Vitamin D is an essential fat-soluble organic molecule that is primarily required for maintaining bone health and immunity. Recently, it has come in the limelight that vitamin D deficiency is associated with an increased risk of type 2 diabetes.
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What are the functions of vitamin D?
The human body can synthesize vitamin D using the ultraviolet-B rays coming from the sun. Daily exposure of 15 – 20 minutes in the sunlight is sufficient for the skin to produce sufficient vitamin D levels. However, too much sun exposure can lead to skin aging and skin cancer.
In addition, vitamin D can be consumed through certain foods, such as egg yolk, saltwater fish, animal liver, cheese, nuts, and vitamin D-fortified milk products and cereals.
Primarily, vitamin D maintains the bones, teeth, and joints by facilitating the absorption of calcium in the body. It is also required for the proper functioning of the nerves, muscles, and immune system.
The short-term effects of vitamin D deficiency (vitamin D level <50 nmol/l) include bone/joint pain, muscle weakness, impaired immunity, depression, etc. However, a prolonged vitamin D deficiency is associated with = more serious complications, including osteoporosis, chronic fatigue, high blood pressure, obesity, Alzheimer’s disease, type 2 diabetes, and even cancer.
How does vitamin D affect type 2 diabetes?
There are many scientific studies and clinical trials claiming that vitamin D plays an important role in improving the sensitivity of insulin, which is a hormone required for maintaining blood glucose levels. It is known that maintaining a vitamin D level of 80 nmol/l or above is appropriate to keep normal glucose homeostasis.
The effects of vitamin D on type 2 diabetes may be driven by multiple mechanisms. For instance, studies have found that the pancreas contains receptors for the active vitamin D metabolite called 1,25-dihydroxyvitamin D, which is required for the synthesis and secretion of insulin by the pancreatic beta cells.
The majority of the studies showing positive management of type 2 diabetes by vitamin D supplementation have claimed that the vitamin helps maintain a normal glycemic status by reducing insulin resistance, the main causative factor for diabetes.
These studies recommend that a daily vitamin D dose of more than 2000 IU is required to keep an optimal level of 25-hydroxyvitamin D (25(OH)D) in the blood (>80 nmol/l). At this level, the risk of diabetes has been found to be the lowest. The blood level of 25(OH)D typically represents the vitamin D status from both sun and food sources.
Studies conducted on elderly people (age: >70 years) have shown that a blood vitamin D level of <50 nmol/l is associated with a two-fold increased risk of diabetes. Moreover, an inverse correlation does exist between the vitamin D status and HbA1C level, which is a well-recognized marker for impaired glucose metabolism.
There are also some secondary effects of vitamin D related to diabetes management. For example, studies have shown that maintaining an optimal vitamin D level over the long-term is associated with weight loss and decreased risk of obesity, both of which decrease the risk of diabetes.
Vitamin D may reduce the risk of obesity in two ways. It can regulate appetite by increasing the blood leptin level, which is required for controlling fat storage and inducing satiety. Moreover, it can reduce the blood level of parathyroid hormone, which in the long run can trigger weight loss mechanisms.
Despite the extensive research on vitamin D and type 2 diabetes, the role of vitamin D in the prevention of diabetes is debatable. Many well-designed, well-controlled clinical trials have clearly demonstrated that vitamin D does not play a significant role in controlling hyperglycemia.
Such studies suggest that people who are non-obese and have vitamin D deficiency get the maximum benefit from vitamin D supplementation in reducing fasting and overall blood glucose levels. In contrast, people who are obese and do not have vitamin D deficiency do not benefit from vitamin D supplementation.
The maximum benefits have been observed when people are supplemented with ≥1000 IU of vitamin D a day for more than 12 weeks.
One recent study conducted on 2423 adults (age: ≥30 years) showed that a daily vitamin D dose of 4000 IU does not prevent type 2 diabetes in people who are at high risk for developing diabetes. The study has involved a wide variety of people with diverse physical features, including age, sex, race and ethnicity, and body mass index, to avoid possible confounding effects.
At the end of the study, findings reveal that the percentage of the people who developed type 2 diabetes in the vitamin D-supplemented group and the control group were similar.
There are other possible explanations for the association between diabetes and low vitamin D levels. It is possible that persons who have sufficient vitamin D levels are more likely to participate in outdoor physical activity, which would also decrease their risk of type 2 diabetes.
Medline Plus. 2019. Vitamin D. https://medlineplus.gov/vitamind.html
Diabetes.co.uk. 2019. Vitamin D and Diabetes. https://www.diabetes.co.uk/food/vitamin-d.html
Scragg R. 2008. Vitamin D and Type 2 Diabetes. Diabetes. https://diabetes.diabetesjournals.org/content/57/10/2565
Dalgard C. 2011. Vitamin D Status in Relation to Glucose Metabolism and Type 2 Diabetes in Septuagenarians. Diabetes Care. https://care.diabetesjournals.org/content/34/6/1284
McMaster. 2018. Vitamin D: A possible ally in the fight against diabetes. www.mcmasteroptimalaging.org/.../vitamin-d-a-possible-ally-in-the-fight-against-diabetes
National Institutes of Health. 2019. NIH-funded trial finds vitamin D does not prevent type 2 diabetes in people at high risk. www.nih.gov/.../nih-funded-trial-finds-vitamin-d-does-not-prevent-type-2-diabetes-people-high-risk