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 the bone.

Various dietary factors can affect calcium bioavailability. Some food components act synergistically to promote calcium absorption. These include:1,2

  • Vitamin D,
  • Lactose,
  • Casein phosphopeptides in milk.

Cow’s milk has good calcium bioavailability (about 30 to 35%). It is estimated that without milk and milk products in the diet, less than half of calcium requirements would be met. In fact, adolescents in Canada who follow a Western diet are unlikely to meet their calcium recommendations 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 compounds, such as oxalates and phytates. Oxalates can be found in leafy greens like spinach and rhubarb, while phytates are commonly found in many cereals and seeds. These compounds bind to calcium and form insoluble salt complexes, thus decreasing calcium absorption.4 For example, cooked spinach contains 129 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. 99 mg) of calcium absorbed in 2% milk. Therefore, one would have to consume about 15 cups of spinach to obtain the same amount of available calcium found in 1 cup of milk (see table below).4

In practice

  • The calcium that milk naturally contains has good bioavailability, thus making it 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 lower than what is advertised due to the calcium settling at the bottom of the container.

In the context of plant-based beverages, while many are fortified with calcium, they may be less dependable sources of calcium.5,6 The added calcium can settle to the bottom of the container despite vigorous shaking.5,6 In fact, a study on fortified soy beverages reported that measured calcium levels averaged only 31% of the calcium content claimed on the label, and even when shaken, only about 59% of this value is obtained.6 Furthermore, the amount of calcium actually absorbed would be even lower when considering bioavailability. In addition, the form of calcium used in fortification may not be as well absorbed as the naturally occurring calcium in cow's milk.5,6 Overall, due to differences in important factors like calcium content and/or bioavailability, plant-based sources often require larger or multiple servings to provide the same amount of calcium as milk. 

 
Explore an updated analysis from the Practice-Based Evidence in Nutrition (PEN) on plant-based beverages for young children.
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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 needs 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 nutrient intake, including the intake of other essential nutrients such as protein, magnesium, phosphorus, and vitamins A, D, B2 (riboflavin) and B12.7,8

 
Curious to discover the role of the dairy matrix and its benefits beyond the sum of its nutrients?
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The following table presents foods based on their relevance as commonly consumed or frequently cited sources of calcium. It also includes foods with recent, methodologically consistent bioavailability data. Calcium sources without corresponding bioavailability data were excluded.

Estimated Bioavailability of Calcium by Dietary Source 4,6,9,12-14

Food Reference amounta Calcium content per reference amount (mg)b Estimated absorption (%) Calcium absorbed (mg) Servings required to equal 240 mL (1 cup) of milk
Milk Products          
Whole milk 250 mL 291 32 93 1
Milk, 2% fat 250 mL 309 32 99 1
Milk, 1% fat 250 mL 322 32 103 1
Skim Milk 250 mL 316 32 101 1
Cheddar cheese 30 g 202 37 75 1
Cottage cheese (2% milk fat) 125 g 139 38 53 2
Yogurt, plain (2-3.9% fat)c 115 g 131-175 24 31-42 2-3
Yogurt, vanilla or fruit flavoured (2-3.9% fat)c 115 g 125-144 24 30-35 3
Greek yogurt, plain (2% milk fat)c 115 g 115-148 24 28-36 3-4
Greek yogurt, vanilla or fruit flavoured (2% milk fat)c 115 g 99-148 24 24-36 3-4
Vegetables          
Bok choy, boiled, drained, shredded 125 mL 84 52 44 2
Brocolli, boiled, drained, chopped 125 mL 33 48 16 6
Kale, boiled, drained, chopped 125 mL 49 41 20 5
Spinach, boiled, drained 125 mL 129 5 6 15
Sweet potato, boiled, without skin, mashed 140 g 38 23 9 11
Fruits          
Rhubarb, cooked 125 mL 184 9 17 6
Nuts & seeds          
Almonds, unroasted or dry-roasted, unblanched 30 g (60 mL) 81 21.2 17 6
Sesame seeds, dried, shelled 30 g (60 mL) 18 20.8 4 26
Sesame seeds, toasted, shelled 30 g (60 mL) 39 20.8 8 12
Legumes          
Beans, kidney, boiled 125 mL 26 19 5 20
Beans, pinto, boiled 125 mL 42 23 10 10
Beans, white, boiled 125 mL 85 23 20 5
Soy beans, boiled 125 mL 93 31 29 3
Breads & Cereal          
Whole bran cereal 30 g 18 22 4 25
Wheat bread 75 g (2 slices) 55 82 45 2
Fortified foods          
Fortified orange juice, calcium citrate malated 250 mL (fortified with added calcium and vitamin D) 310 38 118 1
Fortified soy beverage, calcium carbonatee 250 mL 313 21 66 2
Fortified soy beverage, tricalcium phosphatee 250 mL 313 18 56 2
Tofu, calcium-set, firm/extra firm, calcium sulfatef 85 g 75-225 39 29-88 1-3

aReference amounts for each food are based on the Canadian Food Inspection Agency's nutrition labelling requirements.12

bData on calcium content were sourced from Health Canada's Canadian Nutrient File, unless indicated otherwise.13 

cCalcium content of yogurt varies; presented ranges are based on a 2025 Canadian marketplace scan for regular and Greek yogurts. Refer to product labels. 

dThe data on fortified orange juice should be interpreted with caution, as the Canadian Nutrient File does not specify the type of calcium used for fortification, while the referenced study reports an estimated absorption value specifically for calcium citrate malate. 

eSome plant-based beverages are fortified with calcium – Refer to the label. As per Health Canada regulations, when plant-based beverages are fortified, they must contain 125 mg of calcium per 100 ml; the table above reflects this.14 Keep in mind that added calcium may settle at the bottom of the container – even after vigorous shaking – so these beverages may not be a reliable source of calcium.5,6 Also, the form of calcium used in fortification may not be as well absorbed as the naturally occurring calcium in cow's milk. 

fCalcium content of calcium-set tofu varies considerably by brand and type; presented values are based on a 2022 Canadian marketplace scan for firm/extra firm tofu. Refer to product labels. 

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