Excerpts from Ginny Messina for theveganrd,com on August 29, 2013
If your diet is built around a variety of whole plant foods including at least three servings of legumes (beans, soyfoods and peanuts) per day, and your calorie intake is sufficient, you won’t have any trouble getting enough protein. If you are on a reduced calorie diet or you’re an athlete, you may need more than the three servings of legumes.
Calcium is a little different. It’s not that it’s difficult to meet needs, but you do need to make a bit of effort to include calcium-rich foods in your diet. Both the Institute of Medicine (the government agency that establishes the RDAs) and the World Health Organization recommend 1,000 milligrams of calcium per day for adults (or 1200 for those over 50).
The recommendation is based on the assumption that we absorb about 30 percent of the calcium in our diet—since that’s the absorption rate for calcium from cow’s milk. That is, our biological need for calcium is around 250 to 300 milligrams of calcium per day, but we need to consume as much as 1000 milligrams in order to absorb enough. Or at least that’s true if you get most of your calcium from cow’s milk.
For certain leafy green vegetables, absorption rates are considerably higher. For example, we absorb between 50 and 60 percent of the calcium in cruciferous leafy green vegetables like kale and turnip greens. In contrast, absorption rates for calcium from beans is fairly low at about 17 percent. And for oxalate-rich vegetables like spinach, absorption is extremely low, only around five percent.
So, it’s really better to pay attention to our biological needs and the amount of calcium we’re likely to absorb from a food, rather than the amount that is actually in foods. Unfortunately, we don’t have this information for very many foods.
The table below shows the amount of calcium absorbed from a few foods that are relevant to vegan diets (with cow’s milk included for comparison). And we can use a couple of extreme examples to show just how important the absorption issue is. For example, if you ate three cups of cooked turnip greens as your sole source of calcium, your intake—about 600 milligrams—would be well below the calcium RDA. But you’d still absorb enough to meet biological needs. Alternatively, you could meet the calcium RDA by eating about 4 1/2 cups of white beans, but you’d absorb only around 170 milligrams which is well below biological needs.
I typically get my calcium from a mix of fortified orange juice, tofu, and greens. I mostly aim for the RDA, but on the days when I’m eating a lot of greens, I don’t worry about it quite as much. I don’t exactly micromanage this, but I do pay attention to it. I don’t depend on beans and nuts for my calcium too much because the absorption is so poor, but I know that they contribute at least some calcium to my overall intake.
Getting calcium from fruits and vegetables might have some advantages since diets rich in these foods are linked to improved bone health. This may be because plant sources of calcium are often high in potassium, vitamin C, and vitamin K, all important for keeping bones strong. There has also been interest in the idea that isoflavones in soymilk and tofu might protect bone health, but the findings on this are pretty conflicting.
So, bottom line: Vegans cannot ignore calcium, but it’s relatively easy to get enough. And there may be benefits for everyone to getting calcium from plants.
*amount will vary considerably depending on firmness of tofu
**absorption from collards hasn’t been measured but I think it’s safe to assume that it is at least 50%
References for calcium absorption:
Weaver CM, Plawecki KL. Dietary calcium: adequacy of a vegetarian diet. Am J Clin Nutr 1994;59:1238S-1241S.
Zhao Y, Martin BR, Weaver CM. Calcium bioavailability of calcium carbonate fortified soymilk is equivalent to cow’s milk in young women. J Nutr 2005;135:2379-82.
Heaney RP, Dowell MS, Rafferty K, Bierman J. Bioavailability of the calcium in fortified soy imitation milk, with some observations on method. Am J Clin Nutr 2000;71:1166-9
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