The human body requires about 70 IU of vitamin D per kilogram of body weight. That is in total, from from sun, food and supplements. In the UK , it is very difficult to get that amount , hence many people are deficient. Food contains small amounts, even oily fish , which is probably the best food source, only contains a few hundred IU per serving. The principle source , is or was , sunlight. Our skin can make 10,000 - 20,000 IU in just a 20 min whole body exposure, around midday , in the summer months.
It is best to take a daily supplement in the order of a few thousand units , in order to raise your blood level up to about 120 - 140 nmol/litre. The exact amount is very dependent upon your own response, some people are very sensitive , and will not require as much, other people need more.
Lack of vitamin D has effects throughout the body. Vitamin D , affects each and every cell in your body, it enables each cell to respond to signals , and read the DNA code , approx 2000 genes are either directly or indirectly affected by vitamin D processes in your body. That is approx 10% of your entire genome. it is no wonder , that if you are low on this vitamin you may have all kinds of health problems. Including , calcium absorbtion- leading to bone and joint pain, muscle weakness etc. This particular aspect of deficiency can lead to osteomalacia, which is often mis-diagnosed as fibromyalgia.
The auto immune response can be affected, leading to worsening of auto-immune diseases, low vit D can lead to the inability of the body to fight off infections, etc etc etc.
see some other postings about this here on another thread
www.mumsnet.com/Talk/general_health/2376645-Hospital-wouldnt-test-me-for-vitamin-D
www.mumsnet.com/Talk/general_health/2377129-Quick-Self-Check-for-Vitamin-D-deficiency
Hope this helps
BTBH