Among all the micronutrients, vitamin D is one of the lesser-known vitamins. Most people refer to it as the "vitamin from the sun", which rather short-changes its important role and complex functions in the body.
As we learn more about vitamin D, we are also discovering that it may be linked to cancer prevention. While this is exciting knowledge, we do need to examine all the available evidence within the proper context.
In this article, we will look at the roles and functions of vitamin D, as well as how to obtain adequate amounts of it for good health.
The role of D
To begin with, did you know that vitamin D is not technically a single vitamin? When we talk about vitamin D, we are referring to a group of fat-soluble pro-hormones, or hormone precursors.
There are two main forms of vitamin D that are most relevant to us: vitamin D2 and D3. D2 is found in plants, while D3 is manufactured by the body when exposed to sunlight.
For a long time, it was thought that vitamin D's main role was to support calcium, by promoting the absorption of the mineral by the body so that it can be used to strengthen bones and teeth.
But vitamin D does a lot more than that. Scientists have discovered that at least 2,000 different genes in the body are regulated by vitamin D. This means that D is involved in numerous functions in the body at the cellular level, including cell death and proliferation, insulin production, and the immune system.
Vitamin D also contributes to improving muscle strength and reducing inflammation.
Few, if any, of these functions are known to people. But suffice to say that without adequate levels of vitamin D in the body, a lot of small, but important, processes in our body would not be carried out smoothly.
Vitamin D is a remarkable micronutrient as it is linked to many other health issues that we are only just learning about. In the past few years, research has shown that a lack of the vitamin may be the primary culprit in depression, heart disease, pregnancy problems, birth defects, skin and other cancers, and multiple sclerosis.
Even if you don't suffer from any of these conditions, increasing your vitamin D to optimal levels may just improve your general well-being.
In the US, a number of experts, including those from the Harvard School of Public Health, have urged the government to raise its recommended daily amount (based on US standards) of vitamin D for adults from 200 IU to at least 1,000 IU, possibly more.
Vitamin D deficiency can have a serious impact on bone strength. Children who are severely lacking in vitamin D can develop rickets, a condition where their bones are soft and deformed, due to undermineralisation of the bones.
Adults can also develop a form of rickets, but it is known as osteomalacia. Vitamin D deficiency is also implicated in osteoporosis, a condition of weak and porous bones most commonly seen in elderly women and men.
Vitamin D also plays a role in maintaining healthy body weight, by working in two ways. Firstly, vitamin D is linked to receptors in the brain that keep hunger and cravings in check.
Secondly, D stimulates the body to absorb calcium, which is one of the nutrients that helps our body lose weight by regulating the fatty acid synthase that converts calories into fat.
A 2009 study published in the British Journal of Nutrition found that obese women who were put on a 15-week diet and given 1,200mg of calcium daily, lost six times more weight than women on a diet alone. Therefore, it is important not just to have optimal levels of calcium, but also vitamin D, which helps in calcium absorption.
The link between D and cancer
Some recent studies have suggested that vitamin D may have a protective effect against cancer. Prompted by these encouraging findings, larger scientific studies have been carried out to investigate in greater depth whether vitamin D really plays a role in cancer prevention.
The first hint came from epidemiologic studies, which found that people living in southern latitudes (therefore, exposed to more sunlight) had lower rates of cancer than those living in northern latitudes. This was thought to be due to the higher levels of vitamin D in those exposed to more sunlight.
More directly, a randomised clinical trial looked at the cancer rates of women who took calcium and vitamin D supplements. Compared to a control group on a placebo, the women on supplements had a 60 per cent lower overall incidence of cancer.
However, the researchers cannot say for sure that it was vitamin D that caused this effect, as it was combined with calcium supplements.
Several large studies, including epidemiological studies and clinical trials, have also looked at individual cancers. There are some indications that higher levels of vitamin D intake is associated with a reduced risk of colorectal cancer, such as in the American Cancer Society's Cancer Prevention Study (CPS) II Nutrition Cohort, a pooled analysis of 10 cohort studies and the Third US National Health and Nutrition Examination Survey.
However, the evidence remains inconsistent, with some studies showing a greater reduction of colorectal cancer than others, and some showing no statistical significance.
As for breast cancer, there are also conflicting results. Several studies, including the landmark Women's Health Initiative and the National Cancer Institute's Prostate, Lung, Colorectal and Ovarian (PLCO) Cancer Screening Trial, found no association between higher vitamin D levels and reduced breast cancer risk.
On the other hand, prostate cancer risk has been linked with higher vitamin D levels. One study even showed that men with higher vitamin D levels had an increased risk for aggressive prostate cancer, although it is not enough to confirm a link either way.
A study of more than 120,000 men and women from the Health Professionals Follow-Up Study and the Nurses' Health Study found that those who had higher intake of vitamin D from their diets had a lower risk of pancreatic cancer.
At the same time, other studies showed no protective effect against pancreatic cancer and some even showed that higher D levels were associated with higher cancer risk.
Other less common cancers, such as non-Hodgkin's lymphoma or cancers of the endometrium, oesophagus, stomach, kidney or ovaries, also do not show any concrete link with vitamin D.
By and large, the evidence is still not conclusive enough to make a direct claim that vitamin D can help prevent any sort of cancer. In fact, excessive intake of vitamin D may even tip the scales in the other direction, as some studies have suggested.
Getting enough D
Nonetheless, getting adequate amounts of vitamin D is still important for maintaining normal body functions and good health.
However, chances are that you are not getting as much vitamin D as you should. While the body can make vitamin D from the UVB rays in sunlight, few people get the right kind of rays as they are usually only exposed to sun during early mornings or late afternoons.
Furthermore, fear of skin cancer means that most people use some form of sun protection, and rightly so.
Vitamin D can also be found in foods, although very few foods, like fatty fish, cod liver oil, eggs and meat, naturally contain D. Fortunately, you can also obtain it from fortified foods like milk, juices, yogurt, bread and breakfast cereals (check the food labels to be sure).
If you think that you do not get enough sunlight, or are afraid to, then ensure that you eat plenty of these D-rich foods.
Taking vitamin D supplements puts you at higher risk of toxicity, especially if you are not taking it based on medical advice.
Vitamin D toxicity can increase calcium levels, which leads to the mineral being deposited in the kidneys, heart and lungs. You may experience irregular heart rate, confusion, pain, conjunctivitis, anorexia, fever, chills, thirst, vomiting and weight loss, if you exceed your vitamin D intake.
The amount of vitamin D in multivitamins and other dietary supplements usually ranges from 10 to 50 μg (micrograms). As the recommended daily intake of vitamin D for men and women in Malaysia is between 5 and 10 μg (and only 5 μg for children), you should not exceed what is prescribed or recommended on the packaging of the supplements without consulting your doctor.
If you are suffering from vitamin D deficiency, you need at least 1,000 IU (25 μg) of vitamin D. Higher doses may even be recommended for people with diabetes, hypertension and autoimmune conditions.
Currently, it is not safe to take vitamin D supplements for the purpose of preventing cancer, as there is not enough evidence to back this up. You should only take vitamin D supplements on medical advice, if you are found to have a serious deficiency, which can be diagnosed with a blood test.