🥬 Is Spinach Actually a Good Source of Calcium?

More calcium. I’ve been trying to support my mom’s calcium, and the struggle hasn’t been the calcium itself. It’s been ensuring absorption amidst seemingly endless gut turmoil. That, and finding foods that are rich in calcium, but aren’t dairy.

When we see leafy greens in the list, spinach is one of the first that comes to mind. We’re so used to thinking of it as a health food. (Thank you, Popeye.)

And it does contain calcium. But it’s also full of one of the biggest culprits there is blocking calcium absorption.

So today, I’m taking a look at whether spinach actually ends up being a good source of calcium. And more broadly, how can we tell how the calcium value listed in the label translates to absorption?

The calcium on a food’s label is the total amount it contains. What you actually absorb is a fraction of that. And sometimes that fraction is tiny.

Dairy and fortified foods do pretty well. They give you about 30% of their calcium. A glass of milk’s ~300 mg becomes roughly 90 mg for your body.

This is average. Across the whole global food supply, the amount of calcium people actually absorb is about 31%.

The problem is the foods where the fraction is far lower, especially when you are actively trying to boost the calcium in your body.

Oxalate takes hold of the calcium in your gut and forms a salt that won't dissolve. So that calcium is locked up, can’t be absorbed, and passes straight through you.

It's the single most powerful blocker, about 17 times stronger, gram for gram, than phytate (which is the other blocker and mostly matters in large amounts).

When researchers fed women spinach and milk with the calcium tagged so they could track it, only about 5% of the spinach calcium got absorbed, against about 28% from milk in every single person tested.

An independent European review came to the same conclusion: oxalate-rich foods give around a third of what milk does. You would need roughly 16 servings of spinach (‼️) to match the absorbable calcium in one cup of milk, versus about 2 servings of bok choy.

Kale, bok choy, broccoli, and collards don't have oxalate buildup. In the same kind of test, women absorbed calcium from kale at least as well as from milk. Kale has just as much fiber as spinach, so it isn't the fiber that kills calcium absorption. It's the oxalate.

Fortified foods absorb well because the things calcium gets added to (e.g., juice, water, most plant milks) are naturally low in oxalate and phytate. The form of added calcium ends up mattering most here.

In women drinking calcium-fortified soy milk, the kind made with calcium carbonate absorbed as well as cow's milk, while the kind made with tricalcium phosphate absorbed less.

It's worth making a conscious decision: calcium carbonate is the form to look for.

Mostly no. So that's reassuring.

When researchers had women eat oxalate together with milk, the milk's calcium is absorbed just as well as when they drink it on its own. The oxalate locks up the calcium in its own food matrix. It doesn't affect other foods across your plate or any of your milk intake.

So a handful of spinach in a cheese omelette won't wreck the omelette's calcium. The spinach just doesn't really contribute.

If you're looking up how much calcium is in the foods you eat, here's what's worth understanding.

The calcium numbers on the label (or in the big USDA food database that everyone's numbers trace back to) are total calcium: that is, what the food contains, measured in a lab. These numbers are not indicative of what you actually absorb.

HOWEVER, the National Academies' daily target (1,000–1,200 mg, the higher end after 50) is how much to eat, counted straight from those USDA numbers. It's already set around your body's average absorption, so for most foods there's no extra math.

🏷️ Next time you buy fortified milk or a supplement, read the salt name and pick "calcium carbonate." Carbonate absorbs like cow's milk. Tricalcium phosphate absorbs less.

🥤 Shake fortified plant milk before every pour. The added calcium is a powder that sinks to the bottom of the carton. It only counts if it's dispersed and actually in your glass.

💧 Check the calcium value on your water this week. Some mineral waters are calcium-rich, and that calcium absorbs about as well as milk's. It's printed as mg per liter. A rich one adds a few hundred milligrams a day for free.

Here’s what caught my eye this week:

📄 Having more children didn't weaken women's bones later: In 41,274 Norwegian women tracked over a decade, those with four children broke fewer bones later than those with two. Pregnancy does pull calcium from your skeleton, but this doesn't support the idea that it leaves you permanently worse off.

📄 Extra protein on its own didn't change women's bodies: Across 15 trials in 1,161 women, animal protein beat soy for fat loss and lean mass, but only where women were also cutting calories and strength training. Protein makes those work better. It doesn't replace them.

🔎 "Ferritin face" went from TikTok to Vogue this month: Low iron really can cause grey skin and dark under-eye circles, but so can poor sleep, stress, allergies and age, so you can't read your ferritin levels using only a mirror. (I wish.) Get a ferritin test, then ask what could be causing it: usually heavy periods, tummy troubles like inflammation, or something else driving poor inflammation.

Just sharing here what's new this week. No affiliates, no sponsorships, no freebies. A mention here isn't a recommendation.