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Not All Calcium-Rich Foods are Created Equal

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Many foods contain calcium. In addition to milk and other milk products, plant foods like dark green leafy vegetables, nuts, beans and fortified foods also contain this mineral. While some of these foods boast an impressive calcium content, you might not actually be getting as much calcium from them as you think. Why? Because the body does not absorb calcium from all foods in equal amounts. The degree to which vitamins and minerals in food can be absorbed and used by the body is called "bioavailability." In this article, we'll look at how the calcium bioavailability of various foods impacts the amount of calcium that makes its way into your bones.

What is bioavailability?

Simply put, bioavailability refers to the amount of a nutrient actually absorbed and used by your body. For calcium, this represents the amount of calcium incorporated into your bones. The amount of calcium you consume is much less important than the amount actually absorbed and used by your body.

What's important to know is that different foods have different levels of calcium bioavailability. This is because other components in the food either help or hinder calcium absorption. For example, in addition to calcium, milk also contains vitamin D and lactose, two components that contribute to calcium absorption. Plant sources of calcium, however, tend to contain substances that prevent your body from fully absorbing calcium. These substances, including oxalates and phytates, bind to calcium to form salt complexes that the body can't dissolve, which decreases the amount of calcium that your body can use.

For example, rhubarb appears to be one of the best plant sources of calcium, as it contains 368 mg per cup, while milk contains 310 mg per cup. However, even though cup per cup this vegetable contains more calcium than milk, you would have to eat slightly more than 3 cups of rhubarb for your body to absorb as much calcium as it would get from a single cup of milk. This is because your body can absorb 32.1% of the calcium in milk but only 8.5% of the calcium in rhubarb.

The calcium in some fortified foods, like breads and cereals with added vitamins and minerals, can be absorbed by the body at a similar or higher rate as the calcium in milk, meaning that the body makes good use of the calcium that these foods contain. But since these foods often don’t contain as much calcium as milk, you may not be maximizing your total calcium absorption if you rely on these products alone for your calcium intake. For soy beverages, the added calcium tends to settle at the bottom of the carton, and studies have shown that even vigorous shaking of the container may not redistribute the calcium effectively.

Although the concept of bioavailability may seem complex, you simply need to remember that replacing milk and milk products with calcium-equivalent foods can have a negative effect on your overall nutrition. In fact, getting your calcium from non-milk sources instead of dairy products can also reduce your intake of protein, magnesium, phosphorus, and vitamins A, D, B2 (riboflavin) and B12.

The table below shows the calcium content of various foods, the percentage absorbed by your body, and the number of servings you would have to eat to match the calcium absorbed from one cup of milk.

 Equivalencies of bioavailable calcium

Food Serving size Average calcium content (mg)1 Estimated absorption (%)2-4 Calcium absorbed (mg) Servings required to equal 250 mL (1 cup) of milk
Milk products
Milk (whole, 2%, 1%, skim) 250 mL (1 cup) 310 32.1 99.5 1
Cheddar cheese 50g (1.5 oz) 337 32.1 108.2 1
Yogurt 175 mL (3/4 cup) 272 32.1 87.3 1
Vegetables (cooked)
Bok choy 125 mL (½ cup) 84 53.8 45.2 2 ¼
Kale 125 mL (½ cup) 49 49.3 24.2 4
Broccoli 125 mL (½ cup) 33 61.3 20.2 5
Spinach 125 mL (½ cup) 129 5.1 6.6 15 ¼
Nuts & seeds
Almonds 60 mL (¼ cup) 97 21.2 20.6 4 ¾
Sesame seeds 60 mL (¼ cup) 23 20.8 4.8 20 ¾
Legumes (cooked)
White beans 125 mL (½ cup) 85 21.8 18.5 5 ½
Pinto beans 125 mL (½ cup) 42 26.7 11.2 9
Red kidney beans 125 mL (½ cup) 26 24.4 6.3 15 ¾
Breads & cereals
Whole wheat bread 35 g (1 slice) 26 82.0 21.3 4 ¾
Wheat bran cereal 27 g 19 38.0 7.2 13 ¾
Fortified foods
Orange juice with calcium 125 mL (½ cup)  155 36.3 56.3 1 ¾
Tofu, regular, firm or extra firm, raw (prepared with calcium sulphate) 85 g 171* 31 53 2
Soy beverage with tricalcium phosphate 250 mL (1 cup) 319 24 76.6 1 ¼**

* The calcium content of tofu varies according to the brand and depending on the type of tofu.

** Studies show that as much as 40% of the calcium added to fortified plant-based beverages, such as soy beverages, can settle at the bottom of the container, even when shaken.8 The value of calcium absorbed indicated here does not take this issue into account.

Sources

1. Health Canada. Canadian Nutrient File, 2015. https://food-nutrition.canada.ca/cnf-fce/index-eng.jsp. Accessed April 5, 2019.

2. Weaver CM and Heaney RP. “Food sources, supplements and bioavailability” in Calcium in human health. Totowa, NJ: Humana Press; c2006. Chapter 9, Food sources, supplements and bioavailability; p. 129-142.

3. Weaver CM et al. Choices for achieving adequate dietary calcium with a vegetarian diet. Am J Clin Nutr 1999;70(suppl):543S-548S.

4. Weaver CM and Plawecki KL. Dietary calcium: Adequacy of a vegetarian diet. Am J Clin Nutr 1994;59(suppl):1238S-1241S.

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