Is spinach a source of calcium? Oxalates and mineral absorption
Why only about 5% of the calcium in spinach is absorbed, how kale differs, and better places to get calcium.
· 2 min read · updated
Spinach is often called a good source of calcium, and the tables seem to agree: 100 g of spinach has about 100 mg. But very little of it is absorbed. The reason is oxalates, salts of oxalic acid: they bind calcium and other minerals and make them hard for the body to use (Noonan, Savage, 1999).
What studies found
Heaney and colleagues (1988) compared calcium absorption from spinach and milk in 13 adults: on average 27.6% was absorbed from milk and only 5.1% from spinach. The spinach had slightly more oxalate than needed to bind all its calcium.
That doesn’t make all greens a poor source. Kale is low in oxalate, and its calcium was absorbed even better than milk’s: 41% vs 32% (Heaney, Weaver, 1990).
Calcium in foods
| Food | Calcium per 100 g |
|---|---|
| Cheddar cheese | 707 mg |
| Sardines in oil | 382 mg |
| Sesame seeds | 975 mg |
| Almonds | 254 mg |
| Plain yogurt | 127 mg |
| Whole milk | 123 mg |
| Cottage cheese | 103 mg |
| Kale | 254 mg |
| Spinach | 99 mg |
The table shows content, not how much is absorbed. About five times less of spinach calcium is absorbed than of the same amount of calcium from milk.
Does cooking help?
Boiling cuts soluble oxalates in vegetables by 30–87%: they move into the water, so it’s best drained (Chai, Liebman, 2005). But these studies didn’t measure how much calcium is absorbed from cooked spinach, so it’s still best not to count on it for calcium.
Spinach is still worth eating: it’s rich in vitamin K, folate and carotenoids. Eat it with oil or sour cream — carotenoids are absorbed much better with fat (Brown et al., 2004). Just don’t count on it for calcium.
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- Heaney et al., 1988. Calcium absorbability from spinach. Am J Clin Nutr. PMID 3354496
- Heaney, Weaver, 1990. Calcium absorption from kale. Am J Clin Nutr. PMID 2321572
- Noonan, Savage, 1999. Oxalate content of foods and its effect on humans. Asia Pac J Clin Nutr. PMID 24393738
- Chai, Liebman, 2005. Effect of different cooking methods on vegetable oxalate content. J Agric Food Chem. PMID 15826055
- Brown et al., 2004. Carotenoid bioavailability is higher from salads ingested with full-fat than with fat-reduced salad dressings. Am J Clin Nutr. PMID 15277161