A 45-year-old man came to our OPD after suffering a hairline fracture. He told me that he was most surprised because he had just had a minor fall by his bedside. Besides he had been taking calcium supplements after a blood test and bone density scan had indicated he was low on this essential mineral. Clearly, he had got one piece of the puzzle wrong.
Calcium is an essential mineral that the body needs for building and maintaining strong bones and teeth as well as maintaining the heart’s electrical signals. But it cannot be effectively absorbed from the food we eat without adequate levels of vitamin D3. This vitamin plays a central role in maintaining calcium balance in the body and is vital for bone health, muscle function and several metabolic processes. This interdependence between vitamin D3 and calcium explains why a deficiency in one often leads to issues related to the other, such as weak bones, fractures, or disorders like osteoporosis.
What is vitamin D3?
Vitamin D3, also known as cholecalciferol, is produced naturally in the skin when it is exposed to sunlight, particularly ultraviolet B (UVB) rays. It can also be obtained from certain foods such as fatty fish, egg yolks, and fortified dairy products, or through supplements. Once produced or ingested, vitamin D3 is converted in the liver into calcidiol and then further converted in the kidneys into its active form, calcitriol. This active form acts as a hormone that ensures calcium is absorbed in the intestines and that levels remain balanced in the blood.
How is it different from vitamin D?
Vitamin D is a general term for a group of fat-soluble vitamins, while Vitamin D3 (cholecalciferol) is a specific form of that vitamin. It is considered most effective at raising and maintaining overall vitamin D levels in the blood.
It all happens in the small intestine
The small intestine is the main site of calcium absorption, and vitamin D3 significantly enhances this process. When calcitriol binds to its receptor in the cells lining the intestine, it triggers the production of calcium-binding proteins that actively transport calcium into the bloodstream. Without sufficient vitamin D3, this mechanism becomes inefficient, and even a calcium-rich diet may not be enough to maintain normal calcium levels. As a result, the body begins to draw calcium from bones to keep blood calcium stable, which over time weakens the skeletal structure and increases the risk of fractures.
How vitamin D3 renews bones
Beyond its role in calcium absorption, vitamin D3 also supports bone remodelling — a continuous process in which old bone tissue is replaced by new. It helps regulate the activity of osteoblasts and osteoclasts, the two types of cells responsible for bone formation and breakdown. This balance ensures that bones remain strong yet flexible. A lack of vitamin D3 disrupts this process, leading to soft bones in children (rickets) and brittle bones in adults (osteomalacia). For this reason, vitamin D3 deficiency is one of the leading causes of preventable bone disorders globally.
How vitamin D3 deficiency impacts your health
When vitamin D3 levels are low, calcium absorption falls, leading to muscle weakness, cramps or in severe cases, irregular heart rhythms. Some research also suggests that adequate vitamin D3 levels may help reduce inflammation, support immune function and contribute to better mood and cognitive health, though these effects are still being studied.
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Modern lifestyles have made vitamin D3 deficiency more common. Limited sun exposure due to indoor work, use of sunscreens, pollution and darker skin pigmentation can all reduce the body’s ability to produce vitamin D3 naturally. Therefore, supplementation and dietary sources are often necessary, particularly for older adults, pregnant women, and people living in areas with limited sunlight.
(Dr Gupta is senior consultant, Internal Medicine, Indraprastha Apollo Hospitals, New Delhi)
