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 levels of vitamin D badly affect the bones of our body. In children, it can lead to a disease called rickets. This results in slow bodily growth and deformation of long bones like the leg bones. In adults, low vitamin D levels can cause thinning of bones and the condition is referred to as osteomalacia. It causes fragility of bones and weakens the muscles so much that one can suffer from severe muscle pain. Another disease caused due to low levels of vitamin D is osteoporosis.