It is stated frequently that salmonids, because of their carnivorous nature, logically have a high dietary vitamin B6 requirement in comparison with species with lower protein requirements (Woodward, 1994). To date there is no conclusive evidence to support such a statement. Estimated pyridoxine requirements that have been reported to maximize the weight gain in young fish of nonsalmonid species range from 0.2 mg to 1.1 mg pyridoxine per 100 g protein. Values for salmonids range from 0.5 mg to 0.77 mg pyridoxine per 100 g of protein. Due to the different diets and husbandry conditions it is very difficult to make meaningful comparisons among different requirement studies.
Recent work with tilapia (Shiau and Hsieh, 1997) suggests that the dietary protein level influences the dietary pyridoxine requirement. The dietary pyridoxine requirements for optimal alanine aminotransferase activity were 9.5 mg and 15 mg in a 28% and 36% protein diet, respectively. Growth in the lower protein diet, which was estimated to be below the optimal protein requirement, was not optimal, which confounded the requirement estimate. Theoretically, if the protein had a well balanced amino acid profile that was available to the fish, most of the amino acids would have been utilized for protein synthesis, possibly sparing the amount of vitamin B6 required for catabolism of the amino acids.
Increasing the dietary pyridoxine level from 5 to 50 ppm was found to enhance sea bream (Sparus autraus) growth, feed efficiency and protein efficiency ratio (Baker and Davies, 1995) in a high-protein diet but not a low-protein diet (24 g versus 15 g of protein per MJ GE). Increasing the dietary pyridoxine level improved the apparent net protein utilization at both the low and high dietary protein levels.
Other studies into this question have not shown any interaction between dietary protein level and pyridoxine requirement in fishes (Hardy et al., 1979). There was no interaction between dietary protein and pyridoxine level on weight gain or feed efficiency in juvenile shrimp (Penaeus japonicus) (Giri et al., 1997).
Other studies showed a dose-dependent effect of pyridoxine on plasma glucose concentration in sea bream (Sparus autraus) (Morris and Davies, 1995a). This was thought to be due to the role of vitamin B6 in glycogen metabolism.
