Vitamin D2 and D3 are fat-soluble (absorbed with dietary fat and stored in fat and liver tissue) dietary nutrients, that are also hormones. While there is some controversy, most health professionals agree that vitamin D is technically a hormone (or a prohormone, a precursor to a hormone) and not a nutrient because of how it can be synthesized by the body (thank you sun), is activated by the liver and kidneys, and acts to regulate calcium metabolism. Vitamin D is naturally present in few foods and in small amounts. D3 (cholecalciferol) can be found in animal sources such as salmon and egg yolks, while mushrooms are one of the only sources of plant-derived D2 (ergocalciferol). However, the majority of dietary intake generally comes from fortified foods such as milk and cereals. While neither form is better, D3, which your body can also produce in response to exposure to UV B rays, is more effective in raising vitamin D levels in the blood (2). A glass of milk or a daily supplement is a start, but the best way to raise vitamin D levels is to spend a little more time outside; a daily 15-minute walk should do the trick.

Along with regulating calcium metabolism – so you can build and maintain strong bones and teeth – vitamin D is vital for:

If you are having difficulty recovering from illness or are experiencing fatigue and unexplained muscle pain, low vitamin D levels may be a factor. In addition, there is a growing body of evidence that vitamin D is crucial for cardiometabolic health. Difficulty maintaining a healthy weight and regulating blood sugar levels may be other signs that your vitamin D levels are low.

Several studies have shown an association between obesity, increased body fat composition, and vitamin D deficiency (4). While there are several theories—primarily that obese individuals have less healthy lifestyles, spending less time under the sun and consuming less foods that are naturally high in vitamin D—there isn’t a consensus within the scientific and medical communities. What we do know is that vitamin D requirements are, in part, dependent on body weight as excess fat decreases the activity of the enzymes necessary to convert vitamin D into its active form (1,25-dihydroxycholecalciferol or calcitriol) (5). The more excess body fat you carry around, the more vitamin D per pound of body weight you need to reach the same blood serum levels (6).

Similarly, research has consistently shown that low vitamin D levels are associated with insulin resistance and the progression of type 2 diabetes (7). Like obesity, the association isn’t entirely clear, but we do know that vitamin D deficiency can reduce the activity of the proteins (GLUT-4) that are responsible for insulin regulating the uptake of glucose into muscle and fat cells (8). Low vitamin D levels, obesity, and insulin resistance seem to begin a progressive cycle of metabolic dysregulation: the lifestyle behaviors that promote low vitamin D increase risk for obesity and insulin resistance, increasing weight increases the need for vitamin D, and low vitamin D levels impair your ability to effectively use the insulin your body produces.

On the sunny side, studies have shown that weight loss can significantly increase vitamin D levels without a change in intake, and the greater the weight loss the greater the improvement (9). Increasing vitamin D levels, whether through outside activity or supplementation, may help reverse the cycle of metabolic dysregulation by impeding the formation of new fat cells and promoting higher levels of the fat-burning hormone testosterone (10-12). Vitamin D levels may also indirectly influence appetite through regulation of serotonin, a neurotransmitter that affects your mood and desire for food (13). Furthermore, supplementing with vitamin D3 may improve insulin sensitivity and promote the healthy function of beta cells (cells in the pancreas that make insulin) in individuals who are at high risk for developing type 2 diabetes (14).