Everything You Need to Know About Vitamin D

What Is Vitamin D?

Vitamin D is one of the fat-soluble vitamins (the others are A, E, and K) and we can make it ourselves when we spend time outdoors in the sunlight (ideally 15 or more minutes at a time per day), but we can also obtain it from foods that are fortified with vitamin D (such as milk) and nutraceutical or dietary supplements.

Vitamin D as a hormone

Vitamin D acts like a hormone in the human body through its actions of influencing the kidneys and bone tissue. The kidneys convert vitamin D from its inactive form (cholecalciferol) to its active form (calcitriol).

Calcitriol then binds to receptors, which are present on nearly every cell in the body, to tell them to act.

In the gut, calcitriol tells the intestinal cells (enterocytes) to absorb more calcium from dietary intake.

In the bone matrix, it tells skeletal bone cells to deposit more calcium from the blood in the bone matrix to strengthen bone tissue and prevent conditions like osteoporosis.

What other organs and tissues does Vitamin D influence?

Vitamin D also influences the cells that make sex hormones (in the testes in men, ovaries in women, and adrenal glands in both sexes). [source]

Vitamin D plays a role in parathyroid function, as well. When we have adequate levels of vitamin D in circulation, we will usually absorb adequate levels of calcium from our intestinal tract.

When vitamin D levels are low, we are usually absorbing less calcium from the intestinal tract, which can cause a drop in blood calcium.

When blood calcium levels drop, our parathyroid gland will release parathyroid hormone (PTH), which also signals the body to absorb more calcium from the intestinal tract.

PTH also signals the bone matrix to release more calcium from stored bone, which, over time, if vitamin D and calcium levels are not optimized, can lead to osteoporosis.

How does Vitamin D affect mental health?

Vitamin D activates genes that release neurotransmitters like serotonin and dopamine. These neurotransmitters influence mood and brain development.

Studies have shown that individuals with low vitamin D levels are at higher risk for depression. [source, source, source]

This can be related to a condition called seasonal affective disorder (SAD), in which individuals who do not get enough direct sunlight during the winter months have a marked increase in depressive symptoms, partly attributed to lower vitamin D production in their bodies.

Or, the risk for depression can be directly related lower vitamin D levels available in the blood, which affects the genetic expression of TPH1, TPH2, SERT, and MAO-A [source].

How does Vitamin D affect gut health?

Vitamin D acts on the intestinal immune system by influencing immune cell production and differentiation. [source, source] Differentiation is a process that our immune cells undergo when they become active to either fight a foreign antigen (T cells and B cells) or become a ‘peacekeeper’ (T regs) to suppress other immune cells from overreacting [source, source].

image from: The Specific Roles of Vitamins in the Regulation of Immunosurveillance and Maintenance of Immunologic Homeostasis in the Gut – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/Pivotal-roles-of-vitamins-in-the-maintenance-of-immunologic-homeostasis-in-the-gut_fig1_314167396 [accessed 27 Sep, 2019]

Vitamin D has also been shown to reduce intestinal permeability, a condition in which the ‘gates’ between your intestinal cells are left open or leaky, for longer than normal, which allows inflammatory substances from inside the intestinal tract to absorb into your bloodstream, unregulated. [source, source]

What are sources of Vitamin D?

We get vitamin D from two main sources:

Sun exposure and vitamin D

When humans spend time in the sunlight, the UV rays from the sun penetrate through the top layer of our skin and turn inactive vitamin D (pro-vitamin D3 or 7-dehydrocholesterol) into cholecalciferol (pre-vitamin D3).

If you remember from the beginning of the article, the kidneys turn cholecalciferol into calcitrol, which is the active form of vitamin D in our bodies [source].

It typically takes about 15 or more minutes of sun exposure daily to generate adequate amounts of vitamin D for the average adult.

In the winter months, between October and March, if you live in an area north of the equator, you may need significantly longer exposure times to generate the same amount of vitamin D as you would during spring, summer, and fall months when the earth is closer to the sun.

Diet/supplemental sources of vitamin D

For many people, especially during the winter months, but perhaps year round, vitamin D supplementation has become a necessary therapeutic supplement.

Most supplemental forms of vitamin D are vitamin D3, which is the active form your body can use. However, you may find vitamin D2 supplements, which, as long as you do not possess any genes that down-regulate D2 to D3 conversion, may still provide some benefit to raising vitamin D levels in your body.

Vitamin D3 supplements come in doses ranging from 200IU to 10,000 IU for daily use, but can be found in prescription form as high as 50,000 IU as a weekly dose for invidifduals with severe vitamin D deficiency.

The RDA (recommended dietary allowance) or DRI (daily recommended intake) for vitamin D is between 400 IU- 800 IU per day.

Recent studies have shown this intake level is probably not sufficient for most people, due to our increasingly indoor lives and lack of regular sun exposure for most Americans [source].

Vitamin D Deficiency

What causes vitamin D deficiency?

Vitamin D deficiency is a growing health concern. Deficiency of this critical hormone and nutrient is usually a primary deficiency, which means that the deficiency is because of too little intake of the nutrient, and, in this case, also too little sun exposure to create it ourselves.

Another group of people at risk for vitamin D deficiency due to lack of dietary intake or absorption include individuals with celiac disease and Crohn’s disease.

People at risk for vitamin D deficiency through lack of exposure to the sun are those that spend too much time indoors, those that live very far from the equator, those with darker pigmented skin (skin pigment melanin slows down or prevents more of the UV rays from penetrating the outer layers of the skin), and the elderly.

Other individuals with risk for vitamin D deficiency are those with liver disease and kidney disease, both of which impact the conversion of inactive vitamin D to active vitamin D in the body.

Safe intake levels/recommended intake levels for vitamin D

If you plan to take vitamin D supplements, it is important to know that you can take too much vitamin D.

There is an established safe upper intake of between 4,000 IU – 10,000 IU per day for this nutrient, depending on the source making the recommendation.

Too much vitamin D from supplementation can dangerously increase blood levels of calcium.

Because vitamin D is a fat-soluble vitamin, it can be stored in fat cells in the body, and it is also retained for longer than other vitamins that are water-soluble, which are disposed of more easily by the kidneys.

Safe intake levels established by official health organizations place the range for adults between 800 IU and 2000 IU per day.

Newer research has indicated that a more appropriate recommendation may be around 2000 IU per day for adults.

If you have cardiovascular disease, autoimmune disease, neurological or cognitive decline, gastrointestinal inflammation, or other chronic inflammatory conditions, your healthcare provider may recommend up to 10,000IU per day. Always consult with a knowledgeable healthcare provider before taking any supplemental nutrient.

Below is a table showing some of the research that has been conducted on the use of vitamin D in certain clinical diseases or conditions:

Condition or SymptomDoseSource
Cardiovascular disease risk1000 IU/dayLink https://www.ncbi.nlm.nih.gov/pubmed/20194238
Managing parathyroid hormone levels1000 IU/dayhttps://www.ncbi.nlm.nih.gov/pubmed/23151536
Colorectal cancer1000-2000 IU/dayhttps://www.ncbi.nlm.nih.gov/pubmed/17296473
Blood pressure1000 IU/dayhttps://www.ncbi.nlm.nih.gov/pubmed/19587609 , https://www.ncbi.nlm.nih.gov/pubmed/17586404
Risk for bone fractures400+ IU/dayhttps://www.ncbi.nlm.nih.gov/pubmed/19307517
Insulin sensitivity1300 IU/day - 10,000 IU/dayhttps://www.ncbi.nlm.nih.gov/pubmed/12800453 ; https://www.ncbi.nlm.nih.gov/pubmed/22486948 ; www.ncbi.nlm.nih.gov/pubmed/22005267
Systemic lupus erythematosus symptoms2000 IU/dayhttps://www.ncbi.nlm.nih.gov/pubmed/23204220
Reducing triglycerides3300 IU/day (83 mcg/day)https://www.ncbi.nlm.nih.gov/pubmed/23204220
Risk for multiple sclerosis400+ IU/dayhttps://www.ncbi.nlm.nih.gov/pubmed/14718698
Risk for breast cancer2000+ IU/day https://www.ncbi.nlm.nih.gov/pubmed/17368188 ; https://www.ncbi.nlm.nih.gov/pubmed/19470790
Risk for pancreatic cancer600+ IU/dayhttps://www.ncbi.nlm.nih.gov/pubmed/16985031

Laboratory measurements of vitamin D levels and deficiency

The best way to know if you are taking or making enough vitamin D is to have your vitamin D levels tested by a laboratory.

Most laboratories will test for 25-OH vitamin D, or 25 Hydroxy Vitamin D, which is actually vitamin D2, the inactive form of the vitamin.

This may or may not really be the most accurate way of assessing your true vitamin D availability, especially if you do not efficiently convert vitamin D2 to D3, in the case of liver disease, kidney disease, or other nutrient deficiencies.

Some labs will also test for vitamin D3 levels, and one in particular tests for both intra- and extra-cellular levels of vitamin D3, which can tell you both if you have enough active vitamin D3 in your blood, but also if that vitamin D3 is absorbing into your cells adequately.

Measuring both intra- and extracellular vitamin D3, along with the standard 25-OH vitamin D (vitamin D2) provides the most comprehensive picture of true vitamin D status.

Disease-specific considerations for vitamin D

Vitamin D deficiency has been linked to atherosclerosis and cardiovascular disease. [source]. There are also studies that link vitamin D deficiency to:

  • depression
  • rickets
  • hormonal disturbances in men and women
  • osteoporosis
  • parathyroid dysfunction
  • inflammatory bowel disease (IBD)
  • celiac disease
  • joint pain and inflammation
  • autoimmune diseases
  • thyroid abnormalities
  • fatigue
  • muscle aches and weaknees
  • metabolic disorders and type 2 diabetes
  • obesity

If you have any of the above symptoms or conditions, talk to your healthcare provider about testing your vitamin D levels.

Many of those conditions may also increase your need for supplemental vitamin D. In some cases, the condition listed may be the cause of the vitamin D deficiency (such as obesity), and in others it may result from vitamin D deficiency (such as parathyroid dysfunction).

Some of the conditions listed do not yet have a clear cause and effect established between the vitamin D deficiency and the disease or symptom onset.

Medication and nutrient interactions with vitamin D

It is not very common for vitamin D to interfere with a medication or nutrient, but it is more common for a medication to interfere with adequate vitamin D levels.

Medications that can lower Vitamin D levels in the body include:
• epileptic/anticonvulsant medications
• medications that can decrease intestinal absorption of Vitamin D including bile acid sequestrants (cholestyramine), and some laxatives
• agents that cause fat malabsorption such as orlistat, mineral oil, and the fat substitute olestra can affect the absorption of all fat-soluble vitamins
• long term use of glucocorticoids
• HIV treatment drugs
• some chemotherapy medications


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