You’re probably aware of the relationship between vitamin D and bone health. You may even know that vitamin D is actually considered a hormone, and most cells throughout the body contain receptors for it. For this reason, people are getting excited about its range of roles: eg: modulation of cell growth, neuromuscular and immune function, inflammation reduction. The evidence is mounting though still incomplete; below we outline what is known about its potential to prevent non-skeletal diseases.
- Vitamin D is essential for the health of your bones. It looks like it could also have important roles throughout your body, although we need further research progress to reach a full understanding
- There are varying definitions of deficiency and estimations of how prevalent it is. Nonetheless, it is thought that approximately a billion people world wide may be deficient
- There are a number of subgroups that may be at greater risk of deficiency.
- If you need to, you can increase your vitamin D levels through sensible levels of UV-B exposure and increased dietary intake. In some cases fortified foods and supplements may be necessary in addition to natural food sources. If you have any medical condition that underlies a deficiency, address that as best you can.
The ABC’s of vitamin D
The human body is capable of producing its own vitamin D. When UV-B light acts upon skin cells, the provitamin D3 (a form of cholesterol) contained therein is converted into pre–vitamin D. This in turn is converted into vitamin D, and is then ejected from the cell and bound to a specific protein which carries it to to the liver.
In the liver it is turned into another form before being sent into the bloodstream (we call this the hydroxylated form, 25(OH)D or 25-dehydroxyvitamin D3). This is what is often measured in vitamin D tests, as it is a good indication of levels within the body.
If your body doesn’t produce adequate levels, however, you will need to top yourself up with food and/or supplements. This vitamin D will be absorbed in the small intestine, then transported by chylomicrons (essentially small globules of lipids/fats and some protein) to the liver.
From the liver, 25(OH)D travels to the kidney bound to a protein. In the kidney another reaction happens, converting it to 1,25[OH]2D or calcitriol, the most active form. Then it can finally travel around the body.
If you don’t get adequate vitamin D from the sun or top yourself up through the diet, well…
The Devil is in the Deficiency
Long-term deficiency of Vitamin D may cause:
Vitamin D and musculoskeletal weakness
- We are most certain that deficiency puts people in risk of Metabolic Bone Diseases including bone softening (osteomalacia), rickets in the young, osteoporosis, fractures and bone pain.
- It assists in the absorption of calcium and phosphate, which are key structural building blocks of bone
- It is also thought that deficiency may lead to muscle weakness
Vitamin D and heart, metabolic and hormonal conditions
- High blood pressure has been seen in deficient animals, but there is inadequate evidence from human trials thus far
- There have also been associations- but no proven causative relationship- between deficiency and type 2 diabetes, adrenal diseases, polycystic ovary syndrome, and thyroid diseases such as Hashimoto’s
Vitamin D and cancer
- There are several mechanisms proposed for how deficiency may contribute to cancer, however human studies do not provide great support for the potential of supplements to prevent this
- For example, in non-clinical studies, hindered vitamin D signalling has encouraged formation of thyroid tumours
Vitamin D, infections and autoimmune disorders
- Vitamin D could be beneficial in treatment of infections, but experts have not yet managed to agree on what levels of supplementation to recommend
- A link has been seen between deficiency and type 1 diabetes (where pancreatic cells are attacked by the immune system)
Vitamin D and oxidative stress
- This is a controversial one: in recent years there has been some suggestion that vitamin D may have antioxidant effects, and thereby have a role in the prevention of multiple chronic diseases. However, as of a 2019 review of randomised controlled trials, there is little robust evidence in support of this. Further high-quality studies are required
Vitamin D and neurological conditions
- A couple of observations about the brain have led to the theory that vitamin D may have a role in prevention and treatment of neurological disorders, such as multiple sclerosis, stroke, and Alzheimer’s and Parkinson’s disease.
- eg: Several areas of the brain have vitamin D receptors (eg: hippocampus, hypothalamus, substantia nigra, and thalamus).
- Vitamin D regulates the expression of neural signalling proteins, including neural growth factor (NGF) and neurotransmitters (acetylcholine, dopamine, and GABA).
- However, much more evidence is required to be certain of any precise roles in these conditions.
Vitamin D, depression and anxiety
- There are associations between deficiency and risk of these conditions, but the reason for this relationship is unclear. Furthermore, use of supplements by people experiencing depression and anxiety produces inconsistent results. If it has any value in prevention or treatment, further high quality studies will be critical to prove it.
How to know if you’re vitamin D deficient
The diagnosis of vitamin D deficiency is slightly complicated, by virtue of the fact that there are varying definitions of just what makes someone deficient… Suggested cutoffs range from <20 to <75 nmol/litre of blood (nmol refers to a large number of molecules). For example, the US Institute of Medicine uses a benchmark of <30 nmol/L, based on their linking this level with the risk of metabolic bone disease. Europe has used a benchmark of <25nmol for decades. Meanwhile, some experts suggest higher cut offs as they believe deleterious effects can occur before reaching these lower levels.
Not surprisingly, the standard your blood test is compared against will affect the likelihood of you being diagnosed with deficiency. There is a variety in deficiency rates around the world- anything from 13-100% of studied populations have been found deficient, depending on the study. It’s worth noting also that data obtained from different tests can be of variable quality, so the figures available may not always be reliable.
Regardless of these variations, we can say that deficiency is shockingly common. It is believed almost 1 billion people worldwide have levels <30nmol/L. This is not to say that you will be deficient, but you should consider a blood test if you and your doctor have reason to believe you’re at risk (see below).
Risk factors for vitamin D deficiency
The most obvious causes of deficiency are inadequate sun exposure and/or intake.
A number of health-related factors may also cause deficiency, for example:
- Problems with absorbing it from the gut. This may result from gastric bypass, Crohn’s disease, celiac disease, SIBO and other microbiome changes, pancreatitis, cystic fibrosis, for example, and should be ruled out.
- Some medications such as anticonvulsants, glucocorticoids, antifungals such as ketoconazole and anti-HIV medications can accelerate the breakdown of vitamin D. If this is the case, doctors will be able to determine the best step forward with you.
- Kidney disease, liver disease & alcohol abuse can stop vitamin D from being converted into its most active form
- Obesity may contribute. It is possible that vitamin D can become hidden in adipose tissue, away from the bloodstream. More research in this area is required, however.
- Lymphomas, primary hyperparathyroidism, diabetes are among other conditions that can cause deficiency
Geography also has an impact.
It’s been found that people living in latitudes greater than 37° do not get enough UV-B exposure to synthesise vitamin D during winter. For these populations, dietary intake is critically important. However, studies have shown that deficiency is less common in countries such as Norway, Iceland, and Finland than countries in the United Kingdom, Ireland, Netherlands, and Germany. A theory is that nordic countries are more conscious of ensuring their intake is adequate through food fortification and/or supplement use. On the other hand, deficiency was more common in children residing in mid-latitude countries than southern European countries. Perhaps this is related to the sun exposure that can be obtained from time spent playing outside.
Age is an important factor to consider. Over 70 year olds are more likely to be deficient and require approximately 3 times the sun exposure as a child to synthesise adequate vitamin D.
Ethnicity is generally believed to have an effect, with darker skin ethnic subgroups more often deficient. However, some researchers have indicated that the predominant method of testing levels may skew results, and that perhaps there are similar levels of bioavailable vitamin D across ethnicities.
How to increase vitamin D levels if deficient
First of all, a technical note: Because of the variation in testing procedures and results obtained, you ideally want levels at least a bit above a ‘deficiency’ standard. This will give you a buffer zone.
Get adequate sunlight
According to Australia’s Cancer Council, regular incidental sun exposure is enough to support adequate synthesis within the body. Obviously not everywhere is as sunny as the stereotypical image of Australia, but the following is still a reasonable guideline: when the UV index is 3+ (eg: summer), a few minutes outside is adequate for most people. On the other hand, when the UV Index falls below 3, time outside in the middle of the day with some skin exposed is recommended.
Regardless of your locale, your first port of call should be to consider your personal habits such as time spent inside vs outside, how much skin is covered by clothing, and sunscreen use.
If you’re unable to get adequate sun exposure (and it seems many can’t consistently), look to your dietary intake.
Increase your vitamin D intake
It can be difficult to accurately assess your intake, as there is no authoritative source of vitamin D food composition data, manufacturers often reformulate products, and some supplements may not even be available on food composition databases.
Nonetheless, we do know that fatty fish (eg: salmon, tuna, sardines, swordfish), cod liver oil, egg yolks, mushrooms, beef liver are natural sources.
However, it is estimated that most adults are unlikely to meet the recommendations of the IOM or the Endocrine Society (see below) from natural sources alone.
This is where fortification and supplementation comes in.
Fortification is the addition of a nutrient to foodstuffs. Countries have different policies on fortification. A benefit is, if well-considered amounts are added to basic foodstuffs, vitamin D intake could increase on a population level, while also erring on the side of caution (to avoid toxicity). However, diversity in eating patterns can complicate decisions on what to fortify. Foods fortified with vitamin D may include breakfast cereal, breads, milk (dairy and nondairy), yoghurt, spreads, infant formula, cheese, and orange juice, depending on your location.
In the US and Canada, fortification is mandatory, and it is thought that this is the reason that residents there tend to have higher intakes than most of Europe. An exception to this is the Nordic countries, likely due to the intentional use of supplements as well as fortification mentioned earlier.
Whether fortified foods are available or not, there is also the option to take supplements. They come in the form of either vitamin D2 or vitamin D3. Overall, there’s no clear indication of what type is most effective, but a single Randomised Controlled Trial has pointed to D3 producing longer lasting increases in leves.
Note that, despite ticking many health-related boxes, breast milk contains very little vitamin D. So if you have an infant who is exclusively breastfed, supplements would be valuable.
What are the recommended intake levels?
These vary according to who you ask.
In the last couple of years, there has been controversy regarding adequate intakes, and there have been changes in recommendations. To the best of my knowledge, these are the most recent ones (figures are for daily intakes unless otherwise indicated):
|Australian & NZ NRV||The Endocrine Society*
|US Institute of Medicine (IOM)|
|Adults||5µg (19-50 year olds)
10µg (51-70 year olds)
|37.5 to 50µg||15µg|
|>70 year olds||15µg||–||20µg|
|Deficient adults||–||1.25mg/week or 150µg per
day for 8 weeks
|1.25mg/week or 150µg per
day for 8 weeks
|Maintenance dose for
people with malabsorption
syndromes, obesity, or
taking medications that
disturb vitamin D use
|–||2-3 times normal recommendation||75-150 µg daily|
* Endocrine Society recommendations based on bone health as opposed to other possible functions. New lab technologies have led the endocrine society to lower their recommendations in the last couple of years
Should you consume too much, there’s risk of hypercalcemia, toxicity and interactions with other drugs. The upper limit of intake is 25 µg/day for infants under 12 months, and 80 µg/day for others, according to Australia & New Zealand’s NRV’s. The upper level suggested by the IOM is 100 μg/d (4,000 IU) in persons over the age of 9 y, with lower levels for younger children down to 25 μg/d (1,000 IU).
Results from the The Vitamin D and Omega 3 Trial are expected sometime after 2020. In this trial, 26,000 adults of 50 years or over are receiving vitamin D supplements, omega-3 fatty acids, both, or placebo… and the risk effect of risk of cancer and cardiovascular disease is being studied. Such a large study will no doubt provide interesting insights.
Vitamin D has been receiving increased attention over recent years because it is potentially crucial for the health of many bodily systems, besides your bones. Deficiency appears common- possibly much more common than we might expect given our bodies are able to synthesise it. No need to panic and assume you are deficient, though. If you have reason to believe you’re at risk (based on defined risk factors), you should get a blood test to determine your levels. If you are deficient, you may be able to increase your UV-B exposure or better manage any relevant medical issues. If that is irrelevant or unrealistic, consuming more vitamin D will be necessary, possibly through natural food sources, but likely also with fortified foods and/or supplements.
Getting a balance between sun exposure for vitamin D synthesis and protecting yourself from skin cancer is key. Where do you fall on the continuum for sun exposure?
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