The bioavailability of calcium refers to the fraction of dietary calcium that is potentially absorbable and the incorporation of the absorbed calcium into bone.
Various dietary factors can affect calcium bioavailability. Some food components act synergistically to promote calcium absorption. They include:1,2
- vitamin D,
- lactose,
- casein phosphopeptides in milk.
Cow’s milk has good bioavailability of calcium (about 30 to 35%). It is estimated that without milk and milk products in the diet, less than half of the calcium requirements would be met. In fact, adolescents in Canada who have a Western type of diet are unlikely to meet their recommendations for calcium if they do not consume milk or milk products.3
Plant foods contain many vitamins and minerals that are important for a balanced diet and can be a source of calcium. However, generally speaking, plant foods contain a considerable amount of inhibitory substances, such as oxalates and phytates. These bind to calcium and form insoluble salt complexes, thus decreasing calcium absorption.3 For example, cooked spinach contains 115 mg of calcium per serving (125 mL or ½ cup), but only an estimated 5% (6 mg in absolute value) of it is actually absorbed. This is very little compared to the 32% (i.e. 101 mg) of milk’s calcium absorbed. Therefore, one would have to consume about 8 cups of spinach to obtain the same amount of available calcium found in 1 cup of milk (see table below).
In practice
- The calcium that milk naturally contains has good bioavailability, thus making it is easier to meet calcium requirements.
- Certain compounds contained in plant-based sources decrease the absorption of calcium.
- The calcium content of many fortified foods, such as fortified soy beverages or fruit juices, may be lesser than what is advertised due to the calcium settling at the bottom of the container.
The calcium bioavailability of some fortified foods is comparable with that of milk, but these foods do not always provide the same total calcium content per serving. Studies on fortified beverages, including soy beverages and orange juice, have shown that the fortificant tends to settle to the bottom of the carton and that even vigorous shaking may not be enough to re-suspend the calcium salts.5 Studies with fortified soy beverages have shown that upwards of 40% of the stated calcium content is not available.6
While it is possible to achieve adequate calcium intake and meet calcium requirements with a Western plant-based diet, it is easier and more practical to meet calcium balance when milk and milk products are present in the diet. Furthermore, the contribution of milk and milk products to calcium intake is important and advantageous nutritionally. The replacement of milk and milk products with calcium-equivalent foods has been shown to be detrimental to the overall nutritional profile, including the intake of other essential nutrients such as protein, magnesium, phosphorus, and vitamins A, D, B2 (riboflavin) and B12.7,8
The table below shows the calcium content of selected food sources, the percentage absorbed by the body, and the number of servings required to equal the calcium absorbed from one cup of milk.
Equivalencies of bioavailable calcium6,9-11
| Food | Serving size | Average calcium content (mg) | Estimated absorption (%) | Calcium absorbed (mg) | Servings required to equal 240 mL (1 cup) of milk |
| Milk Products | |||||
| Milk or yogurt, whole, 2%, 1%, skim | 1 cup | 300 | 32.1 | 96.3 | 1.0 |
| Cheddar cheese | 42 g | 303 | 32.1 | 97.2 | 1.0 |
| Vegetables | |||||
| Bok choy | 125 mL (1/2 cup) | 79 | 53.8 | 42.5 | 2.3 |
| Kale | 125 mL (1/2 cup) | 61 | 49.3 | 30.1 | 3.2 |
| Chinese spinach | 125 mL (1/2 cup) | 347 | 8.4 | 29 | 3.3 |
| Broccoli | 125 mL (1/2 cup) | 35 | 61.3 | 21.5 | 4.5 |
| Rhubarb | 125 mL (1/2 cup) | 174 | 8.5 | 10.1 | 9.5 |
| Spinach | 125 mL (1/2 cup) | 115 | 5.1 | 5.9 | 16.3 |
| Nuts & seeds | |||||
| Almonds | 28 g (1 oz) | 80 | 21.2 | 17 | 5.7 |
| Sesame seeds | 28 g (1 oz) | 37 | 20.8 | 7.7 | 12.2 |
| Legumes | |||||
| Beans, white | 110 g | 113 | 21.8 | 24.7 | 3.9 |
| Beans, pinto | 86 g | 44.7 | 26.7 | 11.9 | 8.1 |
| Beans, red | 172 g | 40.5 | 24.4 | 9.9 | 9.7 |
| Breads & Cereal | |||||
| Whole wheat bread | 28 g (1 slice) | 20 | 82.0 | 16.6 | 5.8 |
| Wheat bran cereal | 28 g | 20 | 38.0 | 7.5 | 12.8 |
| Fortified foods | |||||
| Bread with calcium sulphate | 16.8 g | 300 | 43.0 | 129 | 0.74 |
| Orange juice with calcium citrate malate | 1 cup | 300 | 36.3 | 109 | 0.88 |
| Tofu, calcium-set | 126 g | 258 | 31 | 80 | 1.2 |
| Soy beverage (fortified with tricalcium phosphate)* | 1 cup | 300 | 24.0 | 72 | 1.3 |
* There continues to be an issue with calcium fortificants settling to the bottom that is not completely resolved even with vigorous shaking. Studies with fortified soy beverages have shown that upwards of 40% of the stated calcium content may not be available.6
References
- Caroli A et al. Invited review: Dairy intake and bone health: A viewpoint from the state of the art. J Dairy Sci 2011;94(11):5249-62.
- Kwak HS et al. Revisiting lactose as an enhancer of calcium absorption. Int Dairy J 2012;22(2):147-51.
- Weaver CM. Should dairy be recommended as part of a healthy vegetarian diet? Point. Am J Clin Nutr 2009;89(suppl)1634S-7S.
- Miller GD et al. The importance of meeting calcium needs with foods. J Am Coll Nutr 2001;20(2 suppl):168S-85S.
- Rafferty K et al. Calcium fortificants: Overview and strategies for improving calcium nutriture of the U.S. population. J Food Sci 2007;72(9):R152-8.
- Chalupa-Krebzdak S et al. Nutrient density and nutritional value of milk and plant-based milk alternatives. Int Dairy J 2018;87: p. 84-92.
- Fulgoni V 3rd et al. Nutrients from dairy foods are difficult to replace in diets of Americans: Food pattern modeling and an analyses of the National Health and Nutrition Examination Survey 2003-2006. Nutr Res 2011;31(10):759-65.
- Nicklas TA et al. The role of dairy in meeting the recommendations for shortfall nutrients in the American diet. J Am Coll Nutr 2009;28(suppl 1):73S-81S.
- Weaver CM and Plawecki KL. Dietary calcium: Adequacy of a vegetarian diet. Am J Clin Nutr 1994;59(suppl):1238S-41S.
- Weaver CM et al. Choices for achieving adequate dietary calcium with a vegetarian diet. Am J Clin Nutr 1999;70(suppl):543S-8S.
- Weaver CM and Heaney RP. Calcium in human health. Totowa, NJ: Humana Press; c2006. Chapter 9, Food sources, supplements and bioavailability; p. 129-42.
