Vitamin D and bones

Bone tissue is composed of a few basic elements, the first of which is the bone matrix, formed mainly of collagen, which allows the bones to be elastic and strong.

Then there’s a cellular component, represented by three types of cells:

• Osteoblasts, which produce bone tissue and are essential for the process of bone mineralisation, i.e. the fixation of minerals in the bone matrix
• Osteoclasts, responsible for the destruction and resorption of bone tissue
• Osteocytes, which take part in bone remodelling in response to stimuli of various kinds

Bone tissue also has a mineral component: in particular, its phosphorus and calcium content make it particularly hard and resistant.

The structure of the bones includes a compact outer part and a spongy inner part, which makes them lighter, more elastic and less fragile.

Bones are then covered by a thin protective membrane, in which the pain-sensitive nerves are located.

An important characteristic of bone tissue is its tendency to continually change and repair itself, in a process known as bone remodelling: older tissue is destroyed and removed (resorption) and gradually replaced by new tissue (osteogenesis).

In children, adolescents and young people, up to about 20 years of age, during the growth phase, the bones increase in size and become stronger: during this period, the formation of new bone tissue prevails over resorption. Over the next 5-10 years, there’s a consolidation phase, in which peak bone mass is reached.

Under normal conditions, the remodelling process continues regularly until about age 65 in men and menopause in women.

From then on, however, remodelling becomes less and less efficient and resorption takes precedence over the formation of new bone tissue: the bones experience a decrease in mineral concentration and bone mass is reduced. If the loss of mineral content in the bones is too great, osteoporosis occurs, making the bones weak and brittle, increasing the risk of fractures.

The generic term vitamin D refers to a group of molecules that play a fundamental role in the health of the body: they help maintain normal levels of calcium in the blood and contribute to the intestinal absorption of calcium and phosphorus which, together with vitamin D, help maintain normal bones and teeth.

In addition to being involved in the physiological processes of regulation and maintenance of bone tissue, vitamin D comes into play in several biological functions, including that of contributing to the proper functioning of the immune system.

The main forms of this vitamin are vitamin D2, found mainly in mushrooms, and vitamin D3, available in foods of animal origin.

Vitamin D synthesis occurs in the skin through exposure to the sun. The sun’s rays, in fact, trigger a chemical reaction transforming a precursor present in skin cells into vitamin D3. Vitamin D3 produced in this process, like the exogenous form introduced with food and absorbed in the intestine, is not active and therefore cannot exert its biological action. Through the bloodstream, it’s transported by specific proteins first to the liver and then to the kidney, where it’s activated through the action of enzymes. The biologically active form is able to bind to specific receptors expressed on the surface of cells in different tissues and organs, acting as a hormone and regulating various biological functions.

In the case of vitamin D deficiency, also known as hypovitaminosis, the body absorbs less calcium and phosphorus, which can be insufficient to maintain healthy bones, thus increasing the risk of bone disorders.

Vitamin D deficiency also results in reduced blood calcium concentration.

The opposite situation, i.e. vitamin D excess, is quite rare, because it’s difficult to take in excessive amounts from the diet, since foods generally contain small amounts. Care must, however, be taken with the dosage of supplementation: excess vitamin D can cause intoxication symptoms such as vomiting, diarrhoea, contractions and muscle pain.

For women, calcium and vitamin D intake is important for bone health, especially at certain stages of life.

Vitamin D, like calcium, is important during menopause, a time when women may be at increased risk for fractures. In fact, the incidence of fractures due to bone fragility increases with age, especially in women, more than in men.

As already mentioned, the production of vitamin D3 in the skin occurs through the action of sunlight. Normal sun exposure is generally sufficient to meet the daily requirement of vitamin D in adults.

There may, however, be conditions that lead to inadequate exposure to the sun, which can result in lower vitamin D production. For example, the elderly, patients hospitalised for long periods of time, residents of nursing homes and, in general, those who don’t have the opportunity to spend time outdoors are at greater risk of vitamin D deficiency.

There are also certain situations in which, despite adequate exposure to the sun, vitamin D is not produced in sufficient quantities: this is the case for people with a very dark complexion and the elderly (vitamin D production decreases with age). Finally, some liver and kidney disorders may interfere with the transformation of vitamin D into its active form, while some chronic diseases, such as cystic fibrosis, coeliac disease and Crohn’s disease, may decrease its absorption.

As far as dietary intake of vitamin D is concerned, only certain foods contain it. Food sources that contain significant amounts of vitamin D are primarily of animal origin: fish oil, and in particular cod liver oil (the main source of vitamin D), but liver, salmon and other fish products such as mackerel, eel, herring and sardine can also contribute to an adequate intake. In addition, egg yolk, milk and its derivatives, particularly butter and very fatty cheeses contain vitamin D, albeit in more limited quantities. Other foods that may contribute secondarily to a daily intake of vitamin D are mushrooms.

In case of deficiency, supplementation with vitamin D fortified foods and supplements can be used. Where there are risk factors for hypovitaminosis or suspected symptoms, it may be sufficient for the i.e. doctor to request, among other tests, a blood sample for the dosage of serum vitamin D to ascertain whether there is a vitamin D deficiency in the body.

Vitamin D supplementation to counteract vitamin D deficiency may be recommended in the elderly and menopausal women, who often also have osteoporosis and brittle bones.

Dietary supplements may contain different dosages of vitamin D, possibly combined with other components, vitamins and minerals. They’re available in different formulations, such as oral drops, soft capsules and orodispersible films, to be dissolved in the mouth.