With the recent headlines about vitamin D, many of us are thinking that a few minutes in the sunshine may boost our vitamin D levels. However, how much vitamin D we can actually manufacture from the sun depends on where we are in the world, the time of year and the time of day. It is all a matter of the angle that the sun’s rays hit the earth as to whether vitamin D can be produced in our bodies.
For the sun to be at an angle in the sky to support vitamin D manufacture, it needs to be above 50degrees. In the UK, the sun only reaches this elevation through the summer months, in the hours around midday. Because the sun angle changes daily, you need to use the Sun Altitude/Azimuth table to calculate an accurate time-frame for optimal vitamin D manufacture. The table is found at http://aa.usno.navy.mil/data/docs/AltAz.php and details the angle of the sun through-out the day on any given date.
If you go to the Sun Altitude website, drop down past the American form (Form A) to Form B – Locations World Wide, and under today’s date enter London as 0 degrees 5 minutes West, and 51 degrees 32 minutes North. Then add 1 hour East of Grenwich under time zone, in order to allow for British Summer Time. Click ‘compute table’ beneath the form and you will see a whole list of times on 24 hour clock, followed by a list of ‘degrees’. This is the list we are looking for. Scroll down until the figure gets to 50 degrees or higher. I have copied and underlined the relevant bits of the table below. With these calculations you see that the sun is above an angle of 50 degrees between 11am and 3pm today. This time-span will gradually reduce until the end of August, when the sun doesn’t get above an angle of 50 degrees at all. The weather may still be hot, but this has nothing to do with temperature, – it is all about the angle of the suns rays, and even then we only produce vitamin D if it is sunny!
It is not hard to see that there is only a very limited time-frame, even in the summer months, when we can actually manufacture adequate levels of vitamin D to support health. This is where vitamin D supplementation may be of benefit. It is always worth asking your GP for a vitamin D blood test, and from there we can work out how much vitamin D it is suitable to supplement.
However, while the sun is shining aim to get some time outside, with bare arms and legs, to catch some vitamin D-producing sun-rays. You only need to be in the sun for 10 minutes each day to support vitamin D manufacture, but if you burn easily, start with just a couple of minutes and gradually build up to 10 minutes.
Extract from table. We are just referring to the first two columns Time angle of sun 10:40 46.5 124.8 10:50 47.7 127.8 11:00 48.9 130.8 11:10 50.1 134.0 11:20 51.2 137.3 11:30 52.2 140.8 11:40 53.1 144.3 11:50 54.0 148.1 12:00 54.8 151.9 12:10 55.4 155.9 12:20 56.0 159.9 12:30 56.5 164.1 12:40 56.9 168.4 12:50 57.1 172.7 13:00 57.3 177.0 13:10 57.3 181.4 13:20 57.2 185.8 13:30 57.0 190.1 13:40 56.6 194.4 13:50 56.2 198.6 14:00 55.6 202.7 14:10 55.0 206.7 14:20 54.2 210.6 14:30 53.4 214.3 14:40 52.5 217.9 14:50 51.5 221.4 15:00 50.4 224.8 15:10 49.3 228.0 15:20 48.1 231.1