What is vitamin D?

Vitamin D is an important factor for overall health particularly strong and healthy bones. It is also a vital player in ensuring that a number of important organs such as muscles, heart, lungs and brain work well and the immune system is fit to fight against infections.

The body can make its own vitamin D from sunlight. However, adequate vitamin D can also be obtained from supplements and a small amount comes from a few foods. Vitamin D has to be changed by the body a number of times before it can be used.

What does vitamin D do?

Vitamin D is different from other vitamins. While the human body is dependent on various foods for adequate intake of these other vitamins, the body can make its own vitamin D from exposure of skin to sunlight. When the body gets its vitamin D, it turns the vitamin D into a hormone called activated vitamin D or calcitrol. Vitamin D is very important for strong bones and is required to absorb minerals such as calcium and phosphorus. Without enough vitamin D, these minerals cannot be absorbed into the body. Vitamin D is important for general good health, and now researchers are discovering that it may be important for many other reasons outside of good bone health. Some of the functions of the body that vitamin D has been linked with include:

  • Immune system
  • Muscle function
  • Healthy heart and circulation l Healthy lungs and airways
  • Brain development
  • Anti-cancer effects

Doctors are still working to fully understand how vitamin D works within the body and how it affects overall health.

How much vitamin D is needed?

Getting the right amount of vitamin D does not depend on the foods you eat. To get enough vitamin D you need to expose your skin to sunlight regularly and you may also need to take supplements. This makes getting the right amount a little more complex compared to other vitamins and minerals. Public health advice to avoid prolonged sun exposure and to wear sunscreen to protect against skin cancer – whilst scientifically sound to reduce risk of skin damage due to ultraviolet rays – has meant that there is some level of unnecessary alarm. A few minutes of skin exposure to sunlight without the use of sunscreen is safe and helps to generate adequate vitamin D levels. Various organisations recommend different daily requirements for vitamin D, ranging from 200 to 1000 IU (International Units) per day. The NHS guidance for adults and children over 1 years old is to consider taking a daily supplement containing 10 micrograms of vitamin D (400IU). This can vary depending on the colour of the skin, season, geographical location, and clothing.

Has vitamin D deficiency led to health problems?

Vitamin D deficiency has been linked to various diseases including osteoporosis, heart disease, some cancers, autoimmune conditions and poor muscle strength. The evidence to convincingly confirm a definite role for vitamin D deficiency in causing these various conditions is lacking however. The strongest proof about low vitamin D exists for osteoporosis and hence supplementation along with calcium is now routinely prescribed in this condition.

Does vitamin D or its deficiency have a role in the development of thyroid diseases?

Some, but not all, observational studies have found low blood levels of vitamin D in patients with hypothyroidism (underactive thyroid) as well as hyperthyroidism (an overactive thyroid) due to Gravesʼ disease. It is not clear from these studies if low vitamin D is a cause, a consequence or an innocent bystander in the development of these common thyroid conditions. It is possible that low vitamin D may permit the under-performing immune system to facilitate progression of thyroid disease. Equally, it is also possible that people with thyroid diseases may have altered health or lifestyle that leads to a low vitamin D state. For example, patients with an overactive thyroid due to Gravesʼ disease may increase the breakdown of vitamin D into inactive products, whereas those with an underactive thyroid may spend less time outdoors due to tiredness and thus have reduced sun exposure. Only properly conducted scientific trials will be able to answer this question definitively.

Is there previous evidence to link vitamin D deficiency in other autoimmune conditions and to show that supplementation may be beneficial?

Type 1 diabetes mellitus: Researchers in Finland observed that the accelerated increase in occurrence of autoimmune type diabetes has stopped from 2006. They state that the otherwise unexplained 5-fold increase in the incidence of type 1 diabetes prior to 2006 was associated with a corresponding reduction in the recommended dose of vitamin D to one tenth since the 1950s. Since 2003, there has been a vitamin D fortification programme of milk and dairy products in Finland. In a separate observation, researchers found that there was an inverse association between new episodes of insulin-requiring diabetes and levels of serum vitamin D. However, trials of vitamin D supplementation in type 1 diabetes have shown conflicting results.

Multiple sclerosis (MS): Living below 35° latitude for the first 10 years of life reduces the risk of multiple sclerosis by approximately 50%. Among white men and women, the risk of multiple sclerosis decreased by 41% for corresponding increases in blood vitamin D levels. Women who ingested more than 400 IU of vitamin D per day had a 42% reduced risk of developing multiple sclerosis. Recently, in an observational study of 468 MS patients, higher serum vitamin D levels were associated with reduced disease activity and progression. Trials of vitamin D in patients with MS have shown that while some blood and MRI scan abnormalities improve this has yet to be translated into clinically meaningful results.

Should everyone be taking vitamin D supplements?

Most peopleʼs skin will produce vitamin D in the summer but around a fifth of the population will still be deficient during this period. The situation is likely to be worse over winter. As it is not routinely possible to identify these individuals without everyone having a blood test, Public Health England recommends everyone over 1 year old should take a low dose of vitamin D (10 mcg or 400IU), particularly in the winter months. The current advice during the Covid-19 pandemic is for people to consider taking a daily supplement every day and to eat foods rich in vitamin D. This advice has been given to promote good bone and muscle health, especially for people spending more times indoors than normal. The advice to take vitamin D supplements is not given to prevent nor treat Covid-19 as a rapid review undertaken by the National Institute for Health and Care Excellence (NICE) found no evidence to support claims of this.  

People found to have vitamin D deficiency may also be prescribed higher therapeutic doses following the appropriate testing and clinical management. 

Further information

NHS information about vitamin D

COVID-19 rapid evidence summary: vitamin D for COVID-19

Research shows some vitamin supplements may have a small effect on Covid-19 risk 

By Dr Salman Razvi, Clinical Senior Lecturer/Consultant, Institute of Genetic Medicine, International Centre for Life, Newcastle University.

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