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November brings the start of Winter in the Northern Hemisphere, which means sunlight isn’t sufficient enough to trigger the skin to produce Vitamin D, leading to Vitamin D deficiency.
Studies prove that people who live in high altitudes are commonly deficient in Vitamin D, especially during October through March (Winter). In Europe, vitamin D deficiency in winter is a big issue, as demonstrated in 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 commonly known for its role in building bones and promoting healthy muscles (as it is required in calcium and phosphorus absorption). Low vitamin D levels are linked with rickets in children, and osteomalacia in adults, as well as osteoporosis. However, it is now known that Vitamin D is involved in the regulation of many genes, such as those linked with auto-immune diseases including multiple sclerosis, Crohn’s disease, lupus, rheumatoid arthritis, some cancers, and infections. 
Scientists indicate that increased Vitamin D deficiency in northern Europe might explain higher rates of certain diseases including multiple sclerosis, breast cancer, prostate cancer, and bowel cancer in the region.
More studies are emerging linking low Vitamin D levels to autoimmune disease, asthma and other allergies, heart disease, diabetes, cancer, infections, pregnancy complications, depression, and cognitive decline. 
Further, numerous reports have advised that cases of rickets are rising in Europe. Rickets is a traumatic condition in children, with symptoms including muscle spasms, seizures, and soft, deformed bones. Experts fear that people are spending less and less time outside due to the fear of skin cancer from sun exposure. What’s more, they fear that kids are spending less time outside and more time doing inside activities such as gaming, especially during long winters. These factors could be responsible for the increased number of cases of rickets. This is doubly worrisome considering that the condition was assumed to be eradicated in the late 1930s.
What’s with winter sunlight and Vitamin D deficiency?
You may assume that if you simply expose your skin to the sun, your body will go ahead and produce sufficient amounts of vitamin D.
However, you would be mistaken.Not all sunlight is equal when it comes to triggering Vitamin D synthesis. Your skin only produces Vitamin D when exposed to UVB rays, and these are only available when the sun is higher than 45 degrees above the horizon. When the sun is below 45 degrees, the UVB rays are blocked by the atmosphere. This occurs mostly during winter days at latitudes higher than 37 degrees North or South of the equator.
Without UVB rays, the skin simply cannot produce vitamin D, meaning that people living at high altitudes are not exposed to sufficient UVB to make vitamin D during autumn or winter. Other factors that could block UVB rays are pollution, gas, and clouds.
In saying this, even during the summer, the sun remains low in the early morning and late afternoon. This means that you can’t absorb much Vitamin D via sun exposure before 10 am and after 3 pm during summer. A good trick to tell if the sun is low or high is to look at your shadow when outside. If your shadow is taller than you, it suggests the sun is low and, therefore, your body is not producing sufficient vitamin D.
In the northern hemisphere, ‘Vitamin D Winter’ lasts for around four to six months, give or take. Due to little or no UVB availability during this time, people in northern Europe (and similar latitude) are at higher risk of winter vitamin D deficiency. Take London for example, at 51 degrees North, where UVB sunlight is insufficient for about 6 months of the year. Other higher-latitude countries are Denmark, Iceland, Ireland, Sweden, Norway, and Poland.Aside from the latitude issue, other factors contribute to reduced sun exposure, such as:
- Long working hours and more time spent indoors
- Children spending more time on screens versus playing outside
- Increased use of sunscreen and overzealous protection from the sun due to fears of premature aging and skin cancer
It is wise to be aware of the dangers of excessive sun exposure. However, avoiding the sun altogether can also lead to serious health issues, especially considering that sun exposure is the best way to make Vitamin D naturally.
Fatty tissue can store Vitamin D, but these reserves run out quickly if they aren’t constantly replenished, for example, during long winters in northern latitudes.
Furthermore, when Vitamin D production is limited, more sun exposure won’t help after a certain point. This means that even if you sunbathe during summer or spend days in the sun on a southern holiday, you won’t be stockpiling nearly enough Vitamin D to last you until the end of ‘Vitamin D winter’. To maintain sufficient Vitamin D levels during winter, you need to take vitamin D supplements.
Winter months are commonly associated with respiratory infections such as cold and flu. Recent studies show that Vitamin D influences the way your immune system fights infection and disease. It makes sense, therefore, that vitamin D deficiencies are associated with greater risk of cold, flu, wheezing disorders, and asthma.   Other common vitamin D deficiency symptoms are body aches and stiffness, especially during winter.
By now, you’re probably curious about how to get vitamin D in the winter when sunlight is lacking in UVB. Firstly, you can increase your intake of fortified foods and Vitamin D supplements, especially if you live in northern Europe. Boosting your intake of oily fish, cod liver oil, eggs, and mushrooms can help to increase your Vitamin D stores during UVB-poor winters but won’t solve the deficiency outright.
Vitamin D supplements are important for maintaining overall good health as well as supporting the bones and muscles. What’s more, the role of Vitamin D in immune support and the prevention of infections and diseases is becoming increasingly known. Emerging studies provide further insight into how Vitamin D might be closely related to many, if not all, aspects of health, far beyond calcium metabolism and strong bones.
You may be at risk of vitamin D deficiency and related health issues if you live in a ‘Vitamin D Winter’ region. If you live in Europe or countries at a similar latitude, it’s a good idea to get your vitamin D levels checked as winter approaches.
- 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