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Vitamin D, a fat soluble vitamin, is recognized for its favorable effects on human bones and teeth. A proper maintenance of bone structure requires regulation of calcium and phosphorus levels in the blood. Vitamin D aids the absorption of these minerals from the intestine.
It is also studied for its potential benefits in general medical problems like type 1 diabetes, hypertension and heart diseases. In the recent years, vitamin D has also been extensively studied for its beneficial effects on various types of cancers.
Vitamin D is proposed to play a preventive role in many cancers
Several studies have attempted to establish the relationship between vitamin D and cancerous conditions of breast, colon, prostate, skin and pancreas. Most of the studies produced promising results, especially for breast, prostate and colorectal cancers. To understand how vitamin D helps in these diseases, we have to briefly consider how it works in the body.
It is a well-established fact that human body can synthesize vitamin D via the action of sunlight on skin. Ultraviolet B rays of the sun act upon a substance called 7-dehydrocholesterol in the skin and convert it into Vitamin D3 (cholecalciferol). A major share of the vitamin is synthesized by the body and a minor share is obtained from dietary sources such as fish, eggs, dairy products and juices fortified with vitamin D. Animal sources provide vitamin D3 and plant sources provide vitamin D2 (ergocalciferol). But both of these natural forms of vitamin D are in their inactive state. A series of metabolic processes that take place in the liver and kidney convert inactive vitamin D to the active form called calcitriol. This metabolite has more of a hormonal activity. It is this active metabolite that carries out a wide range of functions, one of which is proposed to be the cancer-preventive effect.
The action of calcitriol is mediated through receptors that specifically bind with this metabolite of vitamin D. These are intracellular receptors called Vitamin D Receptors, or VDR, and they are present in many organs. Intestine, kidneys and bones are the main target organs of calcitriol. The important function of calcitriol is the regulation of blood calcium and phosphorus levels. These two minerals are absorbed from the intestine and utilized in bone formation and maintenance by a series of reactions mediated by the vitamin D-VDR complex.
Studies explain the mechanism by which vitamin D exerts its anti-cancer effect
Several studies have identified that in addition to the intestine, kidney and bones, many organs of the human body like brain, heart, skin, breast and prostate glands express vitamin D receptor. The degree to which a tumor cell in these organs expresses the receptor determines the cell’s response to calcitriol. The receptors are expressed to a higher degree in the early stages of cancer and they are poorly expressed during advanced stages and metastasis.
Calcitriol is suggested to inhibit tumor growth and prevent metastasis by three specific actions on the tumor micro-environment:
- It maintains differentiation – When immature cells grow into mature cells, they take the specific structure and functions particular to the organ they are constituted of. This is called differentiation. They grow to resemble the normal cells of the organ and are called well-differentiated. When a tumor cell divides it may form well-differentiated cells (cells resembling the normal cells of the organ) or poorly-differentiated cells (cells that stay immature). Calcitriol helps to maintain differentiation in a tumor tissue.
- It promotes apoptosis –The genetically determined ‘programmed cell death’ that happens through a cascade of events to all normal but old and damaged cells is called apoptosis. This process is essential to remove unwanted cells. Cancer cells avoid this process thus resulting in unlimited cell growth and division. Calcitriol promotes apoptosis and limits tumor growth.
- It inhibits angiogenesis – A growing tumor requires a lot of blood supply. In order to meet this requirement, new networks of blood vessels are continuously formed as the tumor grows. This facilitates the spread of tumor to the nearby tissues and distant organs (metastasis). The process of formation of new blood vessels is called angiogenesis. By inhibiting this process, calcitriol prevents tumor growth and metastasis.