Effect of egg yolk feeding on the concentration and composition of serum lipoproteins in man

The effect of egg yolk consumption on the composition of LDL and on the concentration of HDL subclasses was studied in healthy subjects. Six volunteers consumed a diet low in cholesterol for 10 days and then daily added 6 egg yolks to their diet for another 10 days; the experiment was repeated 1 year later with the same subjects. Egg yolk consumption caused the cholesterol intake to increase by 1600 mg/day, and the fat intake by 7 energy % at the expense of carbohydrates; this increase was due almost exclusively to monounsaturated fatty acids. Upon egg yolk feeding the mean level of serum total cholesterol rose by 13%; the bulk of this rise was due to LDL cholesterol, which increased by 21%. VLDL and IDL cholesterol decreased by 19 and 11%, and serum total triglycerides by 17%. Marked relative increases of 35 and 36% were seen in the cholesterol level of the HDL subfractions with densities of 1.055-1.075 g/ml (HDL1) and 1.075-1.100 g/ml (HDL2), respectively. The HDL2/LDL cholesterol ratio increased by 16%. No change in cholesterol in HDL3 (d greater than 1.100 g/ml) was observed. The increase in cholesterol in HDL isolated by density gradient ultracentrifugation significantly exceeded the increase in cholesterol in heparin-Mn2+ soluble HDL. This suggests the formation of apo E-containing HDL, i.e. HDLc, which has HDL density but is not soluble in heparin-Mn2+. The composition of the LDL particles was significantly altered; the core became enriched in esterified cholesterol at the expense of triglycerides, and the ratio of core components to surface components increased by 7%.