A recent study found that people with obesity have lower blood levels of vitamin D than people of healthy weight. This association of obesity with low vitamin D levels has led to much speculation on whether low vitamin D levels cause obesity or whether obesity causes low vitamin D levels. The interest in this topic is piqued by the possibility that if vitamin D deficiency causes obesity, perhaps treatment could be as simple as providing vitamin D supplementation to enhance weight loss.
What is known about vitamin D’s role in the body?
It’s well known that vitamin D is essential for bone health as well as balancing the minerals calcium and phosphorus, but what is its role in weight management? Approximately 80%-90% of vitamin D in the body is from the skin synthesis of cholecalciferol through ultraviolet B radiation from sun exposure. The normal range of 25-hydroxy vitamin D is measured as nanograms per milliliter (ng/mL). Most experts recommend a level between 20 and 40 ng/mL, but this has been a controversial topic of never-ending debate in the medical literature.
Vitamin D levels and obesity
This has been noted for many years without identifying the underlying reasons beyond the sequestering of vitamin D in adipose tissue, although I’ll discuss other possibilities.
The inverse correlation between vitamin D and obesity has been seen in other diseases, such as cardiovascular disease, hypertension, prediabetes, and insulin resistance, as well as in sarcopenia and aging. Most studies emphasized the correlation between increasing adiposity with vitamin D deficiency in all ethnic and age groups. The causes and potential direct consequences of the vitamin D deficiency state in obesity are not well understood.
Vitamin D and adipose tissue
It’s been proposed that low vitamin D status in obesity might be due to increased vitamin D clearance from serum and enhanced storage of vitamin D by adipose tissue.
In adipose tissue, vitamin D exerts a variety of effects on inflammation, innate immunity, metabolism, and differentiation and apoptosis in many cell types. There is a stronger association between 25(OH)D and visceral fat as compared to subcutaneous adipose tissue, which suggests an influence of inflammation and components of the metabolic syndrome on vitamin D metabolism.
Because vitamin D has anti-inflammatory properties, it’s possible that low vitamin D status contributes to adipose tissue inflammation, a key link between obesity and its associated metabolic complications in obesity. A higher storage of vitamin D in adipose tissue, if accompanied by a parallel increase in the local synthesis of 1,25(OH)2D3 and action, may conceivably modulate adipocyte function as well as the activity of adipose tissue macrophages and hence the level of adipose tissue inflammation. In addition, it seems likely that 1,25(OH)2D3 also regulates the function of macrophages and other immune cell populations within adipose tissue.
It’s well known that vitamin D is stored in body fat, leading to the assumption that this was important in the evolution of vertebrates, including humans, who lived at latitudes where vitamin D could not be made in the winter and vitamin D stores had to be mobilized to maintain vitamin D sufficiency.