Vitamin D promotes calcium absorption in the gut, allowing our bones to be mineralised properly. Because calcium is essential to muscle contraction, vitamin D is also essential for the normal functioning of muscles and other tissues.
Dietician Sophie Medlin, Founder of City Dietitians, discusses vitamin D, including why we need it, where to get it and what to do if you're concerned about vitamin D deficiency.
What do I need vitamin D for?Vitamin D promotes calcium absorption in the gut, allowing our bones to be mineralised properly. Because calcium is essential to muscle contraction, vitamin D is also essential for the normal functioning of muscles and other tissues.
Vitamin D also helps to reduce inflammation in the body and modulates cell growth, neuromuscular and immune function. ‘Modulates’ refers to its ability to ensure there is enough activity without it being too much, which is how it is linked to cancers and autoimmune conditions. Many tissues in the body have vitamin D receptors, indicating that it plays a vital role in many parts of the body.
Can I get vitamin D from food?We can get vitamin D from food – sources include oily fish, red meat, liver, egg yolks and some fortified foods. The problem is, the vast majority of people aren’t eating adequate amounts of these foods regularly. Changes in patterns of eating, moving more towards plant-based foods, coupled with the lack of sunlight means that the vast majority of people don’t meet their vitamin D requirements.
How much vitamin D should I take?The UK government recommends that we take 10 micrograms (μg) which is 400 international units (IU). The reason the government recommends 10μg is because this is from the Recommended Dietary Allowance which is defined as the average daily level of intake sufficient to meet the nutrient requirements of nearly all (97%–98%) healthy individuals. There is a lot of talk about whether the RDAs need to be updated, because we have learnt so much about nutrition and vitamin D in particular since they were published in 1941.
The safe upper limit of vitamin D is set at 100μg (4000IU) for men and women over the age of 18. This means it is considered medically safe to take up to 100μg of vitamin D per day. High dose supplements shouldn’t exceed this amount and if you are taking high dose vitamin D supplements, it is important to only take one supplement that contains vitamin D and try to avoid vitamin D-fortified foods. It is generally a good idea to take less than 100μg per day in supplement form so you have room for more from other sources.
Can I take too much vitamin D?Importantly, we can get too much vitamin D. Vitamin D increases calcium uptake in the gut which can lead to hypercalcaemia (too much calcium). Hypercalcemia, in turn can lead to nausea, vomiting, muscle weakness, mental and neurological disturbances, pain, loss of appetite, dehydration, excess urination, excessive thirst, and kidney stones. In extreme cases, vitamin D toxicity causes kidney failure, a build up of calcium in soft tissues throughout the body (including in blood vessels and heart valves), and even death. Vitamin D toxicity has been shown to be caused by consumption of dietary supplements that contained excessive vitamin D. In the past, this has happened because of manufacturing errors, excessive supplementation by individuals and when supplements were incorrectly prescribed by a healthcare professional.
What are the symptoms of vitamin D deficiency?In children, vitamin D deficiency develops into rickets which is where the bones haven’t absorbed enough calcium to make them rigid, so they remain soft and become misshapen with time. In addition to bone deformities, rickets can cause growth problems, severe pain, seizures, heart problems and dental problems. Although rickets is thought to be a problem that’s in the past, more children are developing it which we think is due to genetic differences in vitamin D metabolism, changes to dietary patterns (e.g. moving to more plant-based eating) and behaviours that lead to less sun exposure, for example, children playing outside less.
In adults, vitamin D deficiency can lead to a condition called osteomalacia, where the bones don’t maintain their calcium levels so they become weaker. Signs and symptoms of this are similar to children including bone deformities, seizures, spasms and dental problems. Early signs of vitamin D deficiency include fatigue, bone pain, muscle weakness, aches and cramps and mood changes like depression.
What kind of vitamin D should I take?Vitamin D is sometimes called calciferol, and in supplements comes in two forms – D2 (ergocalciferol) and D3 (cholecalciferol). Both are absorbed well in the gut but vitamin D3 is the type we get from animal rather than plant sources, is more easily absorbed and lasts longer in your body. Most vitamin D supplements will be vegan even if they’re vitamin D3.
These days vitamin D comes in multiple forms. The standard will be a daily dose of vitamin D in tablet form. You can get a vitamin D supplement that you only need to take once a week which may be convenient if you don’t remember to take supplements well. You can also get vitamin D oral sprays which may suit some people better if they don’t like taking tablets.
It doesn’t matter which you choose, as long as you’re not taking too much and you remember to take it!
Am I at risk of vitamin D deficiency?Everyone in the UK is considered to be at risk of vitamin D deficiency in the winter but some groups are at higher risk. The amount of melanin in your skin affects how much vitamin D you can absorb from the sun. The darker your skin, the higher your risk of vitamin D deficiency.
Age affects our ability to make vitamin D via sunlight. People who are housebound or stay at home most of the time are at greater risk, as are those who cover their skin when they are outside the house. Children who are exclusively breastfed for too long without supplementation are at risk, as are breastfeeding and pregnant mothers.
Some medical conditions increase our risk of vitamin D deficiency including cystic fibrosis, Crohn’s disease and coeliac disease due to lower absorption of vitamin D. Anyone who has undergone weight loss surgery is also at risk for the same reason. People with a BMI over 30kg/m2 are at higher risk because fat cells hold onto vitamin D, making it less available for the body to use. People with kidney and liver disease which affects the amount of an enzyme needed to convert vitamin D into its active form are at higher risk.