The body also gets calcium from the food we eat. When your dietary intake is low, the body will borrow from the bone and may replace later IF the consumption increases.
The amount you see listed on the Nutrition label of a food, is not necessarily the amount you will absorb. The bioavailability depends on the food source because of different food matrix, your metabolism, gut health and availably of other nutrients (magnesium, phosphorous, and vitamins D and K), anti-nutrients and co-factors in the meals. This is true for other nutrients such as iron and magnesium.
Dairy has the bioavailability of 30 - 35%, so if a cup of milk has 300 mg, you will absorb and use only 100 mg. Plant foods have less calcium than dairy, and far lower bioavailability due to endogenous anti-nutrients such as oxalates and phytates that bind to the calcium and inhibit its absorption.
For example, cooked amaranth (thepe, imbuya) contains the most calcium of all the leafy greens at about 130 mg of calcium per half cup (125 ml) and with a 5% bioavailability, you will absorb and use 6,5 mg. So you need about 4 cups of cooked amaranth to match the calcium in 1 cup of milk. This is not to say you should avoid amaranth and other green leafy vegetables, just do not rely on them as significant calcium sources because they will not be as available for the body to use.
Health benefits of Calcium
When talking about calcium’s health benefits we focus mostly on bone health because about 99% of the body’s calcium is stored in bones and the remaining 1% is found in the blood, muscle and other tissues.
When your dietary intake is low, the body will borrow from the bone and may replace later IF the consumption increases.
It is one of the building blocks essential to health - for bone and teeth health, blood clotting, muscle contraction, nerve function and normal heart rhythms.
People at risk of calcium deficiency:
Low intake of calcium, vitamins D & K, magnesium, phosphorus - they work together.
Eating disorders
Poor gut health
Postmenopausal women: oestrogen helps to increase calcium absorption and retain the mineral in bones.
Symptoms of low calcium levels:
Abnormal heart rate
Poor appetite
Tingling or numbness in fingers
Muscle cramps and weakness