Calcium is an integral part of our diet, it is normally absorbed through the gut with the help of vitamin D. The deficiency in dietary calcium would result in decreased absorption through the gut, resulting in decreased excretion of calcium through kidneys and mobilization of calcium from the bones. The calcium metabolism in the body is maintained by vitamin D and parathyroid hormone (PTH), the latter released by parathyroid glands (situated behind thyroid gland) in the neck. On the other hand, Vitamin D is synthesized in the skin, and converted to the active form, 25-hydroxy vitamin D, in the kidneys. Hence sunlight is an important source of Vitamin D.
Dietary calcium deficiency can lower bone mineral density. Increasing the intake of calcium has been shown in postmenopausal women to increase bone density and a smaller but significant benefit in decreasing certain fractures. In patients with Osteoporosis adequate calcium and vitamin D intake would enhance the benefits of drug therapies for Osteoporosis by increasing bone mineral density and decreasing fracture risk. Therefore in patients with calcium insufficiency, osteoporosis treatments can result in decreased mobilization of calcium from the bones and hence hypocalcemia (decreased blood calcium levels).
Although US preventive task force do not recommend calcium supplementation for normal people for bone health, other medical societies recommend adequate calcium intake to optimize skeletal health and also in patients with osteoporosis.
However, there is a concern among general public, if calcium intake through supplements would result in heart disease through deposition in coronary arteries? The 2016 consensus statement from National Osteoporosis foundation (NOF) and American Society for Preventive Cardiology has considered both dietary and supplemental calcium to be safe from cardiovascular perspective.
Higher dietary calcium compared to supplemental calcium is not associated with kidney stone formation. The proposed mechanism being the binding of calcium by oxalate in the diet thus decreasing its absorption, while the supplemental calcium taken between meals is absorbed quickly resulting in higher blood calcium levels and increased urinary excretion thus resulting in more chances of stone formation.
The Institute of Medicine 2011 recommendations for calcium and vitamin D dictate a recommended daily allowance (RDA) of 1000-1200 mg/day (upper limit 2500mg/d) of dietary calcium and 600-800 IU/d (upper limit 4000 IU/d) of Vitamin D respectively in both men and women. This dose of vitamin D is sufficient to achieve 25 hydroxyvitamin D (active metabolite of vitamin D) levels between 20-40ng/ml enough to suppress parathyroid hormone (PTH) and thus mobilization of calcium from the bones. As the elemental calcium content varies in supplements depending on the molecular weight of the calcium salt, it is important to pay attention to the elemental calcium content in these supplements. The RDA of calcium 1000-1200 mg/d means 1000-1200mg of elemental calcium daily.
Calcium is found in a variety of foods. These foods are dairy products (milk, cheese and yoghurt). For example, one cup of milk contains approximately 300mg of calcium, hence 3 cups of milk daily would fulfill and adults daily dietary calcium requirement. The non-dairy sources include soya and almond milk, fruits and vegetables (spinach, broccoli and orange juices), legumes such as beans and fish.
Supplemental calcium is commonly available in the form of calcium carbonate or calcium citrate. The other forms include calcium gluconate, lactate, and hydroxyapatite. Calcium citrate is the preferred form, as it is easily absorbed (with/without food) and also reduces kidney stone formation compared to calcium carbonate which requires stomach acidity for absorption, hence should be taken with food.
In summary, optimal dietary calcium intake with vitamin D is recommended in both men and women for adequate bone health. Dietary calcium sources compared to calcium supplements have more advantages. When using supplements, calcium citrate is the preferred choice.
1 Kilim, HP, Rosen, H. Optimizing calcium and vitamin D intake through diet and supplements.Cleveland Clinic Journal of Medicine. 2018; 85(7):543-550.