My biggest nutrition concern is calcium and bone health. I have heard about oxalates in spinach affecting the absorption of calcium. I just turned 50 and I need to maximize my calcium absorption — how much does it really affect the absorption?
That’s a great nutrition question Dawnette, and thanks for writing. You have heard correctly that oxalates (also known as oxalic acid) can reduce the amount of calcium you absorb from some sources. Oxalates are present in some vegetables, as well as in tea, chocolate, some fruits and some nuts. And yes, although spinach is high in calcium, it is not a very bioavailable source because of its high oxalate content.
The impact of oxalates isn’t a significant factor if you can consume dairy products for calcium, since the amount calcium in dairy products is so abundant. But if you are looking for plant-based sources of calcium, the impact of oxalates can be worth remembering. Since plant sources of calcium tend to have many fewer milligrams of calcium per serving, it makes sense to make sure you are maximizing bioavailability.
How much do oxalates or oxalic acid reduce the absorption of calcium?
To give you some idea of the numbers, let’s look at some research data. One study in animals (not humans, but we can use it for getting an idea) showed that milk calcium was 92% absorbed, while calcium from oxalate-rich greens was only 75% absorbed. Okay, so from that study it looks like calcium from oxalate rich greens is significantly less bioavailable than dairy, but three quarters being absorbed still is pretty beneficial.
I did find a study with human subjects, (premenopausal women to be exact), which showed a much more dramatic effect of oxalate content on calcium absorption. Using some popular Chinese vegetables, the researchers found that calcium bioavailability seemed to correlate well with oxalic acid content. The calcium from mustard greens and cabbage flower leaves was very well-absorbed, the calcium from sweet potatoes was intermediate in bioavailability, and the calcium from spinach and rhubarb was very poorly absorbed (only 23% and 25% as bioavailable as the well-absorbed sources, respectively). Yikes! Only a quarter as absorbable is a big difference.
Another study conducted in Thailand concluded that sesame seeds (which are high in not only oxalate but also phytate, another inhibitory factor) provide virtually no absorbable calcium in an in vitro assay, despite the seeds themselves being quite rich in calcium. The same was found for amaranth (the green vegetable, not the grain. See photo.)
So it would appear that using high-oxalate vegetables like spinach as your main sources of calcium wouldn’t be the best strategy. If you look up which vegetables are highest in calcium, you’ll see some sources topping the list which aren’t very bioavailable (spinach, amaranth, and sesame seeds for example). But there’s plenty of good news too: many vegetables are low in oxalate, and can provide highly bioavailable sources to help you meet your calcium needs.
Vegetables in the brassica family appear to have very well-absorbed calcium, possibly even greater than the calcium in milk. Evidence indicates that broccoli, bok choi, and kale are highly bioavailable calcium sources in humans. Some other bioavailable sources of vegetable calcium include turnip or collard greens, and Chinese cabbage. So if spinach is your go-to leafy green, why not try branching out and experimenting with some other varieties of vegetables? If you aren’t sure what to do with a new vegetable but want to try it, I know who you can ask.
Full disclosure: I cannot feign objectivity here. I love kale and will promote it shamelessly at every opportunity.
High Calcium, Low Oxalate VegetablesTurnip greens: 105 mg per 1 cup chopped Kale: 90 mg per 1 cup chopped Okra: 81 mg per cup, raw Chinese Cabbage (aka bok choi, pak choi): 73 mg per 1 cup shredded Butternut Squash: 67 mg per cup raw cubes Collards: 52 mg per 1 cup chopped Acorn Squash: 46 mg per 1 cup raw cubes
Broccoli: 42 mg per 1 cup chopped Celery: 40 mg per 1 cup chopped
Brussels Sprouts: 37 mg per 1 cup raw
Cabbage: 33 mg per 1 cup shredded
Arugula: 32 mg per 1 cup raw
Now, if you can consume dairy products or calcium-fortified foods, you probably don’t need to worry about the oxalate or oxalic acid content of foods. But if you are vegan, dairy intolerant or allergic, and choose to avoid supplements, it’s a good idea to try to work in as many highly bioavailable plant sources of calcium as you can.
Some other tips to ensure you absorb and retain the most calcium possible from your diet:
- Avoid eating high fiber foods (like wheat bran cereal) together with calcium sources. One exception is the prebiotic fiber inulin, which appears to increase calcium uptake in the intestine.
- Supplement your diet with adequate vitamin D (at least 600 IU for adults, I’d recommend 1000 IU)
- Spread your calcium out, whether from food or supplements. You’ll miss absorbing much of it if you take it more than 500 mg at once.
- High sodium intake increases calcium excretion, so moderate your salt intake by choosing unprocessed foods.