PLA2G12B drives the expansion of triglyceride-rich lipoproteins (TRLs) in vertebrates. This study reveals that PLA2G12B channels lipids from the endoplasmic reticulum (ER) lumen into nascent lipoproteins, promoting efficient lipid secretion and preventing excessive intracellular lipid accumulation. PLA2G12B is calcium-dependent and tightly associated with the ER membrane. Mutant mice lacking PLA2G12B exhibit resistance to atherosclerosis, suggesting an evolutionary trade-off between TRL transport and cardiovascular disease risk. The study identifies PLA2G12B as a key driver of triglyceride incorporation into vertebrate lipoproteins. TRLs are essential for transporting hydrophobic triglycerides in the circulatory system. The microsomal triglyceride transfer protein (MTP) transfers lipids from the ER to Apolipoprotein-B (APOB), which is translated by ER-bound ribosomes. APOB is stabilized by interactions with the ER membrane and lipid transfer by MTP. If lipid availability is insufficient or MTP activity is compromised, APOB is rapidly degraded via the ER-associated degradation (ERAD) pathway. Vertebrate MTP transfers a variety of lipids to APOB, including triglycerides, phospholipids, and cholesteryl-esters. TRLs are predominantly composed of triglycerides, which account for more than 60% of the particle mass. TRLs secreted from the liver are referred to as very-low-density lipoproteins (VLDL), whereas those secreted from the intestine are called chylomicrons (CM). Secreted TRLs are digested or lipolyzed by vascular lipases, giving rise to smaller triglyceride-poor species such as intermediate and low-density lipoproteins (IDL and LDL). Invertebrate MTP can only transfer phospholipids and cholesteryl-esters, but not triglycerides, to nascent lipoproteins. As a result, invertebrates do not secrete TRLs, but instead secrete small triglyceride-poor lipoproteins. The acquisition of triglyceride transfer activity by vertebrate MTP was therefore an essential step in the evolution of TRLs in the vertebrate lineage. In addition to transferring lipids to APOB, MTP can also create APOB-free lipid micelles within the ER lumen called lumenal lipid droplets (LLDs). The biological function of LLDs remains ill-defined. It has been proposed that LLDs may fuse with nascent TRLs, delivering a large bolus of lipids to nascent lipoproteins prior to their secretion. However, these fusion events have never been directly observed. TRL expansion appears to occur in two phases. The first phase involvesPLA2G12B drives the expansion of triglyceride-rich lipoproteins (TRLs) in vertebrates. This study reveals that PLA2G12B channels lipids from the endoplasmic reticulum (ER) lumen into nascent lipoproteins, promoting efficient lipid secretion and preventing excessive intracellular lipid accumulation. PLA2G12B is calcium-dependent and tightly associated with the ER membrane. Mutant mice lacking PLA2G12B exhibit resistance to atherosclerosis, suggesting an evolutionary trade-off between TRL transport and cardiovascular disease risk. The study identifies PLA2G12B as a key driver of triglyceride incorporation into vertebrate lipoproteins. TRLs are essential for transporting hydrophobic triglycerides in the circulatory system. The microsomal triglyceride transfer protein (MTP) transfers lipids from the ER to Apolipoprotein-B (APOB), which is translated by ER-bound ribosomes. APOB is stabilized by interactions with the ER membrane and lipid transfer by MTP. If lipid availability is insufficient or MTP activity is compromised, APOB is rapidly degraded via the ER-associated degradation (ERAD) pathway. Vertebrate MTP transfers a variety of lipids to APOB, including triglycerides, phospholipids, and cholesteryl-esters. TRLs are predominantly composed of triglycerides, which account for more than 60% of the particle mass. TRLs secreted from the liver are referred to as very-low-density lipoproteins (VLDL), whereas those secreted from the intestine are called chylomicrons (CM). Secreted TRLs are digested or lipolyzed by vascular lipases, giving rise to smaller triglyceride-poor species such as intermediate and low-density lipoproteins (IDL and LDL). Invertebrate MTP can only transfer phospholipids and cholesteryl-esters, but not triglycerides, to nascent lipoproteins. As a result, invertebrates do not secrete TRLs, but instead secrete small triglyceride-poor lipoproteins. The acquisition of triglyceride transfer activity by vertebrate MTP was therefore an essential step in the evolution of TRLs in the vertebrate lineage. In addition to transferring lipids to APOB, MTP can also create APOB-free lipid micelles within the ER lumen called lumenal lipid droplets (LLDs). The biological function of LLDs remains ill-defined. It has been proposed that LLDs may fuse with nascent TRLs, delivering a large bolus of lipids to nascent lipoproteins prior to their secretion. However, these fusion events have never been directly observed. TRL expansion appears to occur in two phases. The first phase involves