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The Prediabetes Nutritionist

Vitamin D: What You Need to Know

Here’s what you need to know about the sunshine vitamin, vitamin D.

Vitamin D is a fat-soluble vitamin that functions as a hormone – a compound that is manufactured in one part of the body and travels through the bloodstream to affect another organ or tissue. 

Vitamin D is unique because it is the only vitamin your body can produce upon exposure to ultraviolet light. When ultraviolet B (UVB) rays of the skin hit the skin, they convert the cholesterol precursor of vitamin D (7-dehydrocholesterol) in the skin into an inactive form of vitamin D (cholecalciferol or vitamin D3). 

Vitamin D3 is then converted into 25-hydroxyvitamin D(calcidiol) in the liver and then to 1,25-dihydroxyvitamin D3 (calcitriol) in the kidney. Calcitriol is the active hormone form of vitamin D. 

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Why you need vitamin D

Vitamin D is crucial to maintain the normal range of calcium and phosphorus in the blood. It also helps to deposit calcium and phosphorus in the bones and teeth, keeping them strong and healthy.

Vitamin D regulates cell growth and plays a role in immunity, and may prevent several chronic diseases, including cardiovascular disease, type 2 diabetes, and high blood pressure. Vitamin D may also lower the risk of fibroids, covid infections in Black people, depression, and multiple sclerosis.

How much vitamin D you need

The amount of vitamin D you need depends on your age. Dietary guidelines in the United Kingdom recommend that all infants, children, and adults (including pregnant and breastfeeding women) get 10 micrograms (mcg) of vitamin D daily. In the United States (US), the recommended daily allowance is 15 mcg daily for people aged 1–70 years and 20 micrograms daily for those aged over 70 years.  Pregnant and breastfeeding women in the US are recommended 15 mcg of vitamin D daily. 

Life StageUK recommendationsUS recommendations
Birth to 12 months10mcg (400 IU)10mcg (400 IU)
Children 1-13 years10mcg (400 IU)15mcg (600 IU)
Teens 14-18 years10mcg (400 IU)15mcg (600 IU)
Adults 19-70 years10mcg (400 IU)15mcg (600 IU)
Adults 71 years and older10mcg (400 IU)20mcg (800 IU)
Pregnant and breastfeeding women10mcg (400 IU)15mcg (600 IU)
Mcg = microgram; IU = international unit

One microgram is equivalent to 40 international units (IU) of vitamin D. 

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What are the best sources of vitamin D

Credit: Thesomegirl | Getty Images

There are two forms of vitamin D, vitamin D2 and vitamin D3. Vitamin D2 is a synthetic product derived from UV treatment of plan sterols (ergosterol) and is used in some supplements. Vitamin D3 is the form made in the human body. 

Sunlight is the best source of vitamin D, as, unlike supplements, you can’t get a toxic dose. For Caucasians and those with lighter skin, the skin can produce around 75 mcg (3000 IU) of vitamin D in just 10 minutes of sun exposure. Some of this vitamin D can be stored in the liver and fat cells.

Dietary sources of vitamin D are limited. Fatty fish are considered the richest source of vitamin D. Tuna, eggs, butter, and liver are other natural sources. UV-treated mushrooms are good sources of vitamin D as well as fortified foods. 

It takes plenty of planning to get sufficient vitamin D from food alone. If you work indoors, live in a temperate climate, have a family history of skin cancer, and don’t eat fatty fish daily, you will need a vitamin D supplement. Current research suggests that vitamin D3supplements are most effective at raising vitamin D blood levels and lowering fracture risk.

Food Item and AmountVitamin D (micrograms)% RDA for UK Adults
Salmon, 90g11.0110%
Cod liver oil, 1 teaspoon10.5105%
Mushrooms, UV-treated, 100g5.050%
Milk, whole (fortified) 1 cup3.232%
Orange juice (fortified), 1 cup2.525%
Porridge (fortified), 30g1.313%
Egg, 1 large1.212%
Cheerios cereal (fortified), 20g1.010%

Are you getting enough vitamin D? 

The only way to know if you’re getting enough vitamin D is via a blood test that measures the amount of vitamin D in your blood. Blood tests measure the amount of calcidiol in your blood either in nanomoles per litre (nmol/L) or nanograms per millilitre (ng/mL). One nmol/L is equivalent to 0.4ng/mL. 

  • Vitamin D levels of 50 nmol/L (20 ng/mL) or greater are adequate
  • Vitamin D levels of 125 nmol/L (50 ng/mL) or greater are too high and may cause health problems
  • Vitamin D levels less than 30 nmol/L (12 ng/mL) are too low and may affect your bones and overall health

Some people are at risk of getting insufficient vitamin D

  • Exclusively breastfeed infants: Infants typically have limited sun exposure and breast milk is a poor source of vitamin D
  • Older adults: As you age, your skin’s ability to produce vitamin D reduces and your kidney’s ability to produce the active form of vitamin D decreases. 
  • Darker skin: People with dark skin have increased melanin, which reduces the skin’s ability to produce vitamin D. 
  • People with inadequate sun exposure either due to temperate climates, excess clothing, air pollution, the use of sunscreen or spending too much time indoors. 
  • People with conditions that limit fat absorption such as liver disease, cystic fibrosis or taking weight loss medication: Vitamin D is absorbed alongside fat in the small intestine. If you can’t absorb fat, you’re likely to be vitamin D deficient. 

What are the consequences of vitamin D deficiency? 

Vitamin D deficiency can occur at any time, but when it occurs during infancy and early childhood, it causes rickets. Rickets is characterised by bowed legs, thick wrists and ankles, a curved spine and a chest that protrudes above the breastbone (sternum), skull malformations and pelvic abnormalities. 

Osteomalacia – soft bones – is another consequence of vitamin D deficiency. It typically affects adults and can result from insufficient calcium intake, absorption in the small intestine or loss from the kidneys. Osteomalacia typically occurs in people with kidney, stomach, gall bladder or intestinal disease. 

Credit: Science Photo Library

What are the consequences of excess vitamin D?

Extremely high levels of vitamin D (above 375 nmol/L or 150 ng/mL) is typically caused by excessive use of dietary supplements. 

Excess vitamin D intake can cause nausea, vomiting, muscle weakness, confusion, pain, appetite loss, dehydration, kidney stones, peptic ulcers, pancreatitis and excessive urination and thirst. Extremely high levels of vitamin D in the blood can cause kidney failure, irregular heartbeat and even death. 

The daily upper limit for vitamin D from all sources including food, beverages and supplements are:

  • 0-6 months: 25 mcg (1000IU)
  • 7–12 months: 38 mcg (1500IU)
  • 1–3 years: 63 mcg (2500 IU)
  • 4–8 years: 75 mcg (3000 IU)
  • 9–18 years: 100 mcg (4000 IU)
  • Adults over 19 years including pregnant and breastfeeding women: 100 mcg (4000 IU)

Does vitamin D interact with medications? 

Vitamin D supplements can interact with some medications. The weight-reducing drug, Orlistat (Xenical and Alli) can reduce the amount of vitamin D your body absorbs from food. Cholesterol-lowing statins such as atorvastin, lovastatin and simvastatin may not work correctly if you take high-dose vitamin D supplements. Steroids such as prednisone can lower your vitamin D levels and thiazide diuretics such as Hygroton can excessively raise your blood calcium levels if you take vitamin D supplements. 

What are the best vitamin D supplements?

Evidence suggests that vitamin D3 supplements raise blood vitamin D levels more efficiently than vitamin D2 supplements. Vitamin D supplements in gummies are also better absorbed than pills, capsules, or tablets

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  1. Chauhan, K., Shahrokhi, M., Huecker, M.R. (2021) Vitamin D. Available: 16 August 2021. 
  2. Nair, R., Masseh, A. (2012) VitaminD: the “sunshine” vitamin. Journal of Pharmacology and Pharmacotherapeutics, 3(2): 118–126. 
  3. Sizar, O., Khare, S., Goyal, A., Bansal, P., Givler, A. (2021) Vitamin D deficiency. Available: Accessed: 16 August 2021. 
  4. Marcinowska-Sushowierska, E., Kupisz-Urbanska, M., Lukaszkiewicz, J., Pludowski, P., and Jones, G. (2018) Vitamin D toxicity – A clinical perspective. Frontiers in Endocrinology, 9: 550
  5. Wagner, C.L., Shary, J.R., Nietert, P.J., Wahlquist, A.E., Ebeling, M.D., and Hollis, B.W. (2019) Bioequivalence studies of vitamin D gummies and tablets in health adults: results of a cross-over study. Nutrients, 11(5): 1023
  6. Robien, K., Oppeneer, S., Kelly, J., Hamilton-Reeves, J.M (2013) Drug-vitamin D interactions: a systematic review of the literature. Nutrition in Clinical Practice, 28(2): 194–208.

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The Comments

  • Pamela Quintano
    August 22, 2021

    Thank you Sommi. This article is so complete. My mother is forcing us to take vitamin D3 every day. I understand why now.