1. The rate of blood flow in the portal and hepatic veins, and the net exchange across the gut and liver of volatile fatty acids (VFA), glucose, lactate, pyruvate, amino acids, ketone bodies, glycerol, non-esterified fatty acids (NEFA) and oxygen, were measured in lactating and non-lactating cows (a) in the normal, fed state and (b) before, during and after 6 d of fasting.

2. Blood flow rate through the liver was 52% higher in normal, fed, lactating cows as compared with non-lactating cows, and was decreased by fasting in both groups of cows. Portal blood flow rate increased with an increase in metabolizable energy (ME) intake.

3. Lactating, as compared with non-lactating, cows exhibited lower arterial concentrations of glucose and lactate, higher net portal outputs of VFA and ketone bodies, a higher net hepatic output of glucose, and higher net hepatic uptakes of propionate and lactate. The splanchnic outputs of acetate, glucose and hydroxybutyrate were all apparently greater in the lactating cows.

4. Fasting caused a rapid decrease in the blood concentrations of the VFA and an increase in those of glycerol and NEFA. The portal, i.e. gut, outputs of VFA, lactate, ketone bodies, alanine and (serine+threonine), and the portal uptake of O2, were all decreased by fasting. Fasting for 6 h also decreased the hepatic output of glucose and acetate by 77 and 95% respectively, increased the hepatic uptake of pyruvate, glycerol and NEFA, and doubled hepatic ketone-body output. The splanchnic output of acetate and glucose and the splanchnic uptake of O2 were also decreased by fasting.

5. The net portal outputs of VFA, lactate and hydroxybutyrate, and the net hepatic output of glucose, were all correlated with ME intake in fed and fasted cows. Hepatic glucose output was also correlated with milk yield.

6. The net hepatic uptake of gluconeogenic precursors measured in this study could account for net hepatic glucose output in the fasted cows, but not in the fed cows. The net hepatic uptake of the ketogenic precursors butyrate and NEFA was sufficient to account for the hepatic output of ketone bodies in both fed and fasted cows, but it is unlikely that the hepatic uptake of ketogenic precursors could also account for the observed hepatic output of acetate.

1. In order to exclude the possibility of differences in maternal care which are known to result from typical methods of undernutrition during the suckling period, rat pups were reared artificially on different planes of nutrition away from their mothers.

2. Artificial rearing was accomplished by fitting infant rats with a gastric cannula through which a milk substitute was infused intermittently. Rats were fed thus from 4 to 21 d on a high (ARHI) or a low (ARLO; 44% of ARHI level) plane of nutrition. Underfeeding of the ARLO group was continued till 25 d, after which all rats were given a good-quality pelleted diet ad lib.

3. Compared with mother-reared (MR) litter-mates, ARHI rats showed advanced eye-opening and, at 21 and 25 d, they resisted restraint more strongly.

4. Growth in body-weight of ARHI and MR rats was similar but, when autopsied at 32 weeks, the ARHI rats were shorter (nose–rump length) and had lighter gastrocnemius muscles, adrenals and brains, but heavier epididymal-fat pads.

5. ARLO rats had deficits at 32 weeks compared with ARHI rats in whole body, kidney and epididymal-fat-pad weights, and in tibia length.

6. In a second experiment, ARHI and MR rats were killed at 21 d. All the differences found at 32 weeks were already present at 21 d. In addition, the ARHI pups had enlarged livers and intestines but shorter tibias.

7. The milk substitute, which is one commonly used in such studies, has a low protein and high carbohydrate content compared with rats' milk. This difference probably caused the abnormal organ growth of ARHI rats.

1. Twelve crossbred does (New Zealand White × California) were offered a diet of high protein and metabolizable energy content (249 g/kg dry matter (DM) and 13·6 MJ/kg DM respectively) throughout a 32 d lactation at one of four feeding levels (240, 280, 320 or 360 g/d). Each feeding level was replicated three times.

2. Milk output was measured by weighing the does before and after their one daily suckling period.

3. The lactation was divided into four consecutive 8-d periods. Each doe was placed in a direct calorimeter for 48 h around the mid-point of each of these periods and measurements of energy exchange were made. Nitrogen balance was also measured throughout the study period.

4. Milk samples were taken from a parallel group of animals and the estimates of milk composition were applied to the main group of does.

5. From the second period of lactation onwards nearly all does mobilized body tissue to support milk energy secretion, although there was no loss of weight. Multiple regression analyses were used to examine the apparent efficiency with which metabolizable energy and body-tissue energy were utilized for milk production. Overall, the relationship was described by the equation:

Period of lactation … 2 3 4

Milk E = 0.735 me intake – 0·938Body ER –296 –280 –276

(se 0-020) (se 0-039)

where Milk E is the milk energy output, ME intake is the metabolizable energy intake and Body ER is the body energy retention, all expressed in kJ/kg body-weight0·75 per d.

All does appeared to be in positive N balance throughout lactation on this high-protein diet.

1. Fractional rates of protein synthesis in tissues of young growing rats were estimated by injection of flooding amounts (1·5 mmol/kg body-weight) of [3H]phenylalanine. Rates of 63·6%/d and 90·4%/d respectively were obtained in the skin and bone (tibia) of fed animals. These rates were comparable to that in liver (86·3%/d) but considerably higher than in muscle (16·9%/d).

2. Absolute amounts of protein synthesized in tissues of fed rats were estimated. Together the skin and bones accounted for 25% of whole-body synthesis, a value similar to the contribution of liver (15%) and muscle (25%).

3. In fed rats the ratio, RNA: protein in skin and bone was lower than in liver, but much higher than in muscle. However, the amounts of protein synthesized per unit RNA in skin and bone were higher than in both liver and muscle.

4. After 2 d of starvation the fractional rates of protein synthesis in skin and bone fell by 26% and 31% respectively. This was greater than the fall in liver (17%) but less than in muscle (66%). In bone the fall in synthesis was accompanied by decreases in both RNA: protein and synthesis per unit RNA, but in skin there was a fall in RNA: protein which was partially countered by an increase in the rate of synthesis per unit RNA.