BACKGROUND
in term infants, in utero growth restriction or small-for-gestational-age status at birth  are associated with the development of increased adiposity and impaired insulin sensitivity in later life  <cit> , that may be exacerbated by more rapid catch-up growth in the first 1– <dig> years of life  <cit> .
in comparison, preterm infants grow much more poorly after birth, a term coined ex utero growth restriction, and by term corrected age most are below the 5th weight-for-age centile  <cit> .
this ex utero, postnatal, growth failure is common in preterm infants,  <cit>  and is associated with poorer neurocognitive outcomes in later life  <cit> .
further, it has been shown that neonatal leptin deficiency may contribute to adverse neurodevelopmental outcomes associated with postnatal growth restriction  <cit> .
subsequently, preterm infants have variable amounts of catch-up growth, especially during the first 1– <dig> years of life  <cit> .
this pattern of small body size at term corrected age, followed by increased rates of growth is similar to that seen in sga term infants, and there has been concern that this may lead to increased risk of obesity and metabolic disorders arising from impaired glucose tolerance, such as type ii diabetes, in preterm infants, similar to the increased risk in term sga infants .
we have previously described a rodent model of ex utero growth restriction and the effects of variable amounts of catch-up growth on early metabolic and neurocognitive outcomes  <cit> .
changes in litter size lead to ex utero growth restriction , and in turn, changes in body composition and poorer neurodevelopment.
however, no differences in fasting insulin or glucose in early life were seen  <cit> .
in the present study, we used the same model to assess the effects of ex utero growth restriction and subsequent catch-up growth on longer-term metabolic outcomes including glucose tolerance and insulin sensitivity.
the objectives of our study were to examine the effects of early postnatal growth restriction, followed by varying degrees of postnatal catch-up growth on growth , insulin sensitivity, glucose tolerance, neurodevelopment, and brain myelination.
we hypothesized that early postnatal growth restriction would result in poorer neurodevelopment and lead to improved glucose tolerance and insulin sensitivity.
we further hypothesized that in eugr rats, early catch-up growth would lead to improved neurodevelopment but reduced insulin sensitivity and glucose tolerance compared to eugr rats that did not have early catch-up growth.
RESULTS
growth
growth differed significantly between the normal  and restricted  intake groups by d <dig>  onwards.
by d <dig> the r groups were approximately 20 % smaller than the n groups .fig.
<dig> design of the animal study.
on d <dig>  rat pups were randomized to litters of 10/dam , five males and five females) or 16/dam , eight males and eight females).
on d <dig>  r pups were re-randomized into litters creating catch-up , normal  or reduced growth  groups.
n pups remained in litters of  <dig> pups/dam
on d <dig>  r animals were re-randomized to litters of  <dig> ,  <dig>  or  <dig> , while n pups remained in litters of  <dig> .
the r- <dig> group remained significantly smaller than the n- <dig> group throughout the study.
the weight of the r- <dig> pups “caught-up” with the n- <dig> pups by d <dig> and were statistically indistinguishable from them for the rest of the study.
the r- <dig> group grew intermediate to the n- <dig> and r- <dig> animals until d <dig> , and was similar to the n- <dig> and r- <dig> groups thereafter.fig.
<dig> postnatal weight  from d1- <dig>  by d <dig>  r pups were ~ 20 % smaller than n pups.
values are means ± sem.
*different from n litters, p <  <dig>
by d <dig>  the r- <dig> group remained significantly smaller than the three other groups, which were all statistically similar.
on d <dig>  the r- <dig> rats remained significantly smaller than the other three groups.
this was seen for both males and females .fig.
<dig> postnatal weight  from d11- <dig>  all r groups diverged by d <dig>  by d <dig>  the n- <dig> and r- <dig> groups were similar, the r- <dig> group showed no catch-up growth, and the r- <dig> group caught-up half-way between the n- <dig> and r- <dig> groups.
error bars represent ±  <dig> sem, if not visible they are smaller than the plot symbol
body composition
body composition was assessed in a subset of animals  at d <dig>  there were no significant differences in percentage water, protein, fat, or ash between the four groups .fig.
<dig> postnatal weight  at d <dig>  the r- <dig> rats were significantly smaller than the other three groups, and this was seen in males and females.
*p <  <dig> .
error bars represent ±  <dig> sem
serum hormones
on d <dig>  serum leptin was significantly higher in the n group  than the r group .
serum triglycerides on d <dig> were similar in the n  and r  groups.
serum leptin on d <dig> differed significantly between groups, with the r- <dig> group having the lowest levels .
serum triglycerides on d <dig> were similar among groups, but hepatic triglycerides on d <dig> differed with the lowest level in the r- <dig> and r- <dig> groups and the highest in the r- <dig> group.
serum insulin values did not differ between groups.table  <dig> fasting glucose, insulin, leptin, and triglycerides in the four groups on d60
n
data are expressed as mean ± sem.
p-values represent the overall anova p-values.
adenote significant difference from the n- <dig> group, p <  <dig>
insulin sensitivity
fasting blood glucose on d <dig> was similar in all four groups .
when expressed as the area under the curve , the two catch-up groups, r- <dig>  and r- <dig> , had significantly higher auc than the r- <dig> group , while the n- <dig> group was intermediate between the others  .
similar patterns were seen for the auc between  <dig> and 30 min and between  <dig> and 120 min.
when individual time-points were considered, the r- <dig> and r- <dig> groups had higher blood glucose concentrations than the other groups  at  <dig>   <dig> and 60 min.fig.
<dig> percentage of water, protein, fat, and ash for each study group at d <dig>  error bars represent mean ± sem
when the data were examined as change in glucose concentration from baseline, the area under baseline  between  <dig> and 30 min was significantly greater for the n- <dig> group  than for the r- <dig> group , while the r- <dig>  and r- <dig> groups  were intermediate between the two.
there were no differences in aub among the groups for the time period 30 min to 120 min.
glucose tolerance
fasting blood glucose on d <dig> was significantly affected by sex  and by group .
fasting glucose was lower in the n- <dig>  and the r- <dig>  groups than in the r- <dig> group , with the r- <dig> group being intermediate between them .
the auc was significantly different between groups  and was greater in males than females .
the auc for the n- <dig> group  was significantly greater than both the r- <dig>  and the r- <dig>  groups, with the r- <dig> group being intermediate  .
there were no significant group effects between  <dig> and 30 min, but were seen subsequently during the remainder of the gtt for the time period  <dig> to 180 min.
a similar pattern was seen in aub among the groups.fig.
<dig> blood glucose during an intraperitoneal insulin tolerance test  at d <dig>  fasting blood glucose was similar in all four groups.
error bars represent ±  <dig> sem, if not visible they are smaller than the plot symbol
t-maze test
memory and learning was assessed using spontaneous alternation in a t-maze.
the n- <dig> group scored significantly better ; p <  <dig> ) than any of the other groups , r- <dig>  <dig>  ±  <dig>  , r- <dig>   <dig>  ±  <dig>  successes ).
the effects of group were similar in both sexes .fig.
<dig> blood glucose during an intraperitoneal glucose tolerance test  at d <dig>  fasting blood glucose was significantly affected by sex  and by group .
error bars represent ±  <dig> sem, if not visible they are smaller than the plot symbol
brain histology
the area of mbp-positive fibers in the r- <dig> group appeared smaller than that in the n- <dig> group on d <dig>  but no significant differences could be detected.
these results suggest that myelination within the hypothalamus and corpus callosum may have been completed by d <dig> .fig.
<dig> t-maze test success.
the n- <dig> animals scored significantly better  than any of the other groups .
different letters denote significance
discussion
since poor growth in preterm infants occurs postnatally, we aimed to produce a postnatal model of growth restriction in neonatal rats.
many animal models have been used to examine effects of in utero growth restriction with or without catch-up growth on metabolic outcomes, and though these models have provided great insight into infants born small for gestational age or who experience intrauterine growth restriction  <cit> , they do not represent the type of growth that is experienced by most preterm infants.
further, the effects of growth restriction and subsequent catch-up growth on cognition and metabolism have not been examined concurrently.
we therefore developed a model of post-natal growth restriction in rat pups based on manipulations in litter size, that we have shown leads to reproducible levels of ex utero growth restriction and catch-up growth  <cit> .
this model leads to changes in both milk intake and in growth.
however, it is possible that other factors may also be changed by modifications in litter size, for example dam-pup interactions and pup-pup interactions, as seen in other rodents  <cit> .
the initial intervention in our study was carried out from birth until d <dig> of age, as this period in rats is believed to be equivalent to the third trimester of pregnancy in humans  <cit> , or the period when reduced intake and poor growth are common in premature infants.
the increased milk volume intake that occurs as litter size is decreased in the second intervention represents the increased volume intake that preterm infants who experience catch-up growth encounter after hospital discharge.
further, dams of large litters have been shown to produce milk with unaltered protein composition, and thus litter size manipulation results in modified volume intake without altered milk composition  <cit> .
the current study confirms our previous findings that r- <dig> pups catch-up with n- <dig> pups by d <dig>  r- <dig> pups show partial catch-up by d <dig>  and r- <dig> pups remain smaller than the other three groups  <cit> .
the current study confirms this, but also demonstrates that the r- <dig> group does ultimately show complete catch-up in body weight by d <dig>  the r- <dig> group, however, remained significantly smaller than the n- <dig> group until at least d <dig>
we have previously shown that catch-up growth in r- <dig> pups comes at the cost of changes in body composition with r- <dig> pups having significantly greater percentage body fat, and significantly lower percent lean mass on d <dig>  <cit> .
the current study demonstrates that by d <dig>  body composition in the r- <dig> pups has normalized, and is similar to the n- <dig> pups.
furthermore, although the r- <dig> pups catch-up to the n- <dig> pups by d <dig>  the two groups have similar body composition on d <dig>  just as they have at d <dig>  the early changes in body composition related to catch-up group are therefore not maintained over time.
we have previously shown that the r- <dig> group has lower percentage body fat in d <dig>  by d <dig>  however, the differences in body composition are lost, and all groups have similar percent body fat despite the fact that the r- <dig> rats remain smaller.
once again, early differences in body composition are not sustained over time.
these findings are consistent with the human data, which suggests that although preterm infants with catch-up growth have increased adiposity at term corrected age, those changes are not maintained during the rest of the first year of life  <cit> .
in our previous study there were no differences between groups in fasting insulin or glucose of d <dig>  in the current study we carried out more detailed investigations of glucose homeostasis in older animals.
fasting blood glucose prior to the glucose tolerance test  was significantly greater in the two catch-up groups  than in the groups without catch-up growth .
the difference in fasting blood glucose prior to the insulin tolerance test  failed to reach statistical significance.
insulin sensitivity was higher in the groups without catch-up growth  than in the groups that changed their dietary intakes on d <dig>  and experienced catch-up growth, as shown by their higher auc values.
this occurred even though all groups had similar body composition at the end of the study.
it is possible that early changes in body composition may be responsible for the poorer insulin sensitivity seen in the r- <dig> and r- <dig> groups in later life, or that changes in early dietary intake or growth lead to long-term changes in insulin sensitivity, possibly via epigenetic mechanisms.
growth restriction may result in improved insulin sensitivity in adulthood since it has been suggested that early undernourishment may enhance insulin sensitivity, as well as fatty acid oxidation  <cit> .
it has been shown that children born prematurely have decreased insulin sensitivity immediately after birth, and those who experience greater weight gain remain having lower insulin sensitivity compared to infants born at term  <cit> .
conversely, glucose tolerance by gtt was significantly worse in the n- <dig> group than the r- <dig> group, as shown by auc.
the two catch-up groups were intermediate between the n- <dig> and r- <dig> groups.
the differences in fasting blood glucose among the groups is consistent with the findings that mice who are small at birth and have postnatal catch-up growth are at high risk of glucose intolerance  <cit> ; however, there was no significant group effect in auc for the first 30 min of the gtt, and differences in glucose tolerance were only apparent after 30 min.
growth before weaning, specifically before d <dig>  could be a critical window for later programming.
the developmental origins of disease hypothesis suggests that prenatal development is critical to metabolic adaptation later in life  <cit> .
however, the postnatal environment may be “mismatched” to the early in utero environment, creating a disadvantageous phenotype  <cit> .
cognitive outcomes were worse in the three groups with early growth restriction , and highest in the group with greater early growth .
we thus show that growth restriction, despite catch-up growth, may predispose poor cognition.
though there were no differences in mbp expression at d <dig>  this may be due to the fact that the maximum rate of myelin accumulation in the rat occurs around d <dig>  <cit> .
myelin accumulation does continue into adulthood in the rat, though it occurs at a decreasing rate  <cit> .
several animal studies have shown that dietary restriction during the suckling period results in decreased myelination in early life .
in our study, early postnatal growth restriction and possible undernutrition due to large litter size may be a cause for the developmental impairments seen in the r groups.
we also examined the effects of growth restriction and catch-up growth on serum hormones, specifically insulin and leptin.
neonatal overfeeding of pups by litter size manipulation has been shown to result in a significant elevation of serum insulin concentration and alterations in hepatic enzymes involved in carbohydrate and lipid metabolism  <cit> .
however, we did not find a significant difference in serum insulin concentrations.
interestingly, serum leptin at d <dig> and d <dig> differed significantly between groups, with the r- <dig> group having the lowest levels.
this is consistent with our previous data on d <dig>  <cit> .
the association of low leptin concentrations in the r litters and poor t-maze score suggests that reduced leptin levels may be a mechanism behind the differences seen in cognition.
leptin has recently been proposed to play a role in brain development during the prenatal and neonatal periods  <cit> .
administration of leptin to ob/ob mice, which are leptin deficient, has been shown to increase brain weight, total brain dna, and increase mbp-mrna expression in rodents  <cit> , further suggesting a role for leptin in brain development.
finally, we demonstrated that hepatic triglyceride content was highest in the group with early catch-up growth .
hepatic lipid accumulation may be one of the earliest findings in the metabolic syndrome in humans.
this, combined with the differences in fasting glucose and in insulin sensitivity, suggests that catch-up growth in this model may be associated with increased risk of metabolic syndrome.
CONCLUSIONS
in summary, we have demonstrated that early growth restriction leads to profound and long-lasting adverse effects on neurodevelopment.
catch-up growth occurs after early postnatal growth restriction, and complete catch-up in weight can occur if it begins before d <dig> in the rat .
postnatal growth restriction without catch-up growth  leads to short-term reductions in body adiposity, while postnatal growth restriction with catch-up growth  leads to short-term increases in body adiposity.
neither of these changes in body composition is maintained long-term.
postnatal growth restriction without catch-up growth leads to improved glucose tolerance.
however, insulin sensitivity is reduced if catch-up growth occurs after postnatal growth restriction.
these finding reinforce the concerns that ex utero growth restriction in preterm infants reduces long-term neurocognitive outcomes, and that subsequent catch-up growth may impair insulin sensitivity without improving development.

 in term infants, in utero growth restriction or small-for-gestational-age status at birth are associated with the development of increased adiposity and impaired insulin sensitivity in later life, that may be exacerbated by more rapid catch-up growth in the first 1– years of life. in comparison, preterm infants grow much more poorly after birth, a term coined ex utero growth restriction, and by term corrected age most are below the 5th weight-for-age centile. this ex utero, postnatal, growth failure is common in preterm infants, and is associated with poorer neurocognitive outcomes in later life. further, it has been shown that neonatal leptin deficiency may contribute to adverse neurodevelopmental outcomes associated with postnatal growth restriction. subsequently, preterm infants have variable amounts of catch-up growth, especially during the first 1– years of life. this pattern of small body size at term corrected age, followed by increased rates of growth is similar to that seen in sga term infants, and there has been concern that this may lead to increased risk of obesity and metabolic disorders arising from impaired glucose tolerance, such as type ii diabetes, in preterm infants, similar to the increased risk in term sga infants. we have previously described a rodent model of ex utero growth restriction and the effects of variable amounts of catch-up growth on early metabolic and neurocognitive outcomes. changes in litter size lead to ex utero growth restriction, and in turn, changes in body composition and poorer neurodevelopment. however, no differences in fasting insulin or glucose in early life were seen. in the present study, we used the same model to assess the effects of ex utero growth restriction and subsequent catch-up growth on longer-term metabolic outcomes including glucose tolerance and insulin sensitivity. the objectives of our study were to examine the effects of early postnatal growth restriction, followed by varying degrees of postnatal catch-up growth on growth, insulin sensitivity, glucose tolerance, neurodevelopment, and brain myelination. we hypothesized that early postnatal growth restriction would result in poorer neurodevelopment and lead to improved glucose tolerance and insulin sensitivity. we further hypothesized that in eugr rats, early catch-up growth would lead to improved neurodevelopment but reduced insulin sensitivity and glucose tolerance compared to eugr rats that did not have early catch-up growth.  growth growth differed significantly between the normal and restricted intake groups by d onwards. by d the r groups were approximately 20 % smaller than the n groups.fig. design of the animal study. on d rat pups were randomized to litters of 10/dam, five males and five females) or 16/dam, eight males and eight females). on d r pups were re-randomized into litters creating catch-up, normal or reduced growth groups. n pups remained in litters of pups/dam on d r animals were re-randomized to litters of, or, while n pups remained in litters of. the r- group remained significantly smaller than the n- group throughout the study. the weight of the r- pups “caught-up” with the n- pups by d and were statistically indistinguishable from them for the rest of the study. the r- group grew intermediate to the n- and r- animals until d, and was similar to the n- and r- groups thereafter.fig. postnatal weight from d1- by d r pups were ~ 20 % smaller than n pups. values are means ± sem. *different from n litters, p <  by d the r- group remained significantly smaller than the three other groups, which were all statistically similar. on d the r- rats remained significantly smaller than the other three groups. this was seen for both males and females.fig. postnatal weight from d11- all r groups diverged by d by d the n- and r- groups were similar, the r- group showed no catch-up growth, and the r- group caught-up half-way between the n- and r- groups. error bars represent ±  sem, if not visible they are smaller than the plot symbol body composition body composition was assessed in a subset of animals at d there were no significant differences in percentage water, protein, fat, or ash between the four groups.fig. postnatal weight at d the r- rats were significantly smaller than the other three groups, and this was seen in males and females. *p < . error bars represent ±  sem serum hormones on d serum leptin was significantly higher in the n group than the r group. serum triglycerides on d were similar in the n and r groups. serum leptin on d differed significantly between groups, with the r- group having the lowest levels. serum triglycerides on d were similar among groups, but hepatic triglycerides on d differed with the lowest level in the r- and r- groups and the highest in the r- group. serum insulin values did not differ between groups.table fasting glucose, insulin, leptin, and triglycerides in the four groups on d60 n data are expressed as mean ± sem. p-values represent the overall anova p-values. adenote significant difference from the n- group, p <  insulin sensitivity fasting blood glucose on d was similar in all four groups. when expressed as the area under the curve, the two catch-up groups, r- and r-, had significantly higher auc than the r- group, while the n- group was intermediate between the others. similar patterns were seen for the auc between and 30 min and between and 120 min. when individual time-points were considered, the r- and r- groups had higher blood glucose concentrations than the other groups at and 60 min.fig. percentage of water, protein, fat, and ash for each study group at d error bars represent mean ± sem when the data were examined as change in glucose concentration from baseline, the area under baseline between and 30 min was significantly greater for the n- group than for the r- group, while the r- and r- groups were intermediate between the two. there were no differences in aub among the groups for the time period 30 min to 120 min. glucose tolerance fasting blood glucose on d was significantly affected by sex and by group. fasting glucose was lower in the n- and the r- groups than in the r- group, with the r- group being intermediate between them. the auc was significantly different between groups and was greater in males than females. the auc for the n- group was significantly greater than both the r- and the r- groups, with the r- group being intermediate. there were no significant group effects between and 30 min, but were seen subsequently during the remainder of the gtt for the time period to 180 min. a similar pattern was seen in aub among the groups.fig. blood glucose during an intraperitoneal insulin tolerance test at d fasting blood glucose was similar in all four groups. error bars represent ±  sem, if not visible they are smaller than the plot symbol t-maze test memory and learning was assessed using spontaneous alternation in a t-maze. the n- group scored significantly better; p <  ) than any of the other groups, r-  ± , r-  ±  successes ). the effects of group were similar in both sexes.fig. blood glucose during an intraperitoneal glucose tolerance test at d fasting blood glucose was significantly affected by sex and by group. error bars represent ±  sem, if not visible they are smaller than the plot symbol brain histology the area of mbp-positive fibers in the r- group appeared smaller than that in the n- group on d but no significant differences could be detected. these results suggest that myelination within the hypothalamus and corpus callosum may have been completed by d.fig. t-maze test success. the n- animals scored significantly better than any of the other groups. different letters denote significance discussion since poor growth in preterm infants occurs postnatally, we aimed to produce a postnatal model of growth restriction in neonatal rats. many animal models have been used to examine effects of in utero growth restriction with or without catch-up growth on metabolic outcomes, and though these models have provided great insight into infants born small for gestational age or who experience intrauterine growth restriction, they do not represent the type of growth that is experienced by most preterm infants. further, the effects of growth restriction and subsequent catch-up growth on cognition and metabolism have not been examined concurrently. we therefore developed a model of post-natal growth restriction in rat pups based on manipulations in litter size, that we have shown leads to reproducible levels of ex utero growth restriction and catch-up growth. this model leads to changes in both milk intake and in growth. however, it is possible that other factors may also be changed by modifications in litter size, for example dam-pup interactions and pup-pup interactions, as seen in other rodents. the initial intervention in our study was carried out from birth until d of age, as this period in rats is believed to be equivalent to the third trimester of pregnancy in humans, or the period when reduced intake and poor growth are common in premature infants. the increased milk volume intake that occurs as litter size is decreased in the second intervention represents the increased volume intake that preterm infants who experience catch-up growth encounter after hospital discharge. further, dams of large litters have been shown to produce milk with unaltered protein composition, and thus litter size manipulation results in modified volume intake without altered milk composition. the current study confirms our previous findings that r- pups catch-up with n- pups by d r- pups show partial catch-up by d and r- pups remain smaller than the other three groups. the current study confirms this, but also demonstrates that the r- group does ultimately show complete catch-up in body weight by d the r- group, however, remained significantly smaller than the n- group until at least d we have previously shown that catch-up growth in r- pups comes at the cost of changes in body composition with r- pups having significantly greater percentage body fat, and significantly lower percent lean mass on d. the current study demonstrates that by d body composition in the r- pups has normalized, and is similar to the n- pups. furthermore, although the r- pups catch-up to the n- pups by d the two groups have similar body composition on d just as they have at d the early changes in body composition related to catch-up group are therefore not maintained over time. we have previously shown that the r- group has lower percentage body fat in d by d however, the differences in body composition are lost, and all groups have similar percent body fat despite the fact that the r- rats remain smaller. once again, early differences in body composition are not sustained over time. these findings are consistent with the human data, which suggests that although preterm infants with catch-up growth have increased adiposity at term corrected age, those changes are not maintained during the rest of the first year of life. in our previous study there were no differences between groups in fasting insulin or glucose of d in the current study we carried out more detailed investigations of glucose homeostasis in older animals. fasting blood glucose prior to the glucose tolerance test was significantly greater in the two catch-up groups than in the groups without catch-up growth. the difference in fasting blood glucose prior to the insulin tolerance test failed to reach statistical significance. insulin sensitivity was higher in the groups without catch-up growth than in the groups that changed their dietary intakes on d and experienced catch-up growth, as shown by their higher auc values. this occurred even though all groups had similar body composition at the end of the study. it is possible that early changes in body composition may be responsible for the poorer insulin sensitivity seen in the r- and r- groups in later life, or that changes in early dietary intake or growth lead to long-term changes in insulin sensitivity, possibly via epigenetic mechanisms. growth restriction may result in improved insulin sensitivity in adulthood since it has been suggested that early undernourishment may enhance insulin sensitivity, as well as fatty acid oxidation. it has been shown that children born prematurely have decreased insulin sensitivity immediately after birth, and those who experience greater weight gain remain having lower insulin sensitivity compared to infants born at term. conversely, glucose tolerance by gtt was significantly worse in the n- group than the r- group, as shown by auc. the two catch-up groups were intermediate between the n- and r- groups. the differences in fasting blood glucose among the groups is consistent with the findings that mice who are small at birth and have postnatal catch-up growth are at high risk of glucose intolerance; however, there was no significant group effect in auc for the first 30 min of the gtt, and differences in glucose tolerance were only apparent after 30 min. growth before weaning, specifically before d could be a critical window for later programming. the developmental origins of disease hypothesis suggests that prenatal development is critical to metabolic adaptation later in life. however, the postnatal environment may be “mismatched” to the early in utero environment, creating a disadvantageous phenotype. cognitive outcomes were worse in the three groups with early growth restriction, and highest in the group with greater early growth. we thus show that growth restriction, despite catch-up growth, may predispose poor cognition. though there were no differences in mbp expression at d this may be due to the fact that the maximum rate of myelin accumulation in the rat occurs around d. myelin accumulation does continue into adulthood in the rat, though it occurs at a decreasing rate. several animal studies have shown that dietary restriction during the suckling period results in decreased myelination in early life. in our study, early postnatal growth restriction and possible undernutrition due to large litter size may be a cause for the developmental impairments seen in the r groups. we also examined the effects of growth restriction and catch-up growth on serum hormones, specifically insulin and leptin. neonatal overfeeding of pups by litter size manipulation has been shown to result in a significant elevation of serum insulin concentration and alterations in hepatic enzymes involved in carbohydrate and lipid metabolism. however, we did not find a significant difference in serum insulin concentrations. interestingly, serum leptin at d and d differed significantly between groups, with the r- group having the lowest levels. this is consistent with our previous data on d. the association of low leptin concentrations in the r litters and poor t-maze score suggests that reduced leptin levels may be a mechanism behind the differences seen in cognition. leptin has recently been proposed to play a role in brain development during the prenatal and neonatal periods. administration of leptin to ob/ob mice, which are leptin deficient, has been shown to increase brain weight, total brain dna, and increase mbp-mrna expression in rodents, further suggesting a role for leptin in brain development. finally, we demonstrated that hepatic triglyceride content was highest in the group with early catch-up growth. hepatic lipid accumulation may be one of the earliest findings in the metabolic syndrome in humans. this, combined with the differences in fasting glucose and in insulin sensitivity, suggests that catch-up growth in this model may be associated with increased risk of metabolic syndrome.  in summary, we have demonstrated that early growth restriction leads to profound and long-lasting adverse effects on neurodevelopment. catch-up growth occurs after early postnatal growth restriction, and complete catch-up in weight can occur if it begins before d in the rat. postnatal growth restriction without catch-up growth leads to short-term reductions in body adiposity, while postnatal growth restriction with catch-up growth leads to short-term increases in body adiposity. neither of these changes in body composition is maintained long-term. postnatal growth restriction without catch-up growth leads to improved glucose tolerance. however, insulin sensitivity is reduced if catch-up growth occurs after postnatal growth restriction. these finding reinforce the concerns that ex utero growth restriction in preterm infants reduces long-term neurocognitive outcomes, and that subsequent catch-up growth may impair insulin sensitivity without improving development. 
these results may contribute to better elucidating the causes of poor developmental outcomes in human preterm infants. insulin tolerance test and glucose tolerance test. there is increasing evidence that poor growth of preterm infants is a risk factor for poor long-term development, while the effects of early postnatal growth restriction are not well known. all r groups showed poorer scores in developmental testing when compared with the n- group during t-maze test. although r- were more insulin sensitive than r- and r- all r groups were more glucose tolerant than n- rat pups were separated at birth into normal or restricted intake litters. insulin sensitivity was measured by i.p. in rats, differences in postnatal growth restriction leads to changes in development and in insulin sensitivity. growth restrictioncatch-up growthdevelopmentinsulin sensitivityissue-copyright-statement© the author 2015 by d the r- group caught up to the n- group in weight, the r- group showed partial catch-up growth and the r- group showed no catch-up growth. n pups remained in litters of pups/dam. by d n pups weighed 20 % more than r pups.
 there is increasing evidence that poor growth of preterm infants is a risk factor for poor long-term development, while the effects of early postnatal growth restriction are not well known. we utilized a rat model to examine the consequences of different patterns of postnatal growth and hypothesized that early growth failure leads to impaired development and insulin resistance. rat pups were separated at birth into normal or restricted intake litters. at d r pups were re-randomized into litters of, or pups/dam. n pups remained in litters of pups/dam. memory and learning were examined through t-maze test. insulin sensitivity was measured by i.p. insulin tolerance test and glucose tolerance test.  by d n pups weighed 20 % more than r pups. by d the r- group caught up to the n- group in weight, the r- group showed partial catch-up growth and the r- group showed no catch-up growth. all r groups showed poorer scores in developmental testing when compared with the n- group during t-maze test. although r- were more insulin sensitive than r- and r- all r groups were more glucose tolerant than n-  in rats, differences in postnatal growth restriction leads to changes in development and in insulin sensitivity. these results may contribute to better elucidating the causes of poor developmental outcomes in human preterm infants. keywords growth restrictioncatch-up growthdevelopmentinsulin sensitivityissue-copyright-statement© the author 2015 