Question: “Do you have any recommendations on how to get enough calcium on a low-carb, no-dairy diet? I've read that vegetables provide calcium, but bioavailability is poor.”
The bioavailability of calcium from different vegetables is highly dependent on the specific vegetables.
Cruciferous vegetables have very good bioavailability. It's better than from milk. Spinach has like close to zero bioavailability. It's terrible and you shouldn't even count it. Nuts and seeds have about 20% of the calcium being absorbed. If you compare that to milk — milk is probably going to be like 30% or 40%. Cruciferous vegetables are going to be like 50% or 55%.
The real problem is the volume. If you look at broccoli or kale and you look at how much volume of those foods do you need to eat in order to get 1000 to 1500 milligrams of calcium a day, which is the target, it's a ridiculously high volume.
I'm a bit skeptical that you want to eat more than say 200 or 300 grams measured cooked of those foods a day because they're increasing your iodine requirement. At some point, they become a liability for your thyroid gland. I think it's best to eat two or three servings of those cruciferous vegetables a day, and that's basically maxing out the calcium that you can get from them. You're just not going to get anywhere near the 1000- to 1500-milligram target.
A low carbohydrate, non dairy containing diet is emulating the traditional diets of the Arctic where plant foods were very limited. How did they get their calcium? They crushed up fish bones. They freeze-dried fish bones, they pulverized them, and they ate the bone powder.
Bone meal is a traditional food. Some consider it as a supplement but it is the historic source of calcium in traditional diets that were low-carb.
This Q&A can also be found as part of a much longer episode, here: https://chrismasterjohnphd.com/podcast/2019/03/08/ask-anything-nutrition-feb-23-2019
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