Structured triglycerides (STG) are introduced for clinical purpose by a recently registered lipid emulsion, in which long-chain fatty acids and medium-chain fatty acids are randomly attached to the same glycerol molecule. Studies in animals showed an improvement in nitrogen balance. Studies in patients demonstrate an improvement or at least equivalent effects on the nitrogen balance, compared with LCT or a physical mixture of MCT and LCT. In most of the studies serum triglycerides, and free fatty acid concentrations are lower indicating a faster clearance from the blood; beta-OH-butyrate concentrations are higher but do not reach toxic levels, or influence the serum pH value. No modulatory effects on the immune system by STG can be demonstrated. Hypotheses are discussed of the influence of lipids on protein metabolism. The recently introduced STG open the door for really new structured triglycerides in which on-demand specific fatty acids can be linked to a specific position at the glycerol molecule, with the objective of realising specific clinical effects. (C) 2002, Elsevier Science Ltd. All rights reserved.