Abstract
Background: Vitamin D and calcium are important dietary compounds that affect bone mass, even if other minerals (potassium, zinc, etc.) and vitamins (A, C and K) are also involved. Vitamin D and certain minerals, in fact, play an important role in calcium homeostasis and calcium absorption. Hip fracture incidence is higher in Europe and the United States, where calcium is frequently included in the human diet; while the occurrence of these fractures is lower in developing countries, where diets are often poor in calcium. This condition is named the “calcium paradox”, and may be partially explained by phosphate toxicity, which can negatively affect mineral metabolism. It is important to maintain correct dietary calcium-phosphate balance in order to have a healthy life, reducing the risk of osteoporotic fractures in older people. Vitamin D can also act as a hormone; vitamin D2 (ergocalciferol) is derived from the UV-B radiation of ergosterol, the natural vitamin D precursor detected in plants, fungi, and invertebrates. Vitamin D3 (cholecalciferol) is synthesized by sunlight exposure from 7-dehydrocholesterol, a precursor of cholesterol that can also act as provitamin D3. Dietary intake of vitamin D3 is essential when the skin is exposed for short periods to ultraviolet B light (UV-B), a category of invisible light rays such as UV-A and UV-C. This can be considered the usual situation in northern latitudes during the winter season, or the typical lifestyle for older people and/or for people with very white delicate skin. The actual recommended daily intake of dietary vitamin D is strictly correlated with age, ranging from 5 μg for infants, children, teenagers, and adults—including pregnant and lactating women—to 15 μg for people over 65 years.
Highlights
Vitamins are nutrients characterized by low-molecular weight; these compounds are provided by the diet and play a crucial physiological and metabolic role [1]
In Italy, 1515 women with postmenopausal osteoporosis treated with bisphosphonates were classified as vitamin D deficient or vitamin D replete; the mean bone mineral density (BMD) increase per year in the lumbar spine was 0.22% in vitamin D deficient patients versus 2.11% in vitamin D replete patients; similar differences were determined in the hip [39]
These results show that the addition of both vitamin D
Summary
Vitamins are nutrients characterized by low-molecular weight; these compounds are provided by the diet and play a crucial physiological and metabolic role [1]. Most vitamins cannot be synthesized by humans; for this reason, they must be provided by food sources or dietary supplements [3]. In the kidney, 25(OH)D is converted into 1,25-dihydroxy vitamin D, which is strictly correlated to calcium and phosphate absorption metabolism in the intestine [17], influencing bone cells [18]. Several clinical trials have been performed on older patients to evaluate whether vitamin D supplements can decrease the incidence of fractures [27]. The results obtained in clinical trials have shown (see Table 3) a decrease in fracture incidence in patients receiving vitamin D supplementation [27]. Vitamin D supplementation, with or without calcium, can increase bone mineral density (BMD), decrease bone turnover, and decrease fracture incidence [28]. Appropriate doses of Vitamin D may differ among patients: different genetic polymorphisms, eventual presence of other diseases, and possible use of drugs can affect vitamin D metabolism [29]
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