Vitamin D is an essential fat-soluble substance. It is known as the ‘sunshine vitamin’ because the sun is man’s prime source. Vitamin D is one of the few essential vitamins that is produced by the body with the help of nature.
It occurs in two forms:
- CHOLECALCIFEROL found in foods of animal origin
- ERGOCALCIFEROL produced by the action of light on yeast.
When a vitamin is described as fat soluble, it means that other fat substances must be present in the cells to enable it to be used by the body. A fat-soluble substance cannot be washed out of the system via the urinary tract in the way that water-soluble vitamins are. Vitamin D is stored in the body to be used as and when it is required.
It is measured in micrograms (mcg or µg) and international units (iu).
1 microgram = 40 international units
The prime source of vitamin D for mankind is sunlight. The sun’s ultraviolet rays react on oils under the skin to produce vitamin D. Fair skinned people produce more in this way than dark skin types. A light skin will allow 50-70% penetration by the sun whereas in dark skins, the maximum amount of penetration is only 30%. Rich natural food sources are:
- Oil from the liver of white-fleshed fish such as halibut, cod and shark
- Fatty fish like kippers, sardines, tuna, mackerel, herring and eel
- Dairy produce: eggs, milk, butter, cheese and margarine
If a diet contains fatty fish once a week and a regular selection of dairy foods it should provide enough vitamin D to meet normal bodily requirements. Supplements of cod or halibut liver oil are recommended for children who do not eat a balanced diet or who have faddish or finicky food habits. It can be easier to persuade children to take one capsule per day than to force them to eat foods which they find unpalatable to ensure that they are obtaining adequate supplies of vitamin D.
In some sunny countries, people’s vitamin D requirements may be satisfied completely by sunlight synthesis alone and very little is needed from the diet.
When taken in food, like all fats and oils, vitamin D is emulsified in the upper intestinal tract. This takes approximately 3-5 hours. It is emulsified with bile and other digestive enzymes and then reduced to tiny globules which are absorbed into the blood stream intact and carried to the liver attached to proteins.
Vitamin D formed in the skin (cholecalciferol) is transported to the liver in the same way. The liver processes both cholecalciferol and ergocalciferol, which are then carried on to the kidney where a final conversion takes place.
Most vitamin D is stored in the fatty tissues of the body, with some staying in the liver and kidneys. The amount of cholecalciferol absorbed varies according to skin type.
The absorption of vitamin D from dietary sources can be impaired in the same way as other fat-soluble vitamins, i.e. low protein intake, fat malabsorption and / or impaired liver or kidney function.
Vitamin D is one of the more stable vitamins. Studies have shown that it withstands smoking (of fish), pasteurisation and sterilisation (of milk) and the drying of eggs.
It is possible to obtain a proportion of our vitamin D requirement from exposure to sun. However the use of sun block creams significantly reduces the effectiveness of vitamin D production so a careful balance between exposure and sun protection should be adhered to in order to avoid damage to the skin.
During the winter, a useful level of supplementation would be 1000 iu daily. This would especially apply to those who get little exposure to sunlight during the winter months.
Deficiency symptoms vary considerably according age.
In children, symptoms could include:
- unnatural limb posture
- excessive sweating
- teeth appearing late
- delayed ability to stand up
- knock-knees and/or bowed legs
Adolescents may suffer from:
- aching limbs
- swelling at the knees
- repeated infections
- poor growth
In adults, signs could be:
- bone pain
- muscular weakness
- muscular spasms
- brittle bones
Osteomalacia is the adult version of rickets and often affects women of child bearing age who have become calcium deficient, as a result of repeated pregnancies. The elderly may be particularly prone to osteomalacia if they are housebound due to physical incapacity because they get little sunlight. The areas most affected are the ribs, lower back, pelvis and legs.
Muscular weakness is often experienced, along with difficulty in climbing stairs or getting out of chairs. Deficient individuals sometimes develop a waddling gait or facial twitches because of low blood calcium.
Another group who are at risk of vitamin D deficiency are Asian immigrants. This is thought to be due to insufficient vitamin D in the diet and to inadequate solar exposure because some Asians tend to wear covering national dress. Asians and elderly housebound people are advised to supplement their diet with fish liver oil to guard against possible deficiency.Vitamin D promotes the absorption of the minerals calcium and phosphate in the small intestine and causes calcium to be released into the blood circulatory system.