows and the lutein row — not in the iron row.

Iron — the story everyone knows, and it isn’t true

Spinach contains 2.7 mg of iron per 100 grams, which sounds pretty good. The problem is availability:

• This is non-heme iron, which is absorbed much less well than iron from animal sources.
• The oxalates in spinach bind it and lower absorption even further — estimates speak of single-digit absorption rates.

As for the popular story that a decimal point misplaced in the 19th century created the spinach myth — that story itself is disputed, and later historical research found no basis for it. What is true and relevant: spinach is not a practical source of iron. Anyone who needs iron will find it in red meat, in lentils with vitamin C, or in a supplement — not in a spinach salad.

What does help: vitamin C in the same meal significantly increases the absorption of non-heme iron. Lemon juice on spinach is not just about flavor.

Oxalates — what they are, who should care, and what helps

Oxalic acid is a plant compound that binds divalent minerals — mainly calcium, iron and magnesium — to form insoluble salts. Spinach is one of the most oxalate-rich foods: about 750 mg per 100 grams.

Two consequences:

• The calcium in spinach is barely absorbed. The absorption rate from spinach is around 5%, compared with about 32% from milk. The 99 mg of calcium listed in tables is in practice about 5 mg.
• Kidney stones. In people who have had calcium oxalate stones, large amounts of spinach increase urinary oxalate excretion. The solution is not avoidance but pairing: eat spinach together with a source of calcium — yogurt, cheese, tahini — so that the oxalate binds to calcium in the gut and is excreted in the stool, instead of being absorbed and reaching the kidney. And drink plenty.

What really reduces oxalates: boiling. Studies show a 30-87% drop in oxalate in boiled spinach, as it dissolves into the water. Steaming reduces it much less, and frying hardly at all. If oxalates concern you — boil and discard the water.

Benefits

• Vitamin K in an exceptional amount: 403% of the Daily Value per 100 grams. Essential for clotting, and for directing calcium to bone rather than to the walls of blood vessels.
• Lutein and zeaxanthin: 12,200 mcg per 100 grams — one of the highest sources in food. These carotenoids accumulate in the macula and filter blue light; in observational studies their intake is linked to a lower risk of age-related macular degeneration.
• Folate: 49% of the Daily Value — very significant during pregnancy and the childbearing years.
• Vitamin A as beta-carotene: 52%.
• Natural nitrates. Spinach is rich in nitrate, which the body converts to nitrite and nitric oxide (NO). This pathway has been studied in the context of blood vessel dilation, blood pressure and mechanical efficiency in endurance exercise — the area where beets are better known, but spinach is just as rich in nitrate.
• Magnesium: 19% of the Daily Value.
• Very low energy density: 23 calories per 100 grams.

Downsides and considerations

• Oxalates — as detailed above: calcium that isn’t absorbed, iron that isn’t absorbed, and a consideration with kidney stones.
• Vitamin K and anticoagulants. 403% per 100 grams is about the highest amount in any food. People taking Coumadin (warfarin) don’t need to avoid spinach — they need steady, consistent intake, so that the drug dose is calibrated to it. A sudden bowl of spinach after a month without is the problem, not the spinach.
• Dramatic shrinking when cooked. 500 grams of fresh spinach shrink to about 100 grams cooked. This changes the serving calculation in both directions — it’s easy to eat a huge dose of vitamin K without noticing.
• Nitrate and infants. Infants under three months old are not usually given high-nitrate vegetables, because of the risk of methemoglobinemia. This is not a consideration for older children and adults.
• Canned spinach contains added sodium and loses vitamin C.
• Short storage life. Fresh spinach leaves wilt within days and lose some of their folate and vitamin C.
• Nitrate and cadmium accumulation. Under certain growing conditions spinach can accumulate nitrate and cadmium at high levels; this is a reason to vary leafy greens and not rely on spinach alone.

Practical use

• Eat it both raw and cooked. Raw — to preserve vitamin C and folate. Cooked — fewer oxalates and a more concentrated amount of vitamin A and lutein.
• Always with fat. Vitamin K, vitamin A, lutein and zeaxanthin are all fat-soluble. Olive oil, avocado or walnuts in the salad significantly increase absorption.
• With vitamin C — lemon, red bell pepper, tomato — to improve iron absorption.
• With a source of calcium — yogurt, cheese or tahini — so that the oxalate binds in the gut and not in the kidney. This is especially critical for people prone to kidney stones.
• Boil and discard if oxalates are a consideration — 30-87% of them pass into the water.
• Don’t cook it in iron or aluminum pans for a long time — spinach oxidizes and darkens, and the metallic taste intensifies.
• Vary your greens: kale, Swiss chard, arugula and lettuce — different profiles, and some have much lower oxalates. See the spinach, kale and chard comparison.
• Frozen spinach is an excellent choice. It is blanched and frozen close to harvest, keeps for months, and is significantly cheaper than fresh.

Summary

Spinach is an excellent vegetable — but not for what it is famous for. Its iron and calcium are barely absorbed because of oxalates; if you’re after them, you need another source. What it does offer: vitamin K in an amount that has no substitute (403%), lutein and zeaxanthin at some of the highest levels in food, folate, vitamin A and natural nitrate. Three practical rules: always with fat (the vitamins are fat-soluble), with a source of calcium (the oxalate binds in the gut and not in the kidney), and boil and discard when oxalates are a concern. And for people on Coumadin — don’t avoid it, keep it consistent.