vitamin D production
when the skin is exposed to ultraviolet light , a precursor molecule of vitamon D is formed.
the precursor is carried by the blood to the liver, where it is modified, and then to the kidneys, where the precursor is modified further to from active vitamin D.
If exposed to enough ultraviolet light , humans can produce all the vitamin D they need.
Many people need to ingest vitamin D, however , because clothing and indoor living reduce their exposure to ultraviolet light.
fatty fish (and fish oils) and vitamin D-fortified milk are the best sources of vitamin D.
Eggs, butter, and liver contain smll smounts of vitamin D,but are not considered significant sources because too large a serving size is necessary to meet daily vitamin D requirements.
adequate levels of vitamin D are necessary because vitamin D stimulates calcium and phosphate uptake in the intestines.
these substances are necessary for normal bone metabolism and normal muscle function .