Zhen-Yu Chen

Application of the balance method to determining accumulation, metabolism, and apparent oxidation of linoleic and α-linolenic acids in the pregnant rat

Pregnancy-induced changes in whole-body partitioning of linoleic acid (18:2n-6) and alpha-linolenic acid (18:3n-3) between accumulation, conversion to longer-chain polyunsaturates (LCP), and disappearance (apparent oxidation) were determined over a 13-day balance period. 18:2n-6, 18:3n-3 and n-6 and n-3 LCP levels were measured in food, feces, organs, and carcass. Accumulation was estimated on the basis of paired difference values between one group killed at the beginning and one at the end of the balance period. Actual whole-body accumulation of 18:2n-6 and 18:3n-3 (mg per rat) was similar in pregnant and nonpregnant rats, but when expressed as milligrams per gram weight gain, it was greater in nonpregnant rats. Net accumulation of n-6 and n-3 LCP was significantly greater in pregnant rats whether expressed as milligrams per rat or as milligrams per gram body weight gain. Apparent oxidation of 18:2n-6 and 18:3n-3 expressed as a percentage of intake was similar in both groups, but when expressed as milligrams per gram weight gained, it was significantly greater in the nonpregnant group. We conclude that the balance method provides a useful index of whole-body and organ partitioning of 18:2n-6 and 18:3n-3. This method demonstrates that despite the higher accumulation of LCP during pregnancy, significantly more 18:2n-6 and 18:3n-3 is apparently oxidized than is converted to their respective LCP.

Refeeding after fasting increases apparent oxidation of n-3 and n-6 fatty acids in pregnant rats

The intake, excretion, and accumulation of long-chain fatty acids was analyzed to test the hypothesis that during pregnancy in the rat whole-body partitioning of n-6 and n-3 fatty acids between net accumulation or disappearance (apparent oxidation) is determined by both maternal energy status and fetal development. From midpregnancy to term in ad libitum-fed rats consuming-rodent chow, 30% more saturates and monounsaturates accumulated in the whole body than were consumed, whereas 28% of dietary n-6 fatty acids and 55% of dietary n-3 fatty acids were apparently oxidized. After 48 hours of fasting during midpregnancy (days 13 to 15) followed by refeeding to term, net accumulation of saturates and monounsaturates was equivalent to intake, but whole-body disappearance of n-6 fatty acids exceeded intake by 6% (NS), whereas n-3 fatty acid disappearance exceeded intake by 43%. Thus during refeeding after fasting, dietary n-6 and n-3 fatty acids were apparently completely oxidized and there was actually a net loss of n-3 fatty acids from whole-body stores. Fasting during midpregnancy did not significantly affect fetal weight gain or maternal gestational hyperlipidemia toward term. We conclude that during pregnancy n-6 and n-3 fatty acids are not only required for maternal and fetal structural and storage lipids, but are also used to meet energy requirements, especially during refeeding after fasting.

Gestational hyperlipidemia in the rat is characterized by accumulation of n-6 and n-3 fatty acids, especially docosahexaenoic acid

We have evaluated the relative and quantitative changes in long-chain fatty acids in maternal liver, serum, carcass and conceptus (fetuses plus placentae) during pregnancy in the rat, to ascertain whether previous concern over lower proportions of n - 6 and n - 3 fatty acids in maternal serum could be indicative of suboptimal n - 6 or n - 3 fatty acid status. Gestational hyperlipidemia was characterized by proportional decreases in linoleic, stearic and arachidonic acids but increases in palmitic and docosahexaenoic acids. However, the quantitative amount (microgram/ml) of linoleic, arachidonic and docosahexaenoic acids in serum lipids actually increased 2-5-fold from mid-pregnancy to term. Compared to non-pregnant rats, gestational hyperlipidemia was also associated with a lower proportion but similar quantity of linoleic acid in maternal carcass and adipose stores. We conclude that gestational hyperlipidemia in the rat is characterized by a relative but not quantitative decrease in whole-body stores of n - 6 fatty acids and a marked proportional and quantitative increase in docosahexaenoic acid in maternal organs and in the conceptus.

Short-term energy deficit causes net accumulation of linoleoyl-enriched triacylglycerols in rat liver

Energy depletion by reduced food intake over 4 days resulted in a 73% reduction in total rat liver triacylglycerols (TG). In liver TG of energy‐depleted rats, dilinoleoyl oleoyl glycerol (OLL) and trilinoleoyl glycerol (LLL)) were quantitatively increased by 85% and 147%, respectively. The net increase in linoleoyl‐enriched species could be quantitatively accounted for by the release of linoleate from monolinoleoyl species and its subsequent reacylation into dilinoleoyl species and trilinolein during energy depletion. Hence while palmitate, oleate and some linoleate are being hydrolyzed, presumably for oxidation some linoleate is retained and contributes to the remodelling of hepatic triacylglycerols during energy deficit.

Accumulation of polyunsaturates is decreased by weight-cycling: whole-body analysis in young, growing rats

Whole-body fatty acid analysis in rats has previously shown that 50-70% of dietary linoleate and alpha-linolenate is beta-oxidized to CO2 and that this value increases with refeeding after a single episode of fasting. Our hypothesis was that repeated fasting-refeeding or weight-cycling would increase the beta-oxidation of linoleate and alpha-linolenate thereby depleting their whole-body levels. In rats consuming 3% energy as linoleate and 0.15% energy as alpha-linolenate during a 16 d balance period, 19% of the linoleate consumed accumulated in weight-cycled rats compared with 34% in the free-fed controls (P < 0.01). Similarly, 11% of the alpha-linolenate consumed accumulated in the weight-cycled rats compared with 22% in the controls (P < 0.01). Arachidonate and docosahexaenoate also accumulated to lower extents in the weight-cycled rats than in the controls. In contrast, whole-body accumulation of palmitate, stearate and oleate was not different between the weight-cycled group and the controls when measured as a proportion of intake or relative to weight gain. Thus, whole-body depletion of linoleate and alpha-linolenate did not occur per se but the partitioning of linoleate and alpha-linolenate was significantly altered by weight-cycling resulting in lower whole-body accumulation and higher apparent oxidation of all polyunsaturates especially linoleate and alpha-linolenate.

Early Postnatal Development in the Rat is Characterized by Accumulation of Highly Unsaturated Triacylglycerols

ABSTRACT: Our objective was to determine the quantitative changes of individual triacylglyercol (TG) species in liver and carcass during early postnatal growth. Pregnant rats were killed at d 21 of pregnancy, and neonatal rats were killed at d 3 or 9 after birth. Quantitative changes in fatty acids and TG species of fetal/neonatal liver and carcass were determined using capillary gas liquid chromatography. At postnatal d 3 or 9 compared with fetal d 21, total carcass TG increased 11− to 12-fold, with nonessential fatty acids increasing 8− to 9-fold, n-6 essential fatty acids (EFA) increasing 34− to 44-fold, and n-3 EFA increasing 19− to 29-fold, respectively. Total neonatal liver TG increased 13-fold from fetal d 21 to postnatal d 3, with a 6-fold increase in non-EFA, a 34-fold increase in total n-6 EFA and a 65-fold increase in total n-3 EFA. At postnatal d 3 compared with fetal d 21, larger molecular weight liver TG classes (C56-C64) increased 68-fold, followed by lower molecular weight TG classes C40-C48 (19-fold) with only a 6-fold increase in C50-C54. In liver, highly unsaturated TG classes (C56-C64) accounted for 49% of total TG at postnatal d 3 and consisted mainly of arachidonic, docosahexaenoic, and linoleic acids accompanied by palmitic and oleic acids. During early postnatal development, TG species containing one, two, or even three 20–22 EFA may be structurally important themselves or serve as direct substrates for synthesis of phospholipids. The unique accumulation of C54-C64 TG species, especially in liver, indicates that highly unsaturated TG are a quantitatively important EFA pool during early life.

Linoleoyl-enriched triacylglycerol species increase in maternal liver during late pregnancy in the rat.

In view of the previously reported changes in the fatty acid composition of maternal liver triacylglycerols in late pregnancy, changes in the composition of maternal liver triacylglycerol species were assessed in rats fed a semipurified diet during pregnancy. Between day 18 and day 21 of pregnancy, total maternal liver triacylglycerols increased by 50%. Triacylglycerol species with a total acyl carbon number (C) of 50 or 60 (C50, C60) remained unchanged while C48 and C52–C58 were relatively increased. The individual triacylglycerol species containing one, two or three linoleoyl moieties were incompletely recovered using a polar high temperature gas-liquid chromatography (GLC) column. Nevertheless, at day 21 compared to day 18, the linoleoyl-containing species were relatively increased by 62–463%, while tripalmitin was decreased by 38%. Our data suggest that despite an adequate intake of linoleic acid (25 g/kg in the diet), maternal hepatic triacylglycerol content of linoleic acid decreased during mid-pregnancy but increased significantly toward term possibly in preparation for the transfer of linoleic acid to the neonate during lactation.

Fasting-induced remodelling of hepatic triacylglycerols

Abstract Rat liver triacylglycerol species are known to respond differentially to metabolic and nutritional manipulation but changes in triacylglycerols containing 20–22 carbon polyunsaturates have been difficult to determine. Liver total triacylglycerols were separated by high performance liquid chromatography into subclasses to determine whether their remodelling by fasting induced changes consistent with reacylation causing enrichment by long-chain polyunsaturated fatty acids. Triacylglycerol subclasses and their fatty acid composition were quantitatively determined. The fasting-induced changes in liver triacylglycerols were characterized by the appearance of new subclasses containing 20–22 carbon polyunsaturates at 18–30% but relative and quantitative depletion of more saturated subclasses. Previous studies have not been able to identify whether changes in the polyunsaturated fatty acid content of triacylglycerols during fasting were mainly a function of depletion of saturates and monounsaturates or whether extensive reacylation of subclasses/species specifically enriched in polyunsaturated fatty acids also occurred; our present data suggest that the latter is a significant component of the fasting-induced triacylglycerol remodelling process.

FASTING/REFEEDING HAS LITTLE LASTING EFFECT ON LONG-CHAIN FATTY ACIDS ACCUMULATING IN THE DEVELOPING RAT CONCEPTUS

The objective of this study was to delineate the influence of maternal fasting-refeeding on accumulation of long-chain fatty acids by the developing conceptus in chow-fed pregnant rats. At term, the rat conceptus normally accumulates < 5% of total maternal whole body n - 6 polyunsaturates and < 15% of whole body n - 3 polyunsaturates. In contrast to the ad libitum-fed controls, the conceptus of fasted-refed rats was the main site of accumulation of n - 6 and n - 3 polyunsaturates since the maternal carcass (excluding liver) actually had a lower content of polyunsaturates at the end of the fasting-refeeding period than at the start. The rise in the content of triacylglycerols in the conceptus during fasting was specifically attributable to increased arachidonate, docosatetraenoate and docosahexaenoate; all changes in conceptus triacylglycerols were back to ad libitum control values by the end of the refeeding period (term). Conversely, although conceptus total phospholipids did not change significantly during fasting-refeeding, there was a net increase in the phospholipid content of both palmitate and oleate in conceptus after refeeding. We conclude that the developing rat conceptus is largely resistant to the marked changes in maternal fatty acid intake and retention that occur during fasting and refeeding.