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Vitamin D: the Controversial Vitamin!


This month I wanted to address vitamin D, which comes in for a lot of controversy – particularly in winter.

Strictly speaking, Vitamin D is a hormone that’s made in our skin when exposed to the sun. But it’s also possible to convert it from nutrients in our diet. There is a lot of discussion about how much of vitamin D we should be have, whether supplements are appropriate, whether it can be toxic, and how much sun exposure is safe to get it.

What does Vitamin D do?

Vitamin D is most known for its impact on the bones, because it helps to balance calcium levels in the blood and bones.  Although calcium is needed for bone strength, it is also crucial for other functions such as communication between cells and neurons. The body therefore wants to ensure just the right amount of calcium to circulate in the blood, as well as to build up the bones.

Vitamin D enables the intestines to absorb calcium from the diet. If there isn’t enough to ensure calcium absorption, though, the body prioritises blood over bone calcium levels, and removes calcium from the bones. In adults, this can lead to osteoporosis. In children this can cause a softness of bones called rickets. Famously, King Charles I is thought to have suffered from rickets, and had to wear special calipers to help straighten his legs as he grew up (and for those interested, some of his art collection is currently on display at the Royal Academy of Arts in London!).

Vitamin D and calcium also work together to prevent gut inflammation and regulate the immune system. They have been found to support improved symptoms in Crohns disease, and prevent risk of other auto-immune diseases such as MS and diabetes.  In addition, they produce the feel-good neurotransmitter serotonin, and several scientific studies connect their deficiency with depression, particularly for women in the second half of the menstrual cycle.

So, we have established that vitamin D is essential for (among other things) healthy bones, healthy immune function, and good mood. Let’s now explore how to get enough of this essential nutrient.

How do I get vitamin D?

Ideally, we acquire Vitamin D by exposing our skin to sunlight without burning, at least 2-3 times per week. This method cannot cause overdose in the body. However, in the winter months, when there’s less sun, and we cover up our bodies more, vitamin D levels can fall significantly.

It can be challenging to get enough vitamin D in the UK if you have a darker skin tone, if you’re older, or if you cover up a lot with clothes or sunscreen. Older people have less ability to convert sunlight in the skin, while a mere factor 8 sunscreen is enough to reduce vitamin D production by over 90%.

In winter, therefore, we have to take extra care with our diet. Vitamin D is most abundant in oily fish such as salmon, mackerel and sardines, and in fish livers. Salmon and good quality cod liver oil are therefore ideal to consume on a regular basis throughout the colder months.


Whether in summer or winter, it is also vital to consume co-factors for vitamin D. These are other nutrients which enable it to work correctly. For example, you could be taking plenty of vitamin D and calcium in your diet, but if you don’t consume vitamin K, you could still be at risk of osteoporosis. Vitamin K helps to direct calcium into your bones, rather than your arteries, helping to ensure good cardio-vascular health.

Offal, such as kidneys and liver, is an excellent source of vitamin D, together with co-factor vitamins K2, E and A. Vegetable sources of vitamin K are spinach, broccoli, Brussels sprouts, kale, Japanese natto, some cheeses, and milk kefir.

Two others co-factors are magnesium and zinc, which help the absorption and deployment of vitamin D in the body. Magnesium is easily depleted when we are stressed, but we can get it from foods such as pumpkin seeds, nuts, fish, meat, spinach and cocoa. Good quantities of zinc can be found in red meat, sesame seeds, pumpkin seeds and lentils, and it’s also essential for hormone health.

Controversy: How much is enough?

There has been a lot of disagreement about how much is enough vitamin D – and how much sun is safe.

Although it is absolutely correct to be concerned about the risks of skin cancer from over-exposure to the sun, there are now many scientific studies hypothesising that our increased incidences of skin cancer are the result of reduced non-burning exposure to the sun. Hidden from the sun more than ever, thanks to indoor-based office jobs and indoor habits such as internet and TV, our skins may be more likely to burn or get damaged. and less likely to absorb vitamin D.

Vitamin D levels can be determined via blood tests. The latest UK government report states that 25 ng/mL is  the absolute minimum requirement to prevent diseases in most people, but nutritional therapy looks for the amount which will promote the best possible health.

The USA Endocrine Society practice guidelines recommend that blood levels should be at least 75 nmol/L, with a preferred range of 100–150 nmol/L for maximum bone and muscle health. As winter approaches, I personally like to see my UK clients’ vitamin D levels at about 125 ng/mL, especially if they are not going abroad before the spring.

If you are concerned about your vitamin D levels, you can ask your GP for a test; if he or she is reluctant, you can contact City Assay and order a home test for about £27.

Toxicity: am I at risk?

Too much vitamin D supplementation can cause too high a concentration of calcium in the blood. This in turn can cause bone loss, kidney stones and organ damage. However, evidence for vitamin D toxicity is based on case reports of accidental overdose of fortified foods or supplements, and it is very rare: such reports are associated with blood levels of greater than 300 nmol/L, or (more usually) more than 600 nmol/L.

The Pauling Institute advises that anything up to 10,000 IU per day should be safe for otherwise healthy people. On the whole, one is not likely to suffer vitamin D toxicity unless one takes more than 50,000 IU per day on a long-term basis.

However, I sometimes recommend that certain clients who are very deficient take exactly this amount per week for a limited time, as a therapeutic dose. But I am extremely specific about the number of weeks they should take it, and I also bear in mind that every person is different, and will respond to supplementation differently. For this reason, anyone who is concerned about their Vitamin D levels should seek professional healthcare advice before taking any supplements.

For ordinary vitamin D maintenance in the winter, I often suggest taking Vitamin D3 Complete by Allergy Research, because D3 has been found to be more effective than D2. This brand also contains vitamin A and vitamin K, which can often be reduced when someone takes high doses of vitamin D on its own.

The good news is that in about two months, we will be more able to create vitamin D from exposure to the sun. One way to do this is to go outside for 20 minutes in the first half of the morning, with as much skin bared as you can bear (!).

Please don’t hesitate to contact me if you are concerned about your vitamin D levels. And meanwhile, excellent food sources of vitamin D and its co-factors are, as I’ve mentioned, good-quality fish liver oil, oily fish, organ meats and milk kefir.

With best wishes for a happy end of the winter,


Emma Maitland-Carew MBANT, Registered Nutritionist in Oxford and Bloxham, Oxfordshire


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