INTRODUCTION

Continuing on with our nutrition series, today marks our article on vitamin D. As in previous articles, we will be discussing its role in the body, its recommended intake level given by various regions and the Upper Tolerable Limit, what symptoms may occur if you are getting too little (a deficiency) or too much (a toxicity), as well as the forms and amount we use in our products.

As with previous posts, these will be displayed as a brief, bolded statement which gives the basic information, followed by a more complex, unbolded explanation.

ROLE OF VITAMIN D

Vitamin D is involved in turning gene expression on and off when it binds to them. It is also a key component of maintaining good bone health, and also has an anti-cancer effect, as well as providing support to the immune system. It is also known to reduce rates of depression.

After vitamin D is consumed, it is metabolised in order to convert it into its active form in the body. This active form is called calcitriol, and it is converted to this form in the kidneys from its storage form, calcidiol (which is itself produced in the liver). Calcitriol circulates around the body, and is able to bind to vitamin D receptors which are found on almost every cell in the body. When calcitriol binds to these receptors, it causes a conformational change within the cell, and it is these changes that can activate or deactivate gene expression.

Most people are aware that calcium intake is associated with good bone health, and this is why it is common for milk to be recommended as children grow. However, equally important to bone health is vitamin D. Vitamin D intake is required in order to allow the body to absorb calcium efficiently; insufficient vitamin D intake often leads to insufficient calcium absorption. This is why rickets, a disease caused by vitamin D deficiency, is associated with weak and brittle bones.

Japanese researchers discovered, when looking at cancer risks, that individuals with the highest vitamin D levels exhibited a 22% lower rate of tumours as those with the lowest. It also has further immune benefits via its action of gene expression: it binds to vitamin D receptors on immune cells (eg B-cells and T-cells). This binding modulates the immune system in many ways (largely beyond the scope of this article), but at a basic level, vitamin D binding to these receptors essentially primes T-cells to fight disease. Additionally, we see an increased rate of autoimmune issues in cases of vitamin D deficiency, showing that vitamin D also has a regulatory effect on the immune system, preventing immune cells attacking our own cells.

THE RDA (RECOMMENDED DIETARY ALLOWANCE) AND UTL (UPPER TOLERABLE LIMIT) OF VITAMIN D

The RDA for vitamin D is 5 micrograms (200 I.U.) in the EU, 20 micrograms (800 I.U.) in the US & Canada, and 10 micrograms (400 I.U.) according to the Nordic Nutrition Recommendations. The Upper Tolerable Limit is 100 micrograms (4000 I.U.).

The US & Canadian Institute of Medicine actually provides RDAs for different combinations of age and sex, but the 20 micrograms listed is the highest value given for adults (in this case, for those aged 71+). It is also the intake recommended by the FDA in their Daily Values, which aim to provide a recommended intake for all adults. The IoM’s RDA for adults aged 18-70 is 15 micrograms (600 I.U.).

Notably, a lot of nutritional boards worldwide believe both the RDAs and UTL are far too low. As an example, the Bone Mineral Research Center from Winthrop University Hospital in the US published a paper in which they recommend an intake of 95 micrograms (3800 I.U.) for individuals at or above a certain level of vitamin D in their blood, and 125 micrograms (5000 I.U.) for those below that level, who could be considered to have a deficiency. These intake levels are very close to, and above, the UTL respectively, but don’t seem to display any signs of toxicity as we would expect when going above the UTL. As such, while it is not a formal upper limit, many boards consider a dosage of 250 micrograms (10000 I.U.) to be a more appropriate upper limit.

SIGNS OF VITAMIN D DEFICIENCY

As previously mentioned, vitamin D deficiency can cause immune issues and bone issues, as well as impacting depression. It is also associated with tiredness, impaired wound healing, hair loss and muscle pain.

Vitamin D deficiency can lead to your immune cells attacking other cells in your body incorrectly, aka autoimmune attacks. It can also make you more susceptible to diseases, as your T-cells (cells critical to the immune system) are less primed to fight off infection. This increased susceptibility to infection is thought to be one of the reasons that people are more likely to suffer from colds and the flu in winter, when exposure to the sun provides us with significantly less vitamin D (and in many places in the world, none). This immune suppression also inhibits wound healing, as the process for this involves the same immune cells that vitamin D is responsible for aiding.

Due to inhibited calcium absorption, vitamin D deficiency can lead to bone issues and muscle pain. These bone issues can be so severe as to cause actual bone loss, a symptom particularly common in the elderly. This is because the bone density decreases over time as calcium content remains chronically low. This is one of the reasons that the elderly are more prone to fractures. Unfortunately, studies in which high doses of vitamin D have been given to women with low bone density did not seem to improve this issue, even once vitamin D levels in the blood improved, so it’s vital to not let vitamin D deficiency (and subsequent low bone density) become an issue in the first place!

As mentioned earlier, vitamin D deficiency can be responsible for an increased rate of autoimmune issues. One of the autoimmune diseases that vitamin D can contribute to is alopecia - a condition associated with rickets - where severe hair loss occurs. A study was conducted on this topic, and showed that more severe vitamin D deficiency in people with alopecia was associated with worse rates of hair loss.

Finally, it seems that low vitamin D rates can contribute to depression. This is thought to be why people can be susceptible to Seasonal Affective Disorder, because vitamin D exposure from the sun in winter is much lower than at other times of year. Vitamin D is thought to have an effect on these depression rates because there are a number of areas in the brain associated with depression which are very abundant in vitamin D receptors. The exact way in which vitamin D deficiency brings about symptoms of depression isn’t known, but it is likely due to the fact that when vitamin D binds to the receptors in these areas of the brain, it contributes to the production of a neurotransmitter called serotonin, which is associated with happiness and wellbeing.

SIGNS OF VITAMIN D TOXICITY

While vitamin D toxicity, otherwise known as hypervitaminosis D, is a very rare condition, it is still possible to induce with extremely high supplement doses. Toxicity can be either acute (short-term, very high doses) or chronic (long-term intakes well above the UTL), and can involve symptoms such as nausea & lack of appetite, stomach pain, constipation or diarrhoea, bone loss, and even kidney failure.

Almost all cases of vitamin D toxicity involve the use of supplements. There have been some isolated cases of toxicity solely from sun exposure, but these relate to people in countries like India who spend most of their time outside during hot summers - for example, lifeguards. The average person doesn’t need to worry about sun exposure for the purposes of vitamin D toxicity at all.

Chronic toxicity is caused by long-term ingestion of doses significantly over the Upper Tolerable Limit, starting at around 1250 micrograms (50000 I.U.). Doses such as these can be prescribed by doctors in some cases of severe deficiency, and should not pose a problem in these cases. In fact, intake at this level would need to be sustained for several months before problems began to arise.

Acute toxicity is much rarer, and requires an intake of upwards of 15000 micrograms (600000 I.U.) over a period of several days. This amount, as well as the amount that leads to chronic toxicity, will never be consumed outside of the use of supplements or heavily fortified foods.

Most of the symptoms of vitamin D toxicity stem from the fact that high vitamin D levels allow high levels of calcium to be absorbed and to circulate in the blood. High blood calcium levels are associated with gastrointestinal distress, fatigue, confusion, frequent urination and extreme thirst. It can take a long time for blood calcium levels to drop back to a normal range after such an increase due to vitamin D, so it’s best to avoid extremely high doses in supplements altogether. The gastrointestinal distress can range from simple stomach pain to constipation or diarrhoea, nausea and vomiting.

Notably with the above gastrointestinal symptoms, it is not guaranteed someone experiencing toxicity will exhibit all of them. A small-scale study of ten individuals who had developed excessive calcium levels in their blood following vitamin D supplementation showed three had a loss of appetite, and four felt nausea or vomited. As such, be aware that you may have toxicity without showing all of the symptoms for it.

Another symptom of vitamin D toxicity to be aware of is bone loss. It may seem somewhat contradictory that an adequate level of vitamin D is required for bone health though too much is detrimental, but this is down to interactions between vitamin D and other nutrients. We’ll cover nutrient-nutrient interactions in a future article in more depth, but in this case, vitamin D and calcium are also linked with vitamin K2. Vitamin D allows calcium to be effectively absorbed into the body and to circulate in the blood, while vitamin K2 is responsible for taking that calcium out of the blood and into the bones. Very high levels of vitamin D intake can impair vitamin K2 from doing this, and as such, can lead to insufficient calcium levels in the bones in spite of the high levels in the blood. These insufficient levels in the bones can lead to bone loss and brittleness, so it’s important to ensure that intake of vitamin K2 is sufficiently high when taking large doses of vitamin D.

Finally, vitamin D toxicity can lead to kidney disease. As mentioned previously, calcidiol is converted to the active form of vitamin D - calcitriol - in the kidneys. When there is an excess of vitamin D, the levels of calcidiol sent to the kidneys for conversion to calcitriol increases. Calcidiol has a higher affinity for vitamin D receptors than calcitriol does, leading to an increased level of free active calcitriol in the blood, and concentrated in the kidneys. This leads to an excess of calcium in the kidneys, causing damage which can be long-lasting if toxicity is severe.

WHAT WE USE IN OUR PRODUCTS

In our products, we include 60 micrograms (2400 I.U.) of vitamin D2, and 15 micrograms (600 I.U.) of a vegan source of vitamin D3 which is obtained from lichen. These amounts have been chosen to ensure nutrient needs are met, whilst not getting too close to the Upper Tolerable Limit.

The majority of our vitamin D is in the form of vitamin D2. Some consider D2 to be less effective at raising vitamin D levels in the blood, but with regular intake this does not seem to be the case. Rather, D2 and D3 seem to raise blood levels of vitamin D to similar extents, but upon cessation of intake, blood levels drop quicker following intake of D2 (after 2-3 days) relative to D3 (after ~2 weeks). Given the most common use case of our products is 1-2 meals per day, we are happy to use D2 - especially given we include a good amount above the RDA - for the majority of our vitamin D content. Vitamin D2 is also one step further from calcitriol (active vitamin D), so seems to contribute less to toxicity than D3. As such, despite our high level of vitamin D, our product should be more tolerable to those close to toxicity than if all of our vitamin D was obtained from D3.

We do, however, choose to include some vitamin D3. The amount chosen is equal to the highest recommendation from any nutritional board for adults below the age of 70 - 15 micrograms (600 I.U.). This amount means that vitamin D needs should be met for individuals who have an impairment in converting vitamin D2, or for those who find D2 to be less effective than D3 for raising blood levels of calcitriol.

The overall amount chosen of 75 micrograms (3000 I.U.) was picked in order to: a) take account of the lack of vitamin D synthesis from sunlight exposure in many countries we ship to during the winter months, b) take account of more recent studies that show an intake above the commonly cited RDAs is necessary for many individuals to not be deficient, c) to still provide peace of mind for those who are concerned about exceeding the UTLs, and d) to allow for an optimal ratio of vitamin D to vitamin A - a topic we’ll discuss in our nutrient-nutrient interactions article in future.