Diabetes is a well-known disease that is increasingly affecting millions of people around the world. There are two types of diabetes: Type 1 and Type 2. Type 1 diabetes – also known as insulin-dependent diabetes mellitus or early-onset diabetes – is an autoimmune disease, which is usually developed during an individual’s younger years (Ananya Mandal, no date). The immune system destroys the β-cells found in the pancreas, which are accountable for the production of insulin. Insulin is a hormone that is released after a meal when there is a rise in glucose concentration in the blood. It is responsible for keeping the glucose levels in the blood constant, by causing the cells to absorb glucose from the blood. Insulin can also stimulate the liver to absorb or store any excess glucose in the blood. Therefore, when an individual cannot produce and/or release insulin, their blood glucose levels remain high after a meal (DiabetesUK, no date) (Gebel, 2012).
Type 2 diabetes – also known as dependent diabetes mellitus, or maturity-onset diabetes – is developed gradually over time, and usually associated with obesity (Ananya Mandal, no date). The body is able to produce insulin, but the insulin produced is insufficient to meet the body’s requirement, or the insulin cannot function properly, which means the level of glucose in the blood continues to rise (DiabetesUK, no date). Unlike people diagnosed with Type 1 diabetes, if an individual suffers from Type 2 diabetes they don’t usually notice the symptoms immediately, so some sufferers don’t even find out that they are diagnosed with Type 2 diabetes (NHS, 2017).
There is another type of diabetes called, gestational diabetes mellitus, which is less well-known than Type 1 and Type 2 diabetes. This is developed in women during pregnancy when their body cannot produce sufficient insulin to meet their body’s requirement. Pregnant women are at a higher risk of developing this due to factors such as: weight, history of gestation diabetes in a previous pregnancy, history of gestational diabetes in the family, ethnicity, etc (Todd, 2017). Most of the symptoms of gestational diabetes are similar to pregnancy symptoms such as: frequent urination, dehydration, tiredness, etc. (John D. Jacobson & Linda, 2018). Having gestational diabetes brings complications for both of the mother and baby. This includes increasing the mother’s likelihood of undergoing caesarean section because her child may be too large for natural delivery, or she may have too much amniotic fluid. The mother may also acquire urinary tract infections during pregnancy, and there is also a chance of her developing Type 2 diabetes after delivery. In terms of the infant’s health, the baby may develop jaundice, respiratory problems, and could even lead to death (Alzaim & Wood, 2013) (Kjos & Buchanan, 1999).
Excess glucose in the blood can result into many complications (Ananya Mandal, no date). Firstly, diabetes can cause weight loss. This is because glucose is lost in the urine of a diabetic person, which means that their body uses muscle and fat for energy instead (Dansinger, 2017). Secondly, fatigue is one of the problems a sufferer may experience because their cells are not getting enough glucose. Susceptibility to wound infections is also one of the major problems of a diabetic person because glucose provides nourishment to microorganisms, so microorganism continue to flourish, making it harder to treat the wound (Hirsch, et al., 2008). In addition to that, sufferers may acquire a “diabetic foot”. This occurs when the sufferer doesn’t notice the wounds on their feet due to lack of sensation, caused by the nerve and blood vessel damage. Moreover, high glucose levels may damage and change the size of the blood vessels found in the heart and eyes. Having high levels of glucose in the blood can also cause kidney and nerve damage, so there is a risk of dementia and other cognitive problems.
Vitamin D is a fat-soluble vitamin, which plays a huge role in keeping bones healthy (Wolf, 2004) (Teresa Kulie, 2009). It is mostly referred to as the “sunshine” vitamin because once the skin gets exposed to enough sunlight, the body synthesises vitamin D (Nair & Maseeh, 2012). It is also usually associated with calcium as it essential for the absorption of calcium in bones, which can promote healthier bones later on. Having low levels of vitamin D causes osteomalacia in adults, and rickets in children. It has also been linked to many diseases such as: cancers (e.g. colon cancer, breast cancer, prostate, and ovarian cancer (Cedric F. Garland, et al., 2006) and autoimmune diseases (e.g. sclerosis, arthritis, diabetes, high blood pressure, etc. (Cantona & Mahon, 2004).
Vitamin D deficiency, or hypovitaminosis D may arise due to many reasons. One of the reasons is insufficient exposure to sunlight. Individuals living in countries with shorter days, and those who are institutionalised, or spend most of their time indoors, may be at a higher risk of developing vitamin D deficiency, if they are not taking vitamin D supplements. In addition, the environment can also affect the development of vitamin D deficiency (e.g. areas with more air pollution would have less sunlight reaching the people living in those areas) (Feizabad, et al., 2017)
Another reason for having deficiency of vitamin D is ineffective production of vitamin D in the skin. For example, the skin of an older person can’t generate vitamin D as well as the skin of a younger person. Moreover, inadequate intake of vitamin D from the diet can also affect an individual’s likelihood of acquiring vitamin D deficiency. For example, infants are dependent on their mother’s vitamin D sufficiency as they can obtain vitamin D from breastmilk. Therefore, if the mother is vitamin D deficient, then the infant would be likely to be deficient as well. Furthermore, individuals with certain conditions, diseases and disorders (e.g. kidney or liver disease, hyperparathyroidism, etc.) are potentially at risk of becoming vitamin D deficient. For example, an obese person stores a lot of fat, which means vitamin D can’t circulate in the body as easily because it is a fat-soluble vitamin. (Mikstas, 2018) (MedlinePlus, no date)
As mentioned above, vitamin D deficiency has been associated with many diseases, including diabetes. There are many studies showing the roles of vitamin D in many cellular processes. This includes vitamin D increasing the transcriptional activation of the gene for insulin receptor, which appears to intensify the effect of insulin on glucose. (Maestro, et al., 2002) This suggests that vitamin D deficiency could lead to an elevation of glucose levels in the blood, which could potentially contribute to the onset of insulin resistance.
Another role of vitamin D is to prevent hypermethylation from occurring. DNA methylation is a process of attaching methyl group/s to a DNA molecule (Robertson, 2018). Recent findings show that obese individuals have an increased rate of DNA methylation, which could later lead to the development of diabetes (Wahl, et al., 2016). In addition, an increase in ROS levels can cause hypermethylation. Vitamin D can decrease ROS levels (Baccarelli & Bollati, 2009), thus making vitamin D possible to reduce the possibility of becoming diabetic.
A study in New Zealand shows that when South Asian women with insulin resistance were given 4000 IU of vitamin D per day, they became more sensitive with insulin. This could mean that insulin resistance can be reduced by having appropriate levels of vitamin D (Hurst, et al., 2010). This suggests that vitamin D may be essential in order to avoid developing diabetes.
It is previously stated that individuals exposed to less sunlight are at a higher risk of becoming vitamin D deficient. Countries closer to the equator have higher temperatures because rays of sunlight are directly aimed at the equator (Metcalfe, 2006). According to some studies, individuals living in countries further away from the equator may be more susceptible to developing diabetes. For example, children living in Finland (a country far away from the equator) are more likely to acquire diabetes than the children in Japan (a country, which is closer to the equator) (RE, et al., 1985).
β-cells, which are found in the pancreas, respond to high levels of glucose by releasing insulin (Fu, et al., 2013). The β-cells of a human have a transmembrane carrier protein called, GLUT1, which allows glucose to enter the β-cells. (Vos, et al., 1995). Once glucose enters the β-cells, it enters various of steps in order to form high levels of ATP. Having high levels of ATP would close the ATP-sensitive K+ (KATP) channel, which means that the membrane will be depolarized. Therefore, there will be an increase in amount of Ca2+ inside the cells. When there is an increase of Ca2+, insulin is released because protein motors and kinases are stimulated. (Fridlyand & Philipson, 2010). Looking at diabetic individuals, their β-cells may not be able to release insulin due to having low levels of vitamin D. One of vitamin D’s roles is to regulate Ca2+ ions. (Brewer, et al., 2001)
This suggests that not having enough vitamin D may affect Ca2+ signalling. This could potentially lead to a rise in Ca2+ levels in the cells, which could cause apoptosis of the β-cells (Efanova, et al., 1998) (Wang, et al., 1999). This means, insulin cannot be produced nor secreted, thus diabetes will be developed.
There studies that show an association between vitamin D deficiency and increased risk of gestational diabetes in pregnant women. For example, when gestational diabetic women were compared to women with no gestational diabetes, they found that the women with gestational diabetes had significantly lower levels of vitamin D than those without the disease (Soheilykhah, et al., 2010). From this study, we can infer that the women with gestational diabetes may have developed the diseases because of low levels of vitamin D, which shows how important vitamin D is to reduce the likelihood of obtaining diabetes.
Vitamin D has anti-inflammatory properties (Krishnan & Feldman, 2011), and the fact that diabetes is an inflammatory disease (Donath & Shoelson, 2011) could mean that vitamin D must have positive effects on the β-cells. If β-cells can function properly, then they would be able to produce and secrete insulin, keeping the glucose levels in the blood constant. From this, it can be inferred that vitamin D is important to avoid diabetes.
However, there are some studies that suggest vitamin D may not have a significant role in the development of diabetes. For example, a study comparing mortality rates in Japan and Finland suggest that it may be sex, age, diseases, or disorders (Asao, et al., 2003), instead of the climate, that could be increasing the risk of having diabetes. Findings show that there are more deaths due to diabetes in Japan than in Finland, which could mean that vitamin D is not necessary to overcome or lessen the chance of getting diabetes.
Some of the challenges about the role of vitamin D in diabetes could also be due to the scientific methods used in some studies. For example, most studies regarding the role of vitamin D in diabetes are usually carried out on rats (Papandreou & Hamid, 2015) (Alatawi, et al., 2018) (El-Sayed, et al., 2015). This suggests that the findings from those studies may not apply to humans because humans and rats are different species, which means that we we should be careful with generalising the findings to humans. Moreover, most of the studies conducted to find an association between vitamin D and diabetes have inconsistent results. If there are inconsistent results, then this could mean that there might not be a strong enough link between vitamin D and diabetes to conclude that vitamin D has a significant role in diabetes. In addition to that, there is a study showing that once an individual has been diagnosed with Type 2 diabetes, this cannot be reversed by vitamin D because vitamin D’s influences are disregarded by other factors, which could possibly have a stronger role in diabetes (Mosekilde, 2005). Furthermore, most studies mention that further research may be required when looking into the role of vitamin D in diabetes (Scragg, 2008) (Martin & Campbell, 2011). This means that there is still no definite answer to whether vitamin D can really increase the risk of developing diabetes, thus making it seem like vitamin D is unimportant in diabetes.