The niacin paradox describes the paradoxical situation that results from some important sources of niacin being low in actual niacin. This arises because while some foods are low in niacin, they are high in tryptophan, itself being able to be converted into niacin through the kynurenine pathway. The observation that rats could synthesise niacin, and that the synthesis of that niacin was related to the protein content of the diet lead some researchers to investigate the effects of amino acids on niacin metabolism. From this research it was found that administration of tryptophan to rats increased the urinary excretion of niacin. The realisation that administration of tryptophan was effective at reversing and preventing pellagra confirmed that niacin was likely synthesised from tryptophan. Initially it was thought that the tryptophan to niacin conversion may occur in the intestinal tract through the action of bacteria, but experiments administering injections of tryptophan showed immediate elevations of niacin metabolites.
Around 10 % of dietary tryptophan may be used for niacin synthesis. Niacin is therefore technically not a vitamin as it can be synthesised endogenously if adequate tryptophan is present in the diet. The niacin requirement in man is likely related to the total calorie intake, with higher energy intakes requiring larger intakes of niacin. A figure of around 5 mg of niacin per 1000 kcal is suggested to be the minimum required to prevent pellagra, assuming more than 2000 kcal in total are consumed. The realisation that some foods provide their niacin content as tryptophan has lead some to suggest that instead of absolute niacin concentrations, the niacin equivalents should be used. As a rough approximation the tryptophan concentration of protein is 1 %, and around 1 mg of niacin is obtained from 60 mg of tryptophan. From these figures a rough approximation of the niacin equivalents of food can be made. Some foods such as beef are high in tryptophan and niacin, and as a result are excellent sources of vitamin B3.