Dietary lipid is important both quantitatively and qualitatively to human health, and intestinal processing of exogenous lipid is central to its assimilation. While much is known about luminal digestion of dietary lipid, its intracellular transport and processing are less well understood. These studies will focus on the two different fatty acid-binding proteins (FABPs) which are expressed in the enterocyte, intestinal-FABP and liver-FABP (IFABP and LFABP). While thought to be critical for enterocyte lipid trafficking, this has not been directly demonstrated. Nor is it clear why two homologous proteins that both bind FAs, are co-expressed in the enterocyte. Our earlier kinetics studies demonstrated marked differences between the proteins, suggesting that IFABP and LFABP are likely to have distinct functions We have now compared mice null for these FABPs for the first time, and the remarkable phenotypic differences observed indicate that they indeed have at least some unique roles, not only in enterocyte lipid metabolism, but even more so by leading to markedly different effects on whole body energy homeostasis. Specifically, we find that high fat-fed LFABP-/- mice are hyperphagic and markedly obese, yet do not display glucose intolerance and have improved exercise performance, thus displaying a metabolically healthy obese (MHO) phenotype. The IFABP-/- mouse is also healthier than WT, with improved glucose tolerance and more efficient intestinal lipid secretion. However in marked contrast to the LFABP-/-, the high fat-fed IFABP-/- mouse weighs less and accumulates less body fat and more lean mass than WT mice. Both null mice studied are global knockouts, and while IFABP is normally expressed only in intestine, LFABP is also highly expressed in liver, thus we do not yet know whether the dramatic changes in LFABP-/- mice are due to LFABP in intestine, liver, or both. It is also not yet known how loss of enterocyte IFABP leads to a lean phenotype. Interestingly, we have found changes in intestinal metabolism of fatty acids and monoacylglycerols and in levels of mucosal endocannabinoids (EC), in particular the MG 2-arachidonoyl glycerol, and hypothesize that these tissue-level effects of FABP ablation contribute, at least in part, to the whole body phenotypes. Based on these observations and questions, our aims are to 1) determine whether the hyperphagia and obesity observed in the LFABP-/- mouse are mediated via the EC system; 2) explore the mechanisms by which ablation of IFABP leads to decreased fat mass and body weight and increased lean body mass; 3) use our newly generated LFABP/IFABP double knockout mouse (L/I-DKO) to definitively determine whether the enterocyte FABPs are required for bulk lipid uptake; and 4) define the contributions of liver LFABP vs. intestinal FABP to the MHO LFABP-/- phenotype, and the contributions of intestinal FABPs (L + IFABP) vs. hepatic FABP (LFABP) to the downstream systemic effects of intestinal lipid processing. The proposed studies will use the L/I-DKO mouse, mice generated by conditional deletions of liver LFABP or intestinal LFABP, and adeno-associated viral replacement of LFABP vs. IFABP in the LFABP-/- and L/I-DKO liver, to provide important new information about enterocyte lipid transport and processing, and their relationships to systemic fuel metabolism.