Risk Factors For Vitamin D Deficiency
Obesity is an established risk factor for Vitamin D deficiency. Most morbidly obese people have lower than normal blood levels of 25-hydroxyvitamin D (25-OHD, a measure of vitamin D status), which correlate significantly and negatively with body mass and percentage body fat.
Lower levels of 25-OHD in obesity were seen to be unrelated to skin production of vitamin D3 or dietary absorption of vitamin D2. Instead, they are likely due to decreased 25-OHD bioavailability because of its deposition in various body fat compartments - meaning morbidly obese individuals might need higher doses of vitamin D supplementation than the average population.
Obesity is also the most important known determinant of type 2 diabetes. Interestingly, vitamin D deficiency is also suspected to be a risk factor.
While some studies suggested that the association between 25-OHD and impairments in glucose metabolism may be independent of body size, a detailed investigation of the role of obesity was considered necessary to verify this observation. In 2006, a study published in the journal Diabetes Care specifically examined the relationship between 25-OHD and obesity in relation to BMI and glucose metabolism as measured by HbA1c [A1C] in 7,189 participants from the 1958 British birth cohort aged 45 years.
Their results suggested that vitamin D status may indeed influence glucose metabolism, depending on body size. Interestingly, the decrease in A1C with increasing 25-OHD was the steepest when 25-OHD levels were <65 nmol/l, which fits in well with estimates for optimal 25-OHD status for bone health (75 nmol/l).
In this study population, >80% of obese participants had 25-OHD levels <75 nmol/l. These findings are important for public health, especially if improvement in vitamin D status in obese individuals has the potential to reduce adverse effects of excess weight on glucose metabolism. The study authors concluded that randomized controlled trials using doses of 25-OHD sufficient to improve levels in obese individuals were necessary to determine whether improvements in glucose metabolism could be obtained by vitamin D supplementation.
More recent studies indicate that high 25-OHD levels may be associated with a lower risk of insulin resistance and type 2 diabetes. Vitamin D itself may also impact type 2 diabetes through various mechanisms, such as impaired insulin resistance. Interestingly, many of these pathways are biologically relevant to obesity as well, given that obesity is a risk factor for type 2 diabetes, insulin resistance and low blood 25-OHD levels. The association between blood 25-OHD levels and A1C was stronger in adults with a high BMI relative to those with a low BMI.
Because of decreased bioavailability of vitamin D due to storage in body fat compartments, obesity may interact with vitamin D to influence risk of insulin resistance and type 2 diabetes. However, the possible interaction of vitamin D and obesity with regard to type 2 diabetes and insulin resistance needed to be properly studied.
To examine the effect of obesity on the association between 25-OHD and insulin resistance/type 2 diabetes, researchers from the Drexel University School of Public Health examined data from the National Health and Nutrition Examination Survey (NHANES), a large, nationally representative sample from across the US. Their study results are published in the Oct. 2012 edition of the journal Diabetes Care.
In this sample of adults >20 years of age, researchers found evidence that being overweight or obese modified the associations of 25-OHD with insulin resistance. Further, obese individuals with low 25-OHD had more than 32 times the risk for insulin resistance, much higher than the nearly 20-fold increase among obese individuals with sufficient 25-OHD. The stronger interaction of 25-OHD with obesity in regard to insulin resistance as compared with diabetes may be related to the direct impact of vitamin D deficiency on insulin resistance.
Deposition of fat-soluble vitamin D in adipose tissues, for example abdominal fat, may explain the joint effect of vitamin D insufficiency and obesity on higher risk of insulin resistance. This can result in lower vitamin D bioavailability for influencing pancreatic beta-cell function or activating vitamin D receptors, increasing the risk of adverse glycemic outcomes.
The results of this study are consistent with those reported before, that there was a stronger association between blood levels 25-OHD concentration and A1C in obese compared with normal weight subjects.
These study authors also believe that their results have significant public health significance, since they estimate that nearly half of the cases of insulin resistance in the general population can be explained by interaction between low 25-OHD and high BMI. This further suggests that insulin resistance in obese individuals can be reduced by improving blood levels of 25-OHD - a relatively inexpensive and practical means of reducing diabetes burden. In their opinion, future randomized, controlled trials should focus on the impact of vitamin D supplementation among obese individuals as it may enable them to improve glucose homeostasis and lower their risk for insulin resistance and type 2 diabetes.