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BOR - Papers in Press, published online ahead of print March 31, 2004.
Biol Reprod 2004, 10.1095/biolreprod.103.018986
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BIOLOGY OF REPRODUCTION 71, 359–365 (2004)
DOI: 10.1095/biolreprod.103.018986
© 2004 by the Society for the Study of Reproduction, Inc.

Pregnancy

Nutritional Manipulation of Fetal Adipose Tissue Deposition and Uncoupling Protein 1 Messenger RNA Abundance in the Sheep: Differential Effects of Timing and Duration

H. Budge2, L.J. Edwards3, I.C. McMillen3, A. Bryce3, K. Warnes3, S. Pearce2, T. Stephenson2, and M.E. Symonds1,2

Academic Division of Child Health,2 School of Human Development, University Hospital, Nottingham, NG7 2UH United Kingdom Department of Physiology,3 University of Adelaide, South Australia 5005, Australia


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
A range of epidemiological and experimental studies have indicated that suboptimal nutrition at different stages of gestation is associated with an increased prevalence of adult hypertension, cardiovascular disease, and obesity. The timing of prenatal nutrient restriction is important in determining postnatal outcomes—including obesity. The present study, aimed to determine the extent to which fetal adiposity and expression of the key thermogenic protein, uncoupling protein (UCP)1, are altered by restriction of maternal nutrient intake imposed during four different periods, starting from before conception. Maternal nutrient intake was restricted from 60 days before until 8 days after mating (periconceptional nutrient restriction; R-C), from 60 days before mating and throughout gestation (R-R), from 8 days gestation until term (C-R), or from 115 days gestation until term. Fetal perirenal adipose tissue (PAT) was sampled near to term at ~143 days. UCP1 mRNA, but not protein, abundance in PAT was increased in fetuses in the R-R group (C-C 63 ± 18; R-C 83 ± 43; C-R 103 ± 38; R-R 167 ± 50 arbitrary units (P < 0.05)). In contrast, the abundance of UCP1 mRNA, but not protein, in fetal PAT was decreased when maternal nutrition was restricted from 115 days gestation. The major effect of maternal nutrient restriction on adipose tissue deposition occurred in the C-R group, in which the proportion of fetal fat was doubled, whereas maternal nutrient restriction from 115 days gestation reduced fetal fat deposition. In conclusion, there are differential effects of maternal and therefore fetal nutrient restriction on UCP1 mRNA expression and fetal fat mass and these effects are dependent on the timing and duration of nutrient restriction.

early development, gene regulation


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Over the past decade, extensive epidemiological evidence from a range of populations indicates that the nutritional environment of the fetus is not only important in determining fetal growth but may also be important in determining adult health outcomes [1, 2]. Individuals exposed to nutrient restriction in utero followed by a relative increase in nutrition after birth have a higher incidence of coronary heart disease and obesity in later life [3–5]. Epidemiological findings from the Dutch famine of 1944–1945 have shown that dietary restriction imposed specifically during early gestation had the greatest effect on the subsequent risk of obesity during adulthood [3]. The mechanisms behind this association have not been elucidated but are likely to reflect the consequences of the decrease in fetal nutrient supply.

Experimental animal studies have found that development of the fetal cardiovascular and metabolic systems can be significantly altered by restriction of maternal nutrient intake [4–6]. It has been shown that decreasing maternal nutrition before, and for 1 wk after conception, can result in an earlier activation of the fetal pituitary-adrenal axis and in a concomitant increase in fetal arterial blood pressure in twin fetuses during late gestation [7, 8]. It has also been established in fetal lambs that an increase in fetal glucose supply during late gestation promotes fetal adipose tissue deposition [9]. The extent to which fetal adiposity may be altered by changes in the fetal glucose supply throughout gestation has not, however, been investigated.

In the newborn lamb, the level of expression of uncoupling protein (UCP)1 in neonatal adipose tissue plays an important role in determining survival after birth [10]. Mitochondrial UCP1 plays a key role in the rapid generation of large amounts of heat following exposure of the lamb to the cold extrauterine environment. UCP1 protein first becomes detectable around midgestation and, thereafter, its abundance in fetal perirenal fat, which constitutes >80% of fetal fat mass, gradually increases to peak soon after birth [11]. This increase is dependent on the presence of the fetal thyroid and adrenal glands as well as on an increase in sympathetic stimulation [12–14]. Subsequently, after birth, there is a rapid loss of UCP1 mRNA that is followed by a more gradual reduction in UCP1 protein [15], which has a half life of ~7 days. UCP1 protein is generally no longer detectable in adipose tissue by 1 mo of age. The decrease in the abundance of UCP1 during this period is associated with the replacement of brown adipose tissue by white adipose tissue in the perirenal fat depot [16]. The development of the fetal endocrine systems that regulate UCP1 mRNA expression is, in part, nutritionally regulated but there have been no studies that have investigated the effects of restriction of maternal nutrient intake at different stages of pregnancy on UCP1 mRNA abundance in fetal fat depots. The following study was, therefore, designed to determine whether the timing (i.e., before conception, throughout gestation, or only in late gestation) of maternal and, hence fetal, undernutrition is important in altering fetal adipose tissue deposition and UCP1 mRNA and/or protein abundance.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Two independent studies were undertaken. The first involved nutritional interventions that yielded four different manipulations of fetal nutrient supply (Fig. 1). In the nutritional study (A), maternal nutrient intake was restricted either around the time of conception and/or throughout pregnancy, or from Day 8 of gestation up to term. In the second study (B), maternal nutrient intake was only restricted in late gestation. There were no differences in ewe body weights between studies (study A, 55.7 ± 0.9 kg [control] and 56.6 ± 0.8 kg [nutrient restricted]; study B, 56.7 ± 1.9 kg [control] and 53.5 ± 2.3 kg [nutrient restricted]). Each study protocol was designed in accordance with the Australian Code of Practice for the Care and Use of Animals for Scientific Purposes and approved by The University of Adelaide Standing Committee on Animal Ethics and Experimentation according to Australian legislation.



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  		FIG. 1. Summary of experimental designs of each nutritional intervention relative to the time mating. A) Maternal nutrient restriction (R) in the periconceptional period and/or throughout gestation; (B) Maternal nutrient restriction in late gestation. Period of feeding control diet (C)

Experimental Designs

Study A—Effect of maternal nutrient restriction in the periconceptional period and throughout gestation Sixteen Border-Leicester cross Merino ewes, of similar body weight and parity, that were subsequently confirmed as bearing twins were entered into the study and randomized to one of four nutritional groups. Nutrient restriction was either imposed in the period before and around conception (periconceptional nutrient restriction; R-C), from 8 days after mating throughout gestation (gestational nutrient restriction; C-R), or from before conception and throughout gestation (periconceptional with gestational nutrient restriction; R-R). A further nutritional group experienced maintenance nutrition throughout gestation (C-C; Fig. 1). Sixty days before mating, eight ewes were individually housed in pens and fed control nutrition in the period before and up to 8 days after conception (C-C and C-R groups). They were supplied with 100% of their total energy requirements calculated according to body weight [17] as 80% lucerne chaff (metabolizable energy [ME], 8.3 MJ/kg; crude protein, 193 g/kg) and 20% pelleted feed (ME, 8.0 MJ/kg; crude protein, 110 g/kg). Eight ewes, randomized to periconceptional nutrient restriction (R-C and R-R groups), were provided with 70% of calculated total energy requirements as a proportionate reduction in both lucerne chaff and pelleted feed. All ewes were provided with water ad libitum. There was no significant difference in weight change between groups over this period (C-C, 0.3 ± 0.4; R-C, 2.2 ± 0.8; C-R, 1.0 ± 1.5; R-R, 1.9 ± 0.5 kg). After 60 days of the control or nutrient-restricted diet, ewes were mated, and on the eighth day after mating, ewes from groups randomized to control nutrition from 8 days gestation (C-C and R-C groups) were provided with 100% of their calculated total energy requirements while ewes randomized to gestational nutrient restriction (C-R and R-R groups) and were provided with 70% of their calculated total energy requirements. Thus, control ewes maintained nutritional intake throughout gestation, ewes randomized to be exposed to periconceptional followed by gestational nutrient restriction (R-R) were nutrient restricted from 60 days before mating until delivery, while periconceptional nutrient-restricted ewes received reduced nutrition in the period before conception and up to 8 days after mating. Gestational nutrient-restricted ewes were nutrient restricted from 8 days gestation until delivery. All ewes subsequently gained weight over the rest of gestation although this was reduced in those groups that remained nutrient restricted (e.g., at 13-wk gestation: C-C, 5.8 ± 0.8; R-C, 3.1 ± 1.2; C-R, 1.5 ± 0.8; R-R, 1.5 ± 0.9 kg). Maintenance feed requirements were adjusted to account for increasing gestational age fortnightly [17].

Real-time uterine ultrasound scanning was performed 60 days after mating to confirm twin pregnancy. Then, between 90 and 100 days gestation, pregnant ewes were transported to animal holding rooms and individually housed in pens under controlled lighting providing 12L:12D. At 105–110 days gestation, following 10–15 days acclimatization to this environment, each ewe underwent catheterization of one maternal jugular vein and hysterotomy, with cannulation of a fetal carotid artery and jugular vein of one randomly selected twin of each twin pair. Each ewe was anesthetized under halothane (0.5–4.0% v/v with oxygen; Fluothane; ICI, Melbourne, VIC, Australia). Prophylactic antibiotics (benzylpenicillin, 500 mg; dihydrostreptomycin, 500 mg) were given with local anesthetic (procaine hydrochloride, 40 g; Penstrip Lilium, Troy Laboratories, Smithfield, NSW, Australia) by intramuscular injection after the induction of anesthesia. Catheters were filled with heparinized saline and exteriorized through the ewe's flank. Maternal (5 ml) and fetal blood samples (0.5 ml arterial sample, 3 ml venous sample) were collected every 3 days from 115 days to 142 days gestation. Heparinized blood samples were immediately placed on ice, centrifuged, the plasma separated into aliquots, and stored at –20°C for subsequent analyses. Fetal arterial blood was used for the measurement of PaO2. At 143 days gestation, where term is approximately 147 days, ewes were killed by overdose of intravenous barbiturate (sodium pentobarbitone; Virbac, Peakhurst, NSW, Australia). Fetal lambs were delivered by hysterotomy, weighed, and killed by decapitation, perirenal adipose tissue collected, and stored as below.

Study B—Effect of maternal nutrient restriction in late gestation Ten Merino singleton-bearing ewes, of similar body weight and parity, were randomized to maternal nutrient restriction in late gestation (late R) or control nutrition throughout pregnancy (Fig. 1). Real-time uterine ultrasound scanning was performed 60 days after mating to confirm pregnancy. Ewes were provided with ad libitum feeding at pasture and, between 90 and 100 days gestation, pregnant ewes were transported to animal holding rooms and individually housed in pens under controlled lighting providing 12L:12D. Each ewe underwent catheterization of one maternal jugular vein and hysterotomy, with cannulation of a fetal carotid artery and jugular vein, under maternal anesthesia with prophylactic antibiotics (as described in study A). At 105 days gestation, control ewes were provided with 20 g lucerne and 3 g/kg oats [17], while nutrient-restriction ewes were provided with 50% of these maintenance requirements (10 g lucerne and 1.5 g/kg oats). All ewes were provided with water ad libitum. Fetal sheep were delivered near-term by postmortem hysterotomy between 141 and 145 days gestation, weighed, perirenal adipose tissue collected and stored at –80°C until RNA preparation.

Laboratory Procedures

Fetal blood gas status was determined by immediate analysis of arterial blood using an ABL 520 acid base analyser (Radiometer, Copenhagen, Denmark). Arterial plasma glucose concentrations were determined by enzymatic colorimetric assay.

Total RNA was extracted from fetal perirenal adipose tissue using a modification of the single-step acidified phenol-chloroform extraction method as described by Chomczynski and Sacchi [18]. RNA was electrophoresed through an agarose/formaldehyde gel, transferred onto nylon membrane by capillary action, and hybridized with 32P-labeled oligonucleotide probes to UCP1 mRNA and 18S rRNA based on the method described by Clarke et al. [19]. All gels were performed in duplicate and each included a reference RNA sample of near-term brown adipose tissue for determination of interassay variance, molecular weight markers, and a sample of RNA extracted from the liver of a near-term fetal lamb. As hepatic tissue is known not to contain UCP1, this provided a negative control [20]. The integrity of RNA loaded onto each gel was visualized before hybridization using ethidium bromide. Densities of RNA bands were determined using the Fuji-MacBAS MP2040 system (Fuji Photo Film Co. Ltd., Tokyo, Japan) with MacBAS software (V2.21, Berthold, Australia). The intraassay coefficient of variation, determined by inclusion of separate samples of the same tissue, was 1.1% (n = 5). The interassay coefficient of variance, determined by analyzing five different samples on four occasions, was 8.1% (n = 5).

Protein Analysis

Mitochondrial fractions were prepared from 1 g of frozen adipose tissue [21]. Protein content of each preparation was determined [22] and UCP1 was detected in mitochondrial preparations containing equal amounts of protein, following separation by sodium dodecyl polyacrylamide gel electrophoresis and immunoblotting with enhanced chemiluminescence (ECL; Amersham International, Bucks, UK). The antibody used was raised against purified ovine UCP1 [13]. Densitometric analysis was performed on all membranes following image detection using a Fuji film LAS-1000 cooled CCD camera (Fuji Photo Film Co. Ltd.). All gels were run in duplicate and a reference sample (i.e., from adipose tissue of a 6-h-old sheep) was included on each gel. Confirmation that equal amounts of protein were transferred from each gel to membrane before immunodetection was obtained by Ponceau red staining of all membranes [23].

Statistical Analyses

As twins are exposed to a single maternal environment, inclusion of data in analyses from both fetuses from each set of twins (study A) in analyses would add multiple data from a single maternal intervention and may amplify or confound the apparent effects of the intervention. Therefore, data from only the randomly selected catheterized twin from each twin pair is included in each analysis. The abundance of RNA was defined as the density of the RNA band revealed on phosphoimaging. The abundances of UCP1 mRNA are expressed as a ratio of the abundance of 18S ribosomal RNA (rRNA) detected for each sample on each membrane to correct for variations in gel loading and efficiency of RNA transfer from gel to membrane. UCP1 protein abundances are expressed relative to the reference sample of adipose tissue from a 6-h-old lamb.

As Kolmorgorov-Smirov analysis implied that a normal distribution of values was present, analysis of variance (ANOVA) was used to determine whether there was an effect of experimental intervention on fetal adipose tissue weights and UCP1 abundance. Measurements of plasma glucose and PaO2 (sampled every 3 days from 115 to 142 days gestation) were compared using repeated-measures ANOVA. In study A, where there were more than two nutrition groups, the presence of statistically significant comparisons of variables between groups were determined by multifactorial ANOVA. Data are shown as mean values with standard errors of the mean. A probability of 5% (P < 0.05) was taken to be significant.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Study A: Effect of Maternal Nutrient Restriction in the Periconceptional Period and Throughout Gestation

Maternal nutrient restriction in the period from 60 days before conception had no effect on fetal weight (Fig. 2), perirenal adipose tissue weight (C-C, 17.70 ± 1.35; R-C, 19.73 ± 1.26 g [n = 4 per group]) or perirenal adipose tissue relative to fetal weight as measured at 143 days gestation (Fig. 2). In contrast, nutrient restriction from Day 8 of gestation resulted in lighter fetuses and although perirenal adipose tissue weight was similar to controls (C-R, 20.04 ± 1.13 g), as there was a reduction in fetal weight, relative perirenal adipose tissue weight was significantly increased.



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  		FIG. 2. Effect of maternal nutrient restriction in the periconceptional period and/or throughout gestation on i) fetal weight and ii) relative perirenal adipose tissue weight near to term. Values are means with their standard errors and n = 4 per nutritional group. Controls (C-C), open bars; periconceptional nutrient restriction (R-C), closed bars; gestational nutrient restriction (C-R), shaded bars; periconceptional followed by gestational nutrient restriction (R-R), hatched bars. Significant differences with respect to nutritional group: * P < 0.05; ** P < 0.01; *** P < 0.001

When periconceptional nutrient restriction was continued throughout gestation, although mean body weight was lower than controls, this difference was not statistically significant (Fig. 2). Perirenal adipose tissue weight was similar to controls (R-R, 16.68 ± 1.84 g).

A combination of periconceptional plus gestational nutrient restriction resulted in a marked increase in UCP1 mRNA abundance in fetal perirenal adipose tissue (Fig. 3). Neither periconceptional nor gestational (from Day 8) maternal nutrient restriction alone had a significant effect on UCP1 mRNA abundance. There was no nutritional effect on UCP1 protein abundance, which remained well below values found in newborn sheep in all groups of fetuses (C-C, 54% ± 14%; R-C, 36% ± 4%; C-R, 26% ± 10%; R-R, 45% ± 8% ref [n = 4 per group]). In both ewes and fetuses, the same pattern of changes in plasma glucose with nutritional intervention was apparent (Fig. 4), with a significant reduction when nutrient restriction was imposed throughout the periconceptional and gestational periods. Nutrient restriction during the periconceptional or gestational periods alone had no effect on plasma glucose concentrations. There were no differences in fetal blood gas status between any nutritional groups (results not shown).



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  		FIG. 3. Effect of maternal nutrient restriction in the periconceptional period and/or throughout gestation on uncoupling protein (UCP)1 mRNA in the near to term ovine fetus. Values are means with their standard errors and n = 4 per nutritional group. Controls (C-C), open bars; periconceptional nutrient restriction (R-C), closed bars; gestational nutrient restriction (C-R), shaded bars; periconceptional followed by gestational nutrient restriction (R-R), hatched bars. Significant differences with respect to nutritional group: * P < 0.05



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  		FIG. 4. Effect of maternal nutrient restriction in the periconceptional period and/or throughout gestation on mean i) maternal and ii) fetal plasma glucose concentration as measured between 115 and 142 days gestation. Values are means with their standard errors and n = 4 per nutritional group. Controls (C-C), open bars; periconceptional nutrient restriction (R-C), closed bars; gestational nutrient restriction (C-R), checked bars; periconceptional followed by gestational nutrient restriction (R-R), hatched bars. Significant differences with respect to nutritional group: * P < 0.05; ** P < 0.01

Study B—Effect of Maternal Nutrient Restriction in Late Gestation

Maternal nutrient restriction in late gestation resulted in a 2.5-fold decrease in the abundance of UCP1 mRNA in near-term fetal perirenal adipose tissue (Fig. 5) but had no effect on protein abundance (control, 45% ± 4%; late R, 42% ± 1% ref). This was not accompanied by any change in fetal weight (control, 4.86 ± 0.35; late R, 4.65 ± 0.17 kg), but total perirenal adipose tissue (control, 23.1 ± 1.10; late R, 19.35 ± 1.08 g; P < 0.05) and perirenal adipose tissue weight relative to body weight (control, 4.15 ± 0.10; late R, 3.55 ± 0.20 g/kg) were reduced in fetuses exposed to maternal nutrient restriction in late gestation. Nutrient-restricted fetuses also had lower fetal glucose concentrations compared with controls (control, 1.4 ± 0.1; late R, 0.9 ± 0.1 mmol/L; P < 0.05), but there was no difference in fetal PaO2 between groups (control, 3.12 ± 0.18; late R, 3.58 ± 0.13 kPa).



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  		FIG. 5. Effect of maternal nutrient restriction in late gestation (late R) on uncoupling protein (UCP)1 mRNA in the near to term ovine fetus. Values are means with their standard errors and n = 5 per group. Significant differences between groups: * P < 0.05


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
The major finding of our study is that maternal nutrient restriction commencing from the first week after conception results in a relative increase in adipose tissue deposition despite a moderate fetal growth restriction. In contrast, a combination of periconceptional and gestational nutrient restriction does not compromise fetal growth but results in enhanced UCP1 mRNA, but not protein abundance, in fetal adipose tissue. This increase in UCP1 mRNA may act to promote thermoregulatory adaptation in the newborn lamb [11, 24] following the postpartum endocrine adaptations that promote a doubling in the amount of UCP1 very soon after birth [25]. In addition, those fetuses subjected to reduced nutrient supply only during the final month of gestation had less fat and a reduced abundance of UCP1 mRNA in the fetal adipose tissue. This adaptation may serve to conserve energy availability for those other tissues that have a lower energy requirement for growth when compared with fat. The fetuses in the undernourished group had lower plasma glucose concentrations and these findings support a positive relationship between fetal glucose supply and fetal fat mass over the final month of gestation, as previously shown in studies directly infusing glucose into the fetus [9] as opposed to manipulating nutrition of the fetus. One reason why this relationship is not apparent in twin fetuses that were subjected to prolonged maternal undernutrition may be that fat mass is actually lower in twins compared with singleton fetuses [26].

Gestational nutrient restriction alone resulted in a relative increase in adipose tissue deposition [8]. There was no change, however, in the abundance of UCP1 mRNA or protein within adipose tissue of fetuses subject to gestational nutrient restriction. It is possible that the relative increase in adiposity within these fetuses in conjunction with maintained UCP1 expression would promote heat production in the newborn. An adaptation of this kind could be important in preventing hypothermia in a growth-restricted neonate with an enhanced surface area:volume ratio over which heat loss is greatly increased.

An increase or decrease in mRNA for UCP1 following maternal undernutrition was not, however, accompanied by an immediate change in protein abundance for UCP1. These findings are in accord with previous studies on the effect of chronic maternal cold exposure in which a combination of increased fat mass and mitochondrial protein contribute to greater UCP1 thermogenic potential after, but not before, birth [11]. As a consequence, thermoregulation is substantially improved in the newborn [16]. A number of both catabolic and anabolic hormones promote UCP1 expression in the newborn, including noradrenaline [27], triiodothyronine [14], prolactin [28], and cortisol [12]. The concentrations of each of these hormones normally remains low in the fetal circulation but gradually rise to peak around the time of birth [29–31], coincident with the rapid increase in the amount and thermogenic potential of UCP1 [11]. The effects of prolonged nutrient restriction on the fetal hormonal environment are not well described, but under these conditions, fetal oxygen saturation is normal, consistent with the maintained placental weight [8]. A precocious rise in cortisol and in adrenal sensitivity to ACTH has, however, been described in fetuses exposed to periconceptional nutrient restriction [7], although whether this is sufficient to influence UCP1 mRNA abundance is not known. It is possible that other endocrine adaptations within the adipocytes of these nutrient-restricted fetuses may enhance their sensitivity to cortisol. For example, a more severe period of nutrient restriction between early to midgestation, followed by adequate feeding to term, has been established to promote the abundance of glucocorticoid receptor mRNA abundance in fetal adipose tissue at term [32]. This would be predicted to increase tissue sensitivity to cortisol, which may result in increased UCP1 mRNA expression and promote fat deposition [33].

Irrespective of the precise mechanism by which UCP1 mRNA expression is differentially increased in fetuses whose mothers were nutrient restricted throughout the periconceptional and gestational periods, it is apparent that the adipose tissue of these fetuses had a markedly different response when compared with those fetuses in which a more severe nutrient restriction was targeted to late gestation alone. Both sets of fetuses had similar basal plasma glucose concentrations, but it was only in the latter group that adipose tissue deposition (per kg fetal body weight) was reduced in conjunction with a decrease in UCP1 mRNA abundance. Fetal plasma cortisol concentrations are unaffected by late gestational nutrient restriction [34] but effects on prolactin, thyroid hormones, and catecholamines remain to be determined. It is possible that features of the normal ontogeny of adipose tissue development, which includes sympathetic innervation [35], appearance of prolactin [36], and adrenergic [37] receptor populations, in conjunction with increased 5' monodeiodinase activity [11, 38], may have been promoted by the combination of periconceptional plus gestational nutrient restriction. Programmed adaptations of this type may not be present in adipocytes of fetuses nutrient restricted in late gestation, thereby leading to different structural and functional adaptations in response to the lower plasma glucose concentration. UCP1, 2, and 3 are all expressed in brown adipose tissue and, in rats, there is a decrease in mRNA abundance for all UCPs between 2 and 8 mo of age [39]. The physiological role of UCP2 and 3, are not, however, clearly understood [40, 41], and in the sheep, UCP2 is not detectable in fetal tissues [42].

It has previously been shown that lambs born to chronically cold-exposed mothers possess more adipose tissue with increased total thermogenic potential [16]. Subsequently, at 1 mo of age, these offspring retain more adipose tissue despite similar food intake to controls throughout the postnatal period. The extent to which parallel effects may be observed in the offspring from the current study remains to be investigated. As in human populations, the effects of previous nutrient restriction may be amplified after birth when nutrient availability is no longer limiting and appreciable amounts of adipose tissue are deposited [19]. For example, low-birth-weight sheep have a higher relative fat mass at a body weight of 20 kg when fed to appetite compared with higher-birth-weight offspring [43]. In view of the finding, from the Dutch famine of 1944–1945, that exposure to famine in the first trimester was associated with an increased risk of obesity and more lipogenic lipid profile in adult life [3], the results from the present study provide evidence to support further follow-up studies to elucidate whether the described differences in near-term fetal adipose tissue deposition and UCP1 expression result in enhanced fat deposition in later life.

In conclusion, maternal nutrient restriction commencing the first week after mating results in a growth-restricted fetus with greater adipose tissue deposition. A combination of periconceptional and gestational nutrient restriction has no effect on fetal growth but contributes to enhanced UCP1 mRNA abundance that may act to promote thermoregulatory adaptation in the newborn following UCP1 upregulation around the time of birth [11]. Maternal nutrient restriction during late gestation alone results in a reduced fetal fat mass and lower abundance of UCP1 mRNA in fetal adipose tissue. The extent to which these fetal adaptations in adipose tissue deposition have implications after birth, when UCP1 abundance doubles [11] and the rate of fat deposition then rapidly increases [19], remains to be investigated.


   FOOTNOTES
 
1 Correspondence: Michael E. Symonds, Academic Division of Child Health, School of Human Development, Queen's Medical Centre, University Hospital, Nottingham, NG7 2UH United Kingdom. FAX: 44 115 970 9382; Michael.Symonds{at}nottingham.ac.uk Back

Received: 2 May 2003.

First decision: 2 June 2003.

Accepted: 2 February 2004.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 

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