What is calcium bioavailability and why we are talking about it? Lots of foods are rich in calcium. However, not too much of it is available for our body to absorb, meaning that calcium dietary sources are not created equal. Figuratively speaking, you have to eat as much of green grass as cow to get amount of bioavailable calcium that one glass of milk contains.
Biological role of calcium
Calcium is a mineral necessary for life. It helps our body build and maintain strong bones, teeth and nails, helps our muscles work and our heart beat. Chronic calcium deficiency can lead to rickets and poor blood clotting. It can lead to osteoporosis and cause some other health conditions. When we are getting older, we need to increase it because calcium is washing out of our bodies, so we have to consume more.
Our body cannot produce calcium on its own; the food we eat is the only source of calcium supply. Without any doubts, milk and milk products are champions in calcium content and its bioavailability. But some people are lactose-intolerant or allergic to milk and cannot consume non-fermented dairy products. Other people, such as vegans, intentionally avoid milk products for ethical reasons. So, alternative sources of that vital mineral have to be fruit and vegetables.
Many vegetable sources of calcium exist, including seaweeds such as kelp, wakame and hijiki; nuts and seeds like almonds, hazelnuts, sesame, and pistachio; blackstrap molasses; beans (especially soy beans); figs; quinoa; okra; rutabaga; broccoli; leaves; and kale. But here the problem of calcium bioavailability arises.
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 can be incorporated into bones.
So, a variety of vegetables are rich in calcium, but they may also contain different substances (most often – oxalic acid) that bind calcium forming insoluble salt complexes and making only a small fraction of it available for absorption and utilization in our body. For example, spinach has moderate calcium content, but only about 5% of it is bioavailable. In other words, you have to eat tons of spinach to get enough calcium. The same problem may affect the absorption of calcium from amaranth, collard greens, and chicory greens.
Recommended daily intake for calcium for an average healthy person of 20-50 years old is 1,000mg.
On the other hand, calcium excess from supplements, fortified food and high-calcium diets can cause elevated levels of calcium in the blood (hypercalcemia); impaired kidney function; decreased absorption of other minerals; milk-alkali syndrome, which has serious toxicity and can be fatal. Some studies suggest a correlation between high calcium intake (2000mg per day, which is equivalent to six or more glasses of milk per day) and prostate cancer.
Vitamin D is needed to absorb calcium
The bioavailability of calcium is an important factor to keep in mind when calculating its daily consumption.