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.
Low levels of a vitamin can result from eating a poor diet or not being able to absorb the vitamins you consume. Older adults, vegetarians and people with digestive disorders such as celiac disease or Crohn's disease may have trouble getting enough B-12. Sometimes a vitamin B-12 deficiency occurs for unknown reasons.