November marks the beginning of Vitamin D Winter in the Northern Hemisphere, when sunlight is not strong enough to trigger Vitamin D production in the skin, leading to seriously low levels of the vitamin in the body.
Research shows that people living in high latitudes are deficient in the sunshine vitamin, especially in the winter months (October-March). In fact, Vitamin D deficiency is a real problem in Europe, with a 2016 study concluding that “Vitamin D deficiency is evident throughout the European population at prevalence rates that are concerning and that require action from a public health perspective.” 
Vitamin D is typically known for its classic role in bone building and maintaining healthy muscle function (it is required for the absorption of calcium and phosphorus). Low levels are associated with rickets (in children), osteomalacia (in adults) and osteoporosis. However, it is now known that vitamin D regulates a large number of genes, including some linked with auto-immune disease such as multiple sclerosis, Crohn's disease, lupus and rheumatoid arthritis; certain cancers and infections. 
Scientists suggest that high prevalence of Vitamin D deficiency in northern Europe could explain the higher rates of certain diseases, such as multiple sclerosis, breast cancer, bowel cancer and prostate cancer, in the region.
Indeed, a growing number of studies are linking low levels of vitamin D to autoimmune disease, asthma and other allergies, heart disease, diabetes, cancer, infections, pregnancy complications, depression and cognitive decline. 
What’s more, several reports have warned that rickets is on the rise again in Europe, a devastating condition characterised by muscle spasms, seizures and soft, deformed bones in children. Experts fear long winters and reduced exposure to sunlight (too much time spent indoors, less time playing outdoors and fear of getting skin cancer from too much sun exposure) could be largely responsible for increasing number of cases of rickets, a condition assumed eradicated in the late 1930s.
What’s with winter sunlight and Vitamin D deficiency?
If the sun is shining bright and your skin is exposed, your body will make all the vitamin D it needs. Right? Wrong.
Not all sunlight trigger Vitamin D synthesis. Your skin makes Vitamin D only when it is exposed to UVB rays, which are available when the sun is higher than 45 degrees above the horizon.
When the sun is below 45 degrees, the atmosphere blocks most of the UVB. This happens during most times of the day in winters, at latitudes higher than 37 degrees North or South of the equator. Without this ingredient, it is impossible for the skin to produce vitamin D. For people who live at higher altitudes, there is not enough UVB to make vitamin D in the autumn (fall) or winter. Clouds, glass and pollution also block UVB rays.
For that matter, even in the summer time, the sun stays low during the early morning and late afternoon. That’s why you can’t make much of Vitamin D with sun exposure before 10 a.m. and after 3.00 p.m. even during summer. (Tip: When your shadow is longer than you are tall, it is an indicator that the sun is low and your body is not making enough vitamin D).
In the northern hemisphere, ‘Vitamin D Winter’ generally lasts for around four to six months. With little or no availability of UVB during this time of the year, people in northern Europe and similar latitude are at an increased risk of vitamin D deficiency. For example, London is at 51 degrees North, a distance where UVB sunlight is insufficient for around 6 months of the year. Denmark, Iceland, Ireland, Norway, Sweden, Poland are some other countries that lie at higher latitudes.
However, besides the latitude issue, many other factors contribute to reduced sun exposure:
- Long working hours, more time spent indoors
- Children spending more time on phones and computers versus playing outdoors
- Increased use of sunscreen and overzealous protection from the sun amid fears of premature skin ageing and skin cancer.
Vitamin D can be stored in fatty tissue but these reserves quickly run-down when they aren’t being replenished on a regular basis, a case during the long winter haul in the northern latitudes. In addition, production of vitamin D by the skin is limited and after a certain point, more exposure won’t help. So, even if you have basked in the summer sun or soaked it all up on your holiday down south, your levels are likely to dip long before ‘vitamin D winter’ is over, unless you are taking vitamin D supplements.
Winter months are synonymous with respiratory tract infections such as cold and flu. Interestingly, emerging studies show that vitamin D influences the way your immune system fights infections and disease; and low vitamin D levels are linked with increased risk of cold, flu, wheezing disorders and asthma.   People also tend to complain of more body aches and stiffness during winters, typical symptoms of vitamin D deficiency.
In winter, when sunlight is sparse and devoid of UVB, relying on fortified foods and Vitamin D supplementation may be a necessary strategy for people living in the northern Europe. Increasing your intake of oily fish, good-old cod liver oil, eggs and mushrooms may additionally help, but not enough to offset the deficiency triggered by a serious lack of UVB in winters.
It is not just about preventing aching bones and muscles, topping up your levels with a high-quality supplement is an important step in maintaining your overall health. Vitamin D’s role in immune function and warding off infections and diseases is already being widely recognized. And emerging studies are providing more insights into how vitamin D may be intimately involved in almost all other aspects of our health, that go beyond calcium metabolism and building strong bones.
As winter creeps in, it may be the time to get your vitamin D levels checked, especially if you live in Europe or in a country that happens to be at a similar latitude. So if you are in a ‘Vitamin D Winter’ region, you may be at risk.
- KD Casmena et al. Vitamin D deficiency in Europe: pandemic? Am J Clin Nutr. 2016
- H Wang et al. Vitamin D and Chronic Diseases. Aging Dis. 2017
- http://www.well.ox.ac.uk/aug-10-vitamin-d-influences-over-200-genes. University of Oxford. The Wellcome Trust Centre for Human Genetics.
- Hooman Mirzakhani et al. Vitamin D and the development of allergic disease: how important is it? Clin Exp Allergy. 2015.
- Martineau et al. Vitamin D supplementation to prevent acute respiratory tract infections: systematic review and meta-analysis of individual participant data. BMJ, 2017