1. The effect of including lupin (Lupinus sp.) hulls, maize cobs, wheat bran and lucerne (Medicago sutiva) stems in a basal fibre-free diet on the concentrations and the relative proportions of volatile fatty acids (VFA) in the proximal colon of pigs, 17–18 h after feeding, was studied.

2. Concentrations of total VFA in the proximal colon increased with increasing levels of neutral-detergent fibre (NDF) intake, and this increase was highly dependent on the source of NDF in the diet.

3. Molar proportions of the VFA were significantly affected by the level of NDF intake only in the cases of acetic and butyric acids, whereas the source of dietary NDF had a marked influence on the molar proportions of all acids.

4. The results indicate that the extent of fermentative breakdown of fibre in the pig intestine can be influenced substantially by the type and the level of fibre in the diet.

1. An increase in 1iver:body-weight and in hepatic triacylglycerol content, together with changes in the fatty acid profiles of hepatic phospholipids, were observed as a result of moderate riboflavin deficiency in sucking rat pups. Oxygen consumption by hepatic mitochondria, with palmitoyl L-carnitine as substrate, was not significantly impaired.

2. Mitochondria from interscapular brown adipose tissue, however, showed a marked impairment of O2 Consumption, with palmitoyl L-carnitine as substrate, in the riboflavin-deficient pups. This impairment was also apparent after uncoupling with carbonyl cyanide p-trifluoromethoxyphenylhydrazone, but was not consistently observed after the addition of GDP to suppress uncoupled oxidation. It was much less evident, and did not reach statistical significance, for the mitochondria of brown adipose tissue of the corresponding deficient dams.

3. Binding of 3H-labe11ed GDP by brown adipose tissue mitochondria was unaffected by riboflavin deficiency in the pups, suggesting that the effect on O2 consumption is more likely to be due to impaired integrity of the mitochondrial respiratory chain, than to impairment of the specific capacity for uncoupling of respiration which is characteristic of brown adipose tissue mitochondria. Total cytochrome c oxidase (EC 1.9.3.1) activity of the brown adipose tissue of riboflavin-deficient pups was not significantly reduced.

4. A small but significant impairment was observed in the stimulation ofwhole-body O2consumption by injected noradrenaline in the riboflavin-deficient pups, suggesting that the impairment of brown adipose tissue mitochondrial function may be accompanied by impaired physiological capacity in vivo.

1. Investigations were carried out on the feasibility of using an oral repletion technique in the rat to assess the bioavailability of copper in experimental sources providing no more than 250 μg Cu from any one source.

2. Preliminary studies on the response in plasma Cu ofpartially Cu-depleted rats given repletion doses of 20–50 μg Cu as CuSO4/d on four consecutive days indicated that this index of Cu status was insufficiently sensitive to Cu dose.

3. Incontrast, theactivityofcytochromecoxidase(EC 1. 9.3.1)in theduodenalmucosa ofpartiallycu-depleted rats showed a measurable and uniform response to 10 μg Cu as CuSO4/d given on three consecutive days. Furthermore, when the rats were given 0, 2.5, 5.0 or 10.0 μg Cu/d, the increase in cytochrome c oxidase activity above that of the unsupplemented control group was linearly related to Cu dose. The mean response in cytochrome c oxidase activity in groups of eight rats was therefore used to assess the availability of Cu from experimental sources relative to that of Cu as CuSO4, only 240 μg Cu being required from each experimental material.

4. The assay was used to study the effect of the Cu-antagonist molybdenum on the distribution of available Cu in digesta from sheep given dried grass either untreated (1.6 mg Mo/kg dry matter (DM)) or treated with ammonium molybdate (11.6 mg Mo/kg DM).

5. The relative availability of Cu in untreated dried grass (75%) was substantially higher than in rumen (12%), duodenal (43%) or ileal (28%) digesta. In all cases, addition of Mo to the diet resulted in a substantial reduction in Cu availability.

6. The effects of Mo on availabilitv of Cu are discussed with suecial reference to the Dossible involvement of thiomolybdates in the Cu-Mo antagonism.

1. The results are reported of four feeding experiments in which lactating cows were given fixed rations of hay and high-cereal concentrates at different meal frequencies. In Expt 1 the total ration was given in two and twenty-four meals daily and in Expts 2–4, the concentrates were given in two and five or six meals and the hay was given twice daily. The diets contained 600–900 g concentrate /kg.

2. In all the experiments, more frequent feeding of these low-roughage diets reduced milk fat depression and increased milk fat yield. In each experiment the increase was greater with the diet containing the lower proportion of hay. There was no significant effect on milk yield, the protein or lactose contents of the milk or live-weight gain. Digestibility of dry matter, organic matter and energy was increased in one of the experiments but not in two others in which it was measured.

3. More frequent feeding was calculated to increase the net energy secreted in milk and there was a tendency for it to increase the net energy in live-weight gain with diets containing 600 or 700 g concentrate/kg but to decrease it with diets containing 800 or 900 g concentrate/kg. These results are discussed in relation to theories of energy partition in lactating cows.

4. It is concluded that at fixed feed intakes, the main response to increased meal frequency is likely to be a reduction in milk fat depression and that this will be confined to diets containing not more than about 200 g modified acid-detergent fibre/kg dry matter.

1. The aim of the study was to determine the relative effects of feeding growing pigs with graded amounts of neutral-detergent fibre (NDF) from various sources on the empty wet weight of segments of the pig gastrointestinal tract, on the weight and moisture content of their digesta, and on the pattern of digesta movement in them.

2. Increased NDF intakes were associated with significantly higher wet weights of all gastrointestinal segments and increased lengths of the caecum. The lengths of both the small and the large intestines were unaffected by the ingestion by the pigs of different amounts of NDF from various sources. However, the caecum responded to these increased intakes of NDF by significant increases in length. The source of NDF in the diet was a factor that markedly influenced both the length and the weight of the distal colon. The nature of these increases in weight and length morphologically and their biological significance have not been determined.

3. Despite some significant differences, neither the type nor the level of dietary NDF had any appreciable effect on the dry weight and on the proportion of dry matter (DM) of the contents in the stomach. As the level of NDF intake was increased, more undigested dietary material was found in all segments of the digestive tract of the pigs. The proportion of DM in the residues decreased progressively from the caecal contents to the contents of the distal colon. In most cases the degree to which the level of NDF intake affected the weight of the contents and the proportion of DM in them was highly dependent on the source of NDF in the diet.

4. The distribution of the feed consumed in the morning and in the evening, as measured with stained feed particles and polyethylene beads, was extremely variable. It is concluded that prolonged intakes by pigs of diets containing high levels of fibre may lead to a hypertrophy and hence increased weight of segments of the gastrointestinal tract.

1. The myoelectric activity of the colon was examined in three dogs and three pigs when they were given a basal diet or a basal diet plus indigestible particles (IP), 2 mm in diameter, at 100 g/kg dry matter. The mean retention time was determined using coloured discs as a marker added to the daily meal.

2. Colonic electromyograms of dogs and pigs given IP revealed a 30% reduction in the number of long spike bursts (LSB) when compared with controls. The other components, propulsive migrating spike bursts (MSB) or non-propulsive short spike bursts (SSB), were unchanged. Mean retention time was decreased from 28.6 h to 17.6 h in dogs and from 129 h to 94.2 h in pigs.

3. These changes developed progressively during 3–4 d in both species, suggesting that the reduction in motor activity was an adaptation to the changes in bulk contents.

4. From the decreased motility of the colon linked to the reduction of LSB and paralleled by an increased transit time, it was concluded that one of the functions of the LSB is to impede the passage of digesta.

1. Sixteen Friesian cows were used in Expt 1 to measure the effect of substituting urea-N with fishmeal-N either in early lactation (Part 1) or in mid-lactation (Part 2).

2. In Part 1 (days 15–84 of lactation) the major N constituent of the concentrate was urea (U), urea-N: fishmeal-N in the ratio 2:1 (UF) or 1:2 (FU), or fishmeal (F). In Part 2 (days 84–1 75 of lactation) only urea (UM) and fishmeal (FM) were used.

3. Replacement of urea-N with fishmeal-N significantly (P < 0.05) increased yield of milk protein both in early and in mid-lactation. At both stages of lactation the cows were, by calculation, in positive energy balance. In mid-lactation replacement of urea-N by fishmeal-N significantly depressed (P < 0.001) the concentration of fat in milk.

4. Blood urea concentration decreased with increasing fishmeal inclusion (P < 0.05) from U to FU.

5. In Expt 2 the diets used in Expt 1, Part 1, were offered at a maintenance level of feeding to non-pregnant, non-lactating heifers in a 4 x 4 Latin square design experiment. Digestibility of dry matter, organic matter and cell-wall constituents increased progressively (P < 0.05) with the first two increments of fishmeal inclusion.

6. A major effect of replacing urea-N with fishmeal-N was to increase digestible organic matter intake (DOMI) and differences in DOMI between treatments in Expt 1, Part 1, accounted for observed differences in performance.

1. Studies of iron utilization and haem synthesis were carried out with hepatic mitochondria obtained from copper-deficient and pair-fed control rats.

2. Ferric chloride can be used as Fe substrate for mitochondrial haem synthesis in the presence of succinate. Utilization is further enhanced by the addition of FMN.

3. Ferritin does not support haem synthesis in the presence of succinate alone, but does support haem synthesis when FMN is added.

4. Mitochondria1 haem synthesis is impaired in Cu deficiency when either FeCl3 or homologous ferritin is used as Fe substrate.

5. The results of the present study suggest that impaired haem synthesis in Cu deficiency occurs at a step following the chemical reduction of Fe substrate.

1. When 103Ru-labelled Tris (1, 10-phenanthroline) ruthenium II chloride (103Ru-P) particulate marker in aqueous solution was added to the rumen of four steers given 5.5 kg grass hay/d at two-hourly intervals, the distribution of 103Ru-P marker among rumen particles of various sizes was the same at 4 h, 3 d and 7 d after administration, the concentration of 103Ru-P/g dry matter (DM) was inversely related to particle size and 0.30 of the 103Ru-P was associated with the DM of particles too large to be moved from the rumen at a meaningful rate. Thus, fractional outflow rate (FOR) of 103Ru-P would reflect, but was not a direct measure of, the FOR of the small particle pool in the rumen.

2. When rumen digesta were labelled with 103Ru-P, placed in nylon cloth bags and incubated in vitro with unlabelled digesta, 59% of the 103Ru-P disappeared from the nylon bag in 24 h, and 74% in 48 h. Similar results were obtained when large particles (retained by a 3.2 mm mesh screen during wet sieving) from rumen digesta were subjected to this procedure.

3. In a further experiment, the steers were given the hay in either the long or ground form and drinking water to which 10 g sodium chloride/l were, or were not, added.

4. The FOR of 51CrEDTA in centrifuged rumen fluid was increased (P < 0.05) from 1.78 to 2.10/d by grinding of the hay diet, but was not influenced by the intake of an additional 257 g NaCl/d. The FOR values of 103Ru-P in mixed rumen digesta and organic 35S in micro-organisms were linearly correlated (P < 0.05) and were not affected (P < 0.05) by grinding and salt treatments. On average, the FOR of organic 35S in micro-organisms was 0.41 of that of 51CrEDTA in centrifuged rumen fluid and 0.85 of that of 103Ru-P in rumen digesta respectively.

5. Grinding of the hay did not (P > 0.05) change the proportion of rumen DM (0.476–0.515) or faecal DM (0.107–0.153) retained by the 3.2 mm mesh and larger screens.

6. FOR from the rumen of a given size group of particles was calculated as the ratio, estimated daily flow from the rumen of the size group: rumen pool of the group. With increasing particle size there was a progressive decline in FOR; values of FOR for groups of particles greater than 4.0 mm were negligible. If the rumen DM was considered to behave as two pools, the 3.2 mm mesh screen appeared to be an appropriate division between the large-particle and the small-particle DM pools.

7. FOR of lignin present in mixed rumen digesta was 0.48 of the mean of the FOR values of the particle groups of the small-particle pool, while the FOR of lignin present in the small-particle pool was 0.92 of the mean small-particle FOR.

1.Forty pigs between 23 and 51 d of age were given ad lib. diets containing wheat and one of five protein concentrates: meat meal A, meat meal B, soya-bean meal, milk and lupins (Lupinus augustifolius). Twenty of these pigs were given indigestible markers from 51 to 56 d of age and were killed at 56 d of age.

2.The diets containing meat meals A and B, soya-bean meal and milk contained 2.3 g total methionine/kg and the diet containing lupins contained 2.1 g/kg.

3.A further forty pigs of the same age were given the same diets supplemented with 1 g synthetic methionine/kg.

4.The weight gains and feed conversion ratios of the pigs given the diets containing 2.1–2.3 g methionine and 3.1–3.3 g methionine/kg were not significantly different.

5. The weight gains of the pigs given lupins (2.1 g methionine/kg) were less than those of the pigs given the diets containing 2.3 g methionine/kg.

6. The apparent digestibility of dry matter (DM) and nitrogen was less for the diets containing the meat meals (0.75 and 0.78 respectively) than for those containing the other protein concentrates (0.80 and 0.84).

7.The retention times in the large intestine of the diets containing soya-bean meal and lupins were 965 and 1083 min which were greater than those of the diets containing the other protein concentrates, mean 732 min.

8. The major site of N digestion and absorption for the diet containing milk was the area of the small intestine 25–50% of total length from the pylorus, while for the other protein concentrates the major site was 50–75% of its total length from the pylorus.

9. The digestion and absorption of N in the large intestine was less (3.4%) for the diet containing milk than for those containing the other protein concentrates (7.5–11.3%).

10. The apparent digestibility of the methionine to the ileum for the five diets ranged from 0.74 to 0.86 while the calculated retention of the apparently-absorbed methionine was 1.00. It was suggested that methionine digestibility could be used as an indicator of availability.

11. The calculated retention of apparently absorbed N in the carcass was 0.71 for the pigs given the diet containing milk and 0.51–0.58 for the pigs given the other diets.

1. Sheep were given a diet of 400 g chopped grass hay/d and 500 g crushed barley/d in three equal meals. The mean retention time in the whole gut (MRT) and the rumen retention time (RRT) of chromium-labelled feed particles and of water were estimated from marker concentrations in faecal samples.

2. MRT of Cr-labelled hay (Cr-H), barley hulls (Cr-Bh), crushed peas (Pisum sativum) (Cr-CP) and rapeseed meal (Cr-RSM) were 53.5, 52.3, 44.3 and 37.0 h respectively when estimated from total faecal collections. The estimates obtained from spot samples of faeces were almost identical.

3. RRT of Cr-H, Cr-Bh, Cr-CP, Cr-RSM (Expts 1, 2 and 3) and Cr-labelled cottonseed meal (Cr-CSM) was 35.9, 36.7, 25.3, 18.8, 19.8, 22.8 and 25.3 h respectively.

4. The Cr-labelled feed particles showed marked variations in density in water (Dω). However, both MRT and RRT were closely related to Dω, (r—0.88 and —0.95 respectively).

5. Rumen outflow rates of Cr-RSM in individual animals were related to the rumen outflow rates of water (r 0.76).

1. Groups of rats (n 8) were offered, to appetite, diets containing 10 (LP), 25 (MP) and 45 (HP)% of gross energy as protein energy from 45 d of age to slaughter 50 d later. At 53 d of age, half the rats on each diet were left at 22°while the other half were transferred to 6°. Water balances were measured daily, and digestibilities of energy and nitrogen and the metabolizability of the diets were measured for each rat over a 7 d period at some stage between the age of 74 and 95 d. The rats were slaughtered at day 95 and their carcasses were analysed for protein, lipid, energy and ash contents. Energy expenditure was calculated as the difference between metabolizable energy intake and energy retained.

2. Growth performance was best on the MP diet at both temperatures. At 22° the rate of gain of body-weight and of energy retention, although not of protein, were slightly reduced on the HP diet while overall performance was markedly inferior on the LP diet. At 6° the LP diet, while not so good as the MP diet, led to significantly better all-round growth performance than did the HP diet. Cold increased the energy expenditure of the rats by 50% (109–138 kJ/d); the increase was greater for the LP group than for the HP group.

3. Intrascapular brown adipose tissue hypertrophied in response to cold and to the LP diet.

4. Adrenal gland size was significantly increased by cold exposure and by increasing level of dietary protein concentration.

5. Urine volume was more closely related to the intake of protein than to that of energy. Urinary N concentration for the HP rats was approximately double that for those on the LP diet. Cold-exposed rats had a high water content in their fat-free carcasses, but there were no differences between the dietary treatments.

1. The comparative and absolute growth response of Lactobacillus casei was measured nephelometrically for time periods of 17–23 h in the microbiological assay of folic acid and 5-methyl-tetrahydrofolic acid at concentrations of 0–8 ng/10 ml basal medium at pH 6.8.

2. At concentrations of 0–1 ng/10 ml the comparative growth response to 5-methyl-tetrahydrofolic acid was markedly depressed whereas growth was the same at 2 ng/10 ml and above. Comparative growth was unaffected by the length of assay incubation, depressed growth being due to differences in log-phase growth rates with the rate-plot for 5-methyl-tetrahydrofolic acid being sigmoidal and for folk acid being a rectangular hyperbola with linearity only in the 0–1 ng/10 ml range. The reciprocal rate-plot for folic acid was linear whereas that for 5-methyl-tetrahydrofolic acid was coincidental only in part, giving rise to the same estimate of maximum velocity and substrate concentration for half-maximum velocity, with the exhibition of a strong threshold at low concentration.

3. A previous observation (Phillips & Wright, 1982) that the L. casei growth response to 5-methyl-tetrahydrofolic acid may be significantly less than that to folic acid is confirmed as is the long-established view that the response to both folates may be equal. In the light of current knowledge regarding folate-binding, transport and metabolism by L. casei, it is argued that the intracellular oxidation of 5-methyl-tetrahydrofolic acid to 5, 10-methylene-tetrahydrofolic acid is a rate-limiting step at low substrate concentrations, subsequently giving rise to a threshold growth response peculiar to 5-methyl-tetrahydrofolic acid. Since the rate of L. cusei growth with folk acid is not linearabove 1 ng/10 ml, it is recommended that microbiological folate assays be conducted only in the 0–1 ng/10 ml range and at a pH that elicits the same growth response from L. casei to 5-methyl-tetrahydrofolic acid as to folic acid and other folate monoglutamates.

1. Duplicate groups of rainbow trout (Salrno gairdnert) (mean weight 11 g) were given for 40 weeks one of four partially purified diets that were either adequate or low in selenium or vitamin E or both.

2. Weight gains of trout given the dually deficient diet were significantly lower than those of trout given a complete diet or a diet deficient in Se. No mortalities occurred and the only pathology seen was exudative diathesis in the dually deficient trout.

3. There was significant interaction between the two nutrients both with respect to packed cell volume and to malondialdehyde formation in the in vitro NADPH-dependent microsomal lipid peroxidation system.

4. Tissue levels of vitamin E and Se decreased to very low levels in trout given diets lacking these nutrients. For plasma there was a significant effect of dietary vitamin E on Se concentration.

5. Glutathione (GSH) peroxidase (EC 1. 1 1. 1.9) activity in liver and plasma was significantly lower in trout receiving low dietary Se but was independent of vitamin E intake. The ratios of hepatic GSH peroxidase activity measured with cumene hydroperoxide and hydrogen peroxide were the same for all treatments. This confirms the absence of a Se-independent GSH peroxidase activity in trout liver.

6. Se deficiency did not lead to any compensatory increase in hepatic GSH transferase (EC 2. 5. 1. 18) activity; values were essentially the same in all treatments.

7. Plasma pyruvate kinase (EC 2. 7. 1.40) activity increased significantly in the trout deficient in both nutrients. This was thought to be due to leakage of the enzyme from the muscle and may be indicative of incipient (subclinical) muscle damage.

1. In vitro assay procedures were applied in the measurement of available amino acids in a selection of fish meals representing good- and poor-quality product. Results were assessed by comparing them with results from chick-growth assays.

2. Available methionine and tryptophan were assayed microbiologically with Streptococcus zymogenes, after predigestion of the test samples with papain or pronase. Results for methionine were correlated with chick-growth assays (r 0.86 for papain, 0.91 for pronase; P < 0.01). Compared with the chick assays, papain tended to give lower, and pronase higher, results. Finer milling of the test samples did not influence the pronase values.

3. Results for available tryptophan were also correlated with chick-growth assays (r 0.95 for papain, 0.96 for pronase; P < 0.001). Compared with the chick values, papain gave markedly lower results and pronasem marginally higher ones. Finer milling of the test samples increased the papain values by about 50% but had no effect with pronase.

4. Available lysine was assayed microbiologically with Tetrahymena pyriformis and with a dye-binding procedure (DBL). The results correlated with the chick-growth assays (r 0.99 for DBL, P < 0.001; 0.85 for Tetrahymena, P < 0.01) but both methods overrated the poorer-quality samples.

5. True nitrogen digestibilities and amino acid digestibilities were determined with chickens by the 'ileal analysis' procedure: the amino acid digestibilities were significantly higher and similar to the corresponding availabilities as measured in chick-growth assays.

6. Ball milling a poor-quality fish meal caused a marked fall in its N digestibility, whereas similar treatment of a good-quality meal caused a slight increase. An explanation for this finding is proposed.

7. Strep. zymogenes assays following pronase digestion of the test samples gave precise and acceptably accurate measures of the biologically available methionine and tryptophan in the test samples. For available lysine, Tetrahymena and DBL values for the poorer-quality samples were notably higher than the chick-growth assay; possible reasons for this are discussed. The ileal analysis procedure underestimated true N digestibility.

1. The relation between the level of food intake and gastrointestinal motility and digesta flow in the abomasum and small intestine was studied in sheep fitted with nichrome-wire electrodes in the gut wall, an abomasal and a duodenal catheter and a terminal ileal cannula.

2. Abomasal volume and outflow were calculated from CrEDTA dilution in six sheep and small intestinal transit time by the passage of Phenol Red in ten sheep.

3. The frequency of the migrating myoelectric complex of the small intestine was not altered by the level of food intake but the duration of the periods of irregular spiking activity, the amplitude of abomasal activity and the frequency of duodenal rushes were decreased as the level of food intake was decreased.

4. There was a linear relation between the level of food intake (FI) and abomasal outflow (mean with SEM: 327 (69) ml/h for each kg FI/d; P < 0.01), and abomasal volume (mean with SEM: 344 (50) ml/kg FI per d; P < 0.001), without any significant change in the half-time of marker dilution in the abomasum. Small intestinal transit time decreased with an increase in food intake (mean with SEM: - 54.9 (5.6) min/kg FI per d; P < 0.001).

5.It is concluded that abomasal volume and the rate of digesta flow from the abomasum and along the small intestine are linearly related to the level of food intake.

1. Sixteen sheep, each fitted with cannulas in the rumen and proximal duodenum, were given four diets in the chopped or ground and pelleted form, at fixed intakes at intervals of 2 h. The sheep were closely shorn and exposed to temperatures of 22–25° or 1–4° for four periods of 45 d. Flow of duodenal digesta by reference to the markers CoEDTA and 103Ru-phenanthroline, chewing behaviour and particle size of rumen and duodenal digesta were measured.

2. Apparent digestibility of organic matter (OM) in the gastrointestinal tract was depressed (P < 0.05) by grinding and pelleting the diet, and by exposure of sheep to cold ambient temperatures. This was attributable to depression (P < 0.01) by 0.1 of OM digestion in the reticulo-rumen. No effects on intestinal digestion of OM were observed.

3. Cold ambient temperatures did not affect the content, but increased the rate of digestion for pelleted diets but not for chopped diets, of potentially-degradable cell-wall constituents of ground dietary material incubated in nylon-bags in the rumen.

4. Retention times of markers of the particulate and liquid phases of rumen digesta were not significantly (P < 0.05) affected by ambient temperature, despite significant (P < 0.001) increases in the rate of contraction of the reticulum. Retention time of 103Ru-phenanthroline in the intestines was not affected by cold exposure.

5. Cold exposure was associated with depression (P < 0.05) of volatile fatty acids concentration in the rumen and elevated (P < 0.05) pH. Molar proportions of acetic and isovaleric acid were reduced (P < 0.01), accompanied by increased (P < 0.001) proportions of propionic acid during cold exposure.

6. Cold exposure and pelleting of the diets were both associated with reduction in digesta particle size in the rumen. Duodenal particle size was not affected by cold exposure. Pelleting of the diet markedly reduced (P < 0.001) duration of chewing and number of chews/d during eating and rumination. Cold exposure of sheep resulted in a faster (P < 0.01) rate of eating of the diets.

7. When allowed to express their voluntary feed consumption during a 10 d period, intakes of chopped diets were increased by 0.13 (P < 0.01) by cold exposure, in contrast to lack of significant change in sheep given pellets.

8. The results did not support the hypothesis that the effect of cold exposure on digestion was dependent on the physical form of the diet given at fixed intake, but did indicate that increased voluntary feed consumption was a result of increased clearance of digesta from the rumen throdgh decreased rumen particle size and increased reticulum motility.

1. Eight newborn lambs were given milk-substitutes based on casein (n 4) or a soya-bean-protein isolate (n 4). On the morning before a 10 d collection period, the milk-substitutes were supplemented with 59Fe as ferric chloride and 5Zn as zinc chloride. Faeces and urine were collected daily and determinations of 59Fe and 65Zn were made on these separately. During a second 10 d period the dosing procedure and collection were repeated but the diet of each lamb was reversed.

2. Mean retention of 59Fe in lambs given the casein diet was 0.50 (SE 0.12) and in those given the soya-bean-protein diet 0.06 (SE 0.05). There were large variations among animals but the treatment effect was significant at P < 0.05. Retention of 65Zn was significantly greater than that of 59Fe for all lambs but the effect of dietary treatment was similar to that for Fe. 65Zn retention values for casein and soya-bean-protein diets were 0.84 (E 0.06) and 0.52 (SE 0.03) respectively, and the difference was significant at P < 0.01.

3. There were only very small amounts of 59Fe and 65Zn in urine, representing 0.002–0.003 of the dose.

1. Some metabolic indicators of thyroid hormone activity have been investigated in rats fed on either protein-deficient or energy-restricted diets.

2. Rats were divided into three groups. Control animals were maintained on a diet of protein energy: total energy (P:E) value of 0.20, while the low-protein group (LP) were allowed ad lib. access to food of P:E 0.03. Energy-restricted (ER) rats were given limited amounts of a control diet (P:E 0.20) such that their rate of growth matched that of LP animals.

3. Animals fed on the LP diet had elevated plasma concentrations of both total and free triiodothyronine (T3)concentrations whereas those on the ER regimen showed values below those of controls.

4. The activities of mitochondria1 α-glycerol-3-phosphate dehydrogenase (EC 1. 1.99.5) and of the αglycerol-3-phosphate shuttle system were elevated in the liver of LP rats, but malate-aspartate shuttle operation was reduced. All three activities were reduced in ER animals.

5. Cytochrome c oxidase (EC 1.9.3.1) activity of brown adipose tissue indicated a high rate of thermogenic activity in this tissue in LP rats, but ER animals showed some evidence of below normal function.

6. The results indicate that both the raised plasma T3 of LP rats and the reduced levels observed in ER animals are physiologically significant.

1. The effects of uncoupling of mitochondria1 oxidative phosphorylation on the efficiency of energy conservation during oxidation of amino acids, fatty acids, glycerol, glucose and 101 food proteins have been examined in order to compare how uncoupling at coupling site 1 affects energy yields compared with uncoupling at sites 2 + 3 and uncoupling by proton leakage. The effects of uncoupling by each mechanism on the isodynamic equivalents of carbohydrate, fat and protein at the level of cytoplasmic ATP yield have been estimated.

2. Energy conservation during amino acid oxidation decreases relative to that for glucose as uncoupling by all three mechanisms increases. This effect is least when uncoupling is at site 1 and is associated with a fall in the isodynamicequivalent for protein: glucose of 4%maximally, and a fall in the cytoplasmic ATP yield for glucose of 25% (15–30% when accounting for uncertainty in the choice of proton stoichiometries).

3. Variation in the efficiency of energy conservation for the different amino acids is large for both highly coupled and uncoupled mitochondria but the range of efficiencies for the oxidation of 101 food proteins is relatively small (less than 6% of the mean) for a tightly coupled system. This range increases absolutely (minimally fourfold) and relatively (minimally 44% of the mean value) with severely uncoupled mitochondria but is nearly constant (changes by less than 1% relative to the mean) within the probable physiologically relevant range of uncoupling in the whole body and in the full range of uncoupling at site 1. The rank order position of particular proteins within the range of values is found to change most for gelatin which is oxidized with least energy conservation in a severely (unphysiologically) uncoupled system and most efficiently in a fully coupled system when oxidation of protein is considered to be direct, i.e. not via gluconeogenesis.

4. For medium- and long-chain fatty acids, uncoupling at site 1 elevates the efficiency of energy conservation relative to that for glucose (maximally 4%) whereas uncoupling by other mechanisms decreases this relative efficiency. The pattern of effects for short-chain fatty acids resembles that for the amino acids.

5. The changes in the isodynamic equivalents of protein:glucose and of fat:glucose are small when uncoupling occurs at site I and tend to cancel for a mixed diet but are additive in the effect on food energy values when uncoupling is by the other mechanisms. Hence changes in the efficiency of oxidative energy coupling at site 1 in association with Luft's disease or dietary changes would result in effects which are of little true dietetic significance on the isodynamic equivalents of nutrients at the level of cytoplasmic ATP yield in vivo.