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Fetal Physiological Programming
1Department of Pediatrics, 2Research Centre for Endocrinology and Metabolism, Department of Internal Medicine, 3Cardiovascular Institute and the Wallenberg Laboratory, 4Department of Clinical Immunology, Göteborg University, Göteborg, Sweden
Submitted 26 May 2004 ; accepted in final form 28 October 2004
| ABSTRACT |
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blood pressure; insulin; triacylglycerol; body weight.
The ratio of n-6/n-3 PUFA in the milk affects growth, neurodevelopment, and immunoresponsiveness of the offspring (24). This ratio varies between 5:1 and 15:1 in milk of women from industrialized countries and is to a large extent defined by the maternal diet (10). To date, there are limited data on the long-term metabolic effects caused by variations of dietary fatty acids (FA) in early life. In rats, modification of early dietary PUFA intake has been shown to affect lipid metabolism and liver enzyme activity (7). Deficiency in n-3 PUFA during the first 9 wk of life resulted in hypertension in adult rats, regardless of subsequent supply (25). However, whether the ratio of n-6/n-3 PUFA in the maternal diet is of importance for physiological changes in the offspring in adulthood has not been investigated.
Recently, we have shown that the ratio of n-6/n-3 PUFA in the maternal diet affects serum leptin levels and growth of the offspring. Increased maternal intake of n-3 PUFA led to a decreased growth rate, reduced adipose tissue mass, and lower serum leptin levels in the offspring (16). Furthermore, the ratio of n-6/n-3 PUFA rather than the levels of n-6 PUFA in the maternal milk promoted body weight, growth of inguinal white adipose tissue and adipocyte size in the offspring. Modulation of dietary n-6/n-3 PUFA ratio and/or early leptin levels might have long-term effects on later metabolic parameters. This is of particular interest because it has recently been shown that the early leptin surge was important for long-term development (1, 4).
The aim of the present study was to investigate the long-term metabolic effects of various ratios of the dietary n-6 and n-3 PUFA during the perinatal period.
| MATERIALS AND METHODS |
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Diets. The dams were fed one of three experimental pellet diets (Morinaga Milk Industry, Tokyo) for the last 10 days of gestation and throughout lactation. The diets differed only by lipid composition: 7 wt% soybean oil (n-6/n-3 diet), sunflower oil (n-6 diet), or linseed oil (n-3 diet) (Table 1). The data on major components, salt, and vitamins have been obtained from the manufacturer. The total metabolizable energy of each of these diets was 13.9 MJ/kg. The ordinary chow (rat and mouse standard diet; B and K Universal, Grimston, Aldbrough, Hull, UK) contained 19% protein, 5% fat, 4% crude fiber, and 5.5% ash. The fat composition of ordinary chow is given in Table 1. The total metabolizable energy for this diet was 14.0 MJ/kg.
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Fatty acid analysis. Total serum lipids were extracted according to Folch et al. (11), fractionated on a single SEP-PAK aminopropyl cartridge (Waters, MA), according to the method described previously (17), and PL fractions were collected. The fractions of PL were transmethylated in methanolic-HCl-3N at 90°C over 4 h. The FA methyl esters were separated by capillary gas-liquid chromatography in a Hewlett-Packard 6890 gas chromatograph equipped with a 30 m x 0.25 mm SP-2380 column; film thickness was 20 µm. Helium at 1.1 ml/min was used as a carrier gas. The injector and detector temperatures were 250°C. The column oven temperature was programmed from 60°C to 230°C at a heating rate of 8°C/min up to 155°C, 1.5°C/min up to 180°C, and thereafter 6°C/min. The separation was recorded with Hewlett Packard GC Chem Station software (HP GC, Wilmington, DE). With 21:1 used as an internal standard, the FA methyl esters were identified by comparison with retention times of pure reference substances (Sigma Aldrich Sweden, Stockholm, Sweden). The inter-assay coefficients of variation were 3.0 and 3.5% for palmitic and linoleic acids (n = 10), respectively.
Analytical methods. Plasma glucose was determined by glucose oxidase/PAP assay (PAP refers to peroxidase, aminoantipyrine, and phenol), serum protein by biuret reaction, serum cholesterol by Infinity cholesterol reagent (Sigma Diagnostics, St. Louis, MO), and serum triacylglycerol by glycerol peroxidase/PAP; and all samples were analyzed in duplicate in a biochemical analyzer Cobas MIRA (ABX Diagnostics, Parc Evromedicine, Montpellier, France). Plasma insulin was determined with an enzymatic immunoassay (Rat insulin ELISA kit, Mercodia, Uppsala, Sweden). Leptin concentrations in serum were measured by a rat leptin radioimmunoassay (RIA; Linco Research, St. Charles, MO), and all samples from one experiment were analyzed in duplicate in the same assay.
Hemodynamic measurements. Systemic arterial pressure and heart rates in conscious rats were measured at 28 wk of age by tail-cuff plethysmography (6). The tails were warmed 15 min before measuring. Six tail-cuff measurements were obtained for each data point. The highest and the lowest readings were discarded, and at least three clear readings were averaged to obtain each data point.
Statistical analysis. Values are presented as means ± SD. The data were analyzed by one-way ANOVA followed by Fisher's post hoc paired least significant difference test. A value of P < 0.05 was considered statistically significant.
| RESULTS |
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-linolenic 18:3(n-6) and lower levels of docosahexaenoic 22:6(n-3) acids in the n-6-group compared with the other two diets. Consequently, the ratios of 20:4(n-6)/22:6(n-3) in the n-6-group were higher compared with the n-6/n-3 group. The levels of n-6 FA in serum PL in the n-3 group of rat offspring were significantly decreased, with an increase in the levels of myristic 14:0, palmitic 16:0,
-linolenic 18:3(n-3), and eicosapentaenoic 20:5(n-3) acids compared with the other diet groups. In the n-3 group, the proportions of saturated fatty acid and monounsaturated fatty acid were higher, whereas the level of PUFA was lower compared with the other two groups. The unsaturation index (UI) was elevated in the serum PL of the pups from the n-6/n-3 group compared with both the n-6 and n-3 groups.
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Body weight and food intake. At 3 wk of age, the mean body weight of the male pups of the dams fed the n-3 diet was significantly lower (P < 0.05) than that of the pups suckling the dams on the n-6/n-3, or the n-6 diets (39.0 ± 3.0 g vs. 51.4 ± 4.5 and 50.6 ± 3.6 g, respectively). In the female 3-wk-old pups, a significant difference (P < 0.05) was found in the mean body weight among all three diet groups 41.1 ± 2.9 vs. 53.7 ± 4.2 and 46.5 ± 3.1, respectively. At 8 wk of age, the male offspring in the n-6 group showed a significantly decreased body weight compared with the n-6/n-3 group (Table 3). The mean body weight of the offspring receiving the n-6 diet or the n-3 diet during the perinatal period did not differ until 24 wk of age when the female offspring from the n-6 group had a lower body weight than those from the n-3 group (P < 0.05).
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Circulating protein, triacylglycerol, cholesterol, leptin, glucose, and insulin levels. No differences were observed in the serum levels of protein, glucose, and leptin between the dietary groups in the adult male and female rats at 28 wk of age (Table 4). The triacylglycerol levels were significantly higher in the n-6/n-3 male rats compared with the n-3 and n-6 groups, while there were no differences in the female rats. The cholesterol levels were similar in the male animals and significantly higher in the n-6 female rats compared with females from the n-6/n-3 and n-3 groups (Table 4). The fasting plasma insulin levels in the n-6/n-3 group of adult rats were significantly higher for both males and females compared with those in the other groups. There was no difference in the insulin levels between the n-3 and the n-6 diet groups (Table 4).
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| DISCUSSION |
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The composition of nutrients during early life has been shown to have an impact on later growth and development of the offspring (24). The potential role of PUFA has not been fully explored. Variations in the maternal milk PUFA levels lead to rapid adaptations of the FA compositions of tissue PL in the offspring (5, 16). Although at 3 wk of age there were marked differences of serum PL ratio of n-6/n-3 FA in the n-3, n-6/n-3, and n-6 diet groups (2.5, 8.3, and 17.5, respectively), the concentrations were in physiological range, suggesting that the pups were neither n-6 nor n-3 deficient in any group at 3 wk of age (22). In our previous report, we also found differences in serum leptin levels and body and adipose tissue mass in the pups. The n-6/n-3 group exhibited lower mRNA for leptin in adipose tissue and higher serum leptin levels than the n-3 group, whereas the n-6 group were largely similar to the n-6/n-3 group (16). The lower weight in the n-3 group of suckling offspring might be due to reduced milk yield (2) and/or altered milk FA composition. We did not evaluate directly milk yield due to methodological difficulties; however, there were no differences in serum glucose, protein, triacylglycerol, and cholesterol levels in 3-wk-old pups from different dietary groups, indicating indirectly their similar nutritional status (16). Therefore, we assume that the most likely cause of the decreased body weight at 3 wk of age in the pups of the n-3 group is the result of a different n-6/n-3 ratio rather than difference in milk yield. From 8 wk of age the male and female rats from the n-6/n-3 group gained weight at a higher rate compared with the other dietary groups. Furthermore, the n-3 group caught up in body weight with the n-6 group at 8 wk and 12 wk for the females and males, respectively. The differences in weight gain and other parameters observed in the adult rats might be the long-term consequences of the perinatal n-6/n-3 PUFA ratio, but a relationship to the early leptin levels has to be further investigated. There was no difference in leptin levels between the groups at wk 28 in either gender, but we cannot exclude the possibility that the early disturbances in leptin expression and serum levels might have induced changes in leptin receptors resulting in changed regulation. In a recent study by Bouret et al. (4), neonatal deficiency of leptin in ob/ob mice induced reduced expression of leptin receptors in the hypothalamus, which was not reversible if leptin was administered after the neonatal period. Disturbances in the hypothalamic satiety regulation in parallel to those changes reported by Bouret et al. (4) would result in increased food intake, accelerating body growth. Increased body weight could also be due to changes in food efficiency or energy expenditure, which have to be analyzed in further studies.
The increased weight gain was associated with higher fasting insulin levels in adult rats from the n-6/n-3 group compared with the n-3 and n-6 dietary groups both in male and female rats. These higher insulin levels in combination with higher serum triacylglycerol levels indicate alterations in metabolic state of the n-6/n-3 males. In this context, it has to be considered that in humans the males are more prone to develop metabolic syndrome than females (26). The n-6 females, however, showed higher serum cholesterol levels. Hence, the various dietary programs had diverse outcomes in the adult animals differentiated by gender.
Evidence from chronic insulin infusion studies in rats suggests that hyperinsulinemia induces hypertension (15), and a similar association has also been reported in humans (14). Further studies have to be directed to clamp studies to investigate insulin resistance, since that has been shown in rats to precede the development of hypertension (3), but the precise mechanisms are unknown. The absence of hypertension in the female rats from the n-6/n-3 group despite the higher insulin levels may reflect a protective effect of estrogen, which is known to delay the onset of diet-induced hypertension caused by endothelial dysfunction (21).
Thus an increased body weight with enhanced fasting insulin and triacylglycerol levels and higher blood pressure in adult male rats were linked to specific dietary n-6/n-3 PUFA ratio in the perinatal period and possibly to early leptin homeostasis. These data suggest that the adult metabolism can be programmed by the balance of n-6/n-3 PUFA in the perinatal period, possibly by influencing gene expression. The developmental order of events, as well as the mechanisms involved, remains to be defined.
| GRANTS |
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| ACKNOWLEDGMENTS |
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| FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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