Vitamin D is an extremely hot topic at present, it is everywhere: in newspapers, research articles, blogs and tweets from the medical community. Its deficiency has been linked to several major diseases such as diabetes, cardio-vascular disease, cancer and more recently Covid-19. The interest in vitamin D is growing, so it’s time for us to understand all this excitement a little bit better.
It turned out that I had an opportunity to test my vit D level along with other vitamins and minerals during my last visit to the GP. Having spent all spring and summer in the Mediterranean island of Corsica, mostly outdoors, I expected my vit D level to be in the higher end of the range. To my big surprise, it was not. I was close to the lower end of what is considered to be an adequate range.
This finding triggered a lot of questions and I decided to dive into the research. I realised that for such a trivial question as optimal levels of Vitamin D there is no consensus among members of the medical research community. “The optimal serum 25- hydroxyvitamin D(25(OH)D) concentration for skeletal and extraskeletal health is controversial. Some experts favor maintaining the serum concentration between 20 and 40 ng/mL (50 to 100 nmol/L), whereas other experts favor maintaining 25(OH)D levels between 30 and 50 ng/mL (75 to 125 nmol/L).”
As of now, we don’t have an accepted international standard defining deficient and sufficient vitamin D status, and there is an ongoing international discussion regarding which cut-off values should be used.
In Europe, there is a general agreement that 25(OH)D concentration below 50 nmol/L is considered insufficient. But insufficient for what? That’s another topic of discussion. Typically, the concern is about sufficiency for bone health, however, we also start to find out that vit D is not just about bone health, it also plays a role in our immune system.
As an aside, if you ever test your serum vitamin D, pay attention to the units of measurement before panicking! I noticed that people very often confuse nmol/L with ng/mL, wrongly self-diagnosing severe deficiencies.
So, here is a list of interesting facts I personally found while trying to figure out If I should really worry about my “controversial” level of vid D.
The most exciting of all findings was genetic predisposition. We know that factors affecting circulating 25(OH)D level include sunshine exposure (around 90%) and dietary intake (10%). But research suggests that genetic factors also influence blood 25(OH)D status. Recently an increasing number of candidate genes studies have identified vitamin D modulating genes that are associated with vitamin D status. It means that if vitamin D status is associated with certain genotypes or SNPs, then some people may need a higher or lower level of serum 25(OH)D than general level to minimise health risk.
To become biologically active, vitamin D (received from sun and food) undergoes a series of conversions in the liver and kidneys. The conversion seems to be dependent on the vitamin D concentration and regulated by out endocrine system, based on calcium and phosphate concentrations. To simplify, if you are low in calcium and or phosphate, your body will upregulate the conversion of inactive to active form of vit D. Why? To increase the absorption of these low than optimal amounts of calcium and phosphate in guts (think bone health!). Inversely, to prevent excessive vit D signaling in target organs, our body will transform vit D to biologically inactive water-soluble metabolites which are eventually excreted in the bile (and in stool).
Recently it’s been suggested that gut microbiota is involved in modulating the conversion from inactive to active form of vitamin D. It seems that people with healthy guts are more efficient in this process. I find it fascinating but also logical. We know that there are a lot of vit D receptors in our guts, so it’s plausible to assume that certain gut bacteria might be involved in modulating gene expression of these receptors. For example, several human studies have manipulated the gut microbiome by introducing lactobacillus and have reported increased vit D receptors. It means that probiotic supplements may affect circulating vitamin D levels!!
Vitamin D can be stored in our adipose tissues (our fat cells), but the extent of accumulation and mobilisation during periods of shortage (winter time) of vitamin D is unknown. Perhaps the human body is tightly regulating (rationing) the release of vit D without necessarily creating high circulating levels, especially during winter time.
Interestingly, there is an inverse association between adiposity and blood level of vit D. Obese and overweight people seem to have lower concentration of vit D. One proposed explanation is a decreased availability of stored vit D in body fat. Basically, in obese people, body fat act as a reservoir for storage of fat-soluble vit D but doesn’t properly release it from the reservoir when required. Another explanation is a low time spent outdoors because of lower mobility. It’s also possible that there an endocrine dysregulation in obese/overweight is compromising bioavailability of vid D. And finally, obese people usually have dysbiosis.
On a more technical side, one significant problem of vitamin D research is the analytical method or how it’s being measured. Basically, 25-hydroxy-vitamin D is currently the only preferred form measured by the majority of research. It seems to correlate with overall vitamin D storage. However, there are many other forms of Vitamin D in blood, the activity of which is not well studied, perhaps, due to lack of technology and reliable methods to measure them. It’s been suggested that each member of the family of vitamin D compounds may have a different function.
Humans respond differently to UV radiation and a number of factors (duration of exposure, parts of exposed skins, use of certain medication, sun creams etc.) affect skin synthesis of vitamin D3.
It is controversial whether skin vitamin D synthesis is more efficient in individuals with pale skin compared to individuals with dark skin. However, it is believed to be an evolutionary adaptation resulting from migration to more northern and less sunny climates. Interestingly, countries like Sweden, Norway or Denmark have a significantly higher proportion of foods fortified in vitamin D compared to other countries. And it’s totally understandable.
Unfortunately, our ability to synthetise vitamin D is decreasing with age. That’s why recommendations for elderly people for vitamin D level are higher.
Dose response of vit D supplementation varies greatly from one individual to another. Here, we might have important genetic factors or/and even gut microbiota relationship.
I apologise if some parts of this post were a bit complicated, but it also shows how complicated our bodies are.
One particular accent I wanted to make is on genetics. It seems that genetic predisposition may really have a large impact on 25(OH)D concentration. Based on available research, I selected most cited significantly important variations in certain genes and compared to my own results. To my relief, I couldn’t find any risk allele in my case. Obviously, there is more than 1 gene involved in this, and we are at an early stage of understanding genetic importance in this topic. Needless to say, if you only test 1 SNP (as per majority commercially genetic testing for the general population), it’s relatively pointless. The gap is actually quite big between what’s being currently investigated in research and what’s commercially available. Be careful with interpretation of your genetic results. But again, it’s a different topic of conversation, and i’m really passionate about it.
So, my answer to whether I should worry about my vit D level– “it depends”. Although I’m not in any risk category (age, certain diseases, medications, ethnicity, anthropometric factors, pregnancy etc., which would require a different strategy), I don’t have genetic predisposition and I don’t live in northern part of Europe, however, there are other factors such as optimum gut health, or the amount of calcium/phosphorus in the diet that are also very important for health implication.
As a nutritional practitioner, it’s very important for me to get evidence-based material before making any recommendations. The more I learn, the more I understand that everything in nutrition has a high individual variability, and there is “no one size fits all” approach whether it’s for different diets, interpretation of “healthy range” biomarkers or supplements.