Federal Research Centre of Nutrition,
Biotechnology and Food Safety,
Ustinsky proezd, 2/14, Moscow, 109204, Russian Federation
ABSTRACT. This manuscript contains no answers to any of the questions posed in the title. Instead, the reader is invited to a discussion about what is a physiological need and an adequate intake level of a nutrient, and how to determine them; why an understanding of the signs of clinical deficiency is necessary to recommend a nutrient consumption rate; how to draw the boundary of the metabolic influence of a chemical element; and why, with regard to sulphur, reasoning of all these things inevitably leads to a dead end.
KEYWORDS: thiol, mercaptan, thioether, methionine, cysteine, thiamine, biotin, thioctic (lipoic) acid, recommended dietary intake, adequate intake level.