Did you know that, if your shadow is longer than you are tall, the sun is probably not high enough in the sky for your skin to produce vitamin D?

Thankfully, those of us in the Northern Hemisphere are seeing our shadows shrink.

If we choose to get outside, UVB light from the sun will hit our skin and convert a form of cholesterol into the first stage of Vitamin D, called cholecalciferol.

This cholecalciferol is then converted once by the liver and a second time by the kidney to produce an active form of vitamin D that can do its job.

Vitamin D is not officially a vitamin. It is more a hormone. (A vitamin is something our bodies can’t produce and which we need to consume). As you may remember from your school biology lessons, hormones are messengers which control an array of bodily processes. Vitamin D is no exception.

Some of the processes that Vitamin D (yes, it can’t quite shake off its name) is involved in are: absorption of calcium and phosphate for our bone health (you knew that one), cell proliferation and cell death and regulation of our immune system. There is evidence that our cardiovascular health and response to insulin might also rely on vitamin D.

You might recognise the name cholecalciferol if you already take a supplement, since it is in this form that we consume it (incidentally, this is likely to come from lanolin in sheep’s wool). Ergocalciferol is the plant-based alternative derived from fungi and lichen. Since supplements such as these are still in the inactive form of vitamin D, they rely on our body being able to convert it into the active form.

In the later stages of CKD, the kidneys start to lose their ability to carry out the last step in the process i.e. to synthesise the active form of vitamin D. So, in this case, the active form of the vitamin may be prescribed. In the UK, this is often called alfacalcidol.

In my experience, some people are prescribed a combination of both inactive and active supplements. Consultants all have their personal preferences and they’re not to be messed with!

So, should you be taking a vitamin D supplement if you have CKD?

The decision to commence supplements would be based on a range of blood tests which assess your risk of kidney related bone disease. These will include calcium, phosphate, parathyroid hormone levels and sometimes vitamin D. I say ‘sometimes vitamin D’ because the blood test only tells us the amount of inactive vitamin D, so isn’t necessarily that helpful.

If you’re still at an early stage of CKD, the NHS recommends all adults and children over one year old take 10mcg (400iu) daily during the darker months and that more vulnerable groups would benefit year long. Those with CKD may well fall into this group and a daily supplement of 10mcg all year round will do no harm.

I’m a dietitian, so it would be remiss to totally ignore the food sources of vitamin D such as eggs, offal, meat, oily fish, milk (dependent on the season) and foods fortified with vitamin D such as spreads and cereals. You’ll see some mushrooms marketed as being a source of vitamin D. These have been exposed to UV light and will contain higher levels of ergocalciferol as a result. Getting sufficient from our diet alone is practically impossible. The sunshine, for most of us, is the primary source.

So, nip outside and check your shadow length. Are you likely to be synthesising?



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