Two experiments were conducted to evaluate the association between energy intake and plasma glucose-dependent insulinotropic polypeptide (GIP) concentration, and the association of plasma GIP concentration with gas exchange, carcass characteristics, and growth performance in feedlot cattle. In Experiment 1, I used 60 individually fed backgrounded Angus × SimAngus-crossbred steers (n=30) in a randomized complete block design. Steers (paired by body weight and gain to feed ratio (G:F)) were randomly allocated to one of the following treatments: ad libitum (AI) or restricted intake (RI; the same diet fed at 85% of the AI) of a finishing diet. The diet contained 61 % cracked corn, 9 % corn silage, 15 % DDGS, 5% soyhulls, and 10 % of a protein-mineral-vitamin premix. Measurement of CO2 emission, and consumption of O2, were taken using the Greenfeed system (n=15/treatment) once the steers were fed for at least 140 days. Plasma and gas samples were collected 10 d before slaughter, 1 h before and 2 h after feeding. Plasma glucose, non-esterified fatty acids (NEFA), GIP, and insulin concentration and gasses (O2 and CO2) were analyzed using the MIXED procedure of SAS evaluating the fixed effect of treatment, time (repeated measurement) and their interaction, and the random effect of block. Final body weight and carcass characteristics were analyzed with a similar model, without the time statement and its interaction. Compared with RI, AI steers had greater (P < 0.01) DMI and average daily gain (ADG). Steers on AI tended to have greater final body weight (BW) (P = 0.07) and ribeye area (P = 0.09). There was no treatment effect (P ≥ 0.11) on G:F, subcutaneous (backfat, BF) and intramuscular (IM) fat, O2 consumption and CO2 emission. Plasma glucose concentration of AI steers were greater before and after feeding than RI (P < 0.05). In conclusion, feeding steers ad libitum increased DMI, ADG, and plasma glucose and GIP concentration, but does not affect G:F, BF, IM fat, CO2 emission, and O2 consumption. In Experiment 2, I used 60 individually fed backgrounded Angus × SimAngus-crossbred steer calves during the growing and finishing period in an unbalanced incomplete randomized block design, with a 2 x 2 factorial arrangement of treatments. The calves (blocked by BW and G:F) were randomly allocated to one of the following treatments, restricted-fed or offered diets ad libitum; with or without 4% supplemental fat for 78 d. Animal growth performance, O2 consumption, CO2 emissions, respiratory quotient (RQ), plasma glucose, NEFA, insulin, and GIP concentration were analyzed as Experiment 1. Growth performance was analyzed over days, and hormones and metabolite concentration were compared pre- versus post-feeding. Measurement of CO2 emission, and consumption of O2, were taken using the Greenfeed system and blood samples during the growing and finishing periods. Compared to restricted intake and fat supplemented animals, steers fed ad libitum without fat supplemented fed (AN) had greater (intake by fat interaction; P < 0.01) final BW, DMI, ADG, and G:F. Also, supplementation of fat increased (P < 0.05) plasma GIP, NEFA, and glucose concentration while ad libitum intake increased (P < 0.01) plasma insulin concentration and tended (P = 0.08) to decrease plasma NEFA concentration in the finishing period. In conclusion, steers AN had greater final BW, DMI and ADG, but does not affect G:F, RQ and consumption of O2. Supplementation of fat increased plasma GIP and glucose concentration while restricting intake decrease plasma insulin concentration while increasing plasma NEFA concentration in the growing period. In feedlot cattle, differences in plasma GIP concentration were due to an interaction between dietary fatty acids, energy intake, and time of sampling (relative to feeding).