Several specific dietary components have evoked much interest recently. Thus, the current literature includes several reports of new substrates in clinical investigations. This article focuses on glutamine and the introduction of adjuvant treatment with growth factors. To make such investigations meaningful, two conditions should be established. First, proper techniques should be used to measure the metabolic changes as close as possible to the cellular level. Second, the pathophysiology should be known in order to mechanistically explain possible beneficial effects of new substrates. With that background, this review emphasizes reports on methodology and pathophysiology in addition to those concerning the effects of new substrates.