Calcium and Bioavailability
Bioavailability is simply the amount of a nutrient the body absorbs and uses from a food.
Many people believe that plant foods are reliable sources of calcium that can easily substitute for dairy products, well-known for their particularly high calcium content. However, how much calcium a food contains is not all that matters. It must also be absorbed so that our bones and whole body benefit from it. Generally speaking, plant foods contain a considerable amount of substances like oxalates and phytates that bind to calcium and thus inhibit the latter's absorption.
The most popular example to illustrate this is spinach: cooked spinach contains 129 mg of calcium per 125 ml (1/2 cup), which is higher than almost everything except milk (315 mg) and other dairy products. But only an estimated 5% (6 mg in absolute value) of it is actually absorbed, which is very little compared to 32 % of milk's calcium (101 mg). Therefore, approximately 15 servings of spinach would be required to equal a single serving of milk in terms of absorbed calcium. However, these delicious leafy vegetables are packed with many other vitamins and minerals that are important for a balanced diet.
It is also important to understand that the Dietary Reference Intakes (DRI) have been determined in terms of the actual calcium content of food, but assuming an average 30-40 % absorption rate. That is why dairy products are the ideal food to most easily meet daily calcium requirements. You can get all your calcium from plant sources, but it might turn out to be more difficult – unless you have a very big appetite!
The table below details the calcium contained in selected sources in decreasing order of the amount absorbed by the body.
| Food | Serving size | Calcium content (mg) | Estimated absorption (%) | Calcium absorbed (mg) | Servings required to equal 250 mL (1 cup) of milk |
|---|---|---|---|---|---|
| Whole milk, 2%, 1%, skim | 250 ml (1 cup) | 315 (average) | 32,1 | 101 | 1,0 |
| Milk | |||||
| Fortified soy beverage (fortified with tricalcium phosphate) | 250 ml (1 cup) | 315 | 23 | 73 | 1,4 |
| Vegetables | |||||
| Broccoli, boiled | 125 ml (1/2 cup) | 33 | 53 | 18 | 5,6 |
| Cabbage, boiled | 125ml (1/2 cup) | 25 | 65 | 16 | 6,2 |
| Brussels sprouts, boiled | 125ml (1/2 cup) | 30 | 64 | 19 | 5,3 |
| Chinese cabbage (bok choy), boiled | 125ml (1/2 cup) | 84 | 54 | 45 | 2,2 |
| Kale, boiled | 125ml (1/2 cup) | 49 | 59 | 29 | 3,5 |
| Kohlrabi, boiled | 125ml (1/2 cup) | 22 | 67 | 15 | 6,7 |
| Cauliflower, boiled | 125ml (1/2 cup) | 10 | 69 | 7 | 14 |
| Watercress, raw | 125ml (1/2 cup) | 22 | 67 | 15 | 6,6 |
| Spinach, boiled | 125ml (1/2 cup) | 129 | 5 | 6 | 16,8 |
| Turnip greens, boiled | 125ml (1/2 cup) | 104 | 52 | 54 | 1,8 |
| Turnip (rutabaga), boiled | 125ml (1/2 cup) | 43 | 61 | 26 | 3,9 |
| Radish, raw | 125ml (1/2 cup) | 15 | 74 | 11 | 9,2 |
| Nuts and seeds | |||||
| Unblanched, dry-roasted almonds | 125ml (1/2 cup) | 206 | 21 | 43 | 2,3 |
| Sesame seeds, shelled, toasted | 125ml (1/2 cup) | 89 | 21 | 19 | 5,3 |
| Legumes | |||||
| Beans, white, boiled | 125ml (1/2 cup) | 85 | 17 | 14 | 7,1 |
| Beans, pinto, boiled | 125ml (1/2 cup) | 42 | 17 | 7 | 14,4 |
| Beans, red, boiled | 125ml (1/2 cup) | 26 | 17 | 4 | 25,3 |
Bioavailability of calcium in soy beverages
Heaney et al. conducted a study to evaluate the bioavailability of tricalcium phosphate (1 of the 2 most common forms of calcium used to fortify soy beverages) in comparison to the calcium found in cow's milk. Their results showed that the bioavailability of tricalcium phosphate accounted for 75% of the calcium found in cow's milk.3
Find out more on the bioavailability of calcium in soy beverages ›››
References
- Health Canada. Canadian Nutrient File 2007. 2007.
- Weaver CM and Plawecki KL. Dietary calcium: adequacy of a vegetarian diet. Am J Clin Nutr 1994;59(suppl):1238S-41S.
- Heaney RP et al. Bioavailability of the calcium in fortified soy imitation milk, with some observations on method. Am J Clin Nutr 2000;71:1166-9.
Keywords: calcium
Nutrients in Milk Products Keywords
Related Articles
-
Bioavailability of Calcium in Soy Beverages
Health professionals are increasingly aware of the importance of mineral bioavailability in the foods we eat. Calcium is of particular importance, as it represents one of the most under-consumed nutrients in North America. Because of the current popularity of calcium, many...
Continue reading -
Calcium and Vitamin D: Improved Lipids
A study performed on women-one group that received calcium and vitamin D supplements and one that did not - has provided new data on the correlations between calcium and weight loss. The study showed that calcium and vitamin D supplementation has a beneficial effect on blood lipids and lipoproteins in obese or overweight women.
Continue reading
