Five mid-lactation multicatheterized Jersey cows were used in a 4 × 4 Latin square design to investigate whether the increase in milk N yield associated with diets rich in starch versus fiber could originate from changes in the splanchnic AA metabolism and if these changes depended upon the dietary crude protein (CP) content. Four isoenergetic diets were formulated to provide 2 different carbohydrate compositions [diets rich in starch (350 g of starch and 310 g of neutral detergent fiber/kg of dry matter) versus rich in fiber (45 g of starch and 460 g of neutral detergent fiber/kg of dry matter)] crossed by 2 different CP contents (12.0 vs. 16.5% CP). At the end of each treatment period, 6 hourly blood samples were collected from the portal and hepatic veins as well as the mesenteric artery to determine net nutrient fluxes across the portal drained viscera (PDV), liver, and total splanchnic tissues. Dry matter and calculated energy intake as well as total absorbed energy were similar across treatments. However, the net portal appearance (NPA) of acetate, total volatile fatty acids, and β-hydroxybutyrate were higher with diets rich in fiber versus starch, whereas that of oxygen, glucose, butyrate, and insulin were lower. Concomitant to these changes, the percentage of N intake recovered as total AA (TAA) in the portal vein was lower for diets rich in fiber versus starch (42.3 vs. 51.4%, respectively), without, however, any difference observed in the NPA of the main AA used as energy fuels by the PDV (Glu, Gln, and Asp). Despite a higher NPA of TAA with starch versus fiber diets, no differences in the net hepatic flux of TAA, essential and nonessential AA were observed, resulting in a higher (+22%) net splanchnic release of AA and, hence, a greater (+7%) milk N yield. The net hepatic flux and hepatic fractional removal of none of the individual AA was affected as the main carbohydrate changed from fiber to starch, except for Gly and Lys, which were higher for the latter. After correcting for differences in NPA of TAA, the net hepatic uptake of TAA tended to be lower with starch versus fiber diets. The higher transfer of N from feed to milk with diets rich in starch is not the consequence of a direct sparing AA effect of glucogenic diets but rather the result of lower energy requirements by the PDV along with a higher microbial N flow to the duodenum. A better AA use by peripheral tissues with starch versus fiber diets was also hypothesized but more studies are warranted to clarify this issue.