Editor: Amanita Setari
Background and motivation for the study
My personal interest in this study stems from my early career as a personal fitness trainer and my residence in Tasmania, Australia, where vitamin D deficiency is seasonally endemic. Vitamin D is primarily synthesised within the body from sun exposure but with more and more people working indoors and with the thin ozone layer above Tasmania leading to harsh sunburn, people tend to avoid the sun and subsequently lack sufficient vitamin D. Vitamin D deficiency is also widespread in other developed countries, making it a global concern.
While vitamin D is often cited as being important for bone health, there is evidence to suggest that vitamin D may impact other aspects of health as well. Vitamin D deficiency has been implicated in several pathological conditions. We were motivated by data pointing to vitamin D deficiencies as a risk factor for metabolic diseases, such as type 2 diabetes (resistance to the effects of insulin), and non-alcoholic fatty liver disease (NAFLD). It is therefore important to thoroughly elucidate the ways in which vitamin D deficiency affects us and to determine and provide evidence for the broader relationship between sex and metabolism.
Results and how they were found
We fed both male and female mice different diets that were either low or high fat, combined with either low or high doses of vitamin D. In doing so, we were able to create metabolic disturbances in the mice and observe the effects.
Testing showed that male mice were more prone to obesity and sugar intolerance than female mice. Furthermore, we uncovered that female mice were more likely to develop symptoms of NAFLD, and that vitamin D deficiency could be solely responsible for NAFLD in both sexes. Importantly, ours was the first study to demonstrate that vitamin D deficiency alone is sufficient to increase fat in the liver, leading to questions related to disease prevention.
There is always a need to demonstrate that the same experimental outcomes in our study are applicable to humans. Additionally, the high-fat diet group of mice we intended to keep vitamin D deficient, had unintended vitamin D present in the diet. This led to difficulties in determining the true effect of combining a high-fat diet with the condition of vitamin D deficiency.
What does this mean for the future?
Males and females have different regulatory mechanisms governing metabolism under different circumstances, most likely owing to their respective sex hormones. Understanding the consequences of these differences may help us to appropriately cater our approach to metabolic diseases by personalizing treatment according to sex.
Although previous studies have tested whether vitamin D can reverse metabolic diseases, they have done so unsuccessfully. What has not been assessed rigorously is whether vitamin D supplementation could help prevent metabolic diseases. Therefore, further assessment is needed to determine whether vitamin D has a role in disease prevention.
Finally, as with all research in the biological sciences, there was a dedicated team working on the contents of this paper. I would like to acknowledge Dr. Ellen Bennett, Dr. Graeme Zosky, and Dr. Renée Dwyer for the immense amount of work they contributed.
Research article: The Impact of Sex and 25(OH)D Deficiency on Metabolic Function in Mice. Nutrients. 2017.