Indirect calorimetry to study heat production (HP) and dual energy X-ray absorptiometry (DEXA) for body composition (BC) are powerful techniques to study the dynamics of energy and protein utilization in poultry. The first two chapters present the BC (dry matter, lean, protein, and fat, bone mineral, calcium and phosphorus) of modern broilers from 1 – 60 d of age analyzed by chemical analysis and DEXA. DEXA has been validated for precision, standardized for position, and equations and validations developed for chickens under two different feeding levels. These equations are unique to the machine and software in use. Research in broilers fed exogenous enzymes added alone or mixed as a multi-enzyme composite (protease + glucanase + xylanase + phytase) has shown lower HP and higher protein deposition when protease was evaluated alone or in combination. An unexpected change from protein to fat deposition was seen in the grower phase (around 22d), with a switch back to more protein synthesis in the finisher (\u3e29d). The lower HP let to believe exogenous enzymes reducing the energy for maintenance, the next study resulted in enzymes reducing 6.6% the MEm (metabolizable energy for maintenance) evaluated in retained energy from the body of chicks (16 -27 d). The study was conducted increasing feeding levels and fit by linear regression. The maintenance experiment also showed that under feed deprivation conditions, body directs nutrients to protein synthesis before fat synthesis occurred. Research with broiler breeders resulted in HP increasing continuously along egg production and age from 26 - 59 wk. HP was the highest at 59 wk when the lean tissue was the highest. Respiratory exchange ratio (RER = VO2/VCO2) showed the lowest value at 43 wk suggesting fat utilization is higher at this point of egg production compared to the beginning (26 wk). Lean mass was the lowest at 37, and 50 wk and increased after 50 wk suggesting lean mass being more important than fat during egg production. Hens increased lean tissue after 50 wk suggesting preparation for next clutch as it happens in the wild, so fat is used as fuel for maintenance energy
