Vitamin D and its Enormous Importance for Our Health

Vitamin D is an essential nutrient for the human body.

Unfortunately, vitamin D deficiency is highly prevalent with roughly 50-60% of US adults being deficient.

These numbers increase in darker complexion populations with 70% of Hispanic Americans and almost 80% of African Americans being deficient.    

What is it?

Vitamin D is a group of fat-soluble steroid hormones, meaning not only does it function as a vitamin but also as a steroid hormone in numerous body processes. 

Where do we get it from?

Absorption and production of vitamin D through the skin, from exposure to UVB light, is the key source (roughly 90%) of vitamin D for most people.  Absorption peaks when the sun is at its peak midday.   

The UVB rays from the sun interact with a cholesterol compound known as 7-dehydrocholesterol found in the epidermis converting it to vitamin D3 (cholecalciferol).  Vitamin D3 is then further converted in the liver to 25-hydroxyvitamin D (25(OH)D) and then finally in the kidney to 1,25-dihydroxycholecalciferol or calcitriol.  Calcitriol is the active form of vitamin D. 

A relatively small number of food sources contain vitamin D naturally.  These include wild-caught fatty fish (in particular salmon), cod liver oil, mushrooms grown outdoors in sunlight, and eggs from pasture-raised chickens.  Vitamin D fortified foods include milk (dairy and non-dairy options), yogurt, cheese, butter, orange juice, breads, and breakfast cereals.  Unfortunately, a few of these options can be highly processed or high in added sugars making their consumption less appealing. 

Vitamin D found in animal sources is vitamin D3 while that found in plant sources and fortified foods is vitamin D2.  Vitamin D supplements are either made from lanolin, mushrooms, fish oil, or lichens.  Lichens are a unique combination of fungi and algae and are the only vegan source of vitamin D3.

How much do we need?

It is still controversial how much vitamin D is needed, how it should be given, and what level of serum vitamin D is optimal for immune health and overall health benefits.  Serum concentration of 25(OH)D is the best indicator of vitamin D status.  It reflects vitamin D produced cutaneously and that obtained from food or supplementation.  Serum 25(OH)D levels do not indicate the amount of vitamin D stored in body tissues.  The Institute of Medicine defines deficiency as serum 25(OH)D levels less than 30 nmol/L.  Many believe that ideal levels should be in the range of 50-100 nmol/L.  The Endocrine Society argues that desirable serum concentrations should be greater than 75 nmol/L.

What are the risk factors for deficiency?

Cutaneous production of vitamin D is hindered by many factors.  These include skin tone, higher latitude, time of year, sunscreen usage, and pollution.  Melanin in skin blocks UVB and vitamin D absorption, placing individuals with darker skin complexion at higher risk of vitamin D deficiency.  Less UVB is available in latitudes above the 37th parallel and winter months where days are shorter.

Other risk factors for vitamin D deficiency include older age, malabsorptive syndromes (such as celiac, Crohn’s, cystic fibrosis, surgical alteration of the GI tract), immobilization, obesity, smoking, medications that alter absorption or metabolism (steroids, anti-fungals, anti-epileptics, thiazide diuretics), and chronic kidney or liver disease. 

Higher rates of vitamin D deficiency are found in individuals who are clinically obese.  The complex relationship between body fat and vitamin D makes it difficult to distinguish whether vitamin D deficiency is a consequence or cause of obesity.  However, what is known is that vitamin D3 is stored in adipose tissue.  Excessive body fat appears to hold onto vitamin D, not allowing its use in the rest of the body, resulting in vitamin D deficiency.  Vitamin D deficiency also hinders weight loss.

There is also a broad inter-individual difference in gene expression in response to vitamin D supplementation.  This is likely due to genetic variation.  Certain genetic polymorphisms of the vitamin D receptor are associated with increased risk of cardiovascular disease, cancer, and autoimmune disorders.  Genetic differences in the vitamin D binding protein have also been shown to influence the ratio between circulating total and free vitamin D.

Why is it important?

Vitamin D is essential for bone health. To absorb calcium, your body requires vitamin D. In turn, vitamin D taken with vitamin K2 helps ensure that calcium is transported to the bones where it is needed, rather than accumulating in deposits in the arteries.   

Vitamin D is necessary to maintain adequate phosphate and calcium levels in the blood, thereby helping to develop normal bone, optimal maintenance of muscle contractions, and cellular functions in different parts of the body.   

Vitamin D deficiency in children is a disease called rickets which is characterized by bowing of the legs due to improper bone formation while the growth plates are open. 

Once the growth plates are closed, defective mineralization of existing bone due to vitamin D deficiency is termed osteomalacia. 

Rickets and osteomalacia are caused by vitamin D and/or calcium deficiency.  Low consumption of either calcium or vitamin D causes bone resorption in the attempt to normalize serum calcium levels.  If chronic, this leads to bone structural damage, loss of bone matrix, and ultimately, osteoporosis.  Osteoporosis increases bone fragility and fracture risk.  One in every four men and one in every two women will suffer a fracture from osteoporosis in their lifetime.      

Vitamin D influences the expression of several hundred genes, resulting in its involvement in several body processes, ranging from immunomodulation, sexual function, as well as cell repair or death if damage is too extensive.  Vitamin D receptors are found on nearly all cells of the human body.  Binding of vitamin D to its receptor causes up or down-regulation of many genes. 

Vitamin D’s role in the immune system is reflected in its strong association of deficiency with several autoimmune disorders including psoriasis, type I diabetes, multiple sclerosis, and rheumatoid arthritis.  Poor vitamin D status also correlates with greater susceptibility to viral and bacterial infections such as respiratory infections (including COVID-19 and pneumonia), dental cavities, and sepsis.  Most evidence, to date, suggests that maintenance of a healthy vitamin D status is important for modulating the body’s immune function.

Vitamin D plays a role in maintaining gut integrity and intestinal homeostasis between host and gut microbiota. Vitamin D signaling is shown to increase the viability of intestinal epithelial cells and alleviate intestinal epithelial damage from bacterial lipopolysaccharide.  It also promotes mucosal barrier function.  Moreover, vitamin D induces the production and secretion of antimicrobial peptides by intestinal cells decreasing the risk of bacterial transit across the gut membrane, which is believed to be involved in the pathogenesis of multiple autoinflammatory and metabolic disorders.

It is well recognized that individuals with inflammatory bowel diseases (IBD) are more prone to vitamin D deficiency due to inefficient gut absorption.  Emerging evidence, however, also suggests that the relationship could be bidirectional.  Higher serum levels of vitamin D are associated with reduced risk of Crohn’s disease and ulcerative colitis.

Vitamin D deficiency is a highly prevalent comorbidity in coronary artery disease, heart failure, and atrial fibrillation, the three most frequent cardiovascular diseases worldwide.  It is also linked with worse short- and long-term prognosis.  Vitamin D is involved in heart muscle contractility, endothelial function, and vascular permeability.  It also helps decrease blood pressure, oxidative stress, and inflammation.

The role of vitamin D in reducing the risk of cancer incidence and death has been studied for years.  What has emerged is that vitamin D serum concentrations correlate with the incidence, progression, and death for many types of cancer. Vitamin D may decrease tumor invasiveness and propensity to metastasize, leading to reduced cancer mortality.  There is strong evidence that vitamin D plays a role in the prevention of colon, prostate, and breast cancer.

Vitamin D is important for normal brain development and function in humans, as deficiency can affect cognition.  Neurodevelopmental disorders associated (or possibly induced by) vitamin D deficiency include attention deficit hyperactivity disorder, autism spectrum disorder, and schizophrenia. Vitamin D deficiency has also frequently been associated with depression, dementia of aging, and neurodegenerative diseases such as Alzheimer’s and Parkinson’s disease.  Vitamin D likely exerts its effect on neurocognition through several mechanisms such as induction of neuroprotection, modulation of oxidative stress, regulation of calcium homeostasis, and inhibition of inflammatory processes.  It has also been linked with the dysregulation of dopamine and serotonin neurotransmission.

The relationship between vitamin D and insulin resistance has been a topic of growing interest. Low vitamin D levels appear to be associated with most insulin resistance disorders (including metabolic syndrome, pre-diabetes, and type 2 diabetes) and might be one of the factors accelerating the development of insulin resistance.  Vitamin D appears to play an important role in the regulation of pancreatic insulin secreting beta cells and potentially improving insulin sensitivity.

Both male and female vitamin D levels play a role in fertility and affect outcomes of in vitro fertilization.  Deficiency is also linked with polycystic ovary syndrome and correlates with severity of disease.  Higher levels of vitamin D during pregnancy decreases the risk of preeclampsia, premature birth, and bacterial vaginosis.

Beyond its role in calcium absorption and bone formation, vitamin D appears to have a significant impact on sports performance.  As a steroid hormone, vitamin D alters gene expression resulting in an effect on muscle size, strength, reaction time, endurance, inflammation, and balance coordination.  Studies have shown positive effects on muscle strength, power, and mass; increased force and power output of skeletal muscle tissue; maximization of oxygen uptake; increased size and number of type II muscle fibers; decreased recovery time; and increased testosterone production.

Vitamin D deficiency is linked to chronic pain.  This includes inflammatory joint disease, connective tissue disorders, musculoskeletal, soft tissue rheumatism, osteoarthritis, and nonspecific musculoskeletal back pain.  Research suggests that vitamin D may play a vital role in various cellular activities thought to be protective against chronic pain development and modulation. Briefly, vitamin D acts as a neuroactive steroid, interferes with the creation and role of neurotrophins, influences prostaglandin action, effects inflammatory pathways, and inhibits nitric oxide synthase and T-helper cells.

Due to its enormous importance in our health, if you haven’t already, speak with your healthcare provider about whether you should have your serum vitamin D level checked.