Nitrogen and lipid metabolism in heifers fed at increasing levels of intake. 
The relationship between N and lipid metabolism was investigated in heifers fed five different levels of feed intake (five heifers per treatment group).  Targeted ME intakes were 84, 123, 157, 191, and 225 kcal per kt.75 per day, which were .76x, 1.12x, 1.43x, 1.74x, and 2.05x (times) the estimated ME requirement for maintenance.  After 120 d on trial, the heifers were moved to a confinement building for 7 d, and feces and urine were collected over a 3-d period.  On the 1st d of confinement, blood samples were collected every 15 min for a total of 15 samples.  Because the group fed at 1.43x maintenance was fed improperly during the period in confinement, this group was omitted from the study.  Adipose tissue samples were obtained at slaughter to obtain in vitro measures of lipid metabolism.  As feed intake increased, N retention increased (P less than .05) from 1.7 to 24.3 g/d. Daily urinary N tau-methylhistidine excretion was significantly different between the .76x and 1.74x treatments (769 vs 1,575 mumol/d, respectively).  The fractional breakdown rate of myofibrillar proteins also was significantly different between these two groups of heifers (1.49 vs 2.44%/d, respectively).  Plasma glucose and insulin were lowest (P less than .05) at the lowest level of feed intake.  Conversely, plasma nonesterified fatty acids were lowest (P less than .05) in those animals receiving the highest level of feed intake.  Subcutaneous adipocytes were smallest (93 microns) in the heifers fed at 33% ad libitum intake and largest in heifers fed at 76 or 90% ad libitum intake (115 and 110 microns, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)