Vitamin D has been a hot topic in nutrition and medical science for a few years now. But there is still quite a lot of confusion around its precise benefits, how much we need, and where we should get it. So I'm going to give you a whistle-stop tour of the things I want my clients to know about Vitamin D.
Vitamin D is a fat soluble Vitamin.
That means it is stored in our fat cells. But technically it's actually a seco-steroid hormone.
Vitamin D is best known for the critical role in calcium absorption, bone mineralization, our calcium-phosphorus balance, but it actually has other important cellular and regulatory functions impacting our immunity, hormone balance, nerve and neuromuscular function.
In Australia, most of our Vitamin D (around 90-95%) is acquired through exposure to sunlight.
It's synthesised in our skin, and then metabolised into its active form via our liver and kidneys.
The amount of Vitamin D we can generate through sun exposure depends on how strong the sun is, its zenith angle (determined by season and our geographic location), how much skin we expose, how fair or dark our skin is, how long we expose our skin for, the time of day, our individual skin thickness and our age.
But we can also get Vitamin D through our diet, via:
- Foods that are naturally rich in Vitamin D like fatty fish (sardines, mackerel and salmon) and egg yolk;
- Foods that have been fortified with Vitamin D including milk/dairy, margarine, orange juice and cereals;
- And surprisingly, you can get Vitamin D from mushrooms that have been exposed to UV radiation or sunlight. I wrote about this HERE.
Most people will get only 5%–10% of their vitamin D requirement through diet. So we need to ensure we get enough sun exposure (more on that below) or supplement.
Food sources of Vitamin D provide us with Vitamin D2 (ergocalciferol) while Vitamin D3 (cholecalciferol) is synthesised by our body from sunlight/UVB
For those interested in the physiology, the following diagram, from THIS PAPER, illustrated how our body produces Vitamin D from both sunlight and from Vitamin D-rich foods:
In Australia, Vitamin D status is classified as follows:
- Severe vitamin D deficiency: < 12.5 nmol/L
- Moderate vitamin deficiency: 12.5–29 nmol/L
- Mild vitamin D deficiency: 30–49 nmol/L
- Vitamin D adequacy: ≥ 50 nmol/L at the end of winter. Note that your Vitamin D levels may need to be 10–20 nmol/L higher at the end of summer, to allow for a decrease in your Vitamin D levels over winter.
Considering the abundance of sunshine in Australia you would assume we are all replete with Vitamin D. But that is not the case. Long work hours, indoor leisure activities and public health messages imploring us to "slip slop slap" have all contributed Vitamin D deficiency in Australia. The last Australian Health Survey found that approximately 17% pf Australians are Vitamin D deficient.
Vitamin D deficiency is associated with a wide range of health problems.
When our Vitamin D levels dip below 30 ng/m (75 nmol/L), our body's ability to absorb calcium and phosphorous is impaired. When levels of calcium in our blood decrease, our parathyroid releases parathyroid hormone. This causes secondary hyperparathyroidisaton which in turn stimulates demineralisation of our bones, and increased risk of osteoporosis and fracture.* Elevated parathyroid hormone is also linked with insulin resistance, weight gain, hypertension, and even cardiac impacts like heartbeat arrhythmias.
Most tissues our body have Vitamin D receptors which has prompted investigations into the relationship between Vitamin D status and cancer, immune function, cardiovascular health, depression, Type 2 Diabetes and numerous other health issues. Different health issues are impacted by different levels of Vitamin D.
Whilst there is a degree of agreement about what constitutes Vitamin D deficiency, there is no consensus as to the "ideal" level of Vitamin D to optimise our health
Different studies have investigated the association between different serum levels of Vitamin D and different diseases. This is an ongoing area of scientific inquiry, but preliminary consensus seems to be forming that levels ranging from 60-75 nmol/L may be optimal.
The following diagrams, reproduced from THIS PAPER, summarise the plethora of studies investigating the association between Vitamin D levels and the incidence of diseases ranging from rickets to multiple sclerosis and colon cancer:
We need to tread a fine line when it comes to getting our Vitamin D through sun exposure.
Australia has the highest rates of malignant melanoma in the world. So we need to be careful that the sun exposure we get to boost our Vitamin D levels does not increase our skin cancer risk!
The Cancer Council has released a Position Statement to help you get the balance right. Click HERE to access the Guide. In a nutshell:
- How you balance your sun-protection v sun-exposure is very much dependent on how far from the equator you live
- When the UV Index is 3.0 or above, it is recommended to use one or more forms of sun protection if you are going to be outside for more than a few minutes.
- Where the UV Index is below 3, sun protection is not recommended. During these times, to support vitamin D production it is recommended that people are outdoors in the middle of the day with some skin uncovered on most days of the week. Being physically active while outdoors will further assist with vitamin D levels.
The best way to know your Vitamin D status is to get tested!
This is especially important if you fall into an "at-risk" category. According to the Cancer Council, the following groups of people are at risk of Vitamin D deficiency:
- People with previous skin cancer or at high risk of skin cancer
- People who wear covering/concealing clothing
- Naturally very dark skinned people
- People who spend long hours indoors, including housebound or institutionalised Australians
- Older adults
- Obese people
- Babies and infants of vitamin D deficient mothers
When our doctor tests our Vitamin D status, what they are measuring its major circulating form, 25-hydroxyvitamin D (25(OH)D).
Vitamin D testing is no longer covered by Medicare unless you have a history of Vitamin D deficiency. And if you do have a history of Vitamin D deficiency, make sure your doctor specifically states this on your pathology request form! I was recently slugged with a bill for my Vitamin D test because my GP forgot to state on my pathology request that I have a history of low Vitamin D.
I personally still think it is very worthwhile asking your GP for a Vitamin D test, even if you have to pay for it. And for the record, I think it's stupid that Medicare doesn't cover the cost, considering the significant public health risks of low population Vitamin D status! #micdrop
If your blood test reveals low levels of Vitamin D...
If your blood test reveals low levels of Vitamin D, it's important to understand why. In most cases, people have low Vitamin D levels because they don't get enough sun and don't eat enough Vitamin D-rich foods. But in some cases, deficiency can be caused by issues with your liver or kidneys, or even your digestive system.
Your doctor will most likely advise you about supplementation, and safe sun exposure, and remind you to come back for follow up testing. If you eat and enjoy the foods that contain Vitamin D, consider adding them too.
A word on Vitamin D supplements
Some people may need a supplement to achieve Vitamin D adequacy. But the decision to supplement should really be an informed one, made by agreement with your doctor after you have had a blood test and know your Vitamin D status.
In Australia, the following supplementation is recommended for moderate to severe Vitamin D deficiency:
But *please* don't take large doses of Vitamin D (>5,000 IU) or supplement for prolonged periods unless directed and regularly monitored by your supervising medical practitioner.
Because Vitamin D is a fat soluble Vitamin, it can accumulate to dangerous levels if you megadose with supplements.** Symptoms ofVitamin D toxicity range from constipation, fatigue and depression to severe symptoms including heart arrhythmias, kidney stones and cognitive impacts.
References for my fellow science nerds!
Gies P, Roy C, Javorniczky J, Henderson S, Lemus-Deschamps L, Driscoll C. Global Solar UV Index: Australian measurements, forecasts and comparison with the UK. Photochem Photobiol 2004;79(1):32-9
Nowson CA, McGrath JJ, Ebeling PR, Haikerwal A, Daly RM, Sanders KM, Seibel MJ and Mason, RS, 2012, Vitamin D and health in adults in Australia and New Zealand a position statement, Medical Journal of Australia 196(11): 686-687
Australian Health Survey: Biomedical Results for Nutrients, 2011-12: http://www.abs.gov.au/ausstats/abs@.nsf/Lookup/4364.0.55.006Chapter2002011-12
* In fact, elevated parathyroid hormone can be a warning sign of Vitamin D insufficiency.
** Risk of Vitamin D toxicity is only associated with megadosing with Vitamin D supplements or cod liver oil, not from exposure to sunlight.