All About Vitamin D
All About Vitamin D
Created On
Nov 14 2017
Last Updated
Jun 08 2022

Short Summary: Vitamins, by definition, are not produced by the body. Vitamin D is in fact a hormone produced by UVB rays from the cholesterol in skin.

Most of Vitamin D is stored in liver before it reach the blood through kidneys. It is important for bone health, and absorption of calcium and phosphorus. Recent research shows several other key roles, including in cardiovascular diseases and for fighting over 15 different kind of cancers.

Despite plentiful sunlight, a large population is deficient as people proactively avoid sun.

Large parts of North America (e.g., regions north of Phoenix, AZ or Bakersfield, CA) doesn’t receive the UVB rays from Nov to Mar—as they are blocked by the ozone layer in atmosphere.

Medical professionals recommend maintaining levels of at least 20-30 ng/ml. Fortified foods or supplements are popular methods to make up for any deficiency. An easy and simple at-home Vitamin D test kit can easily check your levels.

Vitamin D toxicity is extremely rare.

Ranges for Vitamin D levels:

  • Deficient: 10-20 ng/ml
  • Insufficient: 20-30 ng/ml
  • Normal: > 30 ng/ml — recommended to maintain 30-40 ng/ml; medical professionals recommend to err on the higher side
  • Overdose: 40-70 ng/ml
  • Toxic: > 150 ng/ml
Note: Measurement units for blood test are: ng/ml (where 1 ng/ml = 2.5 nmol/l)


Source: Evaluation, Treatment, and Prevention of Vitamin D Deficiency: an Endocrine Society Clinical Practice Guideline by Holick et. al. in The Journal of Clinical Endocrinology & Metabolism, Vol 96 (7), 2011, Pages 1911–1930.

What is Vitamin D?

Vitamins, by definition, are compounds that our body does not produce. However, Vitamin D is unique.

UVB rays (290-310 nm wavelengths) in sunlight convert a form of cholesterol in the skin into a form that our body utilizes.

Once ready, it stores in the liver–almost like water in a camel’s hump–until the time it releases into the kidneys. This release gives the final active form that circulates in our blood.

It is crucial for absorption of calcium and phosphorus–minerals essential for maintaining healthy bones, especially in children and elderly.

In recent years, research has shown that Vitamin D is essential for fighting over 15 different kind of cancers.



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Despite abundant sunlight, a large population is Vitamin D deficient. That is because people proactively try to avoid exposure to sun.

Another problem is that a large part of North America (regions north of Phoenix, AZ & Bakersfield, CA & Charlotte, NC) doesn’t receive the UVB rays for almost 6 months a year. In winter months from Nov to Mar as these rays are blocked by the ozone layer.

Fortified foods and supplements are therefore popular sources to make up for any deficiency.

Medical professionals recommend maintaining levels of at least 20-30 nL/ml, which can be tested with a simple blood test using an at home Vitamin D test kit.

Toxicity is extremely rare but those taking the supplements should monitor their levels regularly.

What are the different types of Vitamin D?

  • D3 (chole-calciferol) – this form is naturally produced by the body. It is rarely toxic as higher levels (>20 kIU) are removed by the body. Cholecalciferol is the most common type available in supplements.
  • D2 (ergo-calciferol) – this form is produced in plants and fungi but not in humans. Most prescriptions contain D2. However, ergocalciferol is half as effective as D3.

What are some of the most interesting properties of Vitamin D?

  • It is not really a vitamin but a hormone (by definition Vitamins are essential but not produced by the body; Vitamin D surely is!).
  • It is fat soluble: overweight & obesity will prevent release into body.
  • It is rarely toxic with high dose (does more sunlight cause toxicity?).

What does Vitamin D do?

Vit D is necessary for absorption of calcium and phosphorus in the body. Deficiency can have serious health impact in:

  • Children: Rickets (weak, improper formation of bones).
  • Adults: Osteomalacia (low bone density, unhealthy bone repair, maintenance due to improper Ca, P absorption).
  • Any age: Crucial for preventing and reducing multiple cancers (including cancers of breast, lung, colon, prostate). Also, critical for cardiovascular health, bone fracture & fall, diabetes, stress & fatigue, and other common diseases.

Source: Vitamin D Supplementation, Glycemic Control, and Insulin Resistance in Prediabetics: A Meta-Analysis by Mirhosseini et. al. inJournal of the Endocrine Society, Vol 2 (7), 2018, Pages 687–709.

Other key roles and functions of Vit D:

  • Our bodies have 26 Vitamin D responsive tissues that require adequate levels to activate their functions, which affect over 200 genes in the body.
  • Excess 25-D can be metabolized to 1,25-D3 in other cells and tissues besides kidneys.

How does Vitamin D metabolism happen in the body?

The four key steps that start with sunlight to the final production of Vit D in our body are:

  • First, skin exposure to sun (i.e., UVB rays) activates a type of cholesterol, 7-dehydrocholesterol (7-D).
  • Second, 7-D becomes pre-Vitamin D & immediately becomes Vitamin D3.
  • Third, Vitamin D3 becomes 25-D in liver through hydroxylation and stores in liver (like a camel’s hump storing water).
  • Finally, 25-D is releases, as necessary, to kidneys where it becomes an active and potent form (1,25-D3). This form circulates in the body and maintains the amount of calcium. 1,25-D3 is the right form of Vitamin D to monitor as multiple cells and tissues have receptors to convert 25-D to 1,25-D3 making it critical in other parts of body.

Metabolism happens through regular method and in the cells:

  • Regular metabolism:Liver(25-D) –> into bloodstream –> kidneys(1,25-D3: active, vital form) –> into bloodstream –> intestinal cells (absorption of Ca, P –> healthy bones)
  • Autocrine cell functions (metabolism in the tissues and cells, a role discovered based on research in past 20-25 years): Liver(25-D) –> most organs of body with Vitamin D receptors(1,25 D3: active, vital form) –> utilized by cells & tissues (essential for preventing cancers & other diseases)

Source: Vitamin D Supplementation, Glycemic Control, and Insulin Resistance in Prediabetics: A Meta-Analysis by Mirhosseini et. al. in Journal of the Endocrine Society, Vol 2 (7), 2018, Pages 687–709.

What prevents people from soaking the sun to be healthy?

  • Fear of skin-cancer
  • Aging
  • Wrinkled skin
  • Discoloration of skin

How can I increase my Vitamin D levels? What are the sources of Vitamin D?

The sources of Vitamin D include

  • Sunlight – the simplest source (however, only UVB: 290nm – 315nm wavelength, deep violet)
  • Fish liver oil (cod-liver, salmon)
  • Supplements
  • Tanning beds
  • Fortified food (milk, orange juice, cereals, margarine, etc.)
  • Most food items don’t have Vitamin D or barely have any

Sun as a source of Vitamin D

  • UVB rays [very narrow band of 290-315nm wavelength; visible light is 360 nm(violet) -780 nm(red)] are main source that activate Vitamin D; but can’t penetrate as deep; even window glass will block it.
  • UVA rays (>315nm wavelength) penetrate deeper but do not prompt skin to make Vitamin D; cause wrinkles, aging, skin discoloration.

How long do you need to be in the sun to get vitamin D?

  • Find the time it takes for the skin to turn pink (depends on skin tone, time of day, season, etc.).
  • 25-50% of this time will give 1,000 IU.
  • Try few times a week in summer to get sufficient dose of UVB and let it store in liver.

What is the unit of measure for Vitamin D?

  • In food & other sources: IU (International Unit); 1000 IU = 25 microgram of Vitamin D (for other Vitamins, microgram/IU can be different). This is the unit you see on Vitamin D supplements. A 2,000 daily dose is basically 50 microgram.
  • When measured in the blood: ng/ml (1 ng/ml = 2.5 nmol/l). This is what you will see when tested using an at-home Vitamin D test kit

How much Vitamin D do you need?

Recommended daily allowance (RDA) as per the CDC direction are:

  • 50 years or younger: 200 IU per day.
  • Over 50 years: 400 IU per day.
  • Upper safe limit: 2000 IU/day** Most medical professional recommend 5-10 times higher dose and are asking government to revisit these limits setup in 1997 (note 30 minutes of full Sun exposure gives as much as 20,000 IU!!).

How do I increase my Vitamin D level from each source?

  • From Sun: Standing in sun for 12 minutes at noon with bare arms & legs for a white woman, is roughly equal to 3000 IU. For 30 minutes, is equivalent to 20,000 IU. People with darker skin will need more since the melanin in skin will absorb some of the sun.
  • From Cod-liver: best natural food source at 1360 IU per tablespoon. However, one should watch out for excess Vitamin A in it which can be toxic!
  • Fortified food: 85% of winter supply
  • - US: milk, cereals, orange juice, margarine.

    - Canada: only milk and margarine.

    - Even fewer options for those who are lactose intolerant or vegetarian or vegan.

What foods are high in Vitamin D?

Most common sources of Vitamin D with approximate amounts include:

  • Cod-liver oil: 1360 IU in 1 tablespoon
  • Salmon: 360 IU (3.5 oz, cooked)
  • Mackerel: 345 IU (3.5 oz, cooked)
  • Sardines: 250 IU (1.75 oz, canned in oil)
  • Tuna fish: 200 IU (3 oz, cooked)
  • Milk: 98 IU (fortified, 1 cup; you need 10 cups for 1,000 IU!!)
  • Margarine: 60 IU (1 tablespoon, fortified)
  • Egg: 20 IU (one egg with yolk)
  • Beef: 15 IU (3.5 oz)
  • Cheese: 12 IU (1 oz)
  • Tanning beds: helpful but are known to cause skin cancers & generally not recommended. Otherwise, the same exposure advise as for Sun: 25-25% of time it takes to turn your skin pink.
  • Multivitamins: Okay to use but note overdose of other vitamins can be toxic (e.g., Vitamin A is known toxic).

What can cause low Vitamin D levels?


  • Latitude – i.e., how far from equator. Larger distance means UVB rays go through a longer ozone layer and get absorbed more. Beyond 35 degrees latitude north or south, almost no UVB reach the surface from Nov to March (regions north of Flagstaff AZ, Albuquerque NM, Knoxville TN, Oklahoma city OK, Charlotte NC). For reference, Canadian border is at 49th parallel and Portland OR is near 45th parallel.
  • Altitude – i.e., height above see level. Lower altitudes absorb more UVB and make less of it. People in mountains get more UVB.
  • Season of the year – Winter months get less sun.
  • Time of the day – need UV Index of 3 or more; 10 am – 2 pm are best times.
  • Air pollution – particles absorb UVB and reduce the production.
  • Cloud cover blocks the sun; the US Pacific Northwest region with long rainy periods will get less sun and thus lower production in skin.


  • Sunscreen/sunblock – absorb the sunlight and hence prevent the body in making it. Higher SPF means higher sun block (e.g., SPF8 blocks 92% skin’s production; SPF15 blocks 99%). General advise is to soak in sun for few minutes before applying sunscreen.
  • Melanin – darker skin has higher melanin which absorbs more sunlight and competes with Vitamin D production; African people need almost 10 times more UVB exposure.
  • Age – those at older age have less 7-D in the skin and need almost 25-50% more sunlight.
  • Weight – Vitamin D is fat soluble thus fat cells in skin will dissolve making less of it available for tissues and organs.
  • Clothing – body coverage blocks the sunlight, e.g., Islamic women & men as well as people in cold places with more body coverage will get less sun exposure.

What are the causes of low awareness and deficiency of Vitamin D?

  • People often consider deficiency as a new fad.
  • Several important roles of Vitamin D are discovered only recently in past few decades.
  • Awareness of skin cancer from sunlight is much higher.
  • Perception that inadequate sun exposure only impacts childhood growth.
  • Symptoms are subtle and easy to confuse with other medical conditions, e.g with food sensitivity to something in diet.

What are the causes of low awareness and deficiency of Vitamin D?

  • Muscular weakness (weak muscles)
  • Feeling of heaviness in the legs
  • Chronic musculoskeletal pain (chronic pain in muscles and bones)
  • Fatigue or easy tiring
  • Frequent infections
  • Depression
  • Chronic pains, e.g., back pain, neck pain (defect in bone-hardening with deep, gnawing pain in muscles and bones)

What causes low Vitamin D levels?

  • Low levels in children: due to reduced outdoor activities and sunscreen protection.
  • Pregnant and nursing mothers: due to sharing with baby.
  • Obese and overweight: Vitamin D dissolves in fat and therefore fat cells can absorb and store, making less of it available.
  • Elderly: less Vitamin D precursor in skin (may need up to 4x Sun otherwise can result in weaker bones and fractures).
  • Others: People with Crohn’s disease, cystic fibrosis, gastric-bypass surgery, liver or kidney failure might not be able to absorb fat soluble Vitamin D.

How do you check Vitamin D levels?

The most popular test for Vitamin D is using a simple finger prick blood sample. You can order an at-home Vitamin D test kit that can accurately measure the active form of Vitamin D (1,25-D3) in blood stream. The kit can be ordered online & a blood sample can be provided from home without the need to visit a doctor or any labs. The comprehensive lab report will clearly explain your results.

More comprehensive but expensive test is LC-MS/MS (liquid chromatography tandem mass spectroscopy).

Why should I test for Vitamin D?

  • Despite all the awareness, a significant part of the population is deficient. Without getting tested, it is very difficult to tell the levels. Those working indoors do not get enough exposure to sun and end up being deficient.
  • Symptoms are subtle and can be confused with other medical conditions.
  • It has been shown to play a key role in fighting over 15 different kinds of cancers and new research suggests it is important for other key roles and not just for bone health.
  • Those with darker skin need even more sun since the melanin in skin competes for sunlight. People from Africa, South East Asia, Latin and South America tend to believe deficiency is a problem only for people with fair skin. However, their darker skin color has naturally evolved for high sun exposure and when they live in regions farther away from equator their requirement for sun exposure increases as these regions get less sunlight.
  • During winter, when the sun is not directly above us, sunlight travels a longer path through the atmosphere. Therefore, more of the UVB rays are absorbed by the ozone layer and the remaining sunlight can not produce as much on exposure to skin. In fact, regions north of 35th parallel (north of Phoenix, Bakersfield, Albuquerque, Knoxville, Oklahoma City, Charlotte, Raleigh) get almost no UVB from Nov to Mar.


  • Glass windows, sunscreens block the Vitamin D producing part of the sunlight, filtered light produces almost none of it.
  • Children need calcium and phosphorus for developing their bones require it as key ingredient in absorbing the minerals.
  • Bone density decreases with age, which requires higher levels for maintaining healthy bones.
  • Vitamin D has been shown to reduce fractures among elderly.
  • As fat cells dissolves it, obese and overweight people need more to compensate.
  • Few naturally occurring food items contain Vitamin D. Vegetarian and vegan diets make the choices extremely limited.

It is always a good idea to test your Vitamin D levels before starting any supplements.

What are the annual recommended times for tests?

* November: onset of Vitamin D winter (fall)

* March: end of winter (spring)

How to supplement for Vitamin D?

What physicians normally suggest (although one should confirm with a physician before starting any of these regimens):

  • A 1,000 IU daily dose will increase levels in blood by 10 ng/ml in 3-4 months
  • If levels < 20 ng/ml, then 50 kIU/week for 8 weeks –> then may change to 2,000 IU per day
  • And if levels are 20-30 ng/ml: same as < 20 ng/ml
  • Otherwise, if levels are 30-40 ng/ml: 50 kIU/week for 6 weeks –> then may change to 2,000 IU per day
  • Unborn babies: pre-natal vitamins have 400 IU
  • Breast feeding babies: 400 IU/day (nursing mothers need > 40 ng/ml and may need up to 4,000 IU/day)
  • Younger than 1 year: 400 IU/day
  • 1-2 year old: 1,000-2,000 IU/day (based on weight
  • Over 12 years old: 2,000 IU/day

What are side effects of high Vitamin D levels?

Excess Sun or skin based overdose is naturally protected by the body.

Overdose toxicity of 150 ng/ml can only be achieved by several months of >10,000 IU/day. It is very hard to get this level of overdose.

In reported literature, very few cases of overdose are reported. One case with 150,000 IU/day taken over two years resulted in 500 ng/ml but didn’t have any immediate health conditions after stopping.

What are the overdose symptoms and side effects?

Overdose (>150 ng/ml) results in elevated calcium in blood that can cause:

  • Abdominal pain
  • Constipation
  • Muscle weakness
  • Itching
  • Vomiting
  • Extreme thirst
  • Predisposition to kidney stones and high blood pressure

Who should not take Vitamin D?

High levels are not for:

  • Those with Lymphomas should avoid supplementing it.
  • Patients with granulomatous diseases (Tuberculosis and Sarcoidosis) – keep 20-30 ng/ml.
  • Drugs that lower the levels (drugs for seizures; cimetidine, steroids, certain asthma medicines, and weight loss drugs).
  • It raises Ca level which might increase the risk of kidney stones. Anyone taking the drug hydrochlorothiazide (mild diuretic) for high blood pressure should not supplement as this.

Source: Vitamin D Revolution by Soram Khalsa, M. D.

For more information & references, visit the NIH page on Vitamin D