1. Introduction
Diabetes mellitus is a major health problem. Epidemiologically, individuals with diabetes are expected to double to more than 350 million by 2025 (Zimmet et al., 2003). There is an increasing rate of obesity worldwide that is significantly correlated with diabetes, which can be due to consuming high-calorie foods and low physical activity (James et al., 2001). Hyperglycemia in prediabetes can lead to oxidative stress and the upregulation of proinflammatory factors, which ultimately lead to vascular dysfunction. To prevent the development of comorbidities, it is fundamental to determine the mechanisms involved in the progression from prediabetes to diabetes. Oxidative stress leads to impaired glucose uptake in muscle and fat cells and decreases insulin secretion from pancreatic β-cell (Maddux et al., 2001).
Vitamin A is one of the vital nutrients that should be included in the diet in order to maintain health. It is essential for multiple life mechanisms such as the reproductive system, immune system, vision and differentiation at the cellular levels (Amestin et al., 2017). Moreover, Vitamin A is a very powerful antioxidant that helps maintain the functions of the heart, lungs, and kidneys, along with various other organs in the human body (Coats et al., 2010).
Recently, multiple studies started to investigate the effect of Vitamin A and its derivatives in multiple conditions. An evidence has shown that there is a link between diabetes and vitamin A. This previous study elucidated that Vitamin A can enhance insulin levels that are produced by pancreatic β-cell. In Addition, researchers discovered receptor for vitamin A on the cell surfaces of the beta cells (Amestin et al., 2017). By reviewing the literature, it is evident that Vitamin A deficiency predisposes individuals to type I and type II diabetes. Vitamin A deficiency has been shown to decrease the release of insulin and its synthesis and secretion in both humans and in diabetic animal models. Furthermore, multiple epidemiological studies have demonstrated that there is a link between vitamin A deficiency in both early onset and delayed onset of type I diabetes (Isaia et al., 2001).
Lately, a wide attention has been developed to the protective biochemical function of natural antioxidants contained vitamins, which can prevent of the oxidative damage caused by free radical species (Matos et al., 2018). Little information is available regarding the antidiabetic an antioxidant effect of vitamin A in diabetes mellitus.  As diabetes has been shown to trigger oxidative damage, we hypothesized that Vitamin A might have an improvement effects on glycemic control as well as oxidative stress in diabetes mellitus.