Stay healthy this Winter with vitamin D

Updated: Oct 11, 2021

As we move into the cooler months it’s a good idea to start taking a daily dose of Vitamin D and to continue to take this supplement throughout the winter months. In the summer we are able to make vitamin D from exposing our skin to the sun, but this is not possible in the winter months. Vitamin D Is known for maintaining healthy bones, but its role in the body is much more wide-ranging. So why do we need vitamin D, and what do we need to do to maintain sufficient levels to keep us healthy throughout the year?

What is vitamin D?

Vitamin D is a fat-soluble vitamin which is also known as calciferol, of which there are two main forms:

· Vitamin D3 (cholecalciferol) - found in animal foods such as fatty fish and egg yolks

· Vitamin D2 (ergocalciferol) – found in some plants, mushrooms and yeasts

Vitamin D3 is nearly twice as effective as vitamin D2 at increasing blood levels of vitamin D.

However, whilst we are able to absorb some vitamin D from these foods, the majority of the vitamin D we need comes from exposure to the sun.

Vitamin D and sunshine

Exposure to sun is the most effective way of getting vitamin D, as it is produced through exposure of cholesterol in the skin to UVB rays from the sun. So, during the summer months if you regularly expose your skin to the sun for a few minutes most days, you are likely to be making sufficient vitamin D to meet your needs. However, during the Winter months if we live in the Northern hemisphere, we do not get exposed to enough sunshine and we are therefore unable to make the vitamin D at this time of year.

The graph below shows the seasonal variation in the average serum Vitamin D levels

in a group of British men, which shows how vitamin D levels peak at the end of the summer, in September and fall by approximately 50%, to their lowest levels towards the end of winter.

Graph showing seasonal variation in serum vitamin D levels in a sample of UK men (Hypponen & Power, 2007)

Vitamin D deficiency

In the UK the Scientific Advisory Committee on Nutrition (SACN) have defined deficiency as a blood level of 25nmol/L of 25(OH)D, however, many scientists believe this level is set too low, citing strong evidence that 50 nmol/L would be a more appropriate level for sufficiency (Griffin, 2021). The US Institute of Medicine and the European Food Safety Authority both use >50nmol/L as the level for sufficiency, and the US Endocrine Society sets a higher level of >75nmol/L.

In the UK it is estimated that 55% of the population have serum vitamin D levels below

50 nmol/L, and in Winter, this rises to 69%. Black, Asian, and minority ethnic populations, as well as people who are housebound or unable to go out in summer months, have an even higher risk of insufficiency. A recent study found that 92% of 6,433 UK-dwelling South Asians had serum vitamin D levels below 50 nmol/L.(Darling, 2020).

What are the risks to health of Vitamin D deficiency?

Vitamin D deficiency is associated with a wide range of conditions, including poor bone health, poor immune health, inflammation, metabolic health conditions, cardiovascular disease and cancer. Vitamin D deficiency has also been linked to depression, low mood and sleep disorders.

Bone health – vitamin D regulates the amount of calcium and phosphate in the body, and these nutrients are important for the health of bones, teeth and muscles. A lack of vitamin D can lead to demineralisation of bones, causing rickets in children and in adults, osteomalacia, a softening of the bones, which results in poor bone density and muscular weakness. Vitamin D deficiency can also present as osteoporosis.

Metabolic health - deficiency is associated with metabolic health problems including insulin resistance, obesity and diabetes

Immune health – Vitamin D is needed for the immune system - the body’s first line of defence against infection and disease. Research shows lower levels of Vitamin D to be associated with greater susceptibility to infection, disease and immune-related disorders. Lower levels of Vitamin D are associated with an increased risk of respiratory diseases, including asthma and COPD as well as with viral and bacterial respiratory infections and flu (Gruber-Bzura, B.M. 2018).

Vitamin D and COVID-19

Research shows an association between low vitamin D status and COVID-19 severity. Studies have found low vitamin D status was associated with an increased risk of disease severity (Panagiotou, 2020). A recent systematic review and meta-analysis found that patients with sufficiently high vitamin D3 serum levels before infection were highly unlikely to suffer a fatal outcome. The authors suggest supplementation with vitamin D to reach blood levels above 125nmol/L (50ng/ml) is required in order to strengthen the immune system and reduce risk (Borsche 2021).

Graph showing relationship between vitamin D status and mortality in hospital patients (Borsche, 2021)

Vitamin D and weight loss

Vitamin D may also have benefit in weight loss and in decreasing body fat. In one study half of a group of women who were on a weight loss programme were also given supplementary vitamin D an on average lost 7lbs more than those not taking the supplement (Mason, 2014) A further study of 4600 women over four and a half years found that those taking a vitamin D supplement were less likely to gain weight than those not taking a supplement ( Leblanc, 2012). In a study in men vitamin D was found to increase testosterone and higher levels of testosterone can reduce body fat and help with long term weight loss (Pilz, 2011).

How can you tell if you are deficient in Vitamin D?

The symptoms that might suggest vitamin D deficiency are shown in the box. However, the best way to find out if you are deficient is to get a blood test. You can ask your GP or get a test done privately through an on-line service.

How much Vitamin D should I take during the Winter

In the UK the recommendation is to take a vitamin D supplement over the winter months from October to the end of March. The UK government recommend 600iu of vitamin D, however, this is on the basis of maintaining blood levels at 25nmol/L. In order to maintain levels above 50nmol/L, the following is recommended:

a) If you are not deficient - a supplement of 1000iu of Vitamin D3 should be sufficient to maintain Vitamin D levels throughout the Winter months if you are not deficient in October.

b) If you are deficient – you will need to take advice from your GP on taking a higher dose of Vitamin D3 to address deficiency, and then when serum levels have been raised above adequate levels, you should transfer to 1000iu/day of Vitamin D3 for the remainder of the winter.

Should I take Vitamin D3 with or without Vitamin K2?

Vitamin D increases calcium levels in the body. Vitamin K2 helps calcium to be shunted into the bones. Without the addition of vitamin K2, it is possible for calcium to be deposited in the arteries. For that reason, it is usually recommended that when taking a Vitamin D supplement, you select one which includes a little Vitamin K2.

However, if you are taking any blood-thinning medications, such as warfarin, you should not take Vitamin K2, and you should take Vitamin D3 on its own.


Vitamin D is an essential nutrient and has many health benefits. As most of our vitamin D comes from the sun those of us living in the northern hemisphere need to take a vitamin D supplement between October and March. As well as being important for bone and muscle health, vitamin D is critical for immune health, so it is extremely important to maintain healthy levels of vitamin D as we move towards Winter.


Borsche, L, Glauner, B & von Mendel, J, 2021, COVID-19 mortality risk correlates inversely with vitamin D3 status, and a mortality rate close to zero could theoretically be achieved at 50ng/ml 25(OH)D3: Results of a systematic Review and meta-analysis. (currently in pre-print).

Darling A.L. et al 2020. Very high prevalence of 25-hydroxyvitamin D deficiency in 6433 UK South Asian adults: analysis of the UK Biobank Cohort. Br J Nutr. ( 10.1017/S0007114520002779).

Griffin, G et al, 2021, Preventing vitamin D deficiency during the COVID-19 pandemic: UK definitions of vitamin D sufficiency and recommended supplement dose are set too low. Clinical Medicine Jan 2021, 21 (1) e48-e51; DOI: 10.7861/clinmed.2020-0858

Gruber-Bzura B. M. (2018). Vitamin D and Influenza-Prevention or Therapy?. International journal of molecular sciences, 19(8), 2419.

Hypponen E & Power C. 2007 Hypovitaminosis D in British adults at age 45y: nationwide cohort study of dietary and lifestyle predictors. Am J Clin Nutr 2007;85:860–8.

LeBlanc, Erin S et al. 2012. “Associations between 25-hydroxyvitamin D and weight gain in elderly women.” Journal of women's health vol. 21,10 (2012): 1066-73. doi:10.1089/jwh.2012.3506

Linus Pauling Institute, Vitamin D Accessed 7 October 2021.

Mason C et al 2014. Vitamin D3 supplementation during weight loss: a double-blind randomized controlled trial. Am J Clin Nutr. 2014 May;99(5):1015-25. doi: 10.3945/ajcn.113.073734. Epub Mar 12. Erratum in: Am J Clin Nutr. 2014 Oct;100(4):1213. PMID: 24622804; PMCID: PMC3985208

National Institutes of Health. Vitamin D, Accessed 7th October 2021

Panagiotou G, Tee SA, Ihsan Y, et al. 2020. Low serum 25-hydroxyvitamin D (25[OH]D) levels in patients hospitalized with COVID-19 are associated with greater disease severity. Clin Endocrinol (Oxf);93(4):508-511.

Pilz S, Frisch S, Koertke H, Kuhn J, Dreier J, Obermayer-Pietsch B, Wehr E, Zittermann A. Effect of vitamin D supplementation on testosterone levels in men. Horm Metab Res. 2011 Mar;43(3):223-5. doi: 10.1055/s-0030-1269854. Epub 2010 Dec 10. PMID: 21154195.

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