Calcium Sources, Osteoporosis, And Hard Water

Calcium is of vital importance for blood clotting, nerve transmission, muscle stimulation and maintaining bone structure. The recommended daily intake of calcium is 1000 mg, assuming that 30-40% is actually absorbed by the body. Most people get their calcium from dairy products, which also happens to be a great source of saturated fat, transfat and cholesterol (bad fat). So, what is a better source of calcium?  

Good Sources Of Calcium

Considering plant sources, vegetables in the brassica family have the highest amount of calcium, and the calcium is well absorbed by our body. For instance, bok choy and other Chinese greens (like mustard greens or cabbage), turnip greens, kale and broccoli are all great sources of calcium. Other vegetables like spinach and rhubarb also contain considerable amounts of calcium, however the presence of oxalate in these vegetables reduces our ability to uptake the calcium. Oxalate and calcium form an insoluble salt together, which our body cannot absorb.

Besides vegetables, other foods that contribute to calicum intake are almonds, sesame tahini, calcium-fortified products, and even hard water.

Calcium Every Day

Two servings/cups of mustard greens contain about half the amount of (absorbable) calcium you need on a day. In comparison, that is a bit less than two glasses of milk. Lots of foods contain small amounts of calcium, so on a varied diet, calcium supplementation is not needed. In general, taking calcium supplements is not recommended, only when prescribed, whereas an excess calcium intake can have serious consequences. A main concern regarding calcium supplementation is focused on cardiovascular diseases. Here, calcium supplements potentially contribute to elevated blood calcium levels and could accelerate calcification of the blood vessel in the long term, possibly resulting in a heart attack.

Calcium And Vitamin D

Calcium absorption in the gut occurs by an active and a passive process. The active process is vitamin D dependent, but the passive process is not. In the presence of vitamin D the active calcium absorption in the gut is thus increased. Vitamin D stimulates the cells that line the gut to increase its amount of receptors which uptake calcium from the gut lumen.

Passive absorption is not vitamin D dependent, therefore high calcium intakes can make up for reduced calcium absorption when vitamin D status is low. This does mean that, when calcium intake is low, low vitamin D status becomes a risk for bone health (see below).

Osteoporosis Awareness

Most calcium in the body (about 99%) is found in our bones. It’s a relatively stable pool, however, when calcium levels in our blood are low, calcium is released from the bones. If the release of calcium from the bones exceeds the storage, we lose bone mass, resulting in fragile weakened bones that break more easily. This is called osteoporosis, a condition that affects one out of three women and one out of five men after age 50. Since osteoporosis is so common, it is noteworthy that a balanced diet throughout life with adequate calcium and vitamin D intake, and regular weight-bearing exercise are key in preventing osteoporosis. Since loss of bone mass begins by age 30, many people suffer from low bone mass long before they are aware of the problem.

Hard Water And Kidney Stones

There is no consistent evidence that drinking hard water, with high calcium concentrations, affects kidney stone formation. In fact, both too high and too low calcium intake can cause kidney stones. Other factors that are more important in causing kidney stones are: to little fluid intake, high oxalate intake, consumption of animal protein and sodium/salt.


British Nutrition Foundation. Dietary calcium and health. (2005). Available at:

Christianson, M. S. & Shen, W. Osteoporosis Prevention and Management. Clin. Obstet. Gynecol. 56, 703–710 (2013)

European Food Safety Authority. Scientific Opinion in relation to the authorisation procedure for health claims on calcium and vitamin D and the reduction of the risk of osteoporotic fractures by reducing bone loss pursuant to Article 14 of Regulation (EC) No 1924/2006. (2010). Available at:

European Food Safety Authority. Scientific Opinion on Dietary Reference Values for calcium. (2015). Available at:

Li, K. et al. The good, the bad, and the ugly of calcium supplementation: a review of calcium intake on human health. Clin. Interv. Aging 13, 2443–2452 (2018)

Marini, F. & Brandi, M. L. Pharmacogenetics of osteoporosis. Best Pract. Res. Clin. Endocrinol. Metab. 28, 783–793 (2014)

National Institutes of Health. Calcium. (2018). Available at:

McGirr, C., McEvoy, C. T. & Woodside, J. V. Vegetarian and Vegan Diets: Weighing the Claims. in Nutrition Guide for Physicians and Related Healthcare Professionals 203–212 (Springer International Publishing, 2017). doi:10.1007/978-3-319-49929-1_20

Robertson, W. G. Dietary recommendations and treatment of patients with recurrent idiopathic calcium stone disease. Urolithiasis 44, 9–26 (2016)

Sengupta, P. Potential health impacts of hard water. Int. J. Prev. Med. 4, 866–75 (2013)

Titchenal, C. A. & Dobbs, J. A system to assess the quality of food sources of calcium. J. Food Compos. Anal. (2007). doi:10.1016/j.jfca.2006.04.013

Weaver, C. & Heaney, R. P. Calcium in human health. (Humana Press, 2006)

Weaver, C., Lewis, R. & Laing, E. Adolescence and Acquisition of Peak Bone Mass. Vitam. D 657–677 (2011). doi:10.1016/B978-0-12-381978-9.10037-X

Yang, J., Punshon, T., Guerinot, M. Lou & Hirschi, K. D. Plant calcium content: Ready to remodel. Nutrients 4, 1120–1136 (2012)