Vitamin D: The Iceberg nutrient, more than just bone health...

Most people know vitamin D is important for bones but there's so much more beneath the surface...this is why I like to think of vitamin D as the “iceberg nutrient”, what we can easily see, its role in bone health, is just the tip of the iceberg...

Vitamin D is probably best known for helping our bodies absorb calcium and maintain healthy bones and teeth. Severe deficiency can lead to conditions such as rickets in children and osteomalacia (soft and weak bones due vitamin D or calcium deficiency) in adults.

But vitamin D also has important roles in immune function, muscle function and the regulation of many processes throughout the body.

As we get older, maintaining strong bones and muscles becomes increasingly important. Bone density naturally changes with age, while muscle mass and strength can also decline. Maintaining adequate vitamin D status is therefore one part of the bigger picture when we're thinking about healthy ageing, mobility and independence. Vitamin D also contributes to normal immune function, which is particularly relevant as we age. It isn't a magic bullet for preventing illness, but having adequate vitamin D is an important part of giving your body the nutrients it needs to function properly.

How do we know if we're getting enough?

The most commonly used blood test measures 25-hydroxyvitamin D [25(OH)D], which is the main circulating form of vitamin D and the marker generally used to assess vitamin D status. Your body then converts vitamin D through a series of steps into its biologically active forms. So vitamin D metabolism is more complicated than simply asking how much vitamin D do I take?

Your vitamin D status is influenced by factors including sun exposure, diet, supplementation, skin pigmentation, age, body composition and how your body processes the vitamin. This is one reason why individual circumstances matter. And what about your gut? This is where my interest in vitamin D and gut health comes in. Vitamin D doesn't work independently of the rest of your body, and there is increasing evidence of a two-way relationship between vitamin D and the gut microbiome.

Vitamin D receptors are found in intestinal tissue, and vitamin D signalling is involved in maintaining the integrity and function of the gut barrier and in regulating immune responses within the gut.

At the same time, research suggests that vitamin D status and the composition of the gut microbiome may influence one another. We're still learning exactly how this relationship works, so I wouldn't say that your gut bacteria simply “activate” vitamin D or that increasing microbiome diversity automatically improves your vitamin D status but it is another fascinating example of how gut health doesn't exist in isolation.

Your immune system needs vitamin D too. Vitamin D is involved in the normal functioning of both innate and adaptive immune responses, which are the different arms of your immune defence system. This doesn't mean that taking large doses of vitamin D will prevent infections or cure disease. Rather, it highlights why avoiding deficiency matters. As with many nutrients, more isn't necessarily better. The aim is to have an adequate amount, not to take increasingly high doses in the hope of gaining additional benefits. Because vitamin D is a fat-soluble vitamin, your body can store it. This is useful when sunlight isn't available during the winter months, but it also means that taking very high doses of supplements over a long period isn't a case of “the more, the better”.

Too much vitamin D can cause calcium levels in the blood to become too high (hypercalcaemia), which can lead to symptoms such as nausea, constipation, weakness and excessive thirst and, in more serious cases, kidney and heart problems.

Where do we get vitamin D? There are three main sources: ☀️ Sunlight - our skin can produce vitamin D when it is exposed to UVB radiation. In the UK, however, there is a significant limitation as during the Autumn and Winter months, sunlight isn't strong enough for most people to make sufficient vitamin D through their skin and even during the Summer, vitamin D production varies depending on factors such as skin type, time of day, location, clothing and sunscreen use.

🐟 Food. There aren't huge amounts of vitamin D in most foods, but some useful sources include:

Oily fish such as salmon, sardines and mackerel Egg yolks Some fortified foods Certain mushrooms exposed to UV light Oily fish is particularly useful because it provides vitamin D alongside omega-3 fatty acids and protein and another example of why whole foods can be so valuable.

💊 Supplements. For many people in the UK, supplementation is particularly relevant during the darker months. UK guidance recommends that adults consider taking a daily vitamin D supplement during autumn and winter because it can be difficult to obtain enough vitamin D from sunlight alone. Some people may need supplementation throughout the year depending on their circumstances. If you're concerned about your vitamin D status, your GP or healthcare professional can advise whether testing is appropriate.

For most adults, the NHS advises not taking more than 100 micrograms (4,000 IU) of vitamin D per day unless advised otherwise by a healthcare professional. Higher doses are sometimes prescribed for a defined period to treat deficiency, but these should be medically supervised. Vitamin D testing is easily available as pin prick tests.