Although the name suggests otherwise, vitamin D isn’t actually a vitamin. Vitamins are micronutrients that we must get from our diet because our body can’t create them. But we can create vitamin D, under the right conditions (more on that later).
Unfortunately, most of us are deficient in this incredibly important prohormone and that is having major consequences throughout our bodies.
So, let’s talk about what this so-called vitamin does for us and how we can be sure we have enough of it floating around in our blood…
Functions in the body
Although it is now widely understood that vitamin D is necessary for bone health, its bodily impact reaches much further, from our genetics to our immune system.
When we ingest or create vitamin D, it goes through a two-stage activation process that occurs in the liver as it becomes calcidiol and then in the kidneys (primarily, although there are many cells that can do the final step of activation including the muscles, pancreas, brain and immune cells). The active form is called calcitriol which has hormone-like activity throughout the body and is why vitamin D is considered a prohormone (or committed precursor to a hormone).
Once in its active state, vitamin D directly or indirectly influences up to 5% of our genes. In addition to being able to change which genes are turned on and which are turned off, vitamin D receptors are found in the membranes of many cells, including those in the brain, gonads, skin, vascular smooth muscle, and immune system. This means that vitamin D can trigger messengers that cause a reaction within the cell without genetic involvement.
Although the research is on-going, sufficient levels of vitamin D appear to keep us healthy in (at least) the following ways:
- Bones: increases the absorption of calcium and phosphorus from the intestine and prevents the kidneys from excreting calcium and phosphate (both of which are needed for bone health)
- Cancer: suppresses cancer cells’ ability to continue growing and prevents them from turning off their own suicide function (apoptosis)
- Diabetes: triggers the cells to create insulin receptors and the pancreas to release insulin
- Cardiovascular disease: reduces hypertension by decreasing the levels of parathyroid hormone and renin (two hormones related to increased blood pressure)
- Muscular function: increases the uptake and use of calcium for effective muscle contractions
- Neurodegenerative disorders (including Parkinson’s disease, Alzheimer’s disease and depression): prevents the loss of neurons that produce dopamine (a neurotransmitter that helps control the brain’s reward and pleasure centers)
- Immune system: aids in the development, maturation and/or activation of almost all immune system cells which prevents unnecessary inflammation while also promoting effective self-protection, plus…
- Autoimmune diseases: suppresses the autoimmune response
Although the mechanisms are not as well explained, vitamin D also appears to:
- Support liver regeneration related to chronic and alcoholic liver disease,
- Reduce the risk of death for those with chronic kidney disease (whether on dialysis or not), and
- Decrease the incidence of respiratory conditions such as COPD and asthma likely due to its ability to control inflammation
Bottom line: sufficient amounts of vitamin D are necessary for every system in the body to function optimally, either due to direct interaction with vitamin D or its ability to prevent inflammation.
Causes of deficiency
Most people living in industrialized countries are deficient in vitamin D. This is for three main reasons:
- Our sun exposure is limited and any we do get is under the shroud of sun screen, hats and protective clothing.
- Few foods contain vitamin D naturally and our consumption of vitamin D-fortified foods is inadequate to make up for this plus our limited sun exposure.
- Because it is a fat-soluble vitamin, body fat sequesters vitamin D… so the more body fat a person has, the more of their vitamin D is stored in fat rather than being available for use.
There are also several factors that are not in our control that can increase our need for vitamin D:
- As we get older, our body’s ability to create vitamin D decreases and we become even more dependent on outside sources such as food and supplements. And people with dark skin do not produce as much vitamin D because the melanin in their skin acts as a natural sunscreen.
- Some medical related conditions can cause low vitamin D such as those that produce granulomas or cause malabsorption, taking certain medications (anticonvulsants, glucocoritcoids, antifungals and highly active antiretroviral therapy), and possessing some rare genetic mutations related to the activation or use of vitamin D.
- Because the body can manufacture vitamin D from cholesterol, significantly low levels of serum cholesterol can result in too little vitamin D. (Although this is more theoretical than proven, given that we all seem to have plenty of cholesterol floating around.)
- Some people have one or more genetic SNPs (single-nucleotide polymorphisms) that make it difficult to produce and/or use active forms of vitamin D. These SNPs include DHCR7 (the gene responsible for converting 7-dihydrocholesterol to cholecalciferol in the skin), GC (the gene that encodes for the protein that binds vitamin D making it unavailable for cells), CYPR1 (the enzyme that creates calcidiol), and the VDR genes Bsml, Fokl and Taq1 (the receptors on cells that help carry vitamin D into the cell).
Symptoms of deficiency
Unfortunately, the symptoms of hypovitaminosis D are not unique to vitamin D, but can manifest in any of the conditions or symptoms mentioned above. From easily catching every cold that comes along to osteoporosis, too little vitamin D could be the cause, or at least a contributing factor.
As well, since most of us have at least insufficient (if not deficient) levels of vitamin D, it’s pretty safe to assume that you need more vitamin D even if you have no overt symptoms.
Causes of toxicity
Calcitriol is a self-limiting entity, meaning that the more of it there is in the blood, the less the kidneys and other tissues will create. As well, the body has the ability to completely inactivate calcidiol and calcitriol, and then excrete it from the body.
As a result of these built-in control mechanisms and the fact that most of us don’t get enough vitamin D to start with, toxicity is rare. That being said, the following can result in increased levels of serum vitamin D:
- Administration of hormones such as estrogen and testosterone, and
- A variety of rare genetic mutations related to the activation or use of vitamin D.
Symptoms of toxicity
Although hypervitaminosis D is rare, it can occur. The symptom that tends to garner the most concern, as it is the most common, is hypercalcemia (too much calcium in the blood) which can lead to overcalcification of bone, soft tissues, the heart, blood vessels and kidneys. (Although hypercalcemia can be related to other micronutrient imbalances as well, so it’s worth reading my post about calcium.)
Other symptoms associated with vitamin D toxicity are lack of appetite, dry mouth, a metallic taste, nausea, vomiting, constipation and diarrhea. With long-term over-supplementation, additional symptoms may include excessive thirst, depression, headache, drowsiness and weakness.
Finally, elevated vitamin D levels may cause abnormally elevated blood levels of blood urea nitrogen (BUN), creatinine, aspartate aminotransferase (AST), alanine aminotransferase (ALT) and cholesterol, and elevated urinary levels of calcium, phosphorus and albumin. But if your blood is being tested for all of those things, it’s probably being tested for vitamin D levels as well.
Note: Although the Institute of Medicine says that a blood level of 125 nmol/L puts a person at risk for hypercalcemia, a meta-analysis of 28 trials showed that the serum calcium level did not increase even when calcidiol was higher than 600 nmol/L, unless other contributing factors (such as Williams syndrome) were present.
Body levels of vitamin D are easily tested with a simple blood test that your doctor can perform. Most blood tests will measure your calcidiol level (as opposed to the fully inactive vitamin D or the fully active calcitriol). Although measuring the other forms of the hormone can be important, it is rarely done unless a specific problem is suspected in the activation pathways.
Please note that although most labs indicate a level of 30 nmol/L (or ng/mL) is sufficient, many experts indicate that optimal levels are 75 – 110 nmol/L.
Sources of Vitamin D
Sunlight is the most efficient source of vitamin D. An adult wearing a bathing suit with enough sun exposure to cause a slight pinkness to the skin gets the same amount of vitamin D as does one who ingests 20,000 IU. In addition, vitamin D produced by the skin seems to stay in the body longer than does vitamin D from dietary sources or supplements. As such, responsible sun exposure is the best way to ensure you maintain sufficient levels.
Of course, whether or not the sun is at the right angle for your skin to produce vitamin D is a big factor in its efficacy. For example, people at 52˚N latitude get about one-half of the annual UVB rays as do people at 40˚N latitude. For people above 33˚N latitude and below 33˚S latitude, the angle of the sun is too low during the winter months to produce any vitamin D in the skin, regardless of duration of exposure. So, you must keep this in mind as you consider whether or not your sun exposure is sufficient.
The next source is food. The few foods that are naturally high in vitamin D include cod liver oil, maitake mushrooms, swordfish, salmon, tuna and whole eggs. Foods that are fortified with vitamin D include orange juice, milk, yogurt, margarine and cereals.
Finally, if you are unable to get enough vitamin D from the sun or from food, you can take a supplement. I recommend 5,000 IU per day until your blood levels reach at least 45 nmol/L. Then you can take 2,000 IU per day to maintain sufficient levels. (By the way, this is one of the supplements that I take every day because I live in New York so I get limited sun exposure for most of the year, I don’t eat enough of the foods that have vitamin D in them , and I have several of the genetic SNPs mentioned above.)
Of course, as with all supplements, please consult with your healthcare provider before beginning or increasing your dosage.
— Baeke F, Gysemans C, Korf H, Mathieu C. Vitamin D insufficiency: implications for the immune system. Pediatr Nephrol. 2010;25(9):1597-1606.
— Cantorna MT, Zhao J, Yang L. Symposium 3: vitamin D and immune function: from pregnancy to adolescence vitamin D, invariant natural killer T-cells and experimental autoimmune disease. Proc Nutr Soc. 2012;71(1):62-66.
— Dunne S, Bell JA. Vitamin D’s role in H\health — deterministic or indeterminate? Today’s Dietitian. 2014; 16(7): 48. http://www.todaysdietitian.com/newarchives/070114p48.shtml
— Feldman D, Pike JW, Adams JS, eds. Vitamin D: Two-Volume Set. Waltham, MA: Academic Press, 2011.
— Han YP, Kong M, Zheng S, Ren Y, Zhu L, Shi H, Duan Z. Vitamin D in liver diseases: from mechanisms to clinical trials. J Gastroenterol Hepatol. 2013; 28(S1): 49-55.
— Herr C, Greulich T, Koczulla RA, Meyer S, Zakharkina T, Branscheidt M, Eschmann R, Bals R. The role of vitamin D in pulmonary disease: COPD, asthma, infection, and cancer. Respir Res. 2011; 12(1): 31.
— Zheng Z, Shi H, Jia J, Li D, Lin S. Vitamin D supplementation and mortality risk in chronic kidney disease: a meta-analysis of 20 observational studies. BMC Nephrology. 2013; 14(1): 199.