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.
The second reason for a low vitamin D is that you have primary hyperparathyroidism due to a small tumor in your neck and the body converts all the vitamin D-25 (that your doctor is measuring) to the active Vitamin D-1-25 (that doctors do not measure). This is the blue blob on the graph.
The most common cause of low vitamin D is lack of sun exposure. Low levels of vitamin D are a definite red flag that your good bill of health is being threatened. Vitamin D deficiency is an underlying cause of many poor health conditions, as well as a major factor contributing to the exacerbation of symptoms of other diseases. The causes of low vitamin D can be very complex, but ensuring you get enough may be easier than you think.