vitamin D

Vitamin D update: Why January is a good time to consider your vitamin D levels

January can be a time of mixed emotions. On the one hand it is a fresh start which can bring optimism and a chance to focus on our health, on the other hand it is often associated with the January blues, with the festivities of Christmas over but a long winter still ahead. Many people focus on dietary changes during this time and choose to join in with “Veganuary”, opting for a vegan diet during the month. Increasing plant-based foods is a good way to support health, however certain nutrients can be hard to obtain from a vegan diet, including vitamin D.

In addition, the body’s own vitamin D production relies on exposure to sunlight which is lacking at this time of year. Therefore, it is a relevant time to consider vitamin D status and its importance.

Skip to Key Takeaways

Vitamin D deficiency – who is at risk?

Vitamin D is considered to be deficient if levels are below 25nmol/l however optimum levels are greater than 75nmol/L1–3. Some researchers suggest a higher optimal range – Grassroots Health, a not for profit public health research organisation with a panel of 48 senior vitamin D researchers from around the world, says that “vitamin D research shows that blood levels in the range 40-60 ng/ml (100-150 nmol/L) are safe, effective and will lower overall disease incidence and health care costs.” 4

  • Optimal – greater than 100 with a safe upper limit of 220 nmol/L4
  • Sufficient – 50-100 nmol/L
  • Insufficient – 25-50 nmol/L
  • Deficient – less than 25 nmol/L

NB: Serum vitamin D levels may be described using either ng/ml or nmol/L. To convert ng/ml to nmol/L multiply by 2.5; conversely to convert nmol/L to ng/ml divide by 2.5. For example 100 nmol/L = 40 ng/ml.

The Scientific Advisory Committee on Nutrition’s (SACN) report in 2016 stated that around 30-40% of the population had a plasma 25(OH)D (vitamin D3)  concentration < 25 nmol/L in winter compared to 2-13% in summer (the NHS considers insufficiency to be <50nmol/L). The most at risk population were institutionalised adults who were unable to build up vitamin D reserves during the summer months due to lack of sunlight exposure1. Therefore, it is important to consider vitamin D status particularly in the elderly as skin is less able to manufacture vitamin D as we age and in addition those who may be house bound or in a care home or hospital.

Other studies have shown that the prevalence of vitamin D insufficiency in vegans was higher than in omnivores (73% versus 46%), therefore vitamin D status should be considered in those following a vegan diet5.

Other populations who are at risk include6-7:

  • People with dark skin, as the ability to absorb UV light is reduced with darker skin
  • Obese people, it has been shown that obesity can increase vitamin D deficiency
  • People who have reduced exposure to UV light i.e. those who work indoors all of the time, children who do not play outside and people who cover-up or use high factor sunscreen etc
  • Children under 5 years of age

Sources of vitamin D2 v D3

When the skin is exposed to UVB radiation (from sunlight) a molecule known as 7-dehydrocholesterol (derived from cholesterol) is converted to cholecalciferol, also known as D3. Cholecalciferol can also be obtained from the diet or by supplementation (D3). Cholecalciferol travels to the liver where it is converted to 25-hydroxyvitamin D (25OHD). The kidney then converts 25OHD to 1,25-hydroxyvitamin D (1,25OHD), also known as calcitriol the active form of vitamin D, which possesses hormonal properties and regulates calcium and phosphate balance.

Cholecalciferol or D3 can be obtained directly without the need for UVB radiation either from the diet (sources oily fish, eggs and butter) or via supplementation. Vitamin D3 supplements are generally from animal sources although vegan sources of D3 can be obtained from lichens.

Another plant based form of vitamin D, known as ergocalciferol or D2, can also be obtained from the diet (rich food sources are mushrooms, fortified soya milk and almond milk) or via supplementation. Vitamin D2 can also be converted by the liver to 25-hydroxyvitamin D and then to 1, 25-hydroxyvitamin D by the kidneys6,7.

There has been some debate as to whether there is a difference between supplementing vitamin D3 (cholecalciferol) or Vitamin D2 (ergocalciferol). It appears that vitamin D3 is more effective at increasing levels of 25OHD and therefore calcitriol. Research has also shown that higher serum levels of D2 are associated with lower levels of D3 and therefore a high intake of D2 may reduce availability of D3 and potentially calcitriol7. Other studies have concluded that D3 is more appropriate to sustain adequate 25OHD levels than D2, therefore it is recommended to obtain vitamin D in the form of D39.

Vitamin D functions and Seasonal Affective Disorder

Vitamin D is the most highly researched nutrient – with a search on “vitamin D” returning over 80,000 hits on PubMed. Although research is often small and inconclusive, it is widely believed that vitamin D deficiency plays a significant role in the onset of many conditions.

Seasonal affective disorder or SAD is a type of recurring major depression with a seasonal pattern. According to the Diagnostic and Statistical Manual of Mental Disorders, the criteria for SAD includes having depression that begins and ends during a specific season every year (with full remittance during other seasons), for at least two years and having more seasons of depression than seasons without depression a long period of time . Seasonal pattern disorders occur most frequently in winter although they can also occur in summer10.

A systematic review and meta-analysis concluded that low levels of vitamin D are associated with depression. Many people with SAD have insufficient or deficient levels of vitamin D and research investigating this association suggests that supplementing vitamin D may improve their symptoms10.

Do we need to supplement vitamin D?

SACN recommends all members of the population should obtain:

  • “a reference nutrient intake (RNI) of 10 micrograms of vitamin D per day, throughout the year, for everyone in the general population aged 4 years and older
  • an RNI of 10 micrograms of vitamin D per day for pregnant and lactating women and population groups at increased risk of vitamin D deficiency
  • a ‘safe intake’ of 8.5 to 10 micrograms per day for all infants from birth to 1 year of age
  • a ‘safe intake’ of 10 micrograms per day for children aged 1 to 4 years”

However, those at risk of vitamin D deficiency/insufficiency or who know they are vitamin D deficient may need to increase their intake further1.

Maintenance – in order to support general wellness over the winter months, we recommend a good quality multivitamin and mineral which includes 30ug vitamin D3. However the amount of vitamin D required for individuals to reach optimum levels is specific to them. Grassroots have a vitamin D calculator on their website which gives an indication of how much (i.e. dose) and how long to supplement with vitamin D based on the results of blood tests

Vitamin D deficiency – if tests have demonstrated vitamin D levels are deficient a higher dose of vitamin D should be considered (e.g 4000IU) especially over the winter months to help restore adequate vitamin D levels. High dose vitamin D3 should be taken alongside vitamin K2, to support calcium homeostasis (by directing calcium into bone away from arteries)6. Vitamin D3 levels should be tested after 3-6 months, reducing dose once optimal levels are achieved.

Other ways to support vitamin D levels

Food sources11

D3: oily fish, eggs, butter

D2: Mushrooms, soya and almond milk

Sun exposure

A great way to obtain vitamin D is via exposure to UVB light. This is difficult through the winter months (particularly October to March) however during the summer months it is a good idea to get appropriate levels of sun exposure. To obtain vitamin D from the sun you need to expose your face, legs and forearms to sunlight (between 10am and 2pm) for ten minutes without sun protection on as many days as possible through the summer. Ensuring, though, that the skin does not burn or redden12.

Key Takeaways

  • Vitamin D deficiency is most common in winter, affecting 30-40% of the population, with institutionalised adults being the greatest affected. In addition, many more people have sub-optimal levels of vitamin D. Many more people may have sub-optimal levels of vitamin D.
  • Those at particular risk of vitamin D deficiency include people consuming a vegan diet, people with dark skin, older adults, people who rarely expose their skin outdoors and obese individuals. Children under 5 are also at a higher risk.
  • Vegan sources of vitamin D are in the vitamin D2 form which needs to be converted by the body to vitamin D3. Therefore, it can be difficult for vegans to obtain adequate levels of vitamin D especially during the winter.
  • Vitamin D deficiency is associated with seasonal affective disorder and vitamin D supplementation has been shown to be helpful for these individuals who experience this during the winter months.
  • Vitamin D deficiency also plays a significant role in the onset of many other conditions.

Vitamin D contributes to:

  • The absorption and utilisation of calcium and phosphorus
  • Normal blood calcium levels
  • The maintenance of bones and teeth
  • The maintenance of normal muscle function
  • The normal function of the immune system
  • Vitamin D has a role in the process of cell division
  • Vitamin D is needed for normal growth and development of bone in children
  • Vitamin D helps to reduce the risk of falling associated with postural instability and muscle weakness. Falling is a risk factor for bone fractures among men and women 60 years of age and older


  1. Vitamin D and Health 2016 Ii.; 2016. Accessed December 3, 2019.
  2. New advice on vitamin D – British Nutrition Foundation. Accessed December 3, 2019.
  3. Scientific Opinion on the Tolerable Upper Intake Level of vitamin D. EFSA J. 2012;10(7). doi:10.2903/j.efsa.2012.2813
  5. Ho-Pham LT, Vu BQ, Lai TQ, Nguyen ND, Nguyen T V. Vegetarianism, bone loss, fracture and vitamin D: A longitudinal study in Asian vegans and non-vegans. Eur J Clin Nutr. 2012;66(1):75-82. doi:10.1038/ejcn.2011.131
  6. Murray JPM. Textbook of Natural Medicine. 4th Ed.; 2013.
  7. Bland J et al. Textbook of Functional Medicine.; 2008.
  8. Swanson CM, Nielson CM, Shrestha S, et al. Higher 25(OH)D2 is associated with lower 25(OH)D3 and 1,25(OH)2D3. J Clin Endocrinol Metab. 2014;99(8):2736-2744. doi:10.1210/jc.2014-1069
  9. Oliveri B, Mastaglia SR, Brito GM, et al. Vitamin D3 seems more appropriate than D2 to sustain adequate levels of 25OHD: A pharmacokinetic approach. Eur J Clin Nutr. 2015;69(6):697-702. doi:10.1038/ejcn.2015.16
  10. Melrose S. Seasonal Affective Disorder: An Overview of Assessment and Treatment Approaches. Depress Res Treat. 2015;2015. doi:10.1155/2015/178564
  11. Oseiki H. The Nutrient Bible. 8th Ed. Bio Concepts Publishing Australia.; 2009.
  12. Piotrowska A, Wierzbicka J, Żmijewski MA. Vitamin D in the skin physiology and pathology. doi:10.18388/abp.2015_1104

If you have questions regarding the topics that have been raised, or any other health matters, please do contact me (Helen) by phone or email at any time., 01684 310099

Helen Drake and the Cytoplan Editorial Team

Last updated on 23rd August 2022 by cytoffice


7 thoughts on “Vitamin D update: Why January is a good time to consider your vitamin D levels

  1. Does anyone know whether taking statins has an effect on D levels, being as the aim is to reduce the body’s cholesterol levels? From some of the side effects I’ve noticed in family members, the subsequently reduced cholesterol levels seem potentially to be responsible for those (eg memory issues)

    1. Hi Vanessa,

      I am not aware that statins have an effect on vitamin D levels. There is some research on effects of statins on levels of CoQ10, zinc, selenium and omega-3. Regarding memory issues, cholesterol is needed by the brain and so a reduction in cholesterol could be a relevant contributory factor in memory issues. You might be interested in our blog on cholesterol.


      1. I don’t suppose anyone has even bothered to look into it! The importance of vitamin D seem to be quite a ‘new’ thing, and we’re always told that new information takes an average of 17 years to pass into public health recommendations.

        I also think that the current thinking that ‘the lower the cholesterol levels the better’ precludes any kind of investigation into what other problems may occur even though there is mounting evidence that higher than recommended levels of cholesterol seem to be associated with longevity. Even cholesterol-sceptic Dr Malcolm Kendrick is still picking apart the details as more research appears…

    1. Hi Ab – Throughout life there is a continual process of old worn out cells breaking down and being replaced by new healthy tissue. As we age this process tends to slow down and become less efficient. Most particularly in relation to joints, where excess wear and tear from exercise results in damage to joint cartilage which eventually can turn into arthritis. The key is to have a healthy diet that provides all the nutrients the body needs for health and healthy repair of all tissues and reduction of inflammation and in respect of joints this includes omega 3, collagen from bone broths, Vitamins D and C. Exercise is also important as gentle exercise increases the production of joint lubrication fluids, and avoidance of inflammatory anti-nutrient foods/liquids such as alcohol and foods high in sugar and trans fats.

    1. Hi Kiran – Yes regular alcohol consumption can deplete Vitamin D levels in the body. It will also reduce the levels of vitamins B1, B3, B6 as these are directly or indirectly involved in alcohol metabolism

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