What is vitamin D’s role in obesity?
The main question that has eluded an answer so far is this one: Is vitamin D deficiency only a coincidental finding in obesity due to sequestration of the vitamin in fat, or does it have a role in the development of obesity and its complications, such as cardiovascular disease, type 2 diabetes, and hypertension?
Low vitamin D usually leads to impaired calcium absorption in the intestine and a lower calcium level, and eventually enhanced bone turnover and impaired bone mineral density (BMD).
However, it is known that in obesity there is greater BMD than in those who are lean. This leads to the conclusion that because there is a lack of vitamin D deficiency effects on bone in those with obesity, there is not really a vitamin D deficiency, and it may be that the sequestration in adipose tissue leads to a permanent supply that can maintain bone health.
An alternative explanation is that there is greater skeletal loading in obesity, and elevations in hormones such as estrogen and leptin could compensate for the vitamin D deficiency, leading to greater BMD in obesity.
Several other potential mechanisms besides sequestration of vitamin D in adipose tissue have been identified for low vitamin D and obesity. These include impaired hepatic 25-hydroxylation in nonalcoholic fatty liver disease, less sunlight exposure due to lower mobility and different clothing habits in people with obesity vs. their lean counterparts, and adverse dietary habits. For example, people with higher BMIs spend less time exercising outdoors and are more sedentary in general than their lean counterparts. Therefore, they are less likely to get sun exposure because of a decrease in time spent outdoors. Those with higher BMIs also tend to cover their bodies, showing less skin when outdoors than their leaner counterparts, and thus there is likely to be less conversion to vitamin D via skin and sun exposure in people with obesity.
Some studies suggest that an increased level of parathyroid hormone due to vitamin D deficiency promotes lipogenesis because of greater calcium influx in adipocytes. Another hypothesis is that the active form of vitamin D, 1,25(OH) D, inhibits adipogenesis through actions modulated by vitamin D receptors. These studies are promising, but prospective randomized trials are needed to determine whether vitamin D supplementation is a treatment option in preventing obesity. So far, vitamin D supplementation shows inconsistent results.
To conclude, there is a high prevalence of vitamin D deficiency in obesity, most likely because of dilution and sequestration in greater volumes of fat, blood muscle, and liver in obesity. Low vitamin D levels can’t be ruled out as a cause of obesity because of the research showing some interesting findings in vitamin D receptors in adipose tissue. Vitamin D deficiency in obesity doesn’t seem to affect bone mass but could have deleterious effects on other organ systems.
Weight loss improves obesity and complications, including the risk for cardiovascular disease and type 2 diabetes as well as vitamin D deficiency.
What do the guidelines say?
Treatment of vitamin D deficiency requires higher doses in obesity to achieve the same serum concentration compared with lean persons. Maintenance doses should not differ between those with obesity and lean persons.
The association of vitamin D and obesity remains elusive. Studies need to focus on vitamin D, vitamin D receptors, and actions of vitamin D in adipose tissue to investigate this relationship further.
In the meantime, media attention remains focused on the potential treatment and prevention of obesity with the mighty, all-purpose vitamin D, even though there is scant evidence.
Dr. Apovian is codirector at the Center for Weight Management and Wellness at Brigham and Women’s Hospital and professor of medicine in the division of endocrinology, diabetes and hypertension at Harvard University, Boston. She disclosed conflicts of interest with Abbott, Allergan, Altimmune, Bariatrix Nutrition, Cowen and Company, Curavit, Rhythm Pharma, Jazz, Nutrisystem, Roman, Novo Nordisk, EnteroMedics, Gelesis Srl, Zafgen, Xeno, L-Nutra, Tivity, and Real Appeal.
A version of this article first appeared on Medscape.com.