Calcium and Bioavailability

Bioavailability is the degree to which a nutrient is absorbed and utilized by the body. To meet calcium recommendations, the bioavailability of calcium is an important factor to consider beyond simply the calcium content of foods. 

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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
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
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

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