Tôi có nên uống vitamin D bây giờ vì trời ít nắng, hay là để tốt cho xương hoặc miễn dịch?

It can be easy to think you get plenty of vitamin D when you live in a country bathed in sunshine, but the reality is more complicated.

Almost one in four Australian adults have vitamin D deficiency. Vitamin D supplements are now one of the most commonly used complementary medicines.

So what is vitamin D? And do you need to take it as a supplement?

It functions like a hormone

Vitamin D is a fat-soluble vitamin that plays a crucial role in maintaining overall health. Unlike most vitamins, it functions more like a hormone in the body, and nearly every cell has a receptor for it.

It exists in several forms, but vitamin D3, also known as cholecalciferol, is the most important. Once in the body, D3 undergoes changes – first in the liver and then in the kidneys – to become its fully active form called calcitriol.

Your body is capable of producing its own vitamin D by converting a cholesterol precursor into it, but that requires exposure to ultraviolet radiation (UVB) on your skin.

You can also get it through diet from a few foods including eggs, oily fish and mushrooms – but it’s unlikely to be as much as you need.

What happens when you don’t get enough vitamin D?

Vitamin D’s best-known role is helping the body use calcium. It promotes the absorption of calcium from the gut, ensuring an adequate level in the blood for building strong bones.

Without sufficient vitamin D, your body can’t absorb calcium effectively, which can lead to bone health problems.

In children, severe deficiency causes rickets, a condition where bones become soft. This leads to delayed growth, bone pain, and skeletal conditions, such as bowed legs.

In adults, deficiency can cause a condition called osteomalacia. This results in bone pain, bone tenderness and a higher risk of fractures.

In the long term, low vitamin D contributes to osteoporosis by reducing bone density and increasing the risk of fractures, especially in older people.

Deficiency is also linked to muscle weakness and cramps, and impaired immune function, which results in a higher susceptibility to respiratory infections.

What can cause a vitamin D deficiency?

Insufficient sunlight exposure typically causes vitamin D deficiency.

If you spend all your time indoors, or you work night shifts and sleep during the day, you will get less sunlight exposure and make less vitamin D.

While we get generally get lots of sunlight in mainland Australia, there are regions that for long periods have very low sunlight which can also cause vitamin D deficiency. In very northern and southern latitudes, such as Tasmania, there are only a few hours of sunlight in winter.

For people living at these latitudes, they can not only have a vitamin D deficiency, but they may also suffer from a type of depression called seasonal affective disorder which has been linked to low vitamin D.

Melanin, or skin pigmentation, affects vitamin D production. People with darker skin and people with significant skin disorders, such as psoriasis or severe burns and scarring, can also be at risk of vitamin D deficiency.

Prescription vs over-the-counter supplements

There are various vitamin D supplements in Australia. There are low-dose (20 microgram) and higher-dose (175 microgram) formulations of vitamin D3. There is also a 0.25 microgram formulation of calcitriol, the active form of vitamin D.

Both of the vitamin D3 products are used for treating vitamin D deficiency, while the calcitriol product is used for treating hypocalcaemia (low calcium level) in people with chronic kidney disease.

The low dose vitamin D3 is taken daily whereas the higher dose formulation is taken once a week.

The higher-dose formulation is sold as a pharmacist-only medicine, meaning you’ll need to speak to a pharmacist before they are able to supply it to you.

The calcitriol vitamin D product is only available as a prescription medicine.

Vitamin D3 is also available in multivitamins at lower doses and in products that are combined with calcium or vitamin K.

Are there any dangers in taking vitamin D?

Vitamin D3 is generally well-tolerated. When taken daily, the upper tolerable intake level is 100 microgram.

A regular dose higher than 100 microgram for prolonged periods can cause excessive calcium absorption. This can result in nausea, vomiting, muscle weakness, loss of appetite, dehydration, excessive thirst and kidney stones.

On the flip side, excessive sunlight exposure will not cause vitamin D toxicity, but may increase your risk of skin cancer.

Vitamin D3 supplements may also interact with some cholesterol medications (statins) and alter those medicines’ level in your body.

There are also reports that suggest a potential interaction between vitamin D and a weight-loss medicine orlistat, interactions with steroids, and with the diuretic thiazide.

So do you need a supplement?

Most people only need five to 30 minutes of direct sunlight exposure, several times a week for their body to produce adequate vitamin D.

So unless there is a reason why you are not getting enough sunlight, or you have a skin condition, then you don’t need a supplement.

If you think you might need a supplement, your GP can order a blood test. There are also at-home test kits for vitamin D that have been approved by the Therapeutic Goods Administration.

If you are deficient, consult your local pharmacist who can recommend the right product and quantity for you based on your needs.

Nial Wheate in the past has received funding from the ACT Cancer Council, Tenovus Scotland, Medical Research Scotland, Scottish Crucible, and the Scottish Universities Life Sciences Alliance. He is a Chartered Chemist and fellow of the Royal Australian Chemical Institute. Nial is the chief scientific officer of Vaihea Skincare LLC, a director of SetDose Pty Ltd (a medical device company) and was previously a Standards Australia panel member for sunscreen agents. He is a member of the Expert Panel for the Haleon Pain Management Institute. Nial regularly consults to industry on issues to do with medicine risk assessments, and medicines manufacturing, design, and testing. Nial is often called as a expert witness in sports tribunals involving adverse analytical findings for elite athletes.

Ian Jamie and Wai-Jo Jocelin Chan do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.