Decreased fidelity of DNA polymerases and decreased DNA excision repair in aging mice: effects of caloric restriction.

Hepatic DNA polymerases from calorie restricted and ad libitum 26 month old C57BL/6 mice showed a decline in fidelity of nucleotide incorporation compared with weanling animals. Both alpha and beta polymerases from calorie restricted aged mice exhibited a higher level of fidelity than polymerases from ad libitum aged mice. UV-initiated unscheduled DNA synthesis was significantly higher in hepatocytes from weanling and 18 month old calorie restricted animals compared with cells from 18 month old ad libitum animals, while MMS-initiated unscheduled DNA synthesis did not differ significantly between cells from young and old or ad libitum and calorie restricted animals. These data suggest that calorie restriction could play a significant role in decreasing the age-related decline of cellular mechanisms expected to reduce the rate at which mutations accumulate during aging, and could potentially prolong the onset age of mutation-associated diseases of the elderly.     

The effect of long-term caloric restriction on function of T-cell subsets in old mice

The effect of caloric restriction (from weaning to old age) on CD3-stimulated CD4+ and CD8+ lymphocyte proliferation and calcium mobilization was examined. Young ad libitum (ad lib) fed, old ad lib fed, old calorically restricted, and old calorically restricted mice which were fed ad lib during the last 6 weeks of their life (restricted/refed) were compared in both BDF1 [(C57BL/6 x DBA/2)F1] and C57BL/6 mice. Proliferation of CD4+ cells was lower in old ad lib animals than in young animals; this difference was not seen in CD8+ cells. Those CD4+ cells which did proliferate in old ad lib animals underwent similar cell cycle progression as young cells. In calorically restricted and calorically restricted/refed animals, CD4+ cell proliferation was similar to the young animals, and CD8+ cells showed a higher proliferative capacity than cells from either young or old ad lib mice. Differences in proliferative capacity were not correlated with alterations in transmembrane signaling efficiency as peak [Ca2+]i was reduced in both T-cell subsets in all groups of old mice relative to young mice. Additionally, reduced [Ca2+]i was observed in the CD8+ subset for which there was no deficit in proliferation, and the enhanced proliferation seen in old restricted and old restricted/refed mice did not manifest as increased [Ca2+]i mobilization. The percentage of CD4+ cells from both mouse strains was reduced in all groups of old mice compared with young mice, while the percentage of CD8+ cells was generally similar in young and all groups of old mice. Our studies would suggest that lifelong caloric restriction of mice prevents the age-associated decrease in T-cell proliferative capacity but that the enhanced proliferation of these cells is not due to increased efficiency of transmembrane signaling.     

Effect of age and dietary restriction on protein synthesis by isolated kidney cells

The influence of age and food restriction on kidney protein synthesis was studied in Fischer F344 rats. The rate of total protein synthesis by suspensions of kidney cells declined 60% between 4 and 31 months of age. The rate of protein synthesis by kidney cells isolated from 19-month old rats fed a restricted diet (60% of diet consumed by rats fed ad libitum) was 45% higher than the rate of protein synthesis by kidney cells isolated from 19-month old rats fed ad libitum. The excretion of protein in the urine was measured to assess the effect of the age related decline in protein synthesis on kidney function. A dramatic increase in proteinuria was observed with increasing age, and rats fed the restricted diet excreted significantly less protein in the urine than rats fed ad libitum.     

Free amino acid levels in undernourished developing rat brain

Free amino acid levels in the brains of young ones born to mothers fed a 20% protein diet ad libitum (well nourished), 7.5% protein diet ad libitum (protein restricted) and a 20% protein diet in restricted amounts (pair-fed) were investigated during brain development in the present study. The dietary protein was obtained from a cereal-legume mixture. Protein restricted animals showed increases in the levels of taurine, glycine and glutamic acid and decreases in the concentrations of methionine, leucine, isoleucine, and GABA. The pair-fed animal showed increases only in glutamic acid and glycine and a decrease only in the levels of GABA. The significance of these observations is discussed.     

Age-related change in the degradation rate of ovalbumin microinjected into mouse liver parenchymal cells

Change in the degradation rate of ovalbumin (OVA) microinjected into liver parenchymal cells isolated from mice of various ages was studied. OVA was injected by osmotic lysis of pinosomes and the amount of OVA was determined by immunoblotting using purified antibody to OVA. Cellular activity as judged by rates of protein synthesis and degradation of pulse-labeled proteins was not affected by the injection. To localize injected OVA in the cells, cell extracts were fractionated by differential centrifugation and the amount of OVA and the activity of beta-D-galactosidase as a lysosomal marker enzyme of each fraction were determined. The ovalbumin was mostly found in the soluble fraction, while the beta-D-galactosidase activity was found in the particulate fraction, indicating that the ovalbumin was spread in the cytosol but was not present in the pinosomes or lysosomes. The average half-lives of OVA were 106, 113, and 164 h in the cells from young (3.5-6.5 months), middle-aged (13.5-20.0 months), and old (24.5-30.5 months) mice, respectively. Thus, the half-life of ovalbumin in the cells of senescent mice was about 50% longer than that in the cells of young or middle-aged mice. These results are in good agreement with those of our previous investigation, which showed that the half-life of inactivation of horseradish peroxidase was extended by about 50% in the hepatocytes from old mice (Ishigami and Goto, 1988, Mech. Ageing Dev., 46, 125-133).     

Triiodothyronine improves the primary antibody response to sheep red blood cells in severely undernourished weanling mice

Three experiments were conducted in which weanling mice were fed a nutritionally complete diet either ad libitum or in restricted quantities such that they lost about 30% of their initial weight over a 14-day period. In Experiments 1 and 2, half the animals from each group received dietary triiodothyronine (T3) supplements. In Experiment 3, food-intake-restricted mice were fed graded levels of potassium iodide. Malnutrition reduced the number of nucleated cells per spleen, the number of splenic IgG plaque-forming cells (PFC) per 10(6) cells, and the serum antibody titers against sheep red blood cells. T3 supplements increased antibody titers, the number of nucleated cells per spleen, and both IgM and IgG PFC per 10(6) spleen cells in malnourished mice, but had no effect on well-nourished mice. The beneficial effect of T3 was not a result of improved protein, energy, or iodine status in the malnourished mice.     

Reduction of enhanced mammary carcinogenesis in LA/N-cp (corpulent) rats by energy restriction

Restriction of energy intake significantly reduces mammary tumorigenesis in normal rats exposed to carcinogens. Genetically obese LA/N-cp (corpulent) female rats were given 7,12-dimethylbenz[a]anthracene and fed purified diets ad libitum or restricted to 60% of the ad libitum caloric intake. Phenotypically lean littermates were also fed ad libitum. Obese animals developed large mammary tumors more rapidly than genetically normal rats so that 100% of the animals had tumors in less than 16 weeks. Only 21% of the lean animals developed tumors; the energy restricted obese animals had a tumor incidence of 27%. Although obese rats fed the restricted diet weighed significantly less than those fed ad libitum, percent body fat was not reduced, indicating that lean tissue was affected more. Obese animals were markedly hyperinsulinemic (1003 +/- 193 microunits/ml) and energy restriction reduced this to 328 +/- 41; the lean animals had insulin levels of 12 +/- 2. Tumor-bearing rats had higher insulin levels than rats without tumors. These data suggest that body fatness is not directly associated with risk of carcinogenesis. Lean body mass, adipose tissue mass, and their interaction with insulin in its capacity as a growth factor rather than body fatness per se may be determinants of tumor promotion.     

Energy balance and facultative diet-induced thermogenesis in mice fed a high-fat diet

The present study was aimed at studying energy balance in mice fed a high-fat diet. Albino mice were divided into three groups. One group had free access to the stock diet, whereas the two other groups consumed a high-fat diet. One of the high-fat fed groups was fed ad libitum, whereas the other was offered a restricted amount of the same diet so that its energy intake was comparable to the group of mice given the stock diet. Energy balance measurements, which included indirect calorimetry and carcass analysis, were performed. Brown adipose tissue (BAT) properties were also investigated. The results show that gains in both body weight and fat were higher in mice that had free access to high-fat diet than in mice fed the stock diet. In animals given a restricted amount of the high-fat diet, fat gain increased, whereas protein gain was reduced in comparison with animals fed the stock diet. Unrestricted access to the high-fat diet led to an increase in both energy intake and energy gain. As revealed by both slaughter and indirect calorimetry techniques energy expenditure was, in high-fat fed mice, 40% higher than in animals fed either stock or a restricted amount of high-fat diet. Nadolol was shown to suppress a large part of the elevated metabolic rate seen in mice fed an unrestricted high-fat diet. In those mice, BAT mitochondrial GDP binding was also increased. In summary, the present results confirm that adaptive diet-induced thermogenesis (DIT) develops in mice made hyperphagic by an energy-dense palatable diet.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of fish oil diet on immune response and proteinuria in mice

In this study, we examined the immune response and proteinuria caused by dietary polyunsaturated fatty acids in normal NZW/N and autoimmune NZB/NZW mice. Mice were maintained more than one year on five dietary groups: normal (5% corn oil), calorie-restricted, high fat (20% corn oil), high fat (20% fish oil), and Purina laboratory rodent chow. Normal mice fed with the fish oil diet had a more reduced anti-sheep red blood cells (SRBC) plaque-forming cell (PFC) response and less interleukin-2 (IL-2) enhancement of PFC than did the group with the restricted diet and the young control group. The corn oil (5 and 20%) diet animals also showed reduced PFC response and IL-2 utilization. NZB/NZW mice fed with the fish oil diet showed similar reduced PFC response but had a significantly lower response to IL-2 than did those on the corn oil diets and the restricted diet. The IL-2 production by macrophages from NZW/N mice was reduced in both the fish oil and corn oil diet groups. However, mice fed with the fish oil diet had less proteinuria and good survival rates, similar to the group with the restricted diet. These results suggest that the beneficial effect of the fish oil diet in these animals may be attributed in part to the immunosuppression mechanism.     

The effect of aging and dietary restriction on DNA repair

DNA repair was studied as a function of age in cells isolated from both the liver and the kidney of male Fischer F344 rats. DNA repair was measured by quantifying unscheduled DNA synthesis induced by UV irradiation. Unscheduled DNA synthesis decreased approximately 50% between the ages of 5 and 30 months in both hepatocytes and kidney cells. The age-related decline in unscheduled DNA synthesis in cells isolated from the liver and kidney was compared in rats fed ad libitum and rats fed a calorie-restricted diet; calorie restriction has been shown to increase the survival of rodents. The level of unscheduled DNA synthesis was significantly higher in hepatocytes and kidney cells isolated from the rats fed the restricted diet. Thus, calorie restriction appears to retard the age-related decline in DNA repair.     

The high mobility group of nuclear proteins as biomarkers of age and caloric restriction in rats.

The quantitative levels and phosphorylation states of the high mobility group (HMG) of proteins were investigated in bone marrow, brain, heart, kidney, liver, pancreas, spleen, testis and thymus of three groups of male Fischer 344 rats. Two groups of rats, young ad libitum (Y/AL - 1 1/2 mo.) and old ad libitum (O/AL - 28 mo.), had free access to rat chow, and a third group of old rats were maintained on a caloric restricted intake (O/CR - 28 mo.). The quantities of HMGs 1,2,14 and 17 were significantly reduced in O/AL rats compared with Y/AL rats in all tissues examined, and in many cases, the amount of HMGs of O/CR rats were increased by varying degrees from O/AL animals. In G2-phase nuclei of bone marrow, spleen and testis, phosphorylation of HMG proteins was reduced significantly in O/AL rats, but was enhanced in O/CR animals (especially HMG14). These levels of HMGs in O/CR animals, altered by age and diet dependent factors, reflect a condition which is more reminiscent of Y/AL than O/AL animals.     

Conjugated Linoleic Acid Does Not Improve Insulin Tolerance in Mice*

Objective: To determine if the addition or removal of dietary conjugated linoleic acid (CLA) would alter insulin tolerances in mice from two genetic lines. Research Methods and Procedures: High metabolic rate (MH) and low metabolic rate (ML) mice were assigned to consume 1) a control diet ad libitum, 2) a control diet at a restricted intake, or 3) a diet containing 1% CLA ad libitum. After 9 weeks, an insulin tolerance test was conducted, and a portion of the mice were killed. All remaining mice consumed the control diet ad libitum. Insulin tolerance tests were conducted 11 and 32 days after the diet change, and mice were killed 3 days after each test. Body fatness, fat pad weights, and serum insulin concentrations of mice were determined at each time‐point. Two follow‐up experiments were also conducted. Results: Restricted mice had insulin sensitivities not different than control mice. CLA‐fed MH mice in experiment 1 were resistant (p < 0.001) to insulin on each day measured. CLA‐fed ML mice were slightly resistant (p = 0.08) to exogenous insulin on day 0 of recovery and not different from control mice on day 11 or 32. Glucose response to insulin in MH mice fed CLA in experiments 2 or 3 did not differ from control mice. Discussion: Mice fed CLA did not have improved insulin tolerances compared with control mice. In some cases, dietary CLA may cause insulin resistance. MH mice seem more sensitive to CLA than ML mice.     

Ultrastructural morphometry of matrical changes induced by exercise and food restriction in the rat calcaneal tendon.

The ultrastructural morphometry of collagen fibril populations in 24 calcaneal tendons obtained from 12 Fischer 344 rats were studied to elucidate matrical changes induced by food restriction and/or endurance exercise. Rats were randomly assigned to four equal groups: ad libitum control (AC), ad libitum exercise (AE), restricted diet control (RC) and restricted diet exercise (RE) groups. Beginning from 6 weeks of age, animals in the two food restriction groups were fed 60% of the mean food consumption of ad libitum fed rats. Then, starting from 6-7 months of age, the rats in the two exercise groups performed 40-50 min of treadmill running at 1.2-1.6 miles h-1 every day for a total of 10 weeks. Endurance training did not significantly alter body weight, but food restriction with or without exercise resulted in a significant loss of body weight. In ad libitum fed controls, food restriction alone did not significantly alter the mean collagen fibril CSA, but predisposed a preponderance of small-sized collagen fibrils. Endurance training per se induced a significant (32%) increase in mean fibril CSA (P less than 0.05), but this adaptive response to exercise was prevented by food restriction, as indicated by a 33% decline in fibril CSA (P less than 0.05). These findings demonstrate that dietary restriction modifies the adaptation of tendon collagen morphometry in response to endurance training, and that weight loss is better achieved with food restriction than endurance exercise.     

Protein turnover,synthesis and secretion of albumin in hepatocytes isolated from rats bearing walker 256 carcinoma

Summary Hepatocytes isolated from rats bearing line A of Walker 256 carcinoma (WA) were used to study the turnover of total liver protein and the synthesis of albumin in comparison with ad libitum (AL) and pair-fed (PF) healthy controls. The rates of total protein synthesis by hepatocytes of WA animals were 40 and 90% higher than in AL and PF controls, respectively. The degradation of fast-turnover proteins was not affected by nutrition or by the tumor, whereas the degradation of slow-turnover proteins was slightly but significantly increased—about 24% higher in hepatocytes from WA rats than in PF controls. The combination of the two processes, synthesis and degradation, was in favor of an increased synthesis which explains the increase in liver protein content observed in vivo in WA rats. Dietary restriction did not affect the synthesis and secretion of albumin, whereas the tumor significantly reduced its synthesis by about 30%. The plasma concentration of albumin in WA rats dropped by about the same percentage compared with AL and PF animals.     

Influence of caloric restriction and exercise on tumorigenesis in rats

Underfeeding or caloric restriction have been shown to inhibit the growth of spontaneous, transplanted, or chemically induced tumors in rats and mice. At 40% caloric restriction, growth of 7,12-dimethylbenz(a)anthracene-induced mammary and 1,2-dimethylhydrazine-induced colonic tumors is inhibited significantly even when the restricted diet contains twice as much fat as the control diet. Some inhibitory effects become evident even at 10% caloric restriction. In studies involving high fat diets, we find that rats receiving 20% fat ad libitum exhibit significantly higher 7,12-dimethylbenz(a)anthracene-induced mammary tumor incidence, multiplicity, and weight than rats ingesting the same amount of fat daily, but in a diet containing 25% fewer calories. In a study of intermittent ad libitum and restrictive feedings, chemically induced tumorigenicity varies inversely with feed efficiency. Exercise has also been shown to inhibit tumor growth. Sedentary rats fed ad libitum have a 108% higher incidence of 1,2-dimethylhydrazine-induced colon tumors than rats fed ad libitum but subjected to vigorous treadmill exercise. Caloric flux (either reduced intake or increased outflow) appears to reduce tumorigenicity in rodents.     

Effect of Dietary Carbohydrate Source on the Development of Obesity in Agouti Transgenic Mice**

Objectives: Our objective was to evaluate the effects of a qualitative change in dietary carbohydrate source on body weight and adiposity in a rodent model of diet‐induced obesity. Research Methods and Procedures: We evaluated the effects of high‐fat diets (basal) varying in carbohydrate source in aP2‐agouti transgenic mice. In the ad libitum study, animals were given free access to the basal diet or one of four test diets for 6 weeks. In two of the diets, dietary carbohydrate was derived from a single source: mung bean noodles (MUNG) or rolled oats (ROLL). The remaining diets were designed to mimic commercially available instant oatmeal with added sugar (IO‐S) or flavored instant oatmeal (IO‐F). In the energy‐restricted study, animals were given ad libitum access to the basal diet for 6 weeks. Subsequently, animals were assigned to one of six treatment groups for 6 weeks. One group was continued on the basal diet ad libitum. The remaining groups were maintained with energy restriction (70% ad libitum) on either the basal, MUNG, ROLL, IO‐S, or IO‐F diet. Results: Subcutaneous fat pad mass was significantly higher (p < 0.05) in the energy‐restricted basal and IO‐S groups compared with the energy‐restricted ROLL diet. Similarly, visceral fat pad mass was significantly lower with ROLL and MUNG diets (p < 0.05 for both) compared with basal and IO‐S diets, and the insulin:glucose ratio was reduced (by 23% to 34%, p < 0.05) in these two diets compared with all others. In ad libitum‐fed animals, liver fatty acid synthase expression was 43% to 62% lower (p < 0.05) with ROLL and MUNG diets compared with all others. Discussion: These data suggest that a qualitative change in dietary carbohydrate source modulates body weight and adiposity.     

Differential effects of amount of feeding on cell proliferation and progesterone production in response to gonadotrophins and insulin-like growth factor I by ovarian granulosa cells of broiler breeder chickens selected for fatness or leanness.

Strain differences in reproductive performance were demonstrated between broiler breeder female chickens selected for growth (GL line) or for food conversion efficiency (FC line) and the improvement in reproductive performance due to feed restriction also differed significantly. Feed allowance effects on the maturation of ovarian follicles, the incidence of atresia and egg production differed between the two lines exposed to similar feeding protocols. Feed restriction reduced body weights significantly and to a similar extent in both GL and FC lines. The number of normal and atretic yellow follicles was significantly higher under ad libitum feeding and in GL line than it was in the FC line. In both lines, feed restriction decreased multiple ovulation and increased egg production. In culture, granulosa cells from the three largest follicles (F1, F2 and F3) increased progesterone production in response to LH, FSH and insulin-like growth factor I but responses were different between the GL and FC lines fed either ad libitum or restricted diets. Granulosa cells from the two or three largest follicles in GL and FC (ad libitum) lines produced similar amounts of progesterone in response to LH, FSH and insulin-like growth factor I whereas, in restricted birds, the progesterone production was of the rank order F1 > F2 > F3 in both lines. The responsiveness of the GL line fed ad libitum was higher for LH than for either FSH or insulin-like growth factor I but in the GL line fed a restricted diet, it was high for all the hormones. In the FC line, responses to LH, FSH or insulin-like growth factor I were high in ad libitum-fed birds, but low in birds fed a restricted diet for all hormones. Insulin-like growth factor I combined with LH or FSH significantly increased the progesterone production of granulosa cells from birds fed restricted diets of both lines and this effect increased with increasing follicular size. There was a lack of interaction between insulin-like growth factor I and LH or FSH in the regulation of progesterone production by birds of both lines fed ad libitum. Insulin-like growth factor alone or in combination with LH or FSH increased granulosa cell proliferation in birds fed ad libitum more than it did in birds fed restricted diets. The greater proliferation rate of granulosa cells of chickens fed ad libitum, in response to insulin-like growth factor I alone or in combination with gonadotrophins, leading to the simultaneous differentiation of two or three large follicles with high progesterone production in response to LH or insulin-like growth factor I, accelerates the rate of maturation of follicles. This may also be the major cause of erratic and multiple ovulations in broiler breeder female chickens fed ad libitum. In conclusion, insulin-like growth factor I, alone or in combination with LH or FSH, is an important component in the control mechanisms for follicular development in broiler breeder hens. It is this component that is targeted by feed allowance and inadvertently altered by selection for growth.     

Effects of nutrient intake and number of oestrous cycles on in vitro development of preimplantation pig embryos

The effects of nutrient intake and insemination of gilts at first versus third oestrus on the in vitro development of preimplantation pig embryos were investigated. Standard swine management involves ad libitum feeding of gilts at first oestrus and restricted feeding of gilts at third oestrus. According to previous research, gilts inseminated at first oestrus demonstrate greater embryonic mortality than gilts inseminated at third oestrus, and it is possible that differences in nutrient intake between gilts inseminated at first versus third oestrus affect the viability of eggs or embryos. In the present study, experimental gilts were assigned to three treatments: animals designated 1A were inseminated at first oestrus and fed ad libitum; animals designated 3R were inseminated at third oestrus and were fed a restricted diet; and 3A animals were inseminated at third oestrus and fed ad libitum. Embryos collected from each treatment group were cultured in vitro, and data were evaluated according to cell stage at collection. Comparison of treatments 1A and 3R supported the contention of increased embryo mortality in gilts inseminated at first oestrus under normal management conditions. When cultures were initiated at the one- to two-cell or two- to four-cell stages, the percentage of 1A embryos developing to the morula stage (50.9%, 68.0%) was significantly lower than that of 3R embryos (88.9%, 90.9%; P < 0.05). Comparison of treatments 1A and 3A addressed effects due to the number of oestrous cycles. Significantly more two- to four-cell embryos from gilts inseminated at third oestrus and fed ad libitum reached the morula and expanded blastocyst stages of development (87.0%, 41.3%) compared with embryos from gilts inseminated at first oestrus and fed ad libitum (68.0%, 20.3%; P < 0.05). Finally, the effects of ad libitum feeding were determined by comparing treatments 3A and 3R. These data were inconclusive, as both positive and negative effects were observed. More one- to two-cell embryos from treatment 3R developed to the morula stage (88.9%) compared with 3A embryos collected at the same stage (64.7%), whereas a greater number of 3A embryos in the two- to four-cell category reached the expanded blastocyst stage (41.3%) than 3R embryos (21.2%; P < 0.05). These results support the hypothesis of lower in vitro developmental capacity for embryos collected from gilts inseminated at first oestrus. Furthermore, the findings indicate that differences in embryo viability between gilts inseminated at first versus third oestrus are related to the number of oestrous cycles and possibly to differential nutrition.     

Milk composition and growth in wild and captive Tasmanian bettongs,<Emphasis Type="Italic"> Bettongia gaimardi</Emphasis> (Marsupialia)

Changes in milk composition (total solids, carbohydrate, protein, lipid and calculated gross energy content) during lactation in three groups of Tasmanian bettongs ( Bettongia gaimardi): free-living animals (wild group), captive animals offered a diet of dry dog food and apples ad libitum (ad lib group), and captive animals fed restricted amounts of the same diet (maintenance group) were related to growth rates (measured as body mass and head length) of their young. There were no significant differences in the concentration of milk solids among the three groups, but the wild group had higher lipid concentrations and the maintenance group had higher carbohydrate but lower protein concentrations. For all three groups, milk total solids increased through lactation from ca. 25% to ca. 45% and carbohydrate concentrations decreased from ca. 18% to about 3%. Protein concentrations increased from ca. 5% to ca. 10% in the wild and ad lib groups, but only from 4% to 8% in the maintenance group. Lipid concentrations increased in the wild and ad lib groups from ca. 4% to ca. 18%, but in the maintenance group only to ca. 7%. Calculated gross energy content of milk increased through lactation in the wild and ad lib groups (from ca. 500 kJ.100 ml(-1) to ca. 1,000 kJ.100 ml(-1)), but there was no significant increase in the maintenance group. The volume of milk produced increased to a peak just prior to permanent pouch vacation by the young, when the gross energy output in milk was 120-150 kJ.3 h(-1) in the wild and ad lib groups. On a daily basis this is equivalent to the milk energy output of larger wallabies, and helps to explain the relatively high growth rates of young Tasmanian bettongs. There were significant differences in growth rates among the groups, with the heaviest young always in the ad lib group. Thus differences in milk composition resulting from different planes of nutrition can lead to differences in growth rates of marsupial young.