Vitamin D is a fat-soluble vitamin and is often known as the ‘sunshine’ vitamin as it is the only nutrient, which the body makes from direct sunlight on the skin. Vitamin D obtained from the sun, and equally from food or supplements, needs to be converted in the body to become active. It is converted in the liver to 25-hydroxyvitamin D (25OH D), which is relatively inactive but the precursor to the active form called 1,25-dihydroxyvitamin D (1,25OH2D) or calcitriol, which is produced primarily in the kidneys.
Vitamin D has the properties of both a vitamin and a hormone and plays a number of important roles within the body. It is essential for maintaining normal blood levels of calcium and phosphorus. It aids the absorption of calcium in the gut thereby necessary for the formation and maintenance of healthy bone and teeth. Vitamin D is also required for neuromuscular and immune function as well as inflammation reduction.
As well as the exposure of the skin to sunlight, vitamin D can also be taken in through the diet. It is found in oily fish, eggs, mushrooms and fortified products. Low levels of vitamin D can result from a low dietary intake, inadequate exposure to sunlight or intestinal absorption issues and can result in rickets in children and osteomalacia in adults.
Who is at risk of a vitamin D deficiency?
Older adults can be at risk of suboptimal vitamin D status or deficiency, this is mainly because ageing skin can lose its efficiency in producing vitamin D from the sun’s ultraviolet light. Older adults are also less able to convert vitamin D into its active form.
Individuals with darker skin are at risk due to natural content of melanin in their skin, which can absorb much more ultraviolet rays.
Individuals who don’t tolerate fatty foods well or who have a very-low fat diet may not absorb sufficient vitamin D from food due to vitamin D being fat soluble.
Individuals who are obese can be at risk of vitamin D deficiency as their extra body fat may bind some vitamin D reducing its circulation.
The best sources of vitamin D are listed below:
- Cod liver oil
- Rainbow trout
- Salmon
- Mushrooms
- Sardines (canned in oil)
- Fortified milk
- Fortified soy, almond and oat drinks
- Fortified cereals
- Eggs
- Tuna
- Beef liver
- Cheddar cheese
- Tofu
Whilst a number of foods are now fortified there are limited dietary sources of vitamin D if you follow a vegetarian or vegan diet, therefore getting adequate vitamin D can be challenging. Regardless of your diet over the winter in the UK the Scientific Advisory Committee on Nutrition (SACN) recommends that everyone over the age of 5 should take a daily vitamin D supplement containing 10 micrograms (or 400 IU) of vitamin D.
