Abstract
Beyond vitamin D (VD) effect on bone homeostasis, numerous physiological functions in human health have been described for this versatile prohormone. In 2016, 95% of the world's population lived in areas where annual mean ambient particulate matter (<2.5 μm) levels exceeded the World Health Organization guideline value (Shaddick et al., 2018). On the other hand, industries disperse thousands of chemicals continually into the environment. Further, considerable fraction of populations are exposed to tobacco smoke. All of these may disrupt biochemical pathways and cause detrimental consequences, such as VD deficiency (VDD). In spite of the remarkable number of studies conducted on the role of some of the above mentioned exposures on VDD, the literature suffers from two main shortcomings: (1) an overview of the impacts of environmental exposures on the levels of main VD metabolites, and (2) credible engaged mechanisms in VDD because of those exposures. To summarize explanations for these unclear topics, we conducted the present review, using relevant keywords in the PubMed database, to investigate the adverse effects of exposure to air pollution, some environmental chemicals, and smoking on the VD metabolism, and incorporate relevant potential pathways disrupting VD endocrine system (VDES) leading to VDD. Air pollution may lead to the reduction of VD cutaneous production either directly by blocking ultraviolet B photons or indirectly by decreasing outdoor activity. Heavy metals may reduce VD serum levels by increasing renal tubular dysfunction, as well as downregulating the transcription of cytochrome P450 mixed-function oxidases (CYPs). Endocrine-disrupting chemicals (EDCs) may inhibit the activity and expression of CYPs, and indirectly cause VDD through weight gain and dysregulation of thyroid hormone, parathyroid hormone, and calcium homeostasis. Smoking through several pathways decreases serum 25(OH)D and 1,25(OH)2D levels, VD intake from diet, and the cutaneous production of VD through skin aging. In summary, disturbance in the cutaneous production of cholecalciferol, decreased intestinal intake of VD, the modulation of genes involved in VD homeostasis, and decreased local production of calcitriol in target tissues are the most likely mechanisms that involve in decreasing the serum VD levels.
Highlights
Vitamin D is provided either through exposure to sunlight, which is the main source or through intake from diet, which is supplementary (Hansdottir et al, 2008; Holick, 2006)
We aimed to provide a mechanistic overview of the aftermaths of exposure to air pollution, some environmental chemicals and tobacco smoke related to dysfunctional VD endocrine system (VDES) accompanied with the declined serum levels of two main metabolites of VD including 25‐hydroxyvitamin D (25(OH)D, Calcifediol) and 1α,25‐dihydroxyvitamin D (1,25(OH)2D, Calcitriol)
The former is originated from the ultraviolet B (UVB) irradiation of the ergosterol in plants and fungi, and the latter is found in the dietary sources, in oily fish, and is produced in the skin as well (Holick, 2006)
Summary
Vitamin D is provided either through exposure to sunlight, which is the main source or through intake from diet, which is supplementary (Hansdottir et al, 2008; Holick, 2006). To sustain the human health, the presence of a sufficient level of VD is compulsory. VDD has been identified as one of the important public health concerns globally, and in the Middle East region (Palacios and Gonzalez, 2014). TCDD once per week by SC injection for 20 weeks at daily doses of 0, 1, 10, 100, or 1000 ng/kg BW/day From mating to the birth of offspring, rat groups exposed to 0, 5, 20, or 40 mg PCB/kg diet. Lower levels of thyroid hormones and VD were shown in the exposed group compared with control animals. A clear relationship between serum 1,25(OH)2D, calcium, phosphate, and thyroid hormone levels and hepatic PCB and DDT load have been found in the gray seals. 25(OH)D3 was decreased only at high dose (100 ng/kg BW/day) in Long-Evans rats where levels were decreased 27% compared to control. S-25(OH)D was decreased at the time of delivery
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