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.
Low Vitamin D levels are caused by two very different reasons. Low vitamin D is common in people without any other health problems, but low vitamin D can also be caused by a parathyroid tumor. The connection between parathyroid disease and low Vitamin D can be confusing, even for many doctors. Parathyroid tumors cause low Vitamin D, not the other way around.