Many studies have shown an inverse relationship between 25OHD and obesity. Some possible explanations include the popular fat sink (increased storage in the adipose tissue), sedentary lifestyle of the obese, low sunlight exposure among the obese, genetic changes in obesity or other unknown factors. The concept of a fat sink is popular but no randomized controlled trial ever explored the issue. Animal studies show that the vitamin D is stored in muscle, liver and skin, as well as fat.
Professor Reinhold Vieth, long ago, first objected to the popular theory that fat serves as a sink, trapping vitamin D. From what Dr. Vieth could tell, such a complicated explanation was not needed. Fat people have lower vitamin D levels because what vitamin D they do have is diluted by body weight. There is no need for a complicated fat-soluble sink, which traps vitamin D; volume distribution explains it just fine.
J. Christopher Gallagher, MD, and colleagues of Creighton University Medical Center recently found that Vieth was right. They conducted a randomized controlled trial to discover that differences in serum 25OHD levels after various doses of vitamin D among normal-weight and obese patients are simply due to volume dilution.
The authors examined dose response curves to vitamin D in 163 older women according to BMI and by dividing the 163 women into seven vitamin D dose groups (400, 800, 1600, 2400, 3200, 4000 and 4800 IU/day). After a year, the higher doses increased 25(OH)D by about 35 ng/ml while the lower doses increased it by about 15 ng/ml. They confirmed their previous finding that at 25(OH)D levels around 45 ng/ml, the body begins to change how it metabolizes vitamin D; that is, the liver’s wide open metabolic funnel of vitamin D into 25(OH)D begins to be regulated.
The lower dose groups showed little variation in serum 25OHD changes across all BMI groups. In the medium and high dose vitamin D dose groups, only thin women with a normal BMI show higher changes in vitamin D levels. The authors concluded,
“The dose response curves are so parallel between thin and obese people that the difference in serum 25OHD levels must be due to a volume dilution effect.”