Vitamin D - its roles in the body and how to make sure you are getting enough


As seasons are changing and the northern hemisphere is moving into winter and the southern hemisphere is coming out of spring, it is an important time to check in on your vitamin D levels - otherwise known as the sunshine vitamin.  Approximately 50% of the population worldwide are believed to have a vitamin D deficiency - this includes a large proportion of the Australian population that are vitamin D deficient despite the levels of sunshine.

The Roles of Vitamin D in the body

Vitamin D is a fat soluble hormone but actually behaves more like a hormone as it regulates and directs a large number of processes in the body. Some of the different roles of vitamin D play are outlined here:

  • Immune regulation - vitamin D plays a role in balancing and regulating the immune system and sufficient levels of vitamin D helps improve our body’s own regulation of the system. Therefore, sufficient vitamin D levels are very important to those with autoimmune conditions and also supports us fight everyday infections. It is reported that those with autoimmune conditions do better having vitamin D levels towards the upper end of the reference range.
  • Supports healthy teeth and bone health - vitamin D supports calcium and phosphorus assimilation through the intestinal lining into the blood which is needed for mineralisation of our bone and teeth.
  • Mood and brain health - vitamin D is essential for the production of neurotransmitters which are essential for our mood and cognitive health.
  • Gut health - vitamin D interacts with the gut microbiome and deficiency can cause a decrease in microbiome diversity. Our microbes in our gut help us digest food, synthesise vitamins and so much more.

Sources of Vitamin D and the importance of testing

Vitamin D is available naturally from sunlight exposure and certain food sources but unfortunately these sources may not provide us with the amounts of vitamin D our bodies need reliably and consistently. Chris Kresser, a functional medicine practitioner, states that in his clinical experience people usually need about 2000 - 5000 IU of vitamin D per day from all sources.

As a result, it is essential to regularly test your vitamin D levels and then supplement as required on top of the natural sources you are getting if you need to maintain your levels of this important vitamin. Testing is often available through your GP but it can be paid for privately if required.

As vitamin D is a fat soluble hormone it can be stored in our bodies fat tissues for use at a later time which is useful in some respects but also means that we can end up with toxic levels if we over supplements. This is another reason why it is important to test - especially if supplementing. I personally test my vitamin D levels every 3 to 6 months depending on how stable my levels are and how much I am supplementing.

Read on for more information about each source:


Sunlight (UVB rays specifically) converts a form of cholesterol in the skin to a precursor of vitamin D which in turn gets converted by the liver and kidneys to active vitamin D. This process is dependent on having adequate cholesterol levels in the body and also our liver and kidney function. Chris Kresser reports that in summer mid-day sun with pale skin, 30 minutes of direct sunlight will produce 10-20,000 IU of vitamin D. This is much more than food sources can give you but is a best-case scenario as the following will all reduce the conversion of vitamin D: lower solar angle due to season / time of day, body parts being covered with clothes, inflammation, being overweight and having darker skin.  In addition, sunscreen which protects against the harmful consequences of the sun's rays largely eliminates the ability for sunlight to convert cholesterol to vitamin D plus many of us are spending more time indoors or in cars further reducing our sunlight exposure.

Top tip:

Recently I have discovered an app on my iPhone called “dminder” - this helps you to get vitamin D exposure from the sun without burning your skin based on where you are in the world, your skin type etc.  I am often outside in the morning and the evening rather than the middle of the day and this app has made me realise how little sunlight exposure I am getting that will translate to vitamin D!

Food Sources

There are not many foods high in vitamin D with seafood, especially fatty fish (for example: herring, salmon, sardines, mackerel and tuna), being the best source.  The amounts will vary between fish types, are likely higher in wild versus farmed fish, may depend on the cooking type and will need to be consumed regularly to support vitamin D levels. It is very hard to understand the true levels of vitamin D in food sources as data sources vary, but one example is that 100g of Atlantic herring contains 214IU. Vitamin D is also found in smaller levels in egg yolks, mushrooms and dairy products.

What I love about food sources (or natural supplements) of vitamin D however is that alongside the vitamin D in the food, nature provides other vitamins and minerals in the same food that are needed to absorb the vitamin D into the body and to also prevent against toxicity. Vitamin A is often found in foods alongside vitamin D (e.g. in cod liver oil) and this is not only essential in its own right but it prevents against the toxicity of vitamin D.

Top tip:

Unfortunately food alone is unlikely to be enough to maintain adequate vitamin D levels on their own but regularly consume fatty fish either fresh or in BPA free tins in extra virgin olive to support vitamin D levels and also for the other benefits of oily fish. The small fish further down the food chain for example sardines and mackerel have less build-up of the heavy metal mercury - these fish when purchased fresh are often much better value for money too. I enjoy tinned fish alongside a roasted sweet potato and a salad, I make sardine or mackerel pate or I pan fry or BBQ sardines or mackerel and top with lemon juice once cooked.


This may be essential depending on your vitamin D levels and the amount of vitamin D you are getting through sunlight or food.

A great natural source for supplementation can be cod liver oil which contains both vitamin D and vitamin A. Please note however that good quality fish oils can be very hard to source as manufacturers need specialised equipment to process the fish oil correctly. When you consume fish oil supplements, ideally you should chew them in your mouth first so as to mix them with your saliva which starts breaking down the fat -  if the supplement is good quality and something your body needs these nutrients they should actually taste good! As an alternative, canned Icelandic cod livers which are sourced from clean waters in the arctic, if you can tolerate eating them!

In addition, vitamin D3 supplements are available too. These are often found in conjunction with vitamin K2 as vitamin D and K2 are synergistic and vitamin K2 also helps calcium assimilate into the hard tissues such as bones and teeth. As a fat-soluble vitamin, these should be taken with food. The dose will vary person to person depending on your unique circumstances and the time of year and as mentioned above it is crucial to test.

Top tip:

have your levels of vitamin D tested at least once every 6 months and then supplement based on your lifestyle and the test results. 

By Charlotte Hill Functional Nutritionist This email address is being protected from spambots. You need JavaScript enabled to view it. see Patient Advisory Bio