It’s that time of year again. The days are getting shorter and our opportunity to make vitamin D, the sunshine vitamin, is shrinking. And, at some point in the first week of September, our ability to make vitamin D from sunlight switches off until around the middle of April next year.
It’s all about the angle of the sun in the sky
The biochemical reaction in our skin that makes vitamin D can only do that when the ultraviolet B (UVB) wavelength of sunlight is strong enough. And that only happens when the sun is at a 45º (or higher) angle in the sky.
There is a really easy way to tell if the sun is at 45º (or above). Your shadow is shorter than you are. If your shadow is taller than you, then not enough UVB is getting through the atmosphere to allow us to make vitamin D in our skin.
All of this means that in the south of the UK, we have a rather narrow window of opportunity (around five months from mid April to early September) to make enough vitamin D to last us for the whole of the rest of the year. In the north, that time span is even shorter because the further north you go, the lower the angle of the sun in the sky, even at midsummer.
It’s a vitamin – shouldn’t it come from our diet?
Although it is called a vitamin, vitamin D is actually a steroid that gets converted into a type of hormone by our liver and kidneys. And although we can get some vitamin D from our diet, food sources (oily fish, eggs, liver, kidneys, fortified breakfast cereals and mushrooms that have been exposed to UVB) are limited. The best way to get enough vitamin D through food is by taking a food supplement.
We (and many other organisms on the planet) evolved to make vitamin D from sunlight. Phytoplankton does it and then fish get vitamin D by eating phytoplankton . Chickens also synthesise vitamin D, which is how it gets into eggs . For us, the fact that vitamin D is found in some foods is kind of an “add-on” that helps us get through the winter months when we can’t make it from sunlight.
Unfortunately, most of the people in the UK don’t eat much of the foods that contain vitamin D (with the possible exception of fortified breakfast cereal, much of which is sugar laden and ultra-processed and should be avoided anyway). Very few people eat liver or kidneys these days. Fish consumption in the UK is low, and intakes of oily fish are even lower. Egg consumption is rising, but you would need to eat a lot of eggs every day to get enough vitamin D, which could be quite challenging – on several levels. And although some mushrooms on sale are now being exposed to UVB, most still are not (the ones exposed to UVB are generally brown and will say “source of vitamin D” on the label).
Why is this a problem?
The more we find out about vitamin D, the more we realise just what a keystone substance it is, because it influences every single system and organ of the body.
Every cell in our bodies has connection points called receptors that hook up with hormones, neurotransmitters and things like immune messenger chemicals that zip around inside us creating communication channels. Every second of every day the cells inside us are sending and receiving millions of messages through these receptors. And it turns out that virtually every cell in our bodies has vitamin D receptors (VDRs). Having low levels of vitamin D can have serious health consequences.
So if vitamin D is so central to our health and wellbeing, why aren’t the UK government doing something about it?
Deficiency as a matter of public health policy?
As with so many things relating to nutrition and public health, the UK government follows a narrow and outdated line; that vitamin D is only about bones, teeth, and muscles. And, although they do appear to realise that not getting enough vitamin D is a problem, the action they take to deal with it is pretty poor.
Along with its antiquated view about the functions of vitamin D, the UK government has an equally obsolete opinion on how much vitamin D we need, and what constitutes deficiency.
Every year the government runs a rolling National Diet and Nutrition Survey (NDNS) to evaluate what people are eating and the nutrient content of the food being eaten. Blood samples are also taken to check things like folate levels and vitamin D levels. In a significant proportion of the population, vitamin D levels are consistently far too low. So in 2016 the government issued extremely unenthusiastic and half-hearted advice (much against its usual public health ideology), that people might want to think about taking a vitamin D supplement in autumn and winter. Hardly a call to action.
The Office for Health Improvements and Disparities (previously Public Health England, rebranded in 2021) defines vitamin D sufficiency (ie having enough to maintain health) as having 25 nanomoles (a unit of measurement usually written as nmol) of vitamin D per litre of blood.
But the whole of the rest of the world (national governments, the European Union, international and national non-governmental health agencies and charities, and every other relevant institution) considers 25nmol/L to be a measure of severe deficiency. Pretty much everywhere else in the world thinks that a minimum adequate level for most people is 50nmol/L.
Interestingly, the NHS considers 25 nmol/L to be deficient, possibly because the National Institute of Clinical Excellence (NICE) has issued guidance stating that levels at 25 nmol/L or below increases the risk of deficiency.
So how much do we need?
Once again, the UK government seems to be deliberately setting policy to ensure that the UK population remains deficient in vitamin D. Current advice is to think about taking 10µg (micrograms) a day in the autumn and winter, although individuals with darker skin, and a few other groups should think about taking 10µg daily throughout the year. The trouble is, 10µg is only just enough to keep blood levels of vitamin D at around 25nmol/L (severe deficiency) in people who don’t already have blood levels below that.
So despite evidence from the government’s own data that a lot of people in this country have levels below 25nmol/L, there is no genuine action being taken to ensure that the UK population has appropriate information to protect and support their own health.
What the advice should really say is that everyone over the age of 12 needs to be taking 25µg a day (under 12, stick to 10µg because there is no safety data for higher intakes in children), but anyone with a darker skin, or who doesn’t get much sun exposure, anyone planning a pregnancy, or actually pregnant, or breastfeeding, and anyone over the age of 70 needs to be taking at least 50µg a day from the beginning of September up to mid-April.
As noted previously, nutrition policy in the UK is an abject failure, and it is difficult not to wonder when this level of mismanagement becomes criminal negligence.
T C Callis’ articles are very well researched, and always come with a wealth of references. If you are interested in following up anything in the article, don’t hesitate to contact us at Kent & Surrey Bylines for more information.