Obese people may require alternative vitamin D supplementation, mouse study finds

26 Feb 2019 --- Vitamin D supplementation may be less effective in people with obesity, according to a study published in the Journal of Bone and Mineral Research. The researchers conclude that it may be more effective to treat vitamin D deficiency in obese individuals with calcidiol, rather than with other forms of vitamin D.

Also known as the sunshine vitamin, vitamin D is typically associated with bone health, yet in recent years it has been increasingly linked with body fat accumulation and obesity, with the nature of this relationship currently under investigation by researchers. Deficiency in the vitamin has also been linked to depression, cardiovascular disease, inflammation, diabetes and cancer.

The biological mechanism responsible for vitamin D’s weaker effect in obese subjects, which had previously been unknown, was identified by the researchers. In the study, obese mice had very low levels of the liver enzyme that converts vitamin D into 25-hydroxyvitamin D (calcidiol), which is the major form of vitamin D in the blood.

“The findings are surprising to the field because the dogma has been for many years that lower serum vitamin D in obese subjects was due to increased storage of vitamin D in body fat in this group,” lead author Dr. Jeffrey Roizen, of The Children's Hospital of Philadelphia, tells NutritionInsight. “There are two articles which suggest that there is not enough vitamin D in body fat to account for the observed differences in serum vitamin D between lean and overweight individuals and so the actual cause for the lower serum vitamin D in overweight individuals has remained an unanswered question.”

“Here, we propose a second mechanism with greater biological implications: obesity reduces the ability of the liver to convert vitamin D into 25-hydroxyvitamin D,” adds Dr. Roizen. Typically, the 25-hydroxyvitamin D tests how vitamin D levels are monitored. 

“Our observations show that this early step in activating vitamin D is influenced by obesity and suggest that obesity-related effects on the liver can have clinically important systemic effects on bone and mineral metabolism.” 

Furthermore, while it has often been thought that low vitamin D levels can cause obesity, this work shows that an illness or pathology – such as obesity – can cause low vitamin D, Dr. Roizen adds.

Measuring vitamin D levels
In related news, a recent study from Trinity College Dublin, Ireland and St James’s Hospital has offered a new alternative to the classical method of measuring the vitamin levels is through a 25-hydroxy vitamin D blood test. 

In a first-time finding published in Nutrients, the researchers reported that vitamin D levels can be measured in human hair.

The study notes that the typical vitamin D blood test can be painful, requires expertise and training along with hygienic conditions and equipment, so getting a sample is not always workable. In addition, the blood result represents vitamin D status at a single time point, which is problematic because vitamin D changes with the seasons: it's not uncommon for someone to be sufficient in vitamin D in the summertime and very deficient in the winter. This means that a single snapshot of vitamin D status is not able to provide information on vitamin D all year-round.

Hair, which grows at approximately 1cm per month, could reflect vitamin D status over several months capturing the large seasonal differences in its status.

“The presence of vitamin D in hair could be interpreted as a personal record of a person's vitamin D status. Knowing an individual's long-term vitamin D status through analysis of hair samples may allow for better strategies to maintain stable and adequate vitamin D concentrations over an extended period,” says Dr. Martin Healy, Principal Biochemist from the Biochemistry Department in St. James’s Hospital and co-author.

Further studies on the obesity front found that vitamin D supplementation may be part of an effective strategy to tackle childhood obesity and reduce the risk of serious health problems, such as heart disease, in adulthood. This was according to research presented at the 57th Annual European Society for Pediatric Endocrinology Meeting last October.  

Moreover, the long-awaited results of VITAL, the first randomized clinical trial of a general population large enough to adequately address questions surrounding the effects of vitamin D and omega 3 fatty acids, showed that vitamin D could reduce cancer deaths over time. However, it must be noted that the VITAL team also found that vitamin D did not significantly affect heart attack, stroke or cancer incidence. 

By Laxmi Haigh