Vitamin D: do you know why it’s so important?

The National Diet and Nutrition Survey found that almost 1 in 5 adults in the UK has suboptimal levels of Vitamin D in their blood, and a survey commissioned by the British Nutrition Foundation found that around half of the UK population are not aware of the Government’s recommendations for supplementation of vitamin D. 

Do you know that the whole population should be taking a vitamin D supplement from October to April? 

Do you know what vitamin D does and why it’s so important to supplement?

What does vitamin D do for us?

Vitamin D is a fat-soluble vitamin (which means it needs fat in order to be absorbed) that has several key roles in the body:

  • It promotes calcium* absorption in the gut
  • It helps maintain optimal calcium* and phosphate levels in the blood
  • It reduces inflammation
  • It supports the immune system
  • It has a role in reducing the risk of developing diseases such as osteoporosis, diabetes, hypertension and cancer. 
  • It plays a role in regulating mood. 

*Calcium is a key nutrient involved in keeping our bones, teeth and muscles healthy – I’ll do a separate post on this in the future. 

What happens if we don’t get enough?

In adults, inadequate levels of vitamin D can leave you feeling fatigued, low in mood, and because your immune system is weakened, you may get sick more often. You may also develop muscle or bone aches and pains. 

With chronic or severe deficiency you can develop hypocalcaemia where the calcium levels in your blood become low. This can cause your parathyroid glands to work overtime in an attempt to keep your blood calcium levels normal – something known as hyperparathyroidism. Over time, this can lead to a ‘soft bones’ condition called osteomalacia which can cause bone pain and put you at an increased risk of fractures.

In children, low levels of vitamin D can lead to weak bones and poor growth, as well as potentially a bone deformity called rickets (osteomalacia which affects children). A paediatric dietitian is best placed to discuss this with you if your child has been diagnosed with low vitamin D. 

Sources of vitamin D

Vitamin D is found in small amounts in some foods. However it is now widely known that it’s extremely unlikely that an individual can meet requirements through diet alone. 

Dietary sources include:

  • oily fish e.g. salmon, rainbow trout, seabass, mackerel and sardines
  • mushrooms
  • egg yolks
  • fortified plant-based milks
  • fortified spreads and cereals
  • Beef liver

Non Dietary sources:

Our main source of vitamin D is from sunlight. When the UV rays hit our skin our body is able to synthesise and store the vitamin. This connection between vitamin D and the sun was first made 100 years ago!

In the UK during the summer months, it is advisable to get outside regularly to meet some of our vitamin D requirements through daylight. With summers getting hotter and the need for sunscreen to reduce risk of developing skin cancer this may be getting more difficult to achieve safely on a lunch break!

During the autumn and winter months of October to March, the position of the sun in the sky means that we’re not able to get enough UVB radiation from the sun in order to naturally produce vitamin D which is why in 2016 the UK’s Scientific Advisory Committee on Nutrition (SACN) set Reference Nutrient Intakes (RNI) for vitamin D with recommendations for supplementation. 

How much should we be supplementing?

The current recommendation is:

All children over the age of 1 year, and all adults (including pregnant and breastfeeding women) should take 400IU (10 μg) per day of vitamin D to achieve a vitamin D blood level of above 25 nmol/L.

The reference ranges for vitamin D:

  • < 25 nmol/L  = deficient
  • 25-50 nmol/L = insufficient
  • 50-75 nmol/L = adequate
  • > 75 nmol/L = optimal 

Note that the recommendation for supplementation is to achieve only the very minimum level in your blood. 

There are certain individuals who are at greater risk of deficiency:

  • Those whole are inside a lot or are housebound
  • Those who have their skin covered most of the time
  • Those with darker skin
  • Those who are overweight (there is an association between lower levels in those with a BMI greater than 30)
  • Those over the age of 65
  • Those with certain medical conditions (e.g. coeliac disease, Inflammatory Bowel Disease, and those with diagnosed fat malabsorption)
  • Those on certain medications which can lower levels (e.g. laxatives, steroids, cholesterol lowering drugs, bile acid malabsorption drugs, some weight loss drugs)

It is also very likely that individuals who don’t meet any of the above criteria will need more than the minimum level of supplementation in order to reach ‘optimal’ levels. 

Ideally we would tailor your dose to you, which is something I work with my clients on in the clinic by arranging private testing and providing personalised recommendations. 

Can you take too much vitamin D?


As with many vitamins, minerals or supplements, high amounts can be toxic and result in excessive calcium absorption which can lead to nausea, vomiting and other symptoms and complications including kidney stones. 

For most adults, 4000IU (100 μg) per day is considered safe BUT this should only be taken on the recommendation of your dietitian or GP. Self-dosing is not recommended. 

Note that when it comes to vitamin D from the sun, there’s no risk from getting too much that way, but it’s important to practise safe sun exposure, by using sunscreen to protect the skin from burning. 


Vitamin D is an essential vitamin required to keep your body healthy. Whilst a healthy balanced diet and some sunshine exposure can meet some of our requirements, a daily supplement is recommended during the winter months, or all-year round for certain individuals.

I can check your vitamin D level via a home blood test, and then recommend appropriate dose supplementation based on your results. Alternatively, please visit your GP if you’re experiencing any of the symptoms of deficiency and ask them to consider testing you.


Lin L, Smeeth L, Langan S, et alDistribution of vitamin D status in the UK: a cross-sectional analysis of UK BiobankBMJ Open 2021;11:e038503. doi: 10.1136/bmjopen-2020-038503

Roberts C, Steer T, Maplethorpe N et al (2018) National Diet and Nutrition Survey Results from Years 7 and 8 (combined) of the Rolling Programme (2014/2015 to 2015/2016) . PHE publications. Available at: https://www.gov.uk/government/statistics/ndns‐results‐from‐years‐7‐and‐8‐combined

Buttriss JL, Lanham-New SA. Vitamin D: One hundred years on. Nutr Bull. 2022;47(3):282-287. doi:10.1111/nbu.12575

Buttriss JL, Lanham-New SA. Is a vitamin D fortification strategy needed?. Nutr Bull. 2020;45(2):115-122. doi:10.1111/nbu.12430

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