A Vitamin D Winter


Now the clocks have gone back and the nights are drawing in, this leaves less time to get outside in daylight hours. So perhaps it’s time to consider taking a vitamin D supplement over the Winter months?

Vitamin D is currently a hot topic, as it is believed that vitamin D insufficiency is a world wide health problem. It is thought that more than 50% of the world’s population is at risk of being vitamin D deficient (1). Causes in insufficiency are thought to be from spending more time indoors, applying sunscreen (which inhibits the absorption of the UV-B sunlight required to activate vitamin D in the skin), seasonal variation in weather and skin pigmentation (increase in skin pigmentation lowers the level of Vitamin D activated in the skin) (2). During the Winter months the sun is lower in the sky making it harder for the sun to activate vitamin D in the skin, what is more, we cover up more from the cold and tend to retreat indoors out of the harsh weather.

Role of Vitamin D in the Body

We all know vitamin D plays a crucial role in bone health. Recent studies have also shown it is active in many cells and tissues and has benefits for skin, immunity, mental health and  some recent but inconclusive research suggests a role of vitamin D in cardiovascular disease, diabetes and cancer (3).

Benefits to athletes

Athletes are no different from the general population and a deficiency in vitamin D will likely have detrimental effects on bone health, immune status and impair performance. There has been a suggested link between vitamin D status and muscle function, and vitamin D could play a crucial role in muscle strength and function, particularly in relation to age related decline in muscle mass (4). However supplementing with vitamin D if you are not insufficient is unlikely to improve performance.

Where do you get Vitamin D from?

Sunlight exposure on the skin causes a series of reactions that generates the production of Vitamin D3. Vitamin D3 can be found in some foods but it is not possible to meet daily requirements through dietary intake alone. It has to come from exposing your skin to the sun on a consistent basis and when the sun is at the highest angle in the sky, optimally in the midday sun (5).

Daily Requirements

To prevent vitamin D deficiency the Institute of Medicine (IOM) recommends adults should receive 600IU of vitamin D daily however these guidelines are based on the minimum amount for bone health alone and it is thought that intakes should be higher to support full health. The Endocrine Society recommends for those aged over 18yrs 1500-2000IU supplementation per day is required (6). The IOM has defined an upper safe limit of 4,000IU per day and no more than 10,000IU/d as may cause toxicity (7).

Dietary sources of Vitamin D and content:

Cod Liver Oil (1tsp) = 450IU

Egg yolk = 20IU

Salmon (per ~100g) = 800-1000IU

Mackerel (per ~100g) = 250IU

Fortified foods such as breakfast cereals, butter, cheese, milk and orange juice contain around 100IU per 100g-220g.

Sunlight exposure 10,000-25,000IU in just under 30mins (or the time it takes for your skin to just turn pink) (5).

Under the right circumstances 15minutes of sun exposure on the arms and legs 2-3 time per week can generate all the vitamin D required by the body (8).


The higher you go at altitude the more vitamin D your body can make. Studies have shown a 2-4 fold increase in vitamin D synthesis at 3000m and at 5400m Everest base camp, due to the increase exposure of UVB light at altitude (9). So those living and working regularly at altitude may actually receive sufficient vitamin D without the need to supplement.

Who should supplement with vitamin D?

Unless you spend a considerable amount of time outdoors exposed to the sunshine at ‘peak absorption time’ it could be beneficial to everyone during the Winter to take a vitamin D3 supplement in addition to maximizing intake of dietary sources of vitamin D.

Considering supplementing if;

  1. You do not get outside consistently and expose your skin to 15-30mins of midday sunshine (or between the hours of 10am-3pm).
  2. You are outside but you wear factor 30 or 50 sunscreen.
  3. You cover up and wear a lot of clothing (generally during Winter months).
  4. You have a darker skin pigmentation.
  5. You have a low dietary intake of Vitamin D containing foods (in combination with the above).

What Vitamin D supplements are the best to take and dosage?

Preferably, it is advised that you get a blood test done by your doctor to check your vitamin D status. This is quite an expensive blood test so not all doctors will do this unless they see you as ‘at risk’ (e.g. elderly, pregnant, poor bone density/fractures/breaks). You can check your own levels using a home kit test http://www.vitamindtest.org.uk.

What is the best supplement to take?

Make sure you take Vitamin D3 (cholecalciferol) not D2 as D3 is the active version and is far more easily absorbed by the body when taken orally (3).

As a general daily dose, if you think you may be insufficient but don’t get a blood test it is okay to take 2000IU/d.  This is best taken with a meal/snack containing some fat, as vitamin D is a fat soluble vitamin and hence absorption is further enhanced taken with foods/fluids containing fat. I tend to advice taking vitamin D3 supplements with a glass of milk or yoghurt as the fat in the milk/yoghurt will aid absorption of vitamin D3 but also vitamin D3 will enhance absorption of the calcium from the milk and yoghurt.

Some suggested Vitamin D supplements can be purchased from;



Other vitamin D3 supplements can be purchased from supermarkets or pharmacies.

NB for competitive athletes under anti-doping regulations be sure to check the supplement has been tested for banned substances.




(1) Sunlight, Ultraviolet radiation, vitamin D and skin cancer: how much sunlight do we need?

(2) Sunlight, skin pigmentation and vitamin D: integral components of the vitamin D endocrine system

(3) Vitamin D for Health a Global Perspective 2013

(3) Vitamin D effects of skeletal and extra skeletal health and the need for supplementation 

(4) Vitamin D and Athletic performance: The potential role of muscle

(5) Vitamin D Council 

(6) Endocrine Society Clinical Guidelines: Evaluation, prevention and treatment of vitamin D deficiency 

(7) IOM 

(8) Harvard Health Publications: Time for more vitamin D 

(9) Vitamin D and skin physiology: A D-lightful story


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