Calcium and vitamin D, specifically D3 or cholecalciferol, are essential for maintaining the strength and structural integrity of bones. While other nutrients also play an integral in bone development and strength, it is a deficiency of these two nutrients that have the most marked effect on integrity of the bone. Depending on the extent of the deficiency, it may lead to osteoporosis, osteomalacia (adults) or rickets (children). Both calcium and vitamin D are derived from the diet although the latter is also formed in the skin through sunlight exposure. The bones also serve as a reservoir for calcium. Deficiencies may be linked to dietary factors or underlying diseases that hamper the digestion, absorption or proper utilization of these calcium and vitamin D.
Calcium is one of the main components for calcification of the bones. In conjunction with phosphorus it is deposited as hydroxyapatite crystals as bone mineral. Without it, bone will remain uncalcified as softer and more flexible bone tissue (osseus tissue).
The levels of calcium in the blood are regulated by parathyroid hormone (PTH) secreted by the parathyroid glands and calcitonin secreted by the C-cells of the thyroid gland. Parathyroid hormone responds to low levels of calcium in the blood and increases calcium absorption from the gut and calcium resorption in the bone. Conversely, calcitonin responds to high levels of calcium in the blood by reducing gut absorption of calcium, reducing bone resorption and inhibiting calcium reabsorption in the kidney.
The recommended dietary allowance (RDA) for calcium varies by age. The range extends from 200mg in infants to 1,000mg to 1,500mg per day after the age of 50 depending on the gender and risk factors. For premenopausal women and men younger than 50 years of age the RDA is 1,000mg daily.
Calcium in Food
Calcium is present in most foods although at times it may only be trace amounts. Certain foods are high in calcium and this includes :
- Milk and other dairy products
- Dried beans
- Leafy green vegetables
Certain foods may be fortified with calcium, like breads or cereals.
Vitamin D is made up of several sterol compounds. The most significant of these is vitamin D3 or cholecalciferol which is derived from exposure to sunlight. However, cholecalciferol like the other forms of vitamin D needs to be hydroxylated in the liver and kidney to derive the most biologically active compound known as calcitriol.
Calcitriol is important in the regulation of calcium and phosphorus in the body. It promotes calcium absorption in the gut and acts on the kidney to reduce phosphate excretion. By increasing the calcium and phosphate levels in the blood and tissue fluid, calcification of bone can occur. However, calcitriol can also promote calcium and phosphate release from the bone.
The recommended dietary allowance for vitamin D varies by age from 400IU (10 mcg) for infants under 6 months of age to 800IU (20 mcg) for persons over 70 years. Since this vitamin is derived from sunlight exposure, dietary allowance can be as low as 400IU for adults. Due to vitamin D toxicity, dietary intake should not exceed 2,000 IU (50 mcg) per day although the upper tolerable levels for a person over 9 years of age is 4,000 IU (100 mcg).
Vitamin D in Food
Foods that are rich in vitamin D include :
- Oily fish
It may also be found in fortified milk and dairy products, cereals and certain fruit juices. Read more on vitamin D sources. The main source of vitamin D, however, is from the exposure of the skin surface to sunlight. A compound known as 7-dehydrocholesterol which is naturally present on the skin is then converted to cholecalciferol.
Supplementation is advisable in high risk patients and those undergoing medical treatment for conditions like osteoporosis. Read more about these medication under bone strengthening drugs.
Vitamin D supplementation enhances absorption of calcium from the intestine and stimulates the synthesis of bone matrix protein. Vitamin D is usually supplemented at a daily dose of 400 IU. It is required for the best results with other osteoporosis medications. If vitamin D therapy is not supplemented with calcium, it can cause increase blood calcium levels by mobilizing calcium from bones. In osteoporosis, the absorption of calcium is reduced due to the increase in the blood calcium levels. Vitamin D therapy in combination with calcium supplementation counters this reduced intestinal absorption.
Some studies have shown that vitamin D and its analogues, like cholecalciferol and calcitriol, reduce fracture rates and increase the bone mineral density. Calcitriol is also found to exert direct suppression of parathyroid gland function with subsequent reduction in bone turnover. Increased doses of calcitriol alone can probably increases the bone mineral density but at the increased risk of raised calcium in blood and urine. Vitamin D is approved in several countries for osteoporosis but the FDA has not approved it for osteoporosis.
Supplementation of calcium daily along with vitamin D is found to complement the medical treatment of osteoporosis in most patients. Calcium supplementation reduces the bone turnover, improves bone mineral density and reduces the fracture incidence rates. This benefit with calcium supplementation is evident even in patients with high dietary calcium intake. Calcium carbonate is the most commonly prescribed calcium preparation. The standard calcium carbonate supplementation dose is 1,000 mg. Calcium citrate is another formulation that is available. Absorption of citrate formulation is slightly better than the calcium carbonate but may be more expensive than the carbonate.
Calcium supplements are ideally taken with meals for better absorption. Complementing calcium supplementation with increased dietary intake of calcium is also found to improve the outcome. Anorexia, constipation, nausea, vomiting, abdominal discomfort and raised calcium levels in the blood (hypercalemia) and urine (hypercalciuria) are the major side effects associated with calcium supplementation. Patients with renal stones (kidney stones), low phosphate blood levels and high calcium blood levels are not recommended calcium supplementation.