Do obesity and central adiposity lower serum 25‑hydroxyvitamin D by sequestration and dilution?

Obesity and central adiposity are consistently associated with lower circulating levels of serum 25-hydroxyvitamin D [25(OH)D], representing a common challenge in clinical practice.

Clinical evidence

Epidemiologic and longitudinal data, including findings from the English Longitudinal Study of Ageing (ELSA), establish that abdominal obesity (measured by waist circumference) directly predicts an increased risk of incident vitamin D deficiency, with obese individuals exhibiting more than double the risk compared to normal-weight counterparts. Bidirectional Mendelian randomization studies confirm a clear causal direction: genetically determined indicators of higher adiposity (such as BMI and body fat percentage) reduce serum 25(OH)D levels, whereas a genetic predisposition to lower vitamin D status has little to no causal effect on adiposity.

Mechanistic explanations

The reduction of circulating vitamin D in individuals with obesity is driven by two key physical phenomena:

• Adipose Sequestration: Vitamin D is highly lipophilic and partitions passively into the neutral lipid droplets of adipocytes. As adipose tissue mass expands, it acts as a massive physical "sink," sequestering larger quantities of vitamin D and restricting its mobilization back into the bloodstream.
• Volume of Distribution: Physiologically based pharmacokinetic (PBPK) models demonstrate that increased fat mass and body weight significantly expand the apparent volume of distribution of vitamin D. This larger distribution space dilutes circulating levels, resulting in a blunted rise in serum 25(OH)D following oral supplementation or UV exposure.

Clinical implications

Because of this physical dilution and sequestration, standard supplementation regimens are often insufficient for individuals with high adiposity. Achieving optimal serum 25(OH)D levels in obese patients typically requires clinical dosing strategies that are 2 to 3 times higher than those used for normal-weight individuals.

Bottom line

• Obesity and central adiposity causally lower serum 25-hydroxyvitamin D levels because the highly lipophilic vitamin is sequestered within the expanded adipose tissue reservoir and diluted across a larger volume of distribution, necessitating higher therapeutic doses to treat deficiency.