Three pens of male broiler chicks were raised under standard conditions and fed from 7 to 42 days
of age three isocaloric diets each with 15.8; 19.6 and 19.5% of CP; and 51, 51, and 44% of CHO; and
6.5; 3.0 and 7.7% of fat, and designated as the low protein (LowCP), low lipid (LowL) and low
carbohydrate (LowCHO) diets, respectively. Body weights and feed intake were monitored weekly
and blood samples were collected at the same time for posterior analysis of hormone and metabolite
content. Chickens fed the LowCP diet were characterized by a reduced body weight gain and feed
intake and poorer feed conversion efficiency compared to those fed the LowL and LowCHO diets,
which were very similar in this respect. Plasma corticosterone and glucose levels and creatine kinase
activity were not significantly changed by diet composition. LowCP chickens were characterised by
the lowest plasma T4 and uric acid levels (indicative for reduced protein breakdown and lower protein
ingestion) but highest plasma triglyceride levels (congruent with their higher fat deposition) compared
to the LowL and LowCHO chickens. LowL chickens had on average higher plasma T3 and free fatty
acid levels compared to the LowCP and LowCHO chickens.
In conclusion, a limited substitution of carbohydrate for fat in iso-nitrogenous, iso-energetic diets has no pronounced effects on plasma hormone and metabolite levels, except for the elevation in T3
(may enhance glucose uptake) and free fatty acid levels in the plasma of the chickens fed the LowL
diet. The protein content of the diet has a greater impact on zootechnical performance, and underlying
endocrine regulation of the intermediary metabolism compared to the dietary lipid and CHO fraction.