What is Vitamin D? The Sunshine vitamin

Written by Catherine Saxelby on Wednesday, 14 August 2013.
Tagged: butter, calcium, dairy, fish oil, margarine, vitamins

What is Vitamin D?  The Sunshine vitamin

Formed by the action of sunlight on the skin, vitamin D (chemically known as Cholecalciferol) has been called the ‘bone vitamin’ as it enables calcium and phosphorus to be absorbed to make strong bones. It’s a cross between a vitamin and a hormone and may do a lot more for our immune system and keeping us healthy than we ever realised.

The search for a cure for rickets in the 1920s

Vitamin D was first discovered in the 1920's as a result of the search for a cure for rickets, a disease causing softening of the bones in children. It works by improving the absorption of calcium and phosphorus into the body so it helps to ensure strong bones and teeth as well as maintaining healthy nerve and muscle function.

Vitamin D can be made by the body from sunlight in a reaction that takes place in the skin. The UV rays from sunshine are essential for the reaction - in countries like Australia where there's plenty of sunshine most of the year, in theory our bodies should be able to make the vitamin D via regular sun exposure.

However there are groups who do not get adequate exposure and may be deficient eg elderly people who never go outdoors due to ill-health, people with very dark skin, people who are covered due to religious reasons, babies born to women who are vitamin D deficient themselves.

What it does

  • Builds strong bones and teeth
  • Prevents rickets, a bone disease in children which results in stunted growth and bowed legs
  • Boosts the absorption of calcium and phosphorus
  • Contributes to normal cell division, growth and development.
  • Under study for its role in preventing cancers and boosting our immune system

How much do I need?

The suggested Adequate Intakes for vitamin D per day are:
   (from NHMRC Australia 2006)

5 mcg for babies (0 to 12 months)
5 mcg for toddlers (1 to 3 years)
5 mcg for children (4 to 8 years)
5 mcg for children (9 to 18 years)

5 mcg for women (19 to 50 years)
5 mcg for men (19 to 50 years)

10 mcg for women (51 to 70 years)
10 mcg for men (51 to 70 years)

15 mcg for women (70+ years)
15 mcg for men (70+ years)

5 mcg for pregnant women
5 mcg for breastfeeding women.

       mcg means micrograms which are smaller units than milligrams

Conversion from mcg to International Units (IU) often used on supplements:

  5 mcg = 200IU
10 mcg = 400IU
15 mcg = 600IU

Vitamin D is made on the skin by a reaction involving UV radiation from sunlight. Exposing your arms and legs for 10 minutes a day every second day is generally all that's needed to make enough vitamin D but this varies depending on your type of skin and where you live.

Upper limit:
80 micrograms (mcg) or 320IU for those unable to get exposure to sunlight.

Deficiency and low vitamin D status

Vitamin D deficiency is more common:

  • In the winter months, when many of us cover up, travel to and from work in the dark, and stay in the office during the day
  • In people with naturally dark skin
  • In babies born to mothers who themselves are vitamin D deficient
  • In people with little or no sun exposure such as veiled women, the elderly and those in institutional care
  • In obese individuals as their extreme fat stores takes up all the vitamin D leaving little for the bloodstream, organs and muscles.

These individuals may need a vitamin D supplement to achieve adequate Vitamin D eg. Ostelin (Boots) supplies 25 mcg or 1000IU.

Toxicity

Vitamin D is a fat soluble vitamin and is stored by the body, so toxicity from supplements is always a potential risk.

A maximum dose of 80mcg (3200IU) per day from vitamin D supplements is set as an upper limit to avoid excess. If you take supplements - including calcium supplements for bones or cod liver oil - check you don't exceed this total figure for vitamin D.

Foods for vitamin D

Only a few foods (such as oily fish, egg yolk, milk and butter) naturally contain vitamin D so it's often difficult to get enough vitamin D from diet alone, one of the main reasons for the popularity of cod liver oil in European countries where sunlight is limited.

Margarine and a few brands of milk (eg Anlene) are fortified with added vitamin D, but most people in Australia only get around 25 per cent of their vitamin D from food.

Best food sources

Vitamin D is fat-soluble and is found in:

  • cod liver oil as a liquid or as capsules (not fish oil)
  • oily fish such as salmon, trout, sardines, mackerel (fresh or canned)
  • egg yolks
  • foods fortified with vitamin D such as table margarine, speciality milks eg Anlene, some rice milks, some soy milks
  • liver
  • butter
  • cheese

Mushrooms exposed to UV light or sunlight for a few seconds are a surprising source of vitamin D too.

Food mcg
1 tablespoon (20ml) cod liver oil 42
1 cod liver oil capsule 10
100g can salmon 13
1 tablespoon (20g) margarine 2
1 egg 1
30g cheddar cheese 1
1 glass (250ml) regular milk 0.1
   
Adequate Intake 5
Ostelin Vitamin D tablet 25

Halibut or cod liver oil capsules contain 400 IU of cholecalciferol (vitamin D3) per capsule. However, although these are cheap (less than 1 cent each), they also contain vitamin A (4000 IU), which may not be beneficial (epidemiological studies suggest that vitamin A excess may be associated with an increased fracture risk).

 

How much sunshine do I need?

Vitamin D sun on wheatYou'd think we get plenty of vitamin D in a country where there's plenty of sunshine most of the year such as Australia. But there's increasing recognition that significant numbers of the population do not get enough Vitamin D, especially in winter.

The Medical Journal of Australia recommends daily exposure of the hands, face and arms (around 15 per cent of body surface) to one third of a Minimal Erythemal Dose (MED) of sunlight on most days.

The time taken to achieve this differs depending on your latitude, season, time of day and skin type.

For example, those with moderately fair skin living in Sydney to achieve one third MED would need 6-8 minutes in summer but a lengthier 26-28 minutes in winter at 10am, dropping down to 16 minutes in winter at midday.

If sun exposure is not possible, then a supplement dose of at least 10mcg (400IU) per day is recommended.

It's calculated that exposure of this hands, face and arms should produce around 1000 IU of vitamin D (cholecalciferol).