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What is the Shadow Rule and how does it relate to Vitamin D synthesis?

The Vitamin D Shadow Rule

Spring equinox fell at the weekend; a time of balance between light and dark. The days are gradually getting warmer and the sun has shone brightly over the south of England for the last couple of days.


Spring is definitely here!

The sun has many wonderful benefits, so being out in it and amongst nature supports our health in immeasurable ways. Of course, among the most well known benefits is our body’s ability to synthesise vitamin D from the sun’s rays on our skin. 

At this time of year our stored levels of vitamin D are extremely low. If you’re in the northern hemisphere and have not had a holiday somewhere much further south (which is unlikely at the moment), you have gone quite some time unable to synthesise vitamin D. 

Hopefully, like me, you’ve been supplementing throughout the winter months, but even though the sun and warmth is returning, here’s why you need to continue with that vitamin D supplement a little while longer. 


It’s all in the angle

And that angle is 50° or higher to be precise. This refers to the angle at which the sun sits in the sky above the horizon. In order for us synthesise vitamin D, it’s crucial that the UVB rays reach us through the atmosphere and this only occurs when the sun is at an angle of at least 50°. In the UK this generally occurs between 10.30am and 3.30pm from May to late August, but it’s important to know it’s not for the entire 5 hour duration in the earlier months and of course, we are still in March. 

So how can we know when the sun is at an optimal angle for us to effectively synthesise this pro hormone? Well, luckily you don’t need to do a daily calculation, you just need to look at your shadow!


As a general rule of thumb, if your shadow shorter than you (or indeed any object) then you should be able to synthesise vitamin D (cloud cover and pollution will of course affect this somewhat). If your shadow exceeds your height, then there is significantly less UVB in the sun's rays, a greater amount of the more damaging UVA rays and vitamin D will not be synthesised.

Here’s a handy diagram to help you visualise this:

If your shadow is shorter than your height, then you are able to produce vitamin D


The importance of vitamin D

Covid-19 has certainly raised awareness of this vitamin (which is technically not a vitamin, but a pro hormone) and has seen more people opt for a supplement, but it doesn’t hurt to be reminded of its crucial role in supporting our immune system and skeletal health as well as modulating cell growth and glucose metabolism. 

You can read my article on Vitamin D and its role in relation to Covid-19 specifically here.

Low vitamin D levels are linked to many diseases (believe it or not, skin cancer is amongst them) and if you suspect you are deficient it’s worth getting your levels checked. Some of the symptoms of a deficiency to look out for are:

  • Fatigue
  • Bone pain
  • Muscle weakness, muscle aches, or muscle cramps
  • Mood changes and depression
  • Recurring infections
  • Hair loss
  • Hypertension

So whether you’re experiencing these symptoms or not, ensure your levels remain optimal by keeping up with that daily supplement for now and keep an eye on your shadow length from around May to ensure you’re maximising those UVB rays in the summer months.  

This article also talks about the importance of vitamin K2 and why for the sake of your bone and cardiovascular health, you should always opt for a formula that contains the combination of both D3 and K2 - even better if it's held only in Organic Black Seed oil.

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