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Stay Healthy this Winter with Vitamin D

Updated: Dec 5, 2023

As we move into the cooler months it’s a great idea to start taking a daily dose of Vitamin D for the Winter months. In the summer we are able to make Vitamin D by exposing our skin to the sun, but we don't see the sun enough in the winter months in the Northern Hemisphere, so we have to take a supplement. Over 50% of the UK population is thought to be deficient in Vitamin D, and this figure rises during the Winter. Yet Vitamin D Is essential for maintaining healthy bones, maintaining a strong immune health and reducing risk for metabolic disease. Here we describe why this vitamin is so important and how you can maintain sufficient levels to stay 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 enough 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, showing 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 25 nmol/L of 25(OH)D. There is strong evidence that 50 nmol/L is an appropriate level for sufficiency (Griffin, 2021). The US Institute of Medicine and the European Food Safety Authority both use > 50 nmol/L as the level for sufficiency, and the US Endocrine Society sets a higher level of >75 nmol/L.

NHS laboratories generally use the following definitions:

  • < 25 nmol/L – DEFICIENT

  • 25-50 nmol/L – INSUFFICIENT

  • 50-75 nmol/L – ADEQUATE

  • > 75 nmol/L – OPTIMUM

Factors which may increase risk of Vitamin D deficiency

  • Being of African, African-Caribbean, Asian or Middle-Eastern origin

  • Elderly (65 years and older) 

  • Lack of sun exposure (housebound, covering skin) 

  • Malabsorption syndrome or dietary factors (vegan, lactose free diet, coeliac disease, post bariatric surgery) 

  • Drugs - anticonvulsants, cholestyramine, rifampicin, glucocorticoidsor anti –retrovirals 

  • Endocrine conditions such as hyperthyroidism, hyperparathyroidism,

  • BMI > 30 

  • Fragility fracture, osteoporosis or high fracture risk 

  • Pregnant or breastfeeding

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%. B. 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 and muscle 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. In studies Vitamin D alone, or in combination with calcium, has been found to have a significant effect on bone density.

Studies have also shown that Vitamin D supplementation increases muscle protein synthesis, which improves muscle strength, making it important in fracture prevention in older people, particularly when combined with calcium

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).

Studies also found a low vitamin D status was associated with an increased risk of disease severity (Panagiotou, 2020) in COVID-19. A 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) are needed to strengthen the immune system and reduce risk (Borsche 2021).

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

Metabolic health

Vitamin D deficiency is linked to greater risk of metabolic health problems, such as insulin resistance, obesity and diabetes. A meta-analysis of 28 studies showed that higher serum 25OH vitamin D levels were associated with a 55% reduction in diabetes, a 51% decreased risk of metabolic syndrome, and a 33% lower risk of cardiovascular disease. Low levels of Vitamin D are associated with insulin resistance and beta cell dysfunction and the risk of insulin resistance is approximately 30% higher than normal. Surprisingly, Vitamin D deficiency is also highly prevalent in those with metabolic syndrome in North India, a population which benefits from more sunshine than many countries.

Vitamin D and weight loss

Vitamin D may also have benefit in weight loss and in decreasing body fat. In one study

a group of women on a weight loss programme given a Vitamin D supplement lost 7lbs more on average than those not taking a supplement (Mason, 2014). A further study of 4600 elderly 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?

Symptoms that might suggest you may be deficient in Vitamin D are shown here in the green box. But 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 NHS recommend people take a vitamin D supplement over the Winter months between October and the end of March. The UK government suggest that a daily supplement of 10 micrograms or 400 iu of vitamin D should be sufficient to prevent deficiency in most people. They also recommend not to take more than 100 micrograms (4000 iu) of Vitamin D per day.

In order to maintain levels above the more optimal 50 nmol/L, the following is recommended:

a) If you are not deficient in October - a supplement of 25 micrograms (1000 iu) of Vitamin D3 should be sufficient to maintain Vitamin D levels throughout the Winter months.

b) If you are deficient – you will need to take advice from your GP on taking a higher dose of Vitamin D3 as a short-term measure to address deficiency. Once serum levels are raised to adequate levels, you should transfer to 25 microgams (1000 iu) per 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 a Vitamin D3 supplement 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, and metabolic 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).

Chiu KC, Chu A, Go VL, Saad MF. Hypovitaminosis D is associated with insulin resistance and beta cell dysfunction. Am J Clin Nutr. 2004 May;79(5):820-5. doi: 10.1093/ajcn/79.5.820. PMID: 15113720.

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.

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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

Melguizo-Rodríguez L, Costela-Ruiz VJ, García-Recio E, De Luna-Bertos E, Ruiz C, Illescas-Montes R. Role of Vitamin D in the Metabolic Syndrome. Nutrients. 2021 Mar 3;13(3):830. doi: 10.3390/nu13030830. PMID: 33802330; PMCID: PMC7999005.

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.

Pathania M, Dhar M, Kumar A, et al. (April 30, 2023) Association of Vitamin D Status With Metabolic Syndrome and Its Individual Risk Factors: A Cross-Sectional Study. Cureus 15(4): e38344. doi:10.7759/cureus.38344

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.

Strange RC, Shipman KE, Ramachandran S. Metabolic syndrome: A review of the role of vitamin D in mediating susceptibility and outcome. World J Diabetes. 2015 Jul 10;6(7):896-911. doi: 10.4239/wjd.v6.i7.896. PMID: 26185598; PMCID: PMC4499524.


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