Vitamin D

Vitamin D, also known as the “sunshine vitamin”, belongs to the group of fat-soluble vitamins and is essential for maintaining calcium and phosphorus homeostasis in the intestine and bones. In fact, vitamin D behaves like a hormone, due to its endocrine actions, mainly to protect the musculoskeletal system.

Vitamin D deficiency causes bone disorders such as rickets in children and osteoporosis in adults. Vitamin D is involved in various functions in the body due to the presence of its receptors in almost all the body’s tissues and cells.

Vitamin D is a complex of two vitamins, ergocalciferol (D2) which is found in foods of plant origin (e.g. yeast and mushrooms) and cholecalciferol (D3) which comes from foods of animal origin and is formed on the skin under the influence of ultraviolet rays (UVB). The foods richest in vitamin D are: oily fish (e.g. salmon, mackerel, sardines), egg yolks, fortified foods such as vegetable margarine, milk, orange juice, cereals and certain pastries.

The endogenous composition of vitamin D is influenced by various factors such as skin tone, use of sun cream, length of exposure to the sun, time of year, latitude of location, climate, clothing and age. 80-90% of vitamin D comes from the skin through exposure to the sun, while 10-20% is obtained from food.

The most common symptoms associated with vitamin D deficiency are muscle weakness, bone pain and fatigue. Hypervitaminosis is relatively rare, however it occurs at concentrations above 500 nmol/l and can occur when vitamin D intake exceeds 10,000 IU/day. Manifested by nausea, anorexia, constipation, weight loss, weakness and increased levels of calcium and phosphorus in the blood.

The high prevalence of vitamin D deficiency is a particularly important public health problem, as hypovitaminosis D is considered an independent risk factor for mortality in the general population. Severe vitamin D deficiency leads to reduced calcium and mineral deposition in the bones, resulting in rickets in children and osteomalacia in adults. In addition, a reduction in vitamin D levels leads to an increase in parathyroid hormone (PTH), a hormone produced by the parathyroid gland (secondary hyperparathyroidism), which in the long term leads to osteoporosis and an increased incidence of fractures. Numerous studies have established a link between vitamin D deficiency and an increased risk of autoimmune diseases such as multiple sclerosis and type I diabetes. In addition, low levels of vitamin D appear to be associated with cardiovascular disease and certain cancers such as intestinal and breast cancer, making it clear that this vitamin is not just essential for skeletal health. Around 50% of the world’s population has some degree of vitamin D deficiency. This deficiency may be due to insufficient intake and/or reduced exposure to sunlight, to abnormalities that limit its absorption and to conditions that prevent the conversion of the vitamin into its active metabolites, in particular liver and kidney dysfunctions.

Rickets is a disease that affects children with deformed and undeveloped long bones. At present, rickets occurs in low-income countries in Africa, Asia and the Middle East. Abstinence from meat is a risk factor. Children who have dark skin or little exposure to the sun and who do not eat meat, fish and eggs are at risk of developing rickets. Newborns fed on breast milk, which is low in vitamin D, and who are not exposed to the sun develop rickets at a higher rate and in sunny countries it occurs in older children when they do not consume sufficient quantities of calcium.

Occurs in adults and is characterised by muscle weakness and bone fragility. Osteoporosis is a bone disease characterised by deterioration of bone tissue, causing bones to become very thin and brittle over time. Although osteoporosis is more common in older people, the disease can affect people of all ages.

People at risk of vitamin D deficiency are :

• breast-fed infants
• people of advanced age
• people with limited exposure to sunlight
• people suffering from liver or kidney disease
• people undergoing treatment (long-term use of glucocorticoids, antifungal, antiretroviral and anti-epileptic drugs, etc.)
• patients with malabsorption syndromes due to intestinal diseases (e.g. coeliac disease, Crohn’s disease, surgery to remove part of the intestine, etc. )
• people with dark skin
• people who are overweight or obese

High-risk groups should be screened for vitamin D deficiency. Vitamin D can be absorbed by the body through the diet and after exposure to sunlight. People in high-risk groups may be able to ensure and maintain adequate levels of vitamin D through special food supplements, the dosage of which should be determined by a doctor or qualified dietician. Overdosage of vitamin D can cause toxicity only when oral supplements are used.

• bone health
• strengthens the immune system (e.g. allergic reactions, multiple sclerosis, winter flu/infections, etc.)
• cancer cells
• reduces the risk of cardiovascular disease but appears to have a negative correlation with total cholesterol
• should be avoided in cases of hyperkalaemia or renal osteodystrophy with hyperphosphataemia.

During pregnancy and breast-feeding, no problems have been observed with normal consumption.

Food supplements containing vitamin D3 (cholecalciferol) are derived from animal sources and are therefore not suitable for strict vegetarians, whereas vitamin D2 (ergocalciferol) is derived from plant sources.

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