To D or not to D, that’s the question

Despite its name, vitamin D is a hormone not a vitamin

The definition of a vitamin is a nutrient, essential for our survival that we can’t make in our body. This means vitamins must be obtained from our food.

Although vitamin D is available in a limited range of foods, our bodies make this when sunlight interacts with cholesterol molecules in our skin.

This creates vitamin D2 that is activated in our liver to become vitamin D3. Conversion to the active-D (calcitriol), happens in our kidneys, to support all the health functions it’s so well known for. Because we can manufacture this in our body, it’s not a true vitamin.

In fact, vitamin D is a secosteroid, part of the steroid family of hormones and comes in many different forms. Vitamin D2 and D3 are measured together in a blood test called 25(OH)D. I will refer to this as the storage-D. Active-D is not commonly measured on blood tests. If you ask your GP for this, they will likely tell you it’s not available.

The many health benefits of vitamin D

Alongside the benefits for musculoskeletal health including the building and maintenance of bone, the active form of vitamin D is important for our immune system, protecting us from inflammation, parasites, and disease. It communicates with our DNA helping to switch genes on and off as needed. It’s also involved protecting our cells from oxidative stress and free radical damage when we produce energy.

Vitamin D deficiency is implicated in many autoimmune diseases, along with osteoporosis, metabolic syndrome, heart disease, cancers, chronic fatigue, and viral infections. In the last few years, supplementation has been recommended to provide protection against the Covid-19 virus. This has seen a rise in vitamin D sales from 2.5 billion in 2020 to 2.7 billion in 2022 along with increasing cases of vitamin D toxicity.

Is having insufficient storage-D causing disease?

Given all the known benefits of vitamin D, it’s not surprising that it has a reputation for protecting us from disease. However, the research shows that low levels may be found in healthy or sick people. The difference between these groups is the level of active D. When inflammation is present, active-D levels increases as part of an immune response. In this case, low levels of storage-D are considered a consequence of disease, rather than the cause. This distinction is very important.

Does supplementation reduce disease?

The question isn’t the importance of vitamin D for our health, but whether supplementation reduces the risk of disease. Because storage-D levels decrease when people are sick, it’s been proposed that supplementation will reverse this.

However, interventional studies, where supplemental vitamin D is prescribed have shown mixed results. Some demonstrate short-term improvement, while others have shown increased harm. Overall, the results of these studies have been considered disappointing.

The short-term benefits that are seen when storage-D is increased, may be due to temporarily dampening down the immune response to inflammation. Unless the cause of this inflammation is addressed, the reduction in symptoms wears off, as the underlying disease process continues. Other studies have shown no evidence that vitamin D supplementation reduces the risk of cancer, heart disease or falls in the elderly. Some studies have shown worse outcomes in these cases.

There have been a few mechanistic trials using cell culture that show a benefit of supplementation. For example, adding vitamin D to breast cancer cultures suppresses their growth. However, in real life vitamin D is part of a larger regulatory system. It doesn’t just do one thing or act on its own. It’s involved in the regulation of 2000 genes, turning up the expression of some while down regulating others. It’s impossible to separate this regulation from all the other factors that contribute to our health.

Nutrients work together

We often think of the elements of nutrition as separate pieces and that we can individually manage nutrient levels with targeted supplementation. However, nutrients work in a coordinated manner and are very dynamic, depending on the body’s needs at any one time. The absorption and metabolism of a particular nutrient is often dependent on the availability of other key nutrients and vitamin D is no different.

For example. all the enzymes that activate vitamin D require magnesium as a cofactor. Vitamin D is integral to the building of bone and works alongside magnesium, vitamin K2, calcium and phosphate in this process. Zinc and vitamin A are vital to ensure active-D can bind properly with its receptor. Excess storage-D can interfere with an important vitamin A receptor (RXR) with negative implications for our health. There are many other enzymatic functions that are coordinated with vitamin D, through pathways that are still not fully understood.

There’s a long list of other nutrients that cooperate with vitamin D in protecting our health, including copper, selenium and boron. Randomly selecting nutrients without consideration of our overall nutrient status may lead to imbalances and continuing poor health.

We often think of the elements of nutrition as separate pieces and that we can individually manage nutrient levels with targeted supplementation. However, nutrients work in a coordinated manner and are very dynamic, depending on the body’s needs at any one time. The absorption and metabolism of a particular nutrient is often dependent on the availability of other key nutrients and vitamin D is no different.

Could supplementing with D, harm our health?

Given all the health benefits of vitamin D, increasing our storage levels as insurance against inflammation and disease may seem harmless. But what happens when we do this?

Our body has feedback mechanisms that protect us from the unintended consequences of exceeding our vitamin D requirements from sunshine and food. This safeguard doesn’t function with supplementation.

A serious consequence of too much vitamin D is the build-up of calcium in the blood. This can cause nausea and vomiting, weakness, and frequent urination. Vitamin D toxicity might progress to bone pain and kidney problems, such as the formation of calcium stones. Another consequence is an irregular heart beat (atrial fibrillation) and increased risk of calcium build up in the arteries and soft tissue.

Testing your vitamin D status

Testing storage-D (25 (OH)D) is not adequate on its own to assess vitamin D status as this doesn’t tell us anything about what’s really going on.

Parathyroid hormone (PTH) works to help regulate active-D and increase the absorption of calcium and phosphorous from our digestive tract when more is needed to build bone. When PTH levels rise, this suggest that more active-D may be required. It’s important to review ionised calcium and phosphate levels in this situation. If storage-D is low with normal PTH, then adequate vitamin D is available. If storage-D levels are low, PTH should be tested prior to supplementing.

Magnesium and vitamin A levels should also be checked and rectified if needed, before supplementing; as deficiencies in these will affect vitamin D status. Due to the many complex nutrient interactions with vitamin D, I think it’s beneficial to investigate the reasons why levels may be low before turning to the supplement bottle. This includes checking and addressing inflammation.

The recommended level of storage-D was 30 nmol/L up until 2010. This was increased to 40 and now the recommendations are 50-150 nmol/L. However, many experts believe 30 nmol/L is the ideal level. I wouldn’t recommend going over 50 nmol/L.

Looking to our ancestors for answers

A useful question to ask is how did our ancestors survive with only limited foods supplying vitamin D and without access to supplementation? The fact that it’s not abundant in our food supply and is a hormone synthesised by sunlight on our skin tells us a lot.

There’s a belief that many of us are at risk of deficiency due to limited annual sunlight. However, our ancestors survived in a wide variety of sunlight conditions. The amount of pigmentation in our skin is an indication of how much sun our ancestors were exposed to, with those living in hotter climates protectively developing more melanin and darker complexions. Those without yearlong exposure to sunshine survived on summer exposure levels and the limited amount available from food.

Clothing, sunscreen and pollution as well as reduced sun exposure have been used as justification for supplementation. Again, when we consider that many of our ancestors survived far from tropical climates, with very limited access to the sun for many months of the year, it suggests there may be other mechanisms at play. It’s interesting to note that rickets is a serious health problem in many developing countries that have excellent exposure to sunshine. It’s been well documented that in these populations that diets are deficient in calcium. Improving calcium intakes has had more benefit than vitamin D supplements.

Researchers have identified that storage-D has a seasonal variation, increasing during periods of sun exposure where it is stored in the liver, kidneys, body fat and other tissues for use throughout the rest of the year. On the other hand Active-D, has a diurnal rhythm and the body is constantly deciding how much it needs to produce to maintain our health, at any one time.

Emerging research also suggests we may not need as much sun exposure as previously thought. Ten to fifteen minutes of UVB exposure (even on a cloudy day), on the back of hands and arms, is considered enough to keep our vitamin D stores topped up.

It’s interesting that breastmilk is relatively low in vitamin D. It’s improbable that the sole source of nutrition for new-born babies wouldn’t contain all the essential elements they need for good health. There is a move to have all new-borns supplement with vitamin D, but the evidence for this isn’t strong.


I recommend that most people avoid supplementing with vitamin D. The best thing you can do is get sunshine and eat fatty fish or Cod liver oil. Be careful that you purchase a brand that doesn’t have added vitamin D. Other good sources include beef liver, and eggs.

If you are supplementing with vitamin D and wish to continue, I encourage you to get tested regularly and avoid levels of over 50 nmol/L. Keep an eye on your RBC magnesium and vitamin A and ensure these are at optimal levels. It’s also worth checking ionised calcium and PTH as discussed above. Remember if you are inflamed or have any chronic disease, your vitamin D levels may be low as a consequence of this. Dealing with underlying health problems is the first priority.

Leave a Reply

Your email address will not be published. Required fields are marked *