1. The influence of temperature, moisture content and the presence of glucose on the level of available methionine and cysteine in casein was studied.

2. Differences between total and available methionine and cysteine contents of heated casein (90° for 24 h) were determined by an in vitro method. The maximum losses in total and available methionine content were 22 and 51% respectively. The losses in total and available cysteine content were 24 and 100% respectively.

3. The results indicated that for heated casein the release of amino acids by proteolytic enzymes was less complete than for native casein.

4. The results of rat growth assays suggested that diets containing oxidized casein are less well utilized by rats than those containing native casein. The decrease in body-weight of rats receiving the diets containing oxidized casein could be counteracted by the addition of methionine and 20 g unoxidized casein/kg diet.

5. There was a lower level of some available amino acids (determined after enzymic hydrolysis using pancreatopeptidase E (EC 3.4.4.7), leucine aminopeptidase (EC 3.4.1.1) and prolidase (EC 3.4.3.7)), including those essential for the rat, in oxidized casein as compared with native casein.

6. Cysteic acid, in oxidized casein, probably makes impossible the utilization of the amino acids in its neighbourhood.

7. From the differences in the available amino acid contents of the native, oxidized and heated casein it was concluded that the oxidation of casein causes the formation of complexes in the polypeptide chain, resistant to enzymic hydrolysis, but to a much lesser extent than does heating.

1. The available methionine and cysteine contents of proteins were determined by chemical methods after preliminary enzymic hydrolysis.

2. The values for the available methionine and cysteine contents of pure proteins (casein and bovine serum albumin) estimated by chemical methods were similar to those for the total content determined by the method of Moore, Spackman & Stein (1958).

3. Reductions of 15 and 11% respectively, when compared with unprocessed samples, were found in the available methionine contents of sweetened and unsweetened, condensed milks; of roller-dried milk and whey powders, and of mackerel sterilized at 126°, the reductions were 22, 14 and 19% respectively.

4. The available cysteine content of sweetened, condensed milk was reduced by about 32%, whereas for mackerel sterilized at 115 and 126° it was reduced by 64 and 75% respectively.

5. The contents of total sulphur amino acids for these food products did not differ from those for the unprocessed samples.

6. Values obtained for available S amino acid contents by rat bioassay confirmed the results of the in vitro estimations.

1. The effect on body-weight of field work in Antarctica when travelling by mechanical transport has been studied.

2. A steady loss of weight (mean 6.2 kg) was found in the field, and there was a slow gain on return to less arduous conditions.

3. Changes in skinfold thickness and hip girth suggested that the loss of weight was due to an energy deficit, and the gain was due to an excess of energy.

4. However, although weight was being lost the energy balance was calculated to be positive. This was probably because the expenditure was underestimated as the result of a failure to take into account: (a) the weight and restriction of heavy clothing, (b) the high-protein diet, (c) the difficulty imposed by the terrain, (d) the intensity of the subjects' response to the cold.

5. Daily weight changes were significantly related to climatic conditions.

6. Daily weight changes were significantly related to fluid intakes, but it was not possible to assess with certainty the contribution made by dehydration to the recorded weight loss.

7. Daily fluctuations in weight were greater than those previously found under more standard conditions.

1. A survey was done of 2333 men and women who claimed experience of slimming.

2. Their loss of weight was determined from their maximum stated weight and their present weight. The loss of weight was calculated as the percentage of the maximum weight and was related to the stated age of onset of obesity.

3. The results showed that those people in the survey who had been fat since childhood had lost just as much weight as those people who had become fat as adults.

4. These results suggest that the treatment of early-onset obesity may not be an unrealistic objective.

1. The hydrolysis of glycyl-L-leucine, glycyl-L-tyrosine, tributyrin, sucrose, maltose, soluble starch and α- and β-glycerophosphates by everted segments of rat intestine was estimated separately or in combination.

2. A comparative study showed significant interaction between different substrates which affected their digestion.

3. Two types of interaction were identified: products of hydrolysis (1) affected the hydrolysis of homologous substances, e.g. methionine and alanine inhibited glycyl-L-leucine hydrolysis, maltose reduced glucoamylase (α-1, 4-glucan glucohydrolase; EC 3.2.1.3) activity (intracatenary interactions); (2) interfered with the hydrolysis of a different group of substances, e.g. tributyrin inhibited dipeptidase (glycyl-L-leucine hydrolase; EC 3.4.3.2) and alkaline phosphatase (EC 3.1.3.1), glycyl-L-leucine interfered with the activity of the latter enzyme (intercatenary interactions).

4. Mechanisms of interactions were suggested by the results of a comparison of the extent of inhibition or activation of two enzymes (glycyl-L-leucine hydrolase and alkaline phosphatase) in situ in everted intestinal segments or after solubilization with papain or Triton X-100, and different treatments known to affect allosteric sites of these enzymes.

5. Tributyrin and dipeptides were found to act on alkaline phosphatase as allosteric regulators. A discontinuity of the Arrhenius plot suggested the existence of different enzyme conformations which were re-arranged by tributyrin.

6. Substrate interactions in digestion were found in adult rat, cat, rabbit and hen. Substantial differences were found between classes (Aves and Mammalia), orders (rodents, lagomorphs and carnivores) and between age-groups within an animal strain (in this instance, for the rat).

7. These interactions are thought to be involved in the co-ordination of digestion with intestinal absorption and to regulate the time and site of subsequent hydrolysis.

1. In Expt 1, a pelleted grass-meal diet was supplemented with hydrochloric acid, added to the grass pellets before feeding, at five levels from 0 to 628 mmol/kg dry matter (DM). Each diet was offered ad lib. for 21 d to five sheep in a 5 × 5 Latin-square design.

2. Voluntary food intake decreased rectilinearly with increasing HCl supplementation (P < 0.001), to 42% of the control value for sheep on the high-HCl diet. The decrease in food intake was related both to dietary pH and to the extent of metabolic acidosis induced by the HCl treatment. Although the pH of rumen fluid decreased slightly with increasing HCl supplementation, effects of the HCl treatment on volatile fatty acid concentrations in rumen fluid were not significant (P > 0.05).

3. In Expt 2, palatability and metabolic effects of dietary HCl were studied by comparing its effect when mixed into the pelleted grass meal before feeding, with and without a supplement of an equivalent amount of sodium bicarbonate given intraruminally, or when HCl was given intraruminally while the sheep consumed pelleted grass meal alone. Each of the three treatments was given at two levels of HCl, 280 and 560 mmol/kg DM. At each level of dietary HCl supplementation, the three treatments and the control diet (pelleted grass meal alone) were each given to four sheep, in a Latin-square design, for 11 d.

4. At the low level of supplementation, HCl, when mixed into the pelleted grass meal, reduced food intake by 17%, this effect was not altered by NaHCO3 supplementation, but when HCl was given intraruminally food intake was not reduced. At the high level of HCl supplementation, food intake was reduced by about 40% by each method of HCl supplementation; and NaHCO3 supplementation did not appreciably alter the effect of HCl on food intake, but prevented metabolic acidosis associated with the HCl treatments. Food intakes for the low-HCl treatments were significantly higher than those for the high-HCl treatments (P < 0.01) and the level of dietary HCl×treatment interaction was also significant (P < 0.01). DM digestibility, and the pH and volatile fatty acid concentrations of rumen fluid were not significantly affected by the different treatments.

5. It is concluded that at a low level of HCl supplementation the adverse effects of dietary HCl on voluntary food intake of sheep is determined by palatability associated with low dietary pH, whereas at a high level of HCl supplementation the effect is determined by palatability and by a metabolic response.

1. Five experimental diets were offered ad lib. for 21 d to five sheep in a 5 × 5 Latin-square design experiment as follows: pelleted grass meal alone (control); pelleted grass meal plus hydrochloric acid (470 mmol/kg dry matter (DM)); pelleted grass meal plus 470 mmol HCl/kg DM and either an equivalent amount of calcined magnesite (MgO) (235 mmol/kg DM) or twice the amount (470 mmol MgO/kg DM); pelleted grass meal plus 470 mmol MgO/kg DM.

2. MgO supplementation partly prevented the reduction in food intake caused by HCl, being more effective at the low than at the high level. When fed alone, the high level of MgO had a slight adverse effect on food intake.

3. There was no significant treatment effect on either the pH or volatile fatty acid concentrations of rumen fluid. MgO supplementation was only slightly effective in preventing the metabolic acidosis caused by HCl supplementation, as indicated by blood and urine acid–base measurements.

4. With the MgO-supplemented diets, values for the faecal and urinary excretion of magnesium were approximately 70 and 10% respectively of Mg intake and were not significantly affected by HCl supplementation. For rumen fluid, the water-soluble Mg concentration as a percentage of the total Mg concentration was similar for each treatment, approximately 90%. For faeces, the corresponding value was also similar for each treatment, approximately 30%.

5. MgO supplementation altered the effect of dietary HCl on faecal calcium excretion and on the balance of Ca, but did not alter its effect on urinary Ca excretion.

6. It is concluded that the beneficial effect of MgO supplementation on the intake of the HCl-treated diet was related more to its influence on dietary pH than on conditions in the rumen or the acid–base balance of the sheep.

1. The inclusion of phytate (10 g/kg) in a purified diet containing zinc (15 mg/kg) fed to young male rats significantly reduced growth rate and food intake, and promoted a cyclic pattern of food intake characteristic of an uncomplicated Zn deficiency. The decreased growth rate could be accounted for by the reduced food consumption.

2. Rats maintained on a Zn-deficient diet (0.5 mg Zn/kg) were found to have a cyclic pattern of food intake and a very slight weight gain. The addition of phytate (10 g/kg) to the Zn-deficient diet promoted a net loss of mean body-weight.

3. Rats maintained on the Zn-supplemented diet containing phytate excreted significantly more Zn in their faeces than either pair-fed or ad lib.-fed control rats. Rats given the Zn-deficient diet supplemented with phytate excreted more Zn in their faeces than Zn-deficient control rats.

4. Dietary phytate significantly reduced the average daily accumulation (μg/d) and wholebody retention (relative to dietary intake) of iron, copper, manganese and Zn, whether or not the diet was supplemented with Zn.

5. The addition of phytate to the lumen fluid of ligated loops of rat duodenum maintained in situ significantly inhibited 65Zn absorption, compared with the control systems without added phytate.

6. Other studies using ligated duodenal and ileal loops indicated that Zn is secreted into the gut lumen and approximately one-third of this is normally reabsorbed. Recycling of endogenous Zn may be a significant process in the over-all body economy of this trace element.

7. The absorption of 65Zn added to the diet was significantly reduced by dietary phytate. Dietary phytate also reduced the biological half-life of body 65Zn from 91 to 211 h post-administration, possibly by inhibiting reabsorption of endogenous 65Zn and thus promoting a more rapid loss from the body.

1. Plasma amino acid levels have been estimated at 0, 15, 30 and 45 min after ingestion of doses of (1) an amino acid mixture simulating casein and (2) a tryptic hydrolysate of casein consisting mainly of oligopeptides. Both doses contained the same amount of nitrogen.

2. After ingestion of both preparations, there was a prompt increase in plasma amino acid levels, followed by a decrease. No such change occurred in fasting subjects. There were no significant differences between increments in plasma amino acid levels after ingestion of the amino acid mixture and the corresponding increments after ingestion of the tryptic hydrolysate.

3. Correlations were found between the areas under the curves for individual amino acid concentrations, after ingestion of the two preparations, and the amino acid composition of casein. The results do not suggest that increases in plasma amino acid levels following small doses of protein digestion products are the result of circadian changes, or that such increases are ‘swamped’ by absorption of amino acids from endogenous protein in the lumen of the small intestine.

1. Flow rate of digesta and its components in the upper jejunum, and the absorption of L-histidine and glucose from the jejunum segment were measured in pigs fitted with three simple cannulas. The pigs were fed once daily at 08.30 hours.

2. A maximum flow of digesta was obtained in the period 10.00–10.30 hours; the flow rate decreased with time after feeding, reaching a minimum in the period 22.00–22.30 hours.

3. The absorption rate for L-histidine and glucose increased in a hyperbolic manner with increasing concentrations of infused test material, which ranged from 2.5 to 20 g/l for each material.

4. L-histidine and glucose were absorbed nearly independently when perfused in combination. The absorption rates for glucose were significantly (P < 0.01) greater than the corresponding rates for L-histidine at each concentration of infusate.

5. The absorption of both L-histidine and glucose expressed as a percentage of the amounts in the perfusate decreased with increasing flow rate of perfusate, from 400 to 800 ml/h. The increase in flow rate from 400 to 800 ml/h was associated with a 20% increase in L-histidine absorption rate; there was a 30% increase in glucose absorption rate when the flow rate was increased to 600 ml/h, but no further increase at 800 ml/h.

6. The absorption of both L-histidine and glucose decreased with time after feeding; the absorption rates for L-histidine and glucose measured for the period 10.00–10.30 hours were 126 and 133%, respectively, of those measured for the period 22.00–22.30 hours.

1. The heat loss for individual growing pigs, whose initial body-weight was 25–40 kg, was measured continuously for periods of 14 or 21 d when they were maintained at an environmental temperature of either 20 or 30°. For each temperature the feeding level before and after a period of fasting was held at either 39 or 45 g/kg body-weight per d. Four pigs were given each treatment; food was withheld from three animals for a 4 d period in each experiment, and the fourth pig (control) was fed for the whole experimental period.

2. After the withdrawal of food, the heat loss decreased rapidly and minimum values were recorded during the third and fourth days of fasting. Fasting heat loss was higher at 20° than at 30° and was not significantly dependent on the preceding level of feeding. The mean values obtained at 20 and 30° were 457 and 380 kJ/kg0.75 per d respectively.

3. The heat loss during the periods of feeding was significantly higher (P < 0.05) at 30° than at 20°, and was also higher at the higher level of feeding.

4. The apparent efficiency of utilization of metabolizable energy for maintenance was calculated as 0.80.

5. It was concluded that the critical temperature for a fasting pig weighing 25–40 kg is approximately 25°.

1. A study was made of the metabolism of ɛ-(γ-L-glutamyl)-L[4, 5-3H]lysine (GL) in the rat.

2. The compound was largely absorbed from the intestine and metabolized. Labelled lysine was incorporated into blood proteins.

3. In an in vitro experiment with everted sacs of rat small intestine, GL passed through the intestinal wall unchanged.

4. The results of comparative tests using homogenates of different body tissues indicated that the kidneys were particularly active in hydrolysing GL. Their activity was nine times greater than that of the liver and eighteen times greater than that of the small intestine.

1. Enzymes that may contribute to liquefaction of the cornea in retinol-deficient animals and in man have been studied using rat cornea. The established technique of culturing tissue fragments and determining the activity of collagenase (EC 3.4.24.3) and other enzymes in the medium after different periods of culture was used.

2. A collagenolytic system was detected in the media from cultures of rat corneas. This system probably consists of at least two enzymes, a collagenase and a neutral proteinase.

3. Both proteolytic and specific collagenolytic activity were greater in media from retinol-deficient rat corneas. The hydroxyproline level increased in parallel with the increase in enzyme activity.

4. In the final stages of retinol deficiency the cornea is invaded by granulocytes and other cells of the blood and we suggest that destruction of cornea collagen may be due largely to the activity of the enzymes from these cells.

1. A combination of a mineral balance and a radioactive technique has been used to study the effects of increased levels of dietary calcium and phosphorus on the Ca and P metabolism of young and mature wether sheep previously made Ca-deficient.

2. Ca absorption was directly related to Ca intake until an absorption maximum was reached. Young animals absorbed dietary Ca more efficiently than mature animals and had a much higher maximum rate of absorption. Apparent P absorption was directly related to P intake and maximum P absorption was also higher for the young animals.

3. Excretion of Ca in the urine and into the intestine was not altered by changes in Ca intake. Excretion of P in the urine, however, varied considerably and it is suggested that this process may be largely responsible for P homoeostasis.

4. Ca retention was directly related to Ca absorption. Maximum retention was considerably higher for the young animals than for the mature animals and occurred for each group of animals at exactly the same dietary intake as maximum Ca absorption.

5. Ca and P were retained in a constant ratio of 1.25:1 and the results suggested that P retention was controlled by the rate of Ca retention.

6. Bone turnover was higher for the young than for the mature animals. Animals mobilized skeletal Ca when receiving the Ca-deficient diet and replaced their Ca losses when the dietary Ca was plentiful by changing the relative rates of bone resorption and bone accretion. Although bone resorption was mainly responsible for Ca homoeostasis, bone accretion appeared to play some part in the young animals.

7. Maximum retention of Ca occurred just as the rate of bone resorption become zero and was equal to the rate of bone accretion. The lower maximum retention of Ca for mature animals was due to a lower rate of bone accretion.

8. Results suggested that when bone resorption ceased, Ca absorption was regulated by homoeostatic mechanisms at a level just sufficient to maintain maximum retention, and that the maximum rates of absorption measured may not have been true maximum rates.

9. The consequences of a relationship between bone accretion and maximum Ca retention are discussed with reference to older, pregnant and lactating animals.

1. The preparation of a propionylated protein is described, and the effects of this treatment on amino acid composition and availability are compared with the effects of severe heat treatment (autoclaving) of a protein.

2. Using chemical analyses, changes exceeding 5% for total tyrosine, histidine, methionine and cystine contents were found after propionylation of the protein. Autoclaving of the protein resulted in changes in total serine, lysine, methionine, cystine and tryptophan contents.

3. Microbiological estimates of total amino acid contents were not in close agreement with the chemical estimates for the autoclaved protein.

4. Fluorodinitrobenzene-reactive lysine content was reduced to almost zero by propionylation, and by almost 40% by autoclaving.

5. Both propionylating and autoclaving protein reduced the amount of lysine available to the chick by about half. In contrast, the availabilities of methionine and tryptophan to the chick were unchanged by propionylation, but were reduced to 0.66 and 0.44 respectively, relative to the untreated protein, by autoclaving.

6. Because of the difficulties of obtaining reliable absolute estimates of amino acid availability using chick growth assays, our interpretation of results is mainly based on relative values. The merits of microbiological microbiological estimates of amino acid availability are assessed.

1. The digestibilities of protein and amino acids have been estimated by two different techniques: the analysis of faeces (conventional method) and the analysis of ileal contents (ileal technique).

2. Freeze-dried muscle protein was found by both techniques to be almost completely digested. After autoclaving, the digestibility for the same protein was estimated by the conventional and ileal techniques to be 0.65 and 0.57 repectively.

3. Unmodified lactalbumin was found by both techniques to have a digestibility of about 0.90. Propionylation of the lactalbumin reduced digestibility to 0.82 and 0.79 as indicated by faecal analysis and ileal content analysis respectively.

4. In general, the digestibilities of individual amino acids in any one protein sample were rather uniform, and reflected over-all protein digestibility. For each amino acid, digestibility, as determined by the both methods, was lower for the modified protein than for the corresponding control protein: estimates based on ileal content analyses were consistently lower than those obtained by conventional analyses. The ileal technique was considered to be both more convenient and meaningful.

5. From the results obtained by the ileal technique it appears that reduced digestibility is an adequate explanation for the reduction found in nutritional value of the autoclaved protein. In contrast, for the propionylated protein, reduced digestibility of lysine is only a partial explanation of the low availability of this amino acid as estimated by chick growth assay.

6. In our experiments we found that the type of dietary protein used did influence the amino acid composition of the ileal contents. This was most marked with the least-digestible protein. These findings do not support the views of Nasset (1962).