The sunshine vitamin our bones ‘D’- pend on!

Kirstie Lamb – Researcher and Trainer in Public Health/30 May 2017

Firstly, let me introduce myself! My name is Kirstie and I am the newest member of the clinical team here at X-PERT Health, working as a Researcher and Trainer in Public Health.

Below is my first contribution to the X-PERT blog and is focused on vitamin D and the recent changes to recommendations. If you have any feedback or questions, please feel free to email me at

What is it and how is it produced?

Vitamins are vital nutrients that we require in small amounts for a variety of processes in our body and to keep us healthy. They are typically split into two groups depending on the way they act in our body. These groups are fat soluble vitamins (vitamin A, D, E and K) and water soluble vitamins (e.g. B vitamins and vitamin C).

Unlike many of the essential vitamins we require to function correctly, vitamin D can actually be made in our bodies. However, this process depends on one vital ingredient- sunlight!

The process is quite complex, but can be simplified into a few key steps as follows. Firstly, our skin is exposed to sunlight, or ultraviolet B (UVB) radiation to be precise. Conversion of a molecule known as 7-dehydrocholesterol (7-DHC) to previtamin D in the skin occurs when this UVB is absorbed and this is subsequently converted to another form of vitamin D known as vitamin D3 [1]. This form of vitamin D, alongside a variation known as vitamin D2, can also be supplied through certain foods in our diet which will be discussed later on in this blog. Both vitamin D2 and D3 are transported to the liver and then to the kidney, where the final stages of conversion can take place, producing the active form of vitamin D that we need for several processes in the body.

Why do we need vitamin D?

One of the main roles of vitamin D is to regulate calcium and phosphorus metabolism, which helps to maintain bone mineralisation (the process whereby minerals are laid down on bone) and overall musculoskeletal health, including reducing the risk of fractures, rickets (in children) and osteomalacia (a bone disorder in adults characterised by bone pain and muscle weakness) [2]. A decrease in the concentration of calcium in the blood stimulates an increased production of active vitamin D in the kidney. This in turn helps to increase our calcium concentration by increasing absorption of calcium in our intestines, reabsorption of calcium in the kidneys [3] and the breakdown of bone (a process known as resorption), transferring the calcium stored in our bones into our blood. Therefore, we need to maintain an adequate concentration of vitamin D in order to increase calcium concentrations as and when required. Levels below 25 nmol/L have been suggested to increase the risk of poor musculoskeletal health.

Vitamin D may have additional benefits to our health, above those found for the musculoskeletal system. It has been suggested that adequate vitamin D status may be protective against many other diseases and health problems including, but not limited to, several autoimmune diseases, cardiovascular disease, several types of cancer, mental illness, infertility and adverse pregnancy and birth outcomes [2]. Associations have also been proposed between adequate vitamin D levels and a reduction in risk of developing Type 2 diabetes. A systematic review of 19 studies concluded that individuals with the highest vitamin D status had a 43% lower risk of developing Type 2 diabetes than those with the lowest status [4]. Associations have also been proposed between low vitamin D concentrations, impaired β cell function (pancreatic cells that produce insulin) and greater insulin resistance in pre-diabetic individuals [5]. However, due to limitations in study design, a causal link between low vitamin D concentrations in our blood and defects in insulin action and secretion cannot be inferred (in other words, we cannot definitively say that low vitamin D levels cause these defects, as there may be other factors involved!). Alternatively, it has been suggested that vitamin D may indirectly help enhance insulin secretion by regulating the concentration of calcium in β cells [6]. Again, this is currently just speculation and further studies are required to confirm these findings.

How much vitamin D do we need?

Until recently, dietary reference values (values informing us how much of each nutrient we need) for vitamin D were only available for specific groups at risk of deficiency, including infants under 3 years, pregnant and breast-feeding women, and adults over 65 years, as a preventative measure for several musculoskeletal conditions [7]. It had previously been suggested that, as vitamin D is synthesised in the skin following sun exposure, the amount of vitamin D synthesised in the skin during the summer would be sufficient for the rest of the year. However, this has not been shown to be the case, as vitamin D deficiency is prevalent at worryingly high rates.

Recent changes to the recommendations have been made by the Scientific Advisory Committee on Nutrition (SACN) to ensure that safe vitamin D status is achieved by the UK population and reduce the risk of vitamin D deficiency and its associated problems. The main change made is the recommendation of a reference nutrient intake (RNI), a value set that is likely to meet the needs of the majority of the population (97.5%), for vitamin D of 10 µg per day throughout the year for everyone above 4 years old [7]. This value has been chosen to help the majority of the population maintain blood concentrations of at least over 25 nmol/L, the value below which health problems become more likely. Optimal vitamin D concentration is considered at blood levels over 75 nmol/L.

Studies have suggested that toxicity (the concentration at which vitamin D in the blood can cause damage to our bodies) may occur at concentrations above 375 nmol/L. A consequence of this may be excessively high blood calcium levels, a condition known as hypercalcemia. This leads to calcium being deposited in soft tissues and demineralisation (the removal of minerals) of bones. However, this is very unlikely if you are consuming an appropriate diet and not over-consuming vitamin D supplements.

Dietary Sources

Production through ultraviolet B (UVB) exposure can be influenced by several factors, including time of day, month, altitude, sunscreen use and your skin’s efficiency to synthesise vitamin D. As such, it may be difficult to assess how much vitamin D you are making through sun exposure and, although we are able to store and use vitamin D produced during the summer, some individuals may struggle to meet recommendations during the winter by relying on this method alone. There are also certain groups who are at increased risk of deficiency throughout the year due to reduced sun exposure, including housebound individuals, people who regularly wear skin-covering clothing and high sunscreen users. Minority ethnic groups with darker skin are also at greater risk of deficiency [7].

To ensure adequate vitamin D levels are achieved, dietary sources of vitamin D should also be considered. This would be especially essential for these at-risk populations and when UVB exposure is limited, for example during the winter. Unfortunately, one contributing factor to the high rates of deficiency is that dietary sources of vitamin D rich foods are limited and those containing vitamin D often vary greatly in content. The main sources are of animal origin, including egg yolks, meat (including fat, liver and kidney) and oily fish (e.g. salmon and sardines). 100g of oily fish contains between 5 and 16 µg of vitamin D- yet another reason why we should be eating more oily fish! Mushrooms can also be a rich source of vitamin D if treated correctly and can be enhanced through additional preparation practices. For example, you can naturally increase the levels of vitamin D in mushrooms by exposing them, gill-side up, to sunlight after purchase. Other foods, including breakfast cereals and spreads, may be fortified with vitamin D; however, the nutritional quality of these foods should also be considered, as these could also be high in refined carbs, sugars and trans-fats. Table 1 below displays several different food sources of vitamin D, along with their content per 100g. It is important to note however that this value can vary greatly depending on the brand or where the food is sourced or farmed.

Food Source Vitamin D content (µg/100g)
Kipper 10 µg
Mackerel 8.5 µg
Herring 16.1 µg
Tinned salmon 13.6 µg
Branflakes (fortified, 30g) 4.7 µg
Chicken egg 3.2 µg
Lamb chop 0.8 µg
Pork leg joint  0.9 µg
Lamb kidney 0.6 µg

Table 1. Vitamin D content of various foods. Edited from Spiro and Buttriss (2015)[8]

Whilst supplements may be taken to increase vitamin D status, the evidence for their benefits and risks is mixed and so lifestyle changes such as undertaking more physical activity outside and increased consumption of natural food sources of vitamin D are recommended before and above the use of supplements. Supplements are, however, currently recommended by the NHS for groups mentioned previously who are at a higher risk of deficiency.

It is also important to note that, although some sun exposure is encouraged, exposure time should be considered to prevent sunburn and increased skin cancer risk. Typically, the amount of time needed to make sufficient vitamin D is less than that needed for the skin to burn. Sunbed use to increase vitamin D production is also not encouraged, as this increases the exposure to a more harmful ultra-violet radiation which is associated with increased risk of skin cancer. However, for individuals with restricted exposure to sunlight, UVB lamps may be a good method of producing adequate vitamin D [2].

And so to summarise, in order to try and increase your vitamin D levels, firstly evaluate your diet and identify whether there is room to incorporate more vitamin D containing foods. Secondly, try to go outside when and if you can throughout the year- but don’t stay out in direct sunlight without sunscreen for too long, especially during the summer! If having read this you believe you are in an at-risk group, we would recommend speaking to your GP to check whether or not you are deficient (or at risk of deficiency) and to discuss alternative methods for increasing your vitamin D levels, such as the use of supplements or UVB lamps.


1. Holick, M.F., et al., Vitamin D and skin physiology: a D-lightful story. J Bone Miner Res, 2007. 22(2): p. V28-33.

2. Pludowski P, e.a., Vitamin D effects on musculoskeletal health, immunity, autoimmunity, cardiovascular disease, cancer, fertility, pregnancy, dementia and mortality—A review of recent evidence. Autoimmun Rev, 2013. 12(10): p. 976-989.

3. Blaine, J.C., M.; Levi, M., Renal Control of Calcium, Phosphate, and Magnesium Homeostasis. Clinical Journal of the American Society of Nephrology, 2015. 10: p. 1257–1272.

4. Mitri, J., M.D. Muraru, and A.G. Pittas, Vitamin D and type 2 diabetes: a systematic review. Eur J Clin Nutr, 2011. 65(9): p. 1005-15.

5. Abbasi, F., Blasey, C., Feldman, D., Caulfield, M.P., Hantash, F.M., and Reaven, G.M., Low Circulating 25-Hydroxyvitamin D Concentrations Are Associated with Defects in Insulin Action and Insulin Secretion in Persons with Prediabetes. The Journal of Nutrition, 2015. 145(4): p. 714-719.

6. MITRI, J., and PITTAS, A.G., Vitamin D and diabetes. Endocrinol Metab Clin North Am, 2014. 43(1): p. 205-232.

7. SACN, Vitamin D and Health. 2016.

8. Spiro, A., and Buttress, J.L., Vitamin D:An overview of vitamin D status and intake in Europe. Nutrition Bulletin, 2014. 39: p. 322–350.


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