The simple view of the bonding in methane You will be familiar with drawing methane using dots and crosses diagrams, but it is worth looking at its structure a bit more closely. There is a serious mis-match between this structure and the modern electronic structure of carbon, 1s 2 2s 2 2p x 1 2p y 1.
Lack of sun exposure: Lack of sun exposure is a major cause of low vitamin d level especially if you live in the northern us. Less likely would be lack of vitamin d in food, but also a possibility if you are on a very restrictive diet.
Complications With Certain Medications. According to the University of Maryland Medical Center, vitamin D taken at the same time as cholesterol lowering drugs can cause the drug to be less effective. (Ref 3) As well, bile acid sequestrants, also used to lower cholesterol, can cause low vitamin D levels. Similarly, certain weight loss drugs that reduce the amount of fat your body absorbs from food can cause low vitamin D levels. The medication may affect how readily your body absorbs vitamin D.