Calcium Fortification of Soy milk
Dairy products provide most of the dietary calcium in Western countries, where soy milk is becoming increasingly popular. Natural soy milk contains only 200 mg calcium per liter, which is 6x less than cow milk. Therefore, most commercial soy milks are fortified with extra calcium up to a level 1200 mg/L, which is the same as that of cow milk. Manufacturers of soy milk use this specific level of 1200 mg/L to provide an alternative calcium source to cow milk. However, the question is do we really need such high calcium levels in soy milk? Obviously, manufacturers want to avoid the debate about optimal calcium levels and simple use same levels as found in cow milk, which is traditionally a recommended source of calcium. However, studies have shown that a higher intake of milk and calcium from milk is associated with a higher risk of osteoporosis. The Harvard's Nurses' Health Study, which involved about 57,000 women, showed that women who consumed the most calcium from dairy products had almost double the rate of hip fractures compared to women who received the least calcium from dairy products.
Sources of Calcium in Soy Milk
Most manufacturers use tri-calcium phosphate as calcium source. Other calcium sources are calcium carbonate and vegetable calcium from seaweed (Lithothamnium Calcareum).
In addition, the calcium in the water, used in the soy milk manufacturing process, can be significant. The calcium level of municipal water or well water can range from 0 mg/L up to 600 mg/L.
Bioavailability of Calcium in Soy Milk
A study by Robert Heaney and colleagues entitled "Bioavailability of the Calcium in Fortified Soy Imitation Milk, with some Observations on Method" (2000, The American Journal of Clinical Nutrition) found that calcium from soy milk, fortified with tri-calcium phosphate, has an absorption efficiency of 75% compared to cow milk. Another study by Yangdong Zhao entitled "Calcium Bioavailability of Calcium Carbonate Fortified Soy Milk is Equivalent to Cow's Milk in Young Women" (2005, The Journal of Nutrition) found that calcium carbonate in soy milk had the same bioavailability than calcium from cow milk and had a higher bioavailability than tri-calcium carbonate. The lower bioavailability of tri-calcium phosphate is probably coming from the phosphate, which precipitates the calcium in the intestine. Soy milk contains phytochemicals, such as isoflavones and phytic acid, which may influence calcium absorption. Some studies have shown that isoflavones stimulate estrogen receptors in the intestine and increase calcium absorption, whereas other studies failed to show such effect. Phytic acid is known to inhibit calcium absorption but the low levels (less than 0.01%) found in soy milk are unlikely to have any influence.