Vitamin B complex is a group of vitamins that includes thiamine, riboflavin, niacin, B-6, folate, B-12, pantothenic acid, biotin and choline. Individually, deficiencies in these vitamins may lead to a variety of conditions (depending on which ones are deficient) including unhealthy weight loss, emotional distress, weakness, irregular heartbeat, diarrhea, inflammation of the skin, anemia, and even dementia.
We could also say that vitamin B complex is different from most other vitamins because there is a very wide variety of foods that one can get the vitamins in the complex from (probably on account of the fact that it is made of so many different substances).
Our nervous system requires variety of nutrients to function daily, vitamin B6 helps not only maintain a right levels of cholesterol but also increase the circulation of nutrients to our nervous system needed resulting in decreasing the symptoms of endometriosis, such as memory loss, confusion and anxiety.
To keep people informed of the correct nutrition intake, the Food and Nutrition Board of the National Academy of Sciences/National Research Council publishes the Recommended Daily Allowances (RDA) for vitamins and other nutritional supplements based on scientific researches and clinical findings.
The reason that the set of vitamins skips directly from E to K is that the vitamins corresponding to letters F-J were either reclassified over time, discarded as false leads, or renamed because of their relationship to vitamin B, which became a complex of vitamins.