Vitamin D is also called calcitriol. It is a fat-soluble vitamin that is well known for its role in:
- Absorbing calcium from the small intestine
- Bone mineralisation
- Regulating calcium and phosphorous levels in the blood and bones.
Calcium and phosphorous are critical for building and maintaining healthy bones. But vitamin D has many other roles in the body. These include:
- Immune system function
- Reducing respiratory illness
- Potential protection against diabetes, hypertension, dementia, and even certain cancers.
Vitamin D has many other potential health optimisation considerations in which research is still developing.
Where Do We Get It?
80 to 100% of our Vitamin D comes from sunlight, and deficiency is most often caused by not getting enough sun rather than a lack of dietary intake!
When the UV-B rays from sunlight hit our skin, a modified cholesterol molecule in the skin is converted into vitamin D3. Vitamin D3 is more stable form than active vitamin D and is the form measured in the bloodstream. Vitamin D3 is used as an indicator of vitamin D status – but it is not the final active form that the body uses.
Vitamin D3 in the bloodstream travels to the liver undergoing a chemical conversion to “Calcidiol”, and is one step closer to becoming the active vitamin D.
Finally, the kidneys convert calcidiol into the active Vitamin D (calcitriol. This is the form that the body can use.
As you can see, the synthesis of vitamin D in the body is quite a complicated process. On top of this, it is practically impossible to reach your vitamin D requirement from diet sources alone (unless using supplemental vitamin D).
The Downsides of Deficiency
The most extreme sign of vitamin D deficiency is a disease called rickets. Without adequate calcium and phosphorous (which can’t be absorbed and used without vitamin D), the skeleton cannot form the mineral hydroxyapatite, which gives bones their strength.
When calcium levels are low, calcium stored in the bones is removed from them and put into the bloodstream (serum calcium). This calcium is needed for muscle contraction, brain function, and other processes in the body.
The end result in children is a softening and malformation of bones, bowing of legs, deformed pelvis, and enlarged joints. Some of these effects may be treated through dietary correction, surgery, or braces. Some effects can persist throughout life as the bone formation that would normally have occurred at critical growth stages has been compromised.
Vitamin D deficiency in adults can lead to osteoporosis or osteomalacia. Osteomalacia is a condition in which bones become soft and deformed due to insufficient calcium and phosphorus. Bones are more likely to bend, fracture, or break, and bone pain and muscle weakness are common symptoms.
Although your body might not be functioning properly, you can’t really “feel” if you have low vitamin D levels. Outside of osteomalacia or rickets, the only real indication of your Vitamin D levels is through a blood test.
Vitamin D tests aren’t a standardised test in Australia so if you simply ask your doctor for a “blood test”, vitamin D won’t be included. The test itself doesn’t require additional blood, it just needs to be specifically asked for inclusion in the tests being run.
Your vitamin D status is worth checking as (although this data is slightly old now) Australian census data showed a high likelihood of deficiency in the population. Just under one in four Australian adults (23%) had a Vitamin D deficiency in 2010-2011.
Things to Be Aware of When Trying to Optimise Your Vitamin D
Many lifestyle factors can influence vitamin D status. It’s worth knowing that your geographical location plays a role in your sun exposure and the types of UV radiation you are exposed to.
This means there is a proportionally higher risk in some states, such as Victoria, Tasmania, and the ACT where deficiency rates were between 43-49%. That’s almost HALF the population and your odds of being deficient are higher if you live here.
In comparison, rates in Qld and the Northern Territory for example were only 15-17%. This is most likely due to the geographical location and lifestyle more than dietary factors. There are ~919 more hours of sunlight per year in Townsville, Queensland compared to Hobart, Tasmania. That’s over 2.5 hours more per day on average.
If the majority of your vitamin D comes from the sun, and you have proportionally more sun exposure, it makes sense that deficiency levels are lower in these places.
Anything that reduces UVB radiation exposure will decrease the body’s ability to create vitamin D3. Apart from your geographical location, some other factors include:
- The use of sun-screen
- Time spent outside
- Wearing long clothing
- Environmental factors such as pollution
- Cloud cover
- Skim complexion and age
SPF15 sunscreen decreases vitamin D synthesis to 1/15th or about 7% of what direct UVB exposure would give.
Darker skin has significantly higher melanin content and therefore requires comparatively greater UVB exposure time than paler skin to synthesize the same amount of vitamin D.
Aging decreases the capacity of human skin to produce vitamin D3. As we age there is less of the cholesterol-based vitamin D precursor molecule in the skin, leading to reduced vitamin D synthesis from sun exposure. Studies have shown the vitamin D synthesised is about four times less in a 70-year-old compared to a 20-year-old!
Given so much of the population is deficient, with even more likely to have suboptimal levels, these things that compromise vitamin D production are all worth being aware of.
Rickets and osteomalacia are examples of issues that arise when you’re at the extremes of vitamin D deficiency. However, even if you aren’t clinically deficient, there could still be benefits in optimising your levels.
Census data from a few years ago found that ~5% of Australians were taking vitamin D supplements. Even with supplementation some of these people were still classified as deficient.
What Should My Vitamin D Levels Be?
Blood levels of vitamin D are measured in nanomoles per litre (nmol/L) and status is defined as follows:
<12.5nmol/L severe deficiency
<25 nmol/L moderate deficient
<50 nmol/L insufficiency
>50 nmol/L sufficiency
Sufficiency is currently defined as the point at which further increases will have no additional beneficial effects on bone health. So at a minimum for the healthiest possible bones, you want your levels to be above 50 nmol/L.
However, the cut-off value of 50 nmol/L being optimal is debatable. It is arguably not accounting for additional vitamin D that is being used for other effects in the body.
Some studies have suggested or demonstrated levels of over 80 nmol/L and even up to 200 nmol/L as being optimal targets. This would theoretically allow for all the other potential beneficial roles of vitamin D beyond bone health to be fulfilled.
Should I Supplement to Bring My Levels Up?
Practically, some people find it difficult to get enough sun exposure to sustain healthy vitamin D levels. If this is you, supplementation is probably a good idea.
Not many foods in the diet contain vitamin D. Foods such as fatty fish, eggs, and organ meats may provide small amounts. There are also some fortified food products such as milk, margarine, and juice. Even when eating these foods you will still need sunlight for optimal levels.
The Australian adequate intake guidelines in micrograms (µg) of vitamin D are:
<50 yrs 5.0 µg/day
51-70 yrs 10.0 µg/day
>70 yrs 15.0 µg/day
An “adequate intake” is used, not a specifically recommended daily intake (RDI), as there isn’t enough data to calculate an average requirement. This makes sense, as so many factors can affect your vitamin D requirements.
Anything from skin colour and geographical location, to air pollution and the number of cloudy days, can can change requirements at an individual level. This makes blanket intake recommendations difficult to give.
Supplemental forms of vitamin D are commonly between 400 to 1000IU, and research has shown intakes at these levels are very unlikely to cause harm. For reference, 1 microgram of vitamin D is equal to 40 IU.
Vitamin D supplements can be either vitamin D3 or vitamin D2, although D2 is much less common these days as it less effective at raising blood levels. Most of the supplements you will find available to buy find are D3, but if it comes down to choosing, opt for D3.
Vitamin D toxicity is rare but symptoms can include vomiting, nausea, constipation, weight loss, weakness and kidney stones. Toxicity can occur from excessive supplemental intakes of vitamin D, with symptoms typically occurring in blood concentrations over 200 nmol/L. There is good evidence suggesting that dietary intakes should not be greater than 4,000IU per day if supplementation is ongoing.
The natural formation of vitamin D from sunlight is self-limiting and it is impossible to develop toxicity from this.
You should always get a blood test and the help of a qualified practitioner (i.e. a doctor or dietitian) to check where your levels are before undergoing supplementation.
What Is the Benefit of Having Adequate Vitamin D?
Adequate vitamin D will ensure you’re able to absorb calcium from the diet and optimise bone mineral synthesis. If vitamin D is low, there is insufficient calcium absorption from the diet.
The bones are essentially just storage containers for calcium. Calcium is actually needed throughout the body for lots of functions including nerve conduction, muscle contraction, and hormone control.
If blood levels drop and we aren’t absorbing calcium (via vitamin D) through the diet, the body takes calcium from its stores (the bones). This weakens existing bone and prevents the formation of strong/new bone.
Fracture risk is increased, and long-term, rickets, osteomalacia, or osteoporosis can become real risks.
Studies show that fracture risk with low vitamin D levels can be mitigated with a high enough dose of supplemental vitamin D. Calcium supplementation is also often needed for more significant risk reduction.
A 2021 study found that “individuals with Vitamin-D deficiency were 80% more likely to acquire COVID-19 infection as compared to those who have sufficient Vitamin D levels”. Considering the COVID pandemic, and what might be done to minimise your risk of illness, this information is valuable.
In a more general sense, there have been studies linking certain types of immune function with vitamin D status.
In work published over 40 years ago in 1981, a British doctor hypothesized a sunlight-related “seasonal stimulus” triggered influenza outbreaks, as the flu is always more prevalent in winter months.
Due to the lower levels of sunshine and therefore lower vitamin D status, links were drawn between the vitamin D levels and immunity.
A 2017 meta-review with 10,933 study participants concluded that vitamin D supplementation did reduce the risk of acute respiratory tract infection. This effect was more pronounced in those who had low levels (<25nmol/L) before supplementation and in those who had not been undergoing regular supplementation.
More research is needed before we can definitively claim that vitamin D protects against the flu and other acute respiratory infections. At this point though, it seems safe to conclude that if your levels are low, it is definitely worth addressing.
Are There Benefits Beyond Simply “Not Being Deficient”?
Beyond the benefits above, there may be additional benefits in supplemental vitamin D for reducing premature death or “all-cause mortality”. Most of the reduced mortality risk appears to be coming from a reduction in cancer mortality.
The primary means of converting Vitamin D3 to active vitamin D (calcitriol) is performed via the kidneys. Since this discovery, it has also been found that other cells throughout the body can take circulating D3 and activate it.
Cells in the skin, intestine, breast, and prostate can all activate vitamin D3. This indicates these tissues have a use for vitamin D.
Even the cells of the immune system such as macrophages, monocytes, and dendritic cells can all activate vitamin D3, indicating roles in an immune response.
The active form of vitamin D has various effects in many of these cell types which suggest anti-cancer activity. Cells may be better able to avoid or combat cancer by controlling cell growth, replication, and the spread of cancer.
The ability of all of these other cells to use vitamin D will depend on the levels of vitamin D3 that are available in the blood. If there isn’t enough vitamin D left after bone health is optimised, some of these other functions simply may not have access to the amounts needed.
50 nmol/L is the target range of sufficiency, but this may not be enough to optimally support other body functions beyond optimising bone health.
Don’t rely on food alone for your vitamin D as the amount in foods is likely not enough for you to sustain a sufficient blood level of 50 nmol/L.
It is always a good idea to get a blood test to check your vitamin D levels before supplementing. This being said, there are minimal risks associated with taking sensible amounts of vitamin D (400 to 1,000IU per day).
There are many factors that can impact your body’s ability to synthesize vitamin D, even if you are getting some sun exposure. This includes things like sunscreen, cloudy weather, skin tone, and geographical location.
There may be benefits in supplementing to levels beyond 50 nmol/L. However, research in the area is still very much developing. We cannot yet recommend a specific target at which benefits might be seen. An upper limit appears to be around 200 nmol/L so caution should be used not to exceed this amount if supplementing.
In Australia, vitamin D is a non-standard marker on a blood test. This means you’ll need to ask the doctor for your vitamin D levels to be checked in addition to a standard blood test. Hopefully, this will change in the future given the levels of insufficiency and deficiency in the country.