Absorbing Calcium

Chinese Flower Cabbage aka “Choy Sum” aka Brassica rapa var. The leafy green with the most calcium.

Many vegetables are actually better for preventing bone loss than cow’s milk. Preventing osteoporosis is about more than calcium intake. It is also about calcium absorption and the calcium in many vegetables is more absorbable than calcium from cow’s milk. A number of vegetables also have more calcium per calorie than dairy milk and vegetables have other nutrients for bone health that dairy milk does not.

Brenda Davis RD, is a coauthor of the American Dietetic Association’s Position Paper On Vegetarianism. She is also a vegan and a nutrition book author. Her revised edition of “Becoming Vegetarian” has an excellent chart of calcium from various foods and how much you can expect to absorb. Note. Davis’ chart listed the rates of absorption for vegetables per half cup. The rates may not be the same in different amounts. Highlights from page 103:

Cow’s Milk
1 cup – 300 mg – 32% absorbed, 96 mg net

Chinese Cabbage Flower Leaves, cooked
( aka “choy sum”, Brassica rapa var. parachinensis)
1 cup – 478 mg – 40% absorbed, 192 mg net

*Chinese* Mustard Greens, cooked
1 cup – 424 mg – 40% absorbed, 170 net

Turnip Greens, cooked
1 cup – 198 mg – 52% absorbed, 102 mg net

Bok Choy, cooked
1 cup – 158 mg – 53% absorbed, 84 mg net

Mustard Greens, cooked
1 cup – 128 mg – 58% absorbed, 74 mg net

Kale, cooked ( scotch kale has much more )
1 cup – 122 mg – 49% absorbed, 60 mg net

White Beans, cooked
1 cup – 226 mg – 22% absorbed, 50 mg net

Broccoli, cooked
1 cup – 70 mg – 61% absorbed, 42 mg net

Sesame seeds, without hulls
1 ounce – 37 mg – 21% absorbed, 8 mg net

Tofu, made with a calcium based coagulant
1 cup – 516 mg – 31% absorbed, 160 mg net

Warning: not all vegetables are a good sources of calcium.

Some vegetables have a lot of calcium, but also a lot of oxalic acid which binds up the calcium so people can’t absorb it. Vegetables that are healthy to eat, but that are poor calcium sources are spinach, rhubarb, swiss chard and beet greens.

Spinach, cooked
1 cup 230 mg – 5% absorbed, 12 mg net

Rhubarb, cooked
1 cup 348 mg – 8.5% absorbed, 30 mg net

Lettuce too. Lettuce is low in oxalic acid and has a lot of nutrition per calorie but lettuce is so low calorie the amount you would have to eat to get a significant amount of nutrition is impractical.

1 cup of shredded raw romaine lettuce will only provides 15 mg of calcium gross, without taking amount absorbed into consideration.

I couldn’t find any information about absorbability for collard greens. The USDA online nutrition database lists that 1 cup of cooked, chopped collard greens has 266 mgs of calcium.

Krause’s Food, Nutrition, & Diet Therapy textbook (2000) lists the following amount of oxalates per 100 g:

Spinach (boiled) 750 mg
Collards 74 mg
Kale 13 mg

In other words, without a figure for the absorbability of the calcium in collard greens and assuming that oxalates are the only impediment it looks like collard greens are still a decent calcium source.

I couldn’t find any information about the absorbability of calcium from butternut squash, but that squash does have 84 mg of calcium per 1 cooked cup.

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17 thoughts on “Absorbing Calcium”

  1. “Lettuce too. Lettuce is low in oxalic acid and has a lot of nutrition per calorie but lettuce is so low calorie the amount you would have to eat to get a significant amount of nutrition is impractical.”

    Not if you’re a crazy person (e.g. ion).

    I try to eat a lot of leafy greens, and mustard greens are my favorite. But I had my bone density tested a couple of years ago, and found that it’s below average (although still healthy…for now). That was the kick in the pants I needed to get serious about taking a calcium/vitamin D supplement. Osteoporosis is scary.

    I’ve often wondered, though: A lot of times, the nutrition facts labels on frozen broccoli and frozen greens show calcium levels that aren’t nearly as high as the ones you’ve listed. Why is that?

  2. 1. Not every instance of a class of food is exactly the same. A chunk of broccoli grown in 2009 in poor soil with bad weather that year is not going to have as much nutrition as a chunk of broccoli grown in volcanic soil in a season with good weather.

    2. Measuring by volume ( i.e. 1 cup cooked, boiled etc ) is not precise. Two people stuffing broccoli into a measuring cup will not have exactly the same amount.

    3. Laws governing the listing of whats in food give (intentionally) false impressions. For example, if a food product has less than 1/2 gram of fat per serving food packers are allowed to call it “fat free”. A serving can be any size they like.

    4. Improved methods of testing. It used to be thought that plant foods don’t have any cholesterol until improved testing show that they actually have minuscule amounts:

    5. Human error

    6. There is little incentive for retesting new crops of food. My guess would be that measuring nutrition is something frozen food makers are made to do. They probably pay for tests one time on one crop and then keep recycling those results for years. You can see an example of that sort of thing on food labels for different flavors of the same packaged food. Double Death By Chocolate raisins having the same calories as Lemon Flavored raisins, etc.

    There are probably all sorts of other little objective things as well

  3. Your intake of greens was probably fine, IMHO.

    I had a coworker in college who had osteoporosis in her 20s and broke her foot when she kicked her wall in bed. Her doctor told her it was because of the liter bottle of diet cola she had permanently glued to her hand.

    All the milk in the world wouldn’t have prevented that.

    People concerned about osteoporosis focus on calcium intake, often to the exclusion of other factors. Like drinking a lot of soda, using too much sodium, smoking, lack of adequate stress to the bones etc.

    That is why I like the article I quoted above, it mentions those other things.

  4. Yeah, I figured that most of those factors would come into play, although I didn’t think they would have such a large effect. Then again, maybe it’s a case of a bunch of small effects adding up to a large difference.

    I would guess that the biggest issue would be #1. Calcium content in plants is limited by the calcium content in the soil. Are farmers taking care to add calcium back to the soil when it’s been depleted? I would guess probably not, or at least they don’t think so much about it. For most people, the calcium content of vegetables is not a huge selling point, because most people associate calcium with dairy and dairy only. So maybe the farmers happen to use a fertilizer that happens to add calcium (like bone meal maybe?) and maybe they don’t. (At least, this all seems reasonable to me. But I don’t know what I’m talking about.)

    That raises a new question: Why are we to believe that PCRM’s figures are any more representative of the calcium content of vegetables than the frozen food packagers’ figures are? PCRM is a great organization, but they do have an incentive to give people the best possible impression of the nutritional completeness of vegetables – so I can see that given a choice of test results, they might present the higher ones. The frozen food packagers don’t care so much about that, but at least they don’t have any reason to try to mislead people into thinking that vegetables have less calcium than they actually do.

    Last but not least, where can I get some of them Double Death By Chocolate Raisins? 🙂

  5. When I was 17, I broke my leg when I stumbled getting out of bed. Freaked my parents and me right out, but the doctors said that my bones weren’t so much the problem – I must have just fallen in a way that bones haven’t evolved to handle. (Probably a result of all the crap I had piled on my bedroom floor.)

    I know that calcium’s not the only factor that determines whether you get osteoporosis. But calcium supplements are cheap, and better safe than sorry, right?

  6. Exactly. I take calcium supplements. People overestimate how well they eat and I am no exception.

    My college cowoker had her bones scanned when her foot broke. She had osteoporosis from drinking all of that diet soda.

    The figures from the PCRM are in the article and the article has references at the end. I haven’t checked them out. The figures I quote in my blog post above come from Brenda Davis RD, from her book “Becoming Vegetarian”. She has references in her book and she has a fairly good reputation for being conservative about nutrition.

  7. I spot checked some values against the USDA nutrition database. Some were spot on, some off:


    Someone also told me that lower quality crops end up in processed foods. Could be it along with all of the other factors.

    It doesn’t bother me. A cup of cooked greens is a really small amount if you measure it out. It is easy enough to eat more in terms of “room” and calories.

  8. The amount of greens I eat tends to be limited by the following:

    1. They’re on the expensive side ($3/lb for my mustard greens at the Takoma Park farmers’ market as of two days ago).

    2. It sometimes takes a lot of time to prepare them by removing dirt, bugs, and tough-as-ropes stems (and this tends to be more of an issue for cheaper varieties/sources, so there’s a trade-off between time and money).

    Any suggestions for overcoming either of these?

  9. Try another farmers market, seriously. I’ve been to the Takoma Park Farmers market. The prices there are the same or higher than the supermarkets as well as the co-ops. There are a number of farmers markets in the area.

    Soaking greens in ice water will get rid of a lot of dirt effortlessly. To quicken removing the stems I usually stack them on top of one another, roll them into a cylinder, cut off disks of rolled up greens and discard the disk with the stems.

    This method isn’t perfect, but it speeds up removing the stems.

    I think if you ask on the veg-dc email list someone will know of a local guide to the farmers markets in the area.

  10. I’ve been to other farmers’ markets. I like the College Park and Riverdale ones – they both have some good bargains, and they’re within walking distance for me, which is a bonus. But they’re shut down for the season now. This time of year, there’s Takoma Park, Dupont Circle (most of the same vendors but with higher prices, it seems), and not a whole heck of a lot else – at least, not that’s readily Metro-accessible from where I am.

    What I should really do is stop complaining. Good food (which the Takoma Park market pretty consistently delivers) costs good money, and I can afford it really. I’m just a little bit uneasy lately about the attack of the inflation monster. That, and I’m a cheapskate.

    I’ll try the ice water and rolling-up tricks, though. Thanks.

  11. I will have to try those two markets out.

    The one in TPSS is now to browse, but the prices there are the same or worse than organic greens at the local co-ops.

    The last time I was at the one in College Park ( 1990s ) it seemed like they had the exact same stuff from the distribution points where the supermarkets get their stuff.

    Do they have organic greens there that are cheaper?

  12. Have you noticed fresh turnip greens at any of the farmer’s markets in the D.C. area? It has been a while since I have been to the farmer’s markets. Are most like the one in Takoma Park where the prices are the same or do most have some bargains?

  13. The main vendor at the College Park market is a distributor that sells mostly stuff that is trucked in from who-knows-where (and tastes absolutely vile). They do have a few local items that are clearly marked “homegrown.” But I go there mostly for the other vendors, who all sell local stuff. “Thanksgiving Farms,” run by a crazy old guy who counts back your change to you in German, is my favorite, especially in the late summer, when they have varieties of eggplants, tomatoes, and peppers that I’ve never seen anywhere else. And they have “German honey beets,” which are awesome. Once in a while their things have an insect content that’s rather unpleasantly high, but it’s all cheap enough that I don’t feel bad if I have to throw out one or two things. They don’t advertise their greens (or anything else) as organic. I think they also go to the Greenbelt farmers’ market, but I’ve never been to that one.

    I got some spinach for free once at the Riverdale market. The vendor had a whole lot of it, and it was just starting to go bad, so they were giving it away by the bagful to anyone who would take it. But I don’t remember what I usually pay for greens there. Less than $3/pound, certainly.

    I know Thanksgiving Farms has turnip greens at least sometimes, but they’re done for the season now. I think the lady in Takoma Park from whom I bought my mustard greens last week might also have had turnip greens, but I don’t remember.

    I don’t remember a whole lot about other markets I’ve been to – just that none of them beats Takoma Park, College Park, and Riverdale on the combination of quality, selection, price, and convenience for me. There are a bazillion seasonal Saturday morning markets that I’ve never been to – I’m happy with College Park’s seasonal Saturday morning market, so I haven’t explored any others.

  14. I don’t remember a whole lot about other markets I’ve been to – just that none of them beats Takoma Park, College Park, and Riverdale on the combination of quality, selection, price, and convenience for me.

    Very useful information! I take it that College Park and Riverdale are done for the season. The market at Takoma Park is still running despite winter?

  15. Yes, that’s right. College Park and Riverdale both run from May through November or so. Sometimes (in 2006 and 2008, but not in 2007 I don’t think) the Thanksgiving Farms people continue to show up in College Park through December. Takoma Park is year-round, although many of the vendors and most of the customers do not come in the winter. But there is still plenty of good stuff to be had, especially if you like apples.

  16. I went to the TKPK market today after 2. There wasn’t much there besides apples and cheese. No surprise for the dead of winter. I did find mustard greens in the Giant in TKPK for 79 cents a pound.

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