Pantothenic acid
Up to 50 to 95% of the vitamin B5 (pantothenic acid) found in plant and animal tissues is in the form of coenzyme A and 4‘-phosphopantetheine, an essential element of fatty acid synthesis. Once absorbed by the gut the vitamin is transported to the target tissues and taken up by the cells. Whilst no specific storage organs are known for vitamin B5, high tissue concentrations can be found in cardiac muscle, kidneys, adrenal glands and liver. Coenzyme A acts as a universal acyl group carrier in energy metabolism (ATP synthesis). 4'-phosphopantetheine produces a functional group of fatty acid synthase, a multi- enzyme complex for the synthesis of saturated fatty acids (25). Excess pantothenic acid is mainly excreted via the kidneys, in urine.
The toxicity of pantothenic acid is thought to be very low. In studies, intakes of 10 g per day over several weeks were well tolerated. Neither the US Institute of Medicine (26) nor the European Food Safety Authority (27) has defined tolerable upper intake levels for pantothenic acid.
Vitamin B6
Vitamin B6 is occurs widely in plant-source foods (mainly as pyridoxine) and in animal-source foods (mainly as pyridoxal). Moreover, bacteria in the intestines are capable of synthesizing vitamin B6 and thus increasing the amounts of pyridoxine available. Vitamin B6 consumed in the diet is absorbed from the whole length of the small intestine and carried first to the liver, whence it is transported to peripheral tissues like the muscles (28). There, in the form of pyridoxal-5'-phosphate, it is involved as a coenzyme in numerous enzymatic reactions of the carbohydrate, lipid, amino acid and neurotransmitter metabolism systems. If intake is adequate, the total body stores of vitamin B6, mostly in the form of enzyme -bound pyridoxal-5'-phosphate, are around 100 mg, divided between muscle tissue and liver. Excess ingested vitamin B6 is excreted via the kidneys.
The results of some studies provided evidence that targeted administration of pyridoxine in doses over
100 mg daily could, in the long term, lead to neurological disorders such as signs of paralysis, problems with the sense of temperature or numbness in the extremities (29, 30). One controversial study reported cases of women who had taken 50 mg pyridoxine or even less over a period of almost three years in food supple-ments who complained of neurological disturbances (31). Other studies did not observe such symptoms. Based on current data, the US Institute of Medicine (32) has defined an tolerable upper intake level (UL) for adults of 100 mg pyridoxine per day, whilst the European Commission’s Scientific Committee on Food (33) defined a UL of 25 mg per day for adults.
Biotin
Vitamin B7 (biotin) can be synthesized by many fungus and plant species, as well as by the bacteria in the human gut. The latter is of minor importance for provision with this vitamin, because it is mainly absorbed by the small intestine, while the bacteria are found in the large intestine. After absorption it is carried through the bloodstream to the cells of the target tissues, where it acts as coenzyme in a number of carboxylase reactions (introduction of COOH groups) in the amino acid metabolism system and in the biosynthesis of fatty acids (34).
Biotin appears to have very low toxicity: neither injections and oral administration of 10 mg biotin daily for six months to small children (35), nor injections of 20 mg in adults (36) led to any kind of side effects. Consequently neither the US Institute of Medicine (37) nor the European Commission’s Scientific Committee on Food (38) has defined an tolerable upper intake level for vitamin B7.