Effects of dietary protein source on cholesterol metabolism in neonatal pigs

The purposes of this study were to determine the effects of dietary protein sources used in human infant formulas on cholesterol metabolism, and to examine several hormones as potential mediators of a cholesterolemic response. Thirty (15 male/15 female), two day old piglets were removed from sows and fed diets differing only in casein:whey ratios of either 100% casein, 80% casein:20% whey, or 40% casein:60% whey for three weeks. Piglets fed diets containing increasing amounts of casein had increased total and free (unesterified) serum cholesterol (p < .05). Cholesterol in HDL (high- density lipoprotein) and LDL (low-density lipoprotein) fractions indicated a similar trend though only 100% casein-fed animals had significantly higher cholesterol levels than 40:60 fed animals (p < .05). An opposite effect was seen in the VLDL fraction as 100% casein-fed piglets had lower cholesterol levels than whey-fed piglets (p < .001). Serum glucagon and cortisol concentrations were elevated in the 100% casein-fed piglets (p < .05). Hepatic 3-hydroxy-3-methylglutaryl coenzyme A reductase (HMG CoA reductase) activity was lowest in 100% casein-fed piglets, though protein source did not significantly affect bile acid excretion, serum insulin or thyroid hormone concentrations, or total lipid or cholesterol accumulation in the liver. Feed efficiency varied between dietary treatments, and weight gain was highest in animals receiving greater proportions of whey (p < .001). The role of weight gain on blood lipids was not precisely determined. These results suggest that casein or whey protein fed to piglets during the suckling period affect blood lipid levels, HMG CoA reductase activity, glucagon, cortisol, and weight gain.