In North America and Western Europe only small amounts of vitamin D, a fat-soluble secosteroid hormone, enter the metabolic circle via dietary uptake (e.g. from fish). The main source (about 95%) is vitamin D3 (cholecalciferol) that is photochemically synthesized in the skin by ultraviolet-B radiation or sun exposure. This pro-vitamin cholecalciferol is bound to serum vitamin D-binding protein (DBP) and through a two-step enzymatic pathway involving 25-hydroxylase of the liver and 1a-hydroxylase (CYP27B1) of the kidney and extrarenal tissues, it is converted to the biologically active hormone calcitriol.
Proposed Health Implications of Vitamin D Deficiency from Infancy to Seniority
Polycystic ovary syndrome (PCOS) is among the most common endocrine disorders in women of reproductive age and has a strong genetic component. It is characterized by ovarian dysfunction and its clinical manifestations may include obesity, increased insulin resistance and compensatory hyperinsulinemia, oligo-/anovulation and infertility. Studies regarding vitamin D status in patients with PCOS show an inverse correlation between vitamin D levels and metabolic risk factors, e.g. insulin resistance, BMI, waist-to-hip-ratio, triglycerides, total testosterone and DHEAS and a positive correlation with insulin sensitivity. Clinical trials with either vitamin D supplementation or administration of vitamin D3 analogues showed positive effects on insulin secretion, lipid profile, menstrual cycle and follicular development and a decrease of fasting and stimulated glucose and C-peptide levels. Recently it was hypothesized that vitamin D deficiency is not only in association with obesity but potentially causative.
Endometriosis. An impairment of immunologic mechanism and inflammatory responses has been suggested to be involved in the pathogenesis of endometriosis. Cyclic and early pregnant endometrium is an extrarenal site of vitamin D synthesis and action. In endometriosis patients, the gene encoding for 1a-hydroxylase shows an enhanced expression in ectopic endometrium.
Hypertensive disorders of pregnancy and especially preeclampsia (PE) are the most studied reproductive health outcomes in association with maternal vitamin D status. The syndrome PE is defined as the occurrence of hypertension and proteinuria after 20 weeks of gestation. With a prevalence of 3-5% of all pregnancies worldwide, it is the leading cause of maternal and fetal morbidity and mortality. Seasonal patterns in PE suggest a role for vitamin D and sunlight, because of a higher incidence in winter and a lower incidence in summer. Compared with normal pregnancies, PE is characterized by marked changes in vitamin D and calcium metabolism and as early the 1990’s a role for vitamin D in the pathogenesis of PE was hypothesized.
Some research showed an inverse association with having a primary cesarean section and vitamin D status in 253 women. Severely vitamin D deficient women with levels of 25(OH)D3 < 37.5 nmol/l delivered nearly four times as often by cesarean section than those with 37.5 nmol/l or greater. Vitamin D seems to have a complex relation with fetal growth that may vary by genotype, race and other variables that could not be identified yet. This relation between maternal vitamin D status and fetal birth weight has been considered in a number of observational studies. Breast milk is an ideal nutrient for a newborn but is not sufficient to maintain newborn vitamin D levels within a normal range. Many nursing mothers or their infants require vitamin D supplementation for optimal health.
Vitamin D deficiency is still considered a problem of the past by health care professionals and the public. Populations at risk include infants, children, pregnant and postmenopausal women. A range of possible adverse health outcomes during a women’s reproductive period may relate to low vitamin D status. Besides the classical diseases such as rickets, osteoporosis and osteomalacia, vitamin D deficiency in women might be associated with lower fertility and an increased risk for adverse pregnancy outcomes. Vitamin D deficiency is often clinically unrecognized, however laboratory measurements are easy to perform, and treatment of vitamin D deficiency is inexpensive. Oral supplementation is the best-tolerated and the most effective route of administration. In the past few years, studies have shown that a lack of the vitamin may be the primary culprit in depression, heart disease, pregnancy problems, birth defects, skin and other cancers, and multiple sclerosis. Even if you don’t suffer from any of these conditions, getting more D may still be what the doctor ordered.