Vitamin D plays an essential role in the absorption of certain minerals by the body. More details on the origins of this vitamin, its functions in the body, and the level of reference intakes (PRI) for the different population categories are given below.
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Updated on 22/03/2019
Presentation, food sources and nutritional needs
Definition, functions and roles
Vitamin D has two sources: it is provided by food and synthesised by the body through the skin when subjected to solar or ultraviolet rays.
The vitamin D produced by the skin is stored in the liver, muscle and adipose tissue and is used by the body during the winter.
There are two forms of vitamin D: vitamin D2 or ergocalciferol, which is produced by plants, and vitamin D3 or cholecalciferol, produced by animals. In humans these two forms have the same biological activity since they are converted into 1.25-dihydroxyvitamin D (commonly known as vitamin D), which is their principal active metabolite.
When exposed to the sun's ultraviolet rays, the skin produces cholecalciferol which is then transported to the liver where it is metabolised into 25-hydroxycholecalciferol, then to the kidneys where it takes its active form of 1.25 dihydroxycholecalciferol.
When exposed to the sun's ultraviolet rays, the skin produces cholecalciferol, which is then transported to the liver where it is metabolised into 25-hydroxycholecalciferol, then to the kidneys where it takes its active form of 1.,25 -dihydroxycholecalciferol.
The main function of vitamin D is to increase blood concentrations of calcium and phosphorus by facilitating their intestinal absorption and reducing their renal excretion. Maintaining adequate calcium levels helps ensure:
- optimal mineralisation of mineralised tissue, mainly bones, cartilage and teeth
- effective muscle contraction
- proper nerve transmission
- adequate coagulation
In addition to participating in the regulation of calcium and phosphorus metabolism, vitamin D is also involved in:
- hormone regulation (e.g. insulin, hormones of the pituitary gland, etc.);
- differentiation and activity of immune system cells;
- keratinocyte differentiation.
Levels of vitamin D and sensitivity of the vitamin
There are a limited number of foods that contain significant amounts of vitamin D. In addition, vitamin D is fat-soluble, which means that its concentrations are closely related to the presence of fats in foods. The foods richest in vitamin D are cod liver oil and oily fish such as herrings, pilchards, mackerel, sardines, tilapia, anchovies, trout, perch and salmon.
Population Reference Intakes (PRIs)
The population reference intake for vitamin D was defined assuming that endogenous skin production was nil, in order to meet the needs of almost all of the French population. The PRI is 15 µg/day for adult men and women.
The PRIs for other population groups are currently being assessed.
Intake levels and main food sources
According to the data of the INCA 3 study, mean dietary intakes of vitamin D in the French population are 5.2 µg/day for children aged one to three years, 2.6 µg/day for children aged four to 10 years, 2.9 µg/day for children aged 11 to 17 years, and 3.1 µg/day for adults aged 18 to 79 years. These intakes are higher for men than for women.
In France, the main foods that contribute to vitamin D intakes in the population are fish and dairy products (yoghurt, fromage blanc, cheese, milk), which respectively account for 19% and 25% of vitamin D intakes in adults and 12% and 40% of intakes in children between the ages of 11 and 17.
Risk of deficiency and excess intake
The clinical signs of vitamin D deficiency are osteomalacia and rickets in bones, a drop in muscle tone, tetanic convulsion, seizure (associated with hypocalcaemia) and sometimes anaemia.
Osteomalacia is a disease affecting adults related to a lack of accumulation of mineral elements in the skeleton. When it occurs in young, growing people, it is called rickets. These diseases lead to bone and muscle pain as well as bone deformation.
The body’s capacity to absorb or synthesise vitamin D decreases with age. In the elderly, a low intake of vitamin D is a predisposing factor for bone loss and therefore osteoporosis.
Several age groups are at risk of vitamin D deficiency: these are newborns, infants, pregnant women and the elderly (particularly those in institutional care), who do not get much sunshine and/or have increased needs. Other factors may aggravate the risks of deficiency: dark skin pigmentation, specific diets (those excluding meat, fish, eggs and dairy products), and diseases leading to intestinal malabsorption.
Since vitamin D is fat-soluble, it can accumulate in the body in the event of excess supplementation and then cause various disorders (headaches, nausea, vomiting, weight loss, intense fatigue).