I guess I should copy/paste from the link I posted earlier, since nobody reads links on this thread:
Calcium and Bioavailability
Bioavailability is the degree to which a nutrient is absorbed and utilized by the body. The bioavailability of calcium refers to the fraction of dietary calcium that is potentially absorbable and the incorporation of the absorbed calcium into bone.
To meet calcium recommendations, the bioavailability of calcium is an important factor to consider beyond simply the calcium content of foods.
Various dietary factors can affect calcium bioavailability. Some food components act synergistically to promote calcium absorption. They include:
- 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.
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. For example, cooked spinach contains 115 mg 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).
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.
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 B12.6,7
Scroll down to see the table that 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.