Could winter’s low sun be keeping you low in vitamin D?
Apart from the one day of snow, how many of us have ventured outside for a long period of time over the past couple of weeks? We’re already well into Lockdown number 3 – a drearily long winter of short days, limited sunshine, and prolonged periods stuck indoors. Not only are we experiencing much reduced physical activity and increased weight gain, bet we’re also at risk of being deficient in vitamin D.
The impacts of vitamin D deficiency have been well researched in recent years and Public Health England (PHE) advises all adults and children over five years of age to consider taking a daily supplement of vitamin D daily from October to March.
But what is vitamin D - and how do we get it?
Vitamin D is actually a hormone that exists in two forms: Vitamin D3 (cholecalciferol), which is produced by the body; and, vitamin D2 (ergocalciferol), which is obtained from dietary sources. Vitamin D3 is a more ‘potent’ form than D2.
· The majority of our vitamin D is produced in the skin from exposure to the sun. In the northern hemisphere, we make most of our vitamin D between April and September, when the days are longer and the sun is higher in the sky.
· Although we can get vitamin D from foods such as oily fish (sardines, mackerel, salmon) and eggs, dietary sources of vitamin D are limited and only provide us with around 20 % of our vitamin D requirements.
Dietary sources of vitamin D
Why is vitamin D important for my health?
From bone health and muscle function through to our immune system, the more we learn about vitamin D the more we discover its far reaching benefits on our health and wellbeing.
· Bones: Low vitamin D levels affect the health of our bones. Vitamin D enhances the uptake of calcium from our gut, which is then stored in our bones. When we are low in vitamin D, our body takes calcium from the skeleton to be used elsewhere, compromising our bone health.
· Muscle: Vitamin D plays an important role in muscle function, and research (source 1, source 2) has shown that restoring vitamin D levels in the elderly increases muscle strength.
· Immune system: Vitamin D is critical for our immune health. Vitamin D receptors are present in most immune cells in the body and may protect against illness as well as reduce the risk of infection.
Will vitamin D protect me against COVID-19?
Vitamin D helps fight viral infections. Patients hospitalised with COVID-19 present with very low levels of vitamin D - they also tend to be older and more likely to be housebound. There is no solid evidence that vitamin D protects against COVID-19 but ensuring that you have adequate levels of vitamin D is important for your immune health, helping your body fight all infection - not just COVID-19.
Maintaining healthy levels of vitamin D is also vital in the face of our current vaccination programme; studies have shown that reduced levels of vitamin D can lower our response to the vaccine itself.
How do I make sure I have enough vitamin D?
The majority of people in the UK (approx. 80% of us living at latitudes >35 degrees) do not produce sufficient vitamin D in the autumn and winter months. Those who stay indoors during sunlight hours, cover up in the sun, or have darker skin, are also more likely to have poor vitamin D levels throughout the whole year.
The only way to maintain normal vitamin D in these scenarios is to take a supplement.
In 2020, PHE recommended daily 400 IU of vitamin D3 for adults to protect musculoskeletal health during the COVID pandemic.
Whilst 400 IU might maintain vitamin D levels, higher intakes of up to 20 to 25 mg (800 to 1000 IU) a day may be required to restore adequate vitamin D levels throughout the winter months. However, it is important to avoid high doses of vitamin D so the safe upper tolerable limit for over the counter supplementation is 4000 IU a day.
The bottom line is clear. If you live in the UK, diet and sunlight alone will not maintain your levels of vitamin D at this time of year – even if you are going outdoors every day. To optimise your intake, research is clear. To protect your bone, muscle and immune health, you need to supplement with vitamin D3 daily – at least until spring has sprung and our days are at last longer and sunnier once again.
About our guest author:
Dr Julie Greeves OBE is Professor of Applied Physiology, and she is Honorary Chair at the School of Medicine, University of East Anglia. She has worked for over 20 years researching female health, bone physiology and impact of vitamin D in young adults.
