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Vitamin D Supplementation: When, Where and How Much

By Jo Cunningham


October is when we like to remind everyone that it’s that time of year when we should be taking a Vitamin D supplement.

Why do we need vitamin D?

Vitamin D is a fat-soluble vitamin that promotes calcium absorption in the gut. Also, it helps maintain optimal calcium and phosphate levels in the blood – key nutrients in keeping our bones, teeth and muscles healthy. Moreover, it reduces inflammation, supports the immune system, and reduces the risk of developing several diseases such as osteoporosis, diabetes, hypertension and cancer. Research has also shown that it may play a role in regulating mood. 

What happens if we don’t get enough?

In children, low levels of vitamin D can lead to rickets which is a bone deformity, weak bones and poor growth. In adults, vitamin D deficiency can lead to a condition called Osteomalacia or ‘soft bones’ causing bone pain and an increased risk of fractures.

Other symptoms of low levels of vitamin D include regularly getting sick, general tiredness, and muscle or bone aches and pains. 

Where can we find vitamin D?

Whilst we can get small amounts from our diet, and dietary sources are important, it’s unlikely that we can meet requirements to maintain optimal levels through diet alone. 

Dietary sources include:

  • oily fish like salmon, rainbow trout, mackerel and sardines
  • mushrooms
  • egg yolks
  • fortified plant-based milks
  • fortified spreads and cereals
  • red meat

Our main source of vitamin D is sunlight, where our body can cleverly make it when the UV rays strike our skin. During the summer months when the sun’s rays are strongest in the UK, it’s possible to meet at least some of our vitamin D needs naturally without supplementation. During the autumn and winter months of October to March, the sunlight in the UK doesn’t contain enough UVB radiation for us to be able to naturally produce enough vitamin D. This means we need to rely on dietary sources plus supplementation. 

How much should we be supplementing?

The Department of Health recommends that adults (including pregnant and breastfeeding women) and children over the age of 4 should consider taking a daily supplement containing 10ug (400IU) of vitamin D during the months of October to March. 

All year round supplementation with 10ug (400IU) per day is recommended for the following:

  • children aged 1 to 4 years old
  • those who are inside a lot in the summer or are housebound
  • those who have their skin covered most of the time
  • those with darker skin
  • those with certain medical conditions (e.g. coeliac disease, IBD, and diagnosed fat malabsorption)

When it comes to the very little ones, the Department of Health recommends that babies from birth to 1 year old should have 8.5-10ug (340-400IU) daily throughout the year if they are breastfed or have less than 500mls of formula per day. 

*If your GP has recommended that you take a different amount, you should follow your GP’s advice.

Can I take too much?

As with many supplements, excess amounts can be toxic and in the case of vitamin D it can result in excessive calcium absorption which may lead to nausea, vomiting and other symptoms and complications including kidney stones. 

The recommended 10ug per day is a safe level of intake aimed to meet nutritional requirements. For most adults, 100ug (4000IU) per day is considered safe but this should only be taken after seeking the advice of your GP or dietitian rather than self-dosing. 

There’s no risk of getting too much from exposure to the sun, but it’s important to practice safe sun exposure, protecting the skin from burning. 


Vitamin D is an essential vitamin. A healthy balanced diet and some sunshine exposure can meet most of our requirements. However, a supplement is recommended during the winter months, or all year-round for certain individuals.

Your GP can check your vitamin D level via a blood test, and your GP, pharmacist or dietitian can recommend appropriate dose supplementation based on your results. 

Jo Cunningham is a gut specialist dietitian with in-depth clinical dietetic experience ranging across both NHS and private healthcare settings. Jo is experienced in helping clients to resolve gut symptoms including reflux, bloating, constipation, diarrhoea, and abdominal pain. She also specialises in the management of Irritable Bowel Syndrome (IBS) and in the use of the low FODMAP diet.

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