Evidence Based Nutrition

Vitamin D is a fat-soluble vitamin that is synthesised through our skin after sunlight exposure through UVB radiation. We can also get vitamin D through our diet however this is in very small amounts and hard to get adequate amounts without supplementation. Therefore, the best way is through exposure to sunlight, about 50-90% of our vitamin D comes from sunlight and the rest is through the diet (10). However, those living further north of the equator during the wintertime experience shorter and darker days reducing the availability of getting that sunlight exposure. Decreased exposure to the sun can lead to vitamin D deficiency and those further at risk are the elderly, and those with darker skin due to not going outside as much darker skin requires longer to synthesise vitamin D from UVB exposure as the pigments protect against too much sunlight. In the UK it has been found that over 50% of the adult population have insufficient levels of vitamin D and 16% have severe deficiency during the winter and spring months (9). The current RDA of vitamin D in the UK is 400IU (international units) or 10ug per day for adults, 7ug (280IU) for children 6 months to 3-year-olds and 8.5ug (340IU) for under 6-month-olds (9). Although some evidence suggests the recommended adequate amounts are actually inadequate, these recommendations are enough to prevent osteomalacia and rickets but will not provide optimal vitamin D status therefore, it is thought the RDA should be raised to at least 800IU per day (9, 4). Up to 4,000IU per day of vitamin D supplementation can be taken without any toxic effects, in fact, does of 50,000IU per day is safe and toxicity is very rare, however, I would advise consulting with a professional before taking this level of dosage or stick to no more than 4,000IU (8). 

Winter Deficiency

For many, during the winter vitamin D status lowers and risks becoming deficient due to the reduced sunlight this time of year. Vitamin D deficiency is not rare and impacts most of the world no matter where you live, however, the winter months in countries of the northern hemisphere are at greater risk especially the elderly, those with darker skin, individuals institutionalised or prolonged hospitalised and those with certain malabsorption syndromes such as IBS are at greater risk in these countries (6, 10). Although vitamin D is a fat-soluble vitamin there does not appear to be an obvious storage site for the vitamin (7). Vitamin D can get trapped in fat stores however it can only be released through fat loss when weight is lost or required to be used for energy but so far there is no known path to where that trapped vitamin D can be mobilised and used once released to prevent a deficiency happening (7). Vitamin D deficiency can cause rickets, osteomalacia and muscle weakness and other potential illnesses as the body has been found to have vitamin D receptors everywhere including the muscles, brain, prostate, breast and colon tissues as well as the immune cells (4). Therefore, vitamin D may be more important than we realise so supplementing all year round, especially in the wintertime is becoming a must for public health. The current guidelines are 400IU a day however, this is just to prevent rickets evidence has been found that older adults taking 800IU prevent deficiency as well as reduce the risk of fractures and falls whereas 400IU did not prevent fractures and falls therefore, some experts believe the RDA should be raised to at least 800IU (10). Maybe this should go even further and update the minimum to be 1000IU as this may be more appropriate for optimal vitamin D levels however this has not been backed up by science yet. Also, as the days grow shorter and darker many individuals suffer from the seasonal affective disorder (SAD) or a less severe version known as the winter blues (8). SAD and the winter blues have been associated with low vitamin D levels.

The Winter Blues (SAD)

Seasonal affective disorder (SAD) is characterised by symptoms of depression, anxiety, irritability, appetite change and fatigue that occur during the winter months (2). SAD is more likely to occur in women than men between the ages of 18 to 30 years old and those living in northern latitudes are more susceptible, for example, 20% of people in the UK experience the winter blues and 2% experience SAD however this may not be accurate due to under-reporting of cases (8). This may occur for several reasons it is thought that the reduced daylight may be a possible cause of SAD as light therapy is an effective treatment strategy for those with SAD (3). Randomised control trials have found evidence to support light therapy as an effective strategy, sitting in front of a light box first thing in the morning from fall until spring is a suggested way to combat SAD (3, 8). Also, they are light alarms you can look into that wake you up in a more natural way in the mornings, these alarms can potentially make a big difference in how you feel and can fight SAD – check out Lumie Lights. 

In one study of people with SAD, they found that they had 5% more SERT in the winter which is a protein which assists in the transport of serotonin the higher the SERT levels the less serotonin activity as this affects mood which can then cause a depression like symptoms (1). People with SAD may also have trouble with the overproduction of melatonin which may explain the feelings of tiredness as melatonin is associated with control of the sleep and wake cycle (8). The combination of decreased serotonin and increased melatonin impacts our circadian rhythms making it harder for those with SAD to adjust to the seasonal change in day length (8). 

Another reason suggested as a potential cause of SAD is the lack of vitamin D in the winter months. Two small-scale randomised control trials suggested that short courses of vitamin D supplementation may improve positive psychological well-being in those with SAD as well as those that do not have SAD (5). This could be because vitamin D receptors (VDR) have been found in nearly all tissues of the body including multiple areas of the brain and also the central nervous system (2). A meta-analysis of studies for vitamin D and depression finds that improving vitamin D levels has shown improvements in those with depression and epidemiological studies indicate vitamin D to have therapeutic effects on those with depression, a person would have to increase their blood vitamin D levels to between 50 – 85 nmol/L 25OHD for this therapeutic effect according to research (11). 

Taking adequate vitamin D is important for our physical and mental health, however, there are other ways to combat the winter blues or SAD and using a combination of these things can give you a better chance of reducing symptoms of SAD. 

Taking regular supplementation of vitamin D before the winter as well as during the cold and darker months is a good strategy to include, anything up to 4000IU is encouraged and is safe. Light therapy has shown positive results so look into companies such as Lumie Lights which sell alarm clocks and lights for this specific reason, the alarm clocks simulate the sunrise so waking up feels more natural. Prevent becoming sedentary during the winter it can be easy as the days get shorter and colder to just stay indoors slouching on the sofa, as we all know physical activity is great for mental health and improving mood so get to the gym, go to that sport session, or go outside in the daylight and enjoy a nice walk are all good options for physical activity. Keeping a healthy well-balanced diet also helps to balance your mood and raise energy levels, and spending time outdoors in the sunlight hours when you can. Combining these strategies can help you fight against SAD and enjoy the wintertime again. 

Conclusion

Vitamin D deficiency is a major issue all over the world but those living on the northern side of the equator are more likely to suffer from inadequate vitamin D levels or even deficiency especially during the winter months when the days a shorter and colder. Therefore, it is important that during these months of less sunlight we supplement with vitamin D. The current recommendation is 400IU however, it is thought this should be raised to 800IU or even 1000IU to provide optimum vitamin D levels for the human body. Vitamin D levels become even more important when it has been discovered that low vitamin D levels can affect our mental health as well and is thought to be a contributing factor for those that suffer from the winter blues or SAD. Combat SAD by supplementing with vitamin D, light therapy and keeping physically active which individually have all had positive outcomes in studies. Vitamin D should be taken all year round, but no time is more important than the winter to prevent any deficiency and help maintain our physical and mental health.

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• Everything You Need to Know About Vitamin C
• Vitamin D and the Winter Months
• Everything You Need to Know About Vitamin K
• The Mediterranean Diet
• Everything You Need To Know About Vitamin E

References

1. B. McMahon, S. Andersen, M. Madsen (2014) “Patients with seasonal affective disorder show seasonal fluctuations in their cerebral serotonin transporter binding,” European Neuropsychopharmacology, vol. 24, supplement 2, p. S319.
2. Bertone-Johnson, E. R. (2009) “Vitamin D and the occurrence of depression: Causal Association or circumstantial evidence?” Nutrition Reviews, 67(8) pp. 481–492. 
3. Dumville, J., Miles, J., Porthouse, J., Cockayne, S., Saxon, L. and King, C. (2006) “Can Vitamin D supplementation prevent winter-time blues? A randomised trial among older women.” The Journal of Nutrition, Health & Aging, 10(2) pp. 151–153. 
4. F. Holick, M. (2007) “Vitamin D Deficiency.” The New England Journal of Medicine, 357(3) pp. 266–281.
5. Gloth F.M., Alam W, Hollis B. (1999) Vitamin D vs. broad spectrum phototherapy in the treatment of seasonal affective disorder. J. Nutr. Health Aging, 1: 5–7.
6. Holick, M. F., Binkley, N. C., Bischoff-Ferrari, H. A., Gordon, C. M., Hanley, D. A., Heaney, R. P., Murad, M. H. and Weaver, C. M. (2012) “Guidelines for preventing and treating vitamin D deficiency and insufficiency revisited.” The Journal of Clinical Endocrinology & Metabolism, 97(4) pp. 1153–1158. 
7. Mason, R. S., Rybchyn, M. S., Abboud, M., Brennan-Speranza, T. C. and Fraser, D. R. (2019) “The role of skeletal muscle in maintaining vitamin D status in Winter.” Current Developments in Nutrition, 3(10). 
8. Melrose, S. (2015) “Seasonal affective disorder: An overview of assessment and treatment approaches.” Depression Research and Treatment, 2015 pp. 1–6. 
9. Pearce, S. H. and Cheetham, T. D. (2010) “Diagnosis and management of vitamin D deficiency.” BMJ, 340(1).
10. Sizar, O., Khare, S., Goyal, A. and Givler, A. (2022) “Vitamin D Deficiency.” National Library of Medicine, January. 
11. Spedding, S. (2014) “Vitamin D and depression: A systematic review and meta-analysis comparing studies with and without biological flaws.” Nutrients, 6(4) pp. 1501–1518.