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.
There are many possible causes of vitamin D deficiency. Limited exposure to the sun. You may look out your window and see the sun shining and think that you are safe from this deficiency, but that is not always the case. Even in sunny climates there is an increased prevalence of vitamin D deficiency.