We investigated whether short-term underfeeding could induce adaptative mec
hanisms in portal-drained viscera (PDV) that would allow nutrients to be sp
ared for vital functions in adult ewes. Six ewes (three of them fitted with
catheters in the mesenteric artery and portal and mesenteric veins) were f
ed, in a double 3 x 3 Latin square design (2 weeks per experimental period)
, a regrowth of natural grassland hay at 143 (high; H), 88 (medium; M) and
51 (low; L) % of their energy maintenance requirements. The digestibility o
f the diet was measured in all six ewes and the net portal fluxes of nutrie
nts in the three catheterized ewes. The organic matter content and N digest
ibility of the diet were not affected by underfeeding. Urinary and faecal N
losses and N balance were linearly related to feed intake. Arterial concen
tration of acetate was linearly related to feed intake. Arterial concentrat
ions of the other volatile fatty acids, 3-hydroxybutyrate, lactate, glucose
, NH3, urea and total amino acids were not affected by underfeeding. Arteri
al concentration of non-esterified fatty acids (NEFA) increased with underf
eeding. The portal net release of all volatile fatty acids, 3-hydroxybutyra
te and NH3 were linearly related to intake. The portal net flux of both ess
ential and non-essential amino acids, and thus total amino acids, remained
unchanged between levels H and M, and decreased between levels M and L. A s
ignificant net uptake for glycine and total non-essential amino acids occur
red at level L,. The portal net uptake of glucose, urea, glutamate and glut
amine, and the portal net release of lactate and NEFA were not affected by
underfeeding. Summation of portal energy fluxes indicated that 51 % of the
metabolizable energy intake was recovered in the portal blood with the thre
e levels of intake. In conclusion, no quantitative adaptation to spare ener
gy, in terms of percentage of intake, occurred in PDV of short-term underfe
d ruminants, but the pattern of absorption of energetic nutrients was modif
ied.