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The effect of early diet on hepatic cholesterol metabolism in piglets

University of British Columbia

Plasma total, low density lipoprotein (LDL) and high density lipoprotein (HDL) cholesterol concentrations increase immediately following birth. Interestingly, this increase is greater in breast-fed infants than in infants fed formula. The reason(s) why there are differences in plasma cholesterol concentrations between breast-fed and formula-fed infants is not known. However, this difference may be a consequence of the variations in lipid composition between milk and infant formula. Little is known regarding the specific effects of the lipid component(s) of infant diets on the expression of genes involved in hepatic lipid metabolism. The studies presented in this thesis determined whether the addition of cholesterol, arachidonic acid [20:4(n-6)] and docosahexaenoic acid [22:6(n-3)] to formula, and the positional distribution of fatty acids in formula triglycerides increases plasma cholesterol in formula-fed piglets to levels observed in milk-fed piglets. In study #1, piglets were fed from birth to 18 days of age with either a conventional infant formula (conventional formula) or a formula with synthesized triglycerides (TG) (synthesized TG formula). The conventional infant formula had 70% of the total 16:0, representing 23% of total fatty acids, esterified at the sn-1 and 3 positions of the formula triglyceride. The synthesized TG formula contained a similar percentage of 16:0, representing 23% of total fatty acids, but had 47% of the total 16:0 esterified at the centre (sn-2) position of the formula triglyceride. Each of the conventional and synthesized TG formulae were provided either without (<0.10 mM) or with 0.65mM cholesterol added to formula, 0.52mmol/L as unesterified cholesterol and 0.13 mmol/L as cholesterol oleate. A reference group of piglets was also fed sow milk. In study #1, the levels of hepatic HMG-CoA reductase mRNA, 7-a-hydroxylase (C7H) mRNA, and acetyl CoA carboxylase (ACC) mRNA were higher in the formula-fed than milk-fed piglets, irrespective of the formula cholesterol content or the positional distribution of fatty acids in the formula triglyceride. This was accompanied by lower plasma total and HDL cholesterol concentrations, lower hepatic triglyceride concentrations and lower concentrations of bile acids, cholesterol and phospholipid in bile of the formula-fed than milk-fed piglets. Adding cholesterol to the formula increased hepatic cholesterol concentrations and decreased hepatic levels of fatty acid synthase (FAS) mRNA, but had no effect on the plasma cholesterol concentrations of the formula-fed piglets. Directing 16:0 to the sn-2 position of the formula triglyceride led to lower plasma total cholesterol and triglyceride concentrations, lower concentrations of bile acids in bile, lower hepatic levels of FAS mRNA and activity, and higher hepatic levels of ACC mRNA than in piglets fed the conventional formula. In study #2, piglets were fed the conventional formula either without or with egg phospholipid (9.5g/L) to provide 0.8% 20:4(n-6) and 0.3% 22:6(n-3) of total fatty acids, or sow milk from birth to 15 days of age. Supplementing the conventional formula with egg phospholipid resulted in higher levels of 20:4(n-6) and 22:6(n-3) in liver and bile phospholipid, higher plasma HDL concentrations, higher bile acid and phospholipid concentrations in bile and lower hepatic ACC mRNA levels in the formula-fed piglets. The levels of 20:4(n-6) and 22:6(n-4) in liver and bile phospholipid were also higher in the piglets fed the supplemented formula than in the piglets fed milk. A significant inverse relation was found between the levels of hepatic ACC mRNA and the percentage of 20:4(n-6) in liver triglyceride and the percentage of 22:6(n-3) in liver phospholipid. Egg phospholipid supplementation of formula had no effect on hepatic LDL receptor mRNA or hepatic FAS activity and mRNA in the formula-fed piglets. The piglets fed either the supplemented or the conventional formula had lower levels of plasma cholesterol and higher levels of hepatic HMG-CoA reductase activity and mRNA and C7H mRNA than piglets fed milk. These studies show that early diet, that is, milk compared to formula feeding, results in lower levels of hepatic HMG-CoA reductase activity and mRNA and C7H mRNA accompanied by higher plasma cholesterol concentrations in piglets. Supplementing formula with cholesterol or the preferential esterification of 16:0 at the sn-2 position of the formula triglyceride did not raise plasma cholesterol concentrations and had no effect on hepatic HMG-CoA reductase activity and mRNA or C7H mRNA in formula-fed piglets. Supplementing formula with egg phospholipid, increased bile and liver phospholipid 20:4(n-6) and 22:6(n-3), decreased the levels of hepatic ACC mRNA and increased the concentrations of bile acids and phospholipid in bile. These findings suggest that milk-fed piglets have lower rates of hepatic cholesterol synthesis, lower rates of conversion of cholesterol to bile acids and the lipid present in sow milk and formula may be metabolized differently. These findings are significant in that they raise the question as to whether or not this effect of early diet will continue through to adulthood and influence metabolic response to diet fat.

Thesis/Dissertation

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2009-06-18

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http://hdl.handle.net/2429/9455

Interdisciplinary Studies

University of British Columbia

UBCV

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