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Received Nov 23, 2017; Revised Jan 24, 2018; Accepted Feb 20, 2018
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1. Introduction
Type 2 Diabetes Mellitus (T2DM) is a metabolic disease and major lifestyle disorder caused by either the absolute or relative insulin deficiency. T2DM is characterized by impaired glucose tolerance, chronic hyperglycemia, and altered insulin secretion [1]. It affects more than 6% of overweight urban Ghanaian adults, predominately women [2–4]. Accumulating evidence from various studies has linked vitamin D status to insulin secretion and insulin resistance [5–7]; however, the relationship between vitamin D deficiency and glycemic control remains conflicting.
Vitamin D is a steroid hormone known for its essential role in maintaining calcium homeostasis, promoting and maintaining bone health, and improving immune function [8, 9]. Vitamin D deficiency is considered a public health problem around the world. In 2008, it was estimated that 1 billion persons present with vitamin D insufficiency or deficiency [10]. Vitamin D is obtained through exposure to ultraviolet B (UVB) sunlight as well as nutritional sources. Despite the high UVB sunlight exposure in tropical countries, studies suggest vitamin D deficiency is prevalent and is further influenced by age and gender [11, 12].
Menopause, the cessation of menstrual cycle caused by reduced secretions of estrogen and progesterone, is defined as 1 year without menses, occurring between the ages of 45–55 [13]. Oestrogen increases the activity of 1-α-hydroxylase (expressed in the kidneys) responsible for the activation of vitamin D and upregulates the vitamin D receptor (VDR) [14]. During menopausal stages, there is a gradual reduction in amount of oestrogen produced by the ovaries [15]; this decline in oestrogen production is thought to promote vitamin D deficiency. The ensuing vitamin D challenge is related to decrease in number of vitamin D receptors [14, 16, 17].
Aging in women and the subsequent drop in oestrogen levels are thus associated with decline in vitamin D levels. We have previously established a high prevalence of vitamin D deficiency among diabetics and nondiabetics in Ghana [4]. The interplay between sunlight exposure, lifestyle habits, and serum vitamin D levels cannot be disregarded. Besides, these...