The National Center for Health Data statistics advises that almost one in three Americans has vitamin D blood levels below 20 nanograms per milliliter (ng/ml) that is needed for bone health. Some experts say even higher levels are needed.

Figuring out why your Vitamin D levels are low could take some investigation. You can get the vitamin from a few whole foods and by taking supplements (many doctors recommend taking 800- 1000 IU of vitamin D3 a day). The best source of Vitamin D is produced by the body in a complex process that starts when ultraviolet B (UVB) rays part of the light spectrum are absorbed by the skin. The liver the kidneys, are involved in bio chemical steps that eventually result in a bio available form of Vitamin D that the body can use. A review in a Swedish medical journal identified some interesting facts that could affect your levels of Vitamin D. Here are a few that you might not have heard before.

1. The air pollution where you live. Carbon particulates in the air from the burning of fossil fuels, wood, and other materials scatter and absorb UVB rays. Ozone absorbs UVB radiation.

2. Your weight. Fat tissue sops up vitamin D and therefore being overweight may affect the bioavailability of vitamin D.

3. Your age. As you age the body is less efficient at making the bio available form of Vitamin D and therefore levels drop off.

4. The health of your gut. The vitamin D that is consumed in food or as a supplement is absorbed in the part of the small intestine immediately downstream from the stomach. Stomach juices, pancreatic secretions, and bile from the liver, the integrity of the wall of the intestine — they all have some influence on how much of the vitamin is absorbed. Therefore, conditions that affect the gut and digestion, like Celiac disease, IBS, Colitis, chronic Pancreatitis, Crohn’s disease, and Cystic fibrosis, can reduce vitamin D absorption. It should also be noted that the imbalance of Gut Flora due to antibiotic use and poor diet could also affect the health of the intestine and therefore the absorption of Vitamin D.

5. The health of your liver and kidneys. Some types of liver disease can reduce absorption of vitamin D because the ailing liver isn’t producing normal amounts of bile. With other types, steps essential to vitamin D metabolism can’t occur — or occur incompletely. Levels of the bioactive form of vitamin D tend to track with the health of the kidneys, so in someone with kidney disease, bioactive vitamin D levels decrease as the disease gets worse, and in end-stage kidney disease, the level is undetectable.