Mammals store metabolic energy as triacylglycerols (TGs) in adipose tissue. TG synthesis is catalyzed by the evolutionarily unrelated acyl-CoA:diacylglycerol acyltransferase (DGAT) enzymes DGAT1 and DGAT2， which catalyze the same reaction and account for nearly all TG synthesis. The reasons for their convergent evolution to synthesize TGs remain unclear. Mice lacking DGAT1 are viable with reduced fat stores of TGs， whereas DGAT2 KO mice die postnatally just after birth with >90% reduction of TGs， suggesting that DGAT2 is the predominant enzyme for TG storage. To better understand the functional differences between the DGATs， we studied mice fed chow or high-fat diets lacking either enzyme in adipose tissue. Unexpectedly， mice lacking DGAT2 in adipocytes have normal TG storage and glucose metabolism on regular or high-fat diets， indicating DGAT2 is not essential for fat storage. In contrast， mice lacking DGAT1 in adipocytes have normal TG storage on a chow diet but moderately decreased body fat accompanied by glucose intolerance when challenged with a high-fat diet. The latter changes were associated with the activation of ER stress pathways. We conclude that DGAT1 and DGAT2 can largely compensate for each other for TG storage but that DGAT1 uniquely has an important role in protecting the ER from the lipotoxic effects of high-fat diets.