As co-enzymes, the B vitamins are essential components in most major metabolic reactions. They play an important role in energy production, including the metabolism of lipids, carbohydrates, and proteins. B vitamins are also important for blood cells, hormones, and nervous system function. As water-soluble substances, B vitamins are not generally stored in the body in any appreciable amounts. Thiamin and riboflavin are all essential coenzymes in energy production. Thiamin is converted quickly into thiamin pyrophosphate, which is required for glycolytic and Krebs cycle reactions. Thiamin also appears to be related to nerve impulse transmission. Riboflavin is a component of the coenzymes FAD and FMN, which are intermediates in many redox reactions, including energy production and cellular respiration reactions. Vitamin B-6 is a coenzyme in amino acid metabolism. It is necessary for the metabolism of homocysteine and the conversion of tryptophan into niacin. Vitamin B-6 dependent enzymes are also needed for the biosynthesis of many neurotransmitters, including serotonin, epinephrine, and norepinephrine. Pantothenic acid is a component of coenzyme A and of phosphopantetheine, and is therefore essential for Krebs cycle operation. Vitamin A (retinol) is a fat-soluble vitamin essential for vision, growth, reproduction, cell division, and the integrity of the immune system. Zinc is an essential trace element involved in most major metabolic pathways. General signs of human zinc deficiency indicate that zinc has important functions in maintaining immune function, reproduction, healthy skin, and growth. Adrenal and spleen glandulars are added too for their supportive roles in stress management and immune health.