What is Vitamin D?

Vitamin D is a fat-soluble vitamin which can either be absorbed from the diet and supplements or synthesised by the body.

Since its association with better Covid-19 outcomes1, Vitamin D has received a lot of media attention. But what is Vitamin D, what does it do in the body, and most importantly, how can you make sure you’re getting enough?

I’ll answer all of these questions and more, to tell you everything you need to know about this very important vitamin:

What is Vitamin D?

Vitamin D is a fat-soluble vitamin, which means it is absorbed with fats in the diet and then stored in fatty (adipose) tissue and in the liver. It can either be obtained from the diet and supplements or synthesised by the body - more about that later. You may also see it referred to as calciferol. There are two forms of Vitamin D: D2 (ergocalciferol) and D3 (cholecalciferol). Vitamin D is classed one of the 13 ‘essential vitamins’, which are considered to be vital for your body to function optimally.

Benefits of Vitamin D

So, what is all the fuss about? What does Vitamin D do for the body?

Its most well-established and well-known benefits2 are for the absorption/utilisation of calcium and phosphorus, keeping blood calcium levels balanced and consequently supporting bone health. Vitamin D also contributes to the maintenance of teeth and muscle function.

But most media interest is directed towards its recognised benefits for immune health, as it contributes to the normal functioning of the immune system.

In fact, science is uncovering a myriad of different roles for Vitamin D in the body, from cardiovascular health to anti-inflammatory properties. It’s a ‘must-have’ for our bodies! But how do we obtain this wonder vitamin?

Sources of Vitamin D

We now know how valuable Vitamin D is for the body, but let’s find out more about the best sources of Vitamin D.

As we learned earlier, there are two types of Vitamin D: Vitamin D2, which is found in plant sources, and Vitamin D3, which mostly comes from animal sources including humans. Human beings can synthesise Vitamin D3 when sunlight (more specifically UVB radiation) is absorbed by their skin. This is why you may have heard Vitamin D referred to as the ‘sunshine vitamin’.

In terms of Vitamin D rich foods, the shocking truth is that there are not many foods that are naturally high in Vitamin D. There are some dietary sources, but they are foods which many people, especially children, may not regularly eat, such as salmon, herring, and every child’s favourite (not!): cod liver oil! There’s some Vitamin D in egg yolks and mushrooms, and some foods, like cereals, dairy products, orange juice, and soya milk, are fortified with added Vitamin D. However, it’s not widely available in our diet.

Due to the scarcity of this vitamin in nature and, in many countries, the scarcity of sunlight for much of the year, for many people, the easiest way to ensure they are getting enough Vitamin D is to take a supplement. See microbiologist Dr Kate’s top tips on how to choose the best supplement.

Should I take a Vitamin D supplement?

We can see from the previous section that obtaining an optimum intake of Vitamin D each day in your diet is not as easy as you might think. That’s why taking a Vitamin D supplement is recommended by the NHS3 especially for breastfeeding mums. Let’s look at other reasons why some people might be at higher risk of having less than optimal Vitamin D levels.

The majority of people spend less time outdoors than our ancestors did, reducing the sun exposure which our body needs to synthesise its own Vitamin D. When we do go outside on sunny days, the risk of skin cancer means it is very necessary to apply sunscreen, which can reduce Vitamin D absorption by more than 90%4. The body also produces its own natural sunscreen, a substance called melanin, which darkens the skin. High levels of melanin in the skin interfere with vitamin synthesis and consequently, people with darker skin need more sun exposure to generate the same amount of Vitamin D5. Finally, for many of us, sun exposure is a rarity! In the Northern Hemisphere, many months of the year are devoid of sunny days, and when it does make an appearance, the sunlight and UV rays are weaker, so people living in these locations may always struggle to make enough Vitamin D from sunlight6.

We shouldn’t stop wearing sunscreen and we can’t all move to sunnier climes, so taking a Vitamin D supplement is an easy way to remove any worries about getting enough Vitamin D. Thankfully, there are a wide range of supplements available, though if you are vegan or vegetarian do check where the Vitamin D is sourced from as some supplements will use animal sources.

Optibac Probiotics offer a range of products containing Vitamin D, suitable for the whole family: Babies & Children, which is a vegetarian supplement and suitable from birth; Kids Gummies, which are vegan and suitable from age 3 onwards, and Gut Health Gummies, which are vegan and suitable from 12 years.

Adult Gummies probiotics
Optibac Gut Health Gummies friendly bacteria with Vitamin D

What are the signs of Vitamin D deficiency?

If it’s so difficult to get enough Vitamin D from sunshine and your diet, and you haven’t been taking a supplement, how do you know if your Vitamin D levels are low?

Depending on how severe the deficiency is, and because of the wide variety of roles Vitamin D plays in the body, the signs of low Vitamin D can be quite varied. You might experience noticeable and ongoing fatigue, low mood, muscle weakness and aches, and bone pains. Rather than guessing, however, if you’re experiencing these issues, you should always speak to your doctor first as there can be other causes for these symptoms. If you or your doctor/healthcare professional suspect your Vitamin D levels are low, they can do a simple Vitamin D status blood test.

How much Vitamin D do I need?

To avoid deficiency and support your body’s Vitamin D requirements, how much Vitamin D do you need to take each day?

The FDA recommends a daily value (DV) of 10mcg per day from birth to 12 months old, 15mcg daily from one to four years of age, and 20mcg per day from the age of four years onwards3.

It can be difficult to determine how much Vitamin D you or your children are receiving from their diet. Formula milk should have added Vitamin D but check the label, and as previously mentioned, some foods are fortified with Vitamin D so should give the amount per portion. But to be certain of getting enough Vitamin D, many people prefer to take a supplement, then they don’t need to worry.  You may see supplements containing much more than the recommended daily values You might also see supplements giving the amount of Vitamin D in international units (IU), but it’s simple to convert as one microgram of Vitamin D is equal to 40 IU.

Speak to your doctor or health care professional for advice if you want to take more than the recommended daily amount.

Can I take Vitamin D every day?

You may have read that you can overdose on Vitamin D, so are perhaps wondering if Vitamin D supplements are safe, if it’s OK to take Vitamin D every day, or if it's bad for you to take too much Vitamin D.

It’s safe and actually recommended to take Vitamin D supplements daily3, especially if you’re in one of the higher risk categories outlined above. However, as Vitamin D is a fat-soluble vitamin which can be stored in bodily tissues, and it’s more difficult for the body to excrete excess amounts, it is advisable to ensure that your combined daily supplement intake falls within the recommended guidelines and is well under the safe upper limits.  Both the recommended daily amount and the safe upper limits vary from country to country and there is much debate as to the exact amount which should not be exceeded, but the following guidelines are widely accepted as standard6:

0-6 months - 25 mcg (1,000 IU)

7–12 months - 38 mcg (1,500 IU)

1–3 years - 63 mcg (2,500 IU)

4–8 years - 75 mcg (3,000 IU)

9–18 years - 100 mcg (4,000 IU)

19+ years - 100 mcg (4,000 IU)

These are just general guidelines however, and opinion on these limits does vary. Ultimately, if you are in any doubt as to whether it’s appropriate for you to take Vitamin D, or how much is appropriate for your individual needs, please consult your doctor or health care professional.


When should I not take Vitamin D?

Everyone needs Vitamin D for their bodies to function; however, there are circumstances where it is advisable to seek medical advice before taking supplements. Certain medications might interact with Vitamin D supplements, and those with certain medical conditions might need to avoid Vitamin D supplements or take them under medical supervision.

If you’re taking medication or have a serious or chronic medical condition, then you should speak to your doctor or health professional before taking any vitamin supplement.


This FAQ was answered by Kerry Beeson BSc (Nut. Med.) Nutritional Therapist

Hopefully this article has answered some of your questions about the amazing Vitamin D nutrient. Check out the other articles in this series: Added Nutrients

You might also enjoy the following articles:

Benefits of Alpha-Linolenic Acid

Microbiologist’s Top Tips for Winter Health

7 little steps to improve your health

There are more related articles on the Probiotics Learning Lab:

Vitamin D and your baby’s microbiome

Probiotics aid Vitamin D absorption


  1. Dror AA, Morozov N, Daoud A, Namir Y, Yakir O, Shachar Y, et al. (2022) Pre-infection 25-hydroxyVitamin D3 levels and association with severity of COVID-19 illness. PLoS ONE 17(2): e0263069. https://doi.org/10.1371/journal.pone.0263069
  2. EU Register of nutrition and health claims made on foods (v.3.6). (2022). Retrieved 4 March 2022, from https://ec.europa.eu/food/safety/labelling_nutrition/claims/register/public/?event=search
  3. FDA Frequently Asked Questions for Industry on Nutrition Facts Labeling Requirements (2022). Retrieved 8 March 2022, from https://www.fda.gov/media/99069/download
  4.  Holick MF. Vitamin D: importance in the prevention of cancers, type 1 diabetes, heart disease, and osteoporosis. Am J Clin Nutr. 2004; 79:362-71
  5. Bonilla, C., Ness, A. R., Wills, A. K., Lawlor, D. A., Lewis, S. J., & Davey Smith, G. (2014). Skin pigmentation, sun exposure and Vitamin D levels in children of the Avon Longitudinal Study of Parents and Children. BMC public health, 14, 597. https://doi.org/10.1186/1471-2458-14-597Wacker, M., & Holick, M. F. (2013). Sunlight and Vitamin D: A global perspective for health. Dermato-endocrinology, 5(1), 51–108. https://doi.org/10.4161/derm.24494
  6. Office of Dietary Supplements - Vitamin D. (2022). Retrieved 8 March 2022, from https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/