Vitamin D is important for both skeletal health, due to its role in healthy bone growth and mineralisation, and non-skeletal health, where it has been linked to improved immunity. It is found in two forms, D2 and D3.
D3 is found in animal products and is synthesised from sunlight, whereas D2 is the less effective plant-based source.
Animal sources of Vitamin D include:
- Egg yolk
- Oily fish
- Red meat
The richest source of Vitamin D typically comes from oily fish, with some containing the full daily recommendation in one portion.
Plant-based sources include:
- Fortified plant milk
- Fortified tofu
- Mushrooms, of which portobello, maitake, morel, button, and shiitake mushrooms are all high in Vitamin D
The lack of Vitamin D in plant products makes deficiency a relevant concern for those adhering to a vegan diet. This is exacerbated by D2 being less effective, which is due to D2 being excreted quicker and the D3 form being preferable in some mechanistic stages in the body, such as during conversion to the active form.
Vitamin D deficiency
As our main source of Vitamin D is the sun, there are increasing concerns around deficiencies during the winter in the UK.
In 2012, more than 900 cases of rickets, the Vitamin D deficiency disease, were reported, despite previously being considered to no longer be a risk for UK children. These new cases are particularly prevalent in inner-city children.
Osteomalacia is a less well-known vitamin D deficiency disease that affects bone remodelling and mineralisation in adults, resulting in bone pain and muscle weakness.
There is not currently a consensus on an optimal serum Vitamin D level, with various researchers and organisations suggesting measurements between 50 mmol/l and 80 mmol/l. The Scientific Advisory Committee on Nutrition (SACN) produced a detailed report on vitamin D in 2016, in which they stated that less than 25 mmol/l is indicative of deficiency (the full report can be found here)
In the UK, we only convert sunlight to Vitamin D between March and October, and it is estimated that we would need to spend 35 minutes in sunlight everyday between 11am and 3pm during these months to get our required Vitamin D. Given most people spend much of the working day inside, this is largely unattainable for many.
Currently, the average intake in the UK is 3.5 μg per day, which is significantly below the updated 2016 recommendation of 10 μg per day.
Fortification is currently not mandatory in the UK, and there are a lot of questions surrounding its efficacy. An example of biofortification is seen in the ‘Vitamin D mushrooms’ which are fortified with Vitamin D by being grown in the light.
Some of the data does suggest that fortification does not always work in the same way, and it may be impacted by the matrix or food product it is in. As Vitamin D is a fat-soluble vitamin, absorption will likely be reduced in individuals suffering from fat malabsorption or following a very low-fat diet, and in these instances, the matrix is likely to be more important.
Fortification was investigated in the largest and most powerful study to date of Vitamin D in the UK, in the form of a 12-week study over 2 winters (the full study can be found here). The study recruited 335 Caucasian and southern Asian healthy participants, who consumed D2 and D3 fortified biscuits and juice. The results showed there was no difference in uptake between the juice and the biscuit.
However, though D2 increased the level of the Vitamin D marker in the blood, D3 had a much greater impact and caused levels to at least double. The study, therefore, concluded that D3 is more successful than D2 at reducing deficiency. Notably, there is also some suggestion that D2 may negatively impact D3 when both are consumed, due to competition for the same biochemical pathway.
Vitamin D Supplementation
Vitamin D supplements containing D2 and D3 are widely available and are likely to be beneficial to ensure you are getting the required 10 μg per day, particularly during the winter months.
However, it is worth noting that a meta-analysis of studies investigating the effect of Vitamin D supplements on bone mineral density showed very mixed results, with only a small benefit seen at the femoral neck. This suggests that the use of Vitamin D supplements to help improve bone mineral density in patients with osteoporosis may not be worthwhile.
Nonetheless, encouraging supplementation in the UK population, particularly in at-risk groups, is a worthwhile strategy. Specific subgroups thought to be at risk of Vitamin D deficiency in the UK are;
- Pregnant and breastfeeding women
- Southern Asian women
- Those with higher levels of melanin in the skin
- Individuals who have emigrated from countries where Vitamin D can be synthesised from the sun all year round.
From a public health perspective, encouraging both increased dietary consumption and supplementation in the UK population will help to prevent further increases in deficiencies.