It is well-known that calcium is essential for the human body - for strong bones and teeth, to fob off osteoporosis (porous bones), to keep the heart, muscles and nerves functioning well and even for blood to clot. What is less known is that, for calcium to do its good work, Vitamin D must pitch in too. In other words, even copious amounts of calcium won't help your body unless you combine it with a judicious consumption of Vitamin D.

"The crucial calcium-Vitamin D connect is medically well-established though few people are aware of it," explains Dr Girish Varshney, Head, Department of Medicine, Fortis Hospital, Noida (Uttar Pradesh). "It is especially important for post-menopausal women to know this as they are particularly vulnerable to osteoporosis and osteomalacia (softening of bones). This information is also crucial for infants and growing children, because a calcium/Vitamin D deficiency at this age could cause deformed bones characteristic of rickets. Vitamin D is also instrumental in controlling the movement of calcium between bone and blood and vice versa."

Vitamin D, also known as the 'sunshine vitamin' (because the body manufactures it on exposure to sunshine), is a hormone-like, fat-soluble vitamin. It regulates the formation of bones and the absorption of calcium and phosphorus from the intestines. Doctors say that 10-15 minutes of exposure to sunlight thrice a week is sufficient for the human body to produce the required dosage of Vitamin D. This does not vary from summer to winter. That is, the same amount of exposure is sufficient in all seasons. And this is the exposure required for a normal adult. In turn, the vitamin ratchets up the body's calcium absorption capacity by as much as 80 per cent.

The Osteoporosis Society of Canada recommends that adults aged between 19 and 50 years require 400 international units (IUs) of Vitamin D per day. Adults over 50 should receive 800 IUs. An exposure of 10-15 minutes to sunlight is enough to get a day's requirement for an adult. Explains gynecologist and obstetrician Meeta Prabhu of Max Hospital, "Lactating and expecting women need about 400-500 IUs of Vitamin D per day. However, since it is difficult to get enough Vitamin D from food alone, supplements like multivitamins - which provide 400 IUs of Vitamin D per pill - or calcium supplements (most of which are Vitamin D-enriched) may also be taken."

According to Prabhu, some common Vitamin D-enriched foods include milk fortified with Vitamin D (which contains 100 IUs per 250 ml glass), margarine, eggs, chicken liver, salmon, sardines, herring, mackerel, swordfish and fish oils (halibut/cod liver oils).

Prabhu adds that bone loss in post-menopausal women occurs mostly in the winter due to plummeting Vitamin D levels in the blood. "In winter, an additional intake of Vitamin D from fortified foods/supplements is, therefore, strongly recommended. Winter intakes of about 15 micrograms of Vitamin D per day helps prevent bone loss." The vitamin supplements can be had in the form of tablets, granules, tonics and syrups.

Interestingly, in northern countries, where there is less sunshine, or where tradition dictates that the body be almost completely covered (such as in parts of the Islamic world), there is a higher incidence of Vitamin D deficiency. 
However, an excess of Vitamin D too can be detrimental and can trigger off loss of appetite, weight loss, nausea, headache, depression and calcium deposits in the kidneys. Writes renowned US-based dermatologist Dr Hal Skinner in his research paper, "The Skin Story" (2005), that excessive Vitamin D could result in increased calcium absorption from the intestinal tract. "This may lead to an enhanced calcium resorption from the bones, raising calcium levels in the blood. Elevated blood calcium may then cause calcium deposition in soft tissues such as the heart and lungs. This can disrupt the organs' functioning, leading to kidney stones, vomiting and muscle weakness."

Naturally occurring Vitamin D in foods is usually animal-derived and contains Vitamin D3 (cholecalciferol). Since this is not an option vegans (who shun all animal products, including milk) would choose, they get their Vitamin D from sunlight or through fortified foods such as soya milk, margarine, breakfast cereals and vitamin supplements that are made from yeast or other fungi. Fortified vegan products contain Vitamin D2 (ergocalciferol). Vegetarians have an advantage over vegans as the former do not avoid milk/dairy products which are one of the main sources of vitamin D.

Doctors also advise that breast-feeding vegan women must take care to ensure that they supplement their Vitamin D intake during winter. Parents too are advised to include Vitamin D fortified foods or supplements if they wean their infants during the winter months, especially if the children are dark-skinned. The exposure to sun for both light- and dark-skinned babies remains the same. 
According to Delhi-based physician and cardiologist Dr J S Mathur, the most significant supply of Vitamin D (for omnivores as well as vegans) comes from the action of ultra-violet B light on sterols on the skin. "Most people, including infants, require little or no extra Vitamin D from food if they are regularly exposed to sunlight. Bright sunlight is not essential; even sky shine on a cloudy summer day will stimulate formation of some D in the skin, while a short summer exposure in the open air will increase blood levels of the vitamin two- to three-fold." Wintertime supplies of Vitamin D depend on whether the previous summer's exposure has created adequate deposits in the liver.

Dr Michael Holick, author of 'The UV advantage' and one of the world's most respected authorities on Vitamin D and the health benefits of natural sunlight, has established the breadth of activity that this vitamin possesses. He says that it not only regulates calcium metabolism and bone health, but also cell growth. "People living in higher latitudes, " he writes, "are more prone to Vitamin D deficiency and developing common cancers and dying of them, such as cancer of the colon, prostate, breast and even ovaries. And it is now established that this is partly due to the body's inability to manufacture enough activated Vitamin D to help regulate cell growth."

In fact, scientists are now probing the use of Vitamin D to reduce the risk of no fewer than 17 different types of cancer, ranging from colon, breast and prostate cancers to ovarian, oesophageal, renal and bladder cancers. And they are increasingly veering around to the view that Vitamin D may even augment treatment outcomes in people already diagnosed with cancer. In fact, the general figures given by US dermatologists suggest that, in the US, about 50,000-70,000 people die prematurely from cancer each year due to an insufficient Vitamin D intake.

Emerging research suggests that Vitamin D also has applications in promoting bone strength as well as in mitigating auto-immune conditions such as multiple sclerosis, Type One diabetes, and rheumatoid arthritis. Other potential benefits include promoting dental and skin health, and helping to prevent stroke, metabolic syndrome and musculo-skeletal pain.

Skinner writes, "There's a growing body of evidence that higher levels of Vitamin D - through supplements, diet or prudent sun exposure - may provide an array of health benefits by preventing common chronic diseases." And there is also evidence that low Vitamin D levels contribute to cancer and auto-immune disease. "If we lived as we evolved - in the open or nearer the equator - we would synthesize Vitamin D from sunlight equivalent to 100 micrograms or more per day and dietary intake would be irrelevant," he writes.