Historically, consumers were discouraged from consuming whole egg due to the relatively high content of dietary cholesterol in egg yolk. Yet, previous studies showed no relation between the whole egg intake and blood cholesterol concentration [26
]. Several scientific researchers suggest a reconsideration of recommendations to limit dietary cholesterol and egg cholesterol consumption due to conflicting evidence [29
]. The findings from the current two studies indicate that the dietary cholesterol found in whole eggs may not be well absorbed and does not acutely affect plasma total cholesterol concentration.
While fasting state lipid and lipoprotein concentrations mainly reflect body homeostasis, their post-prandial responses reflect the capacity to handle an acute dietary fat and cholesterol load [31
]. Experimentally, more accurate assessments of dietary cholesterol absorption may be made by measuring changes in cholesterol concentrations in TRL fractions versus plasma because changes in cholesterol in TRL fractions may represent newly absorbed dietary cholesterol [4
]. Using the TRL technique, our results of limited dietary cholesterol absorption from whole eggs in apparently healthy young men (Study 1) and young and middle-aged women (Study 2) are consistent with results from healthy men who consumed different amounts of dietary cholesterol (0, 142, 284, and 710 mg) from cooked egg yolk [19
]. The presence of phosphatidylcholine and sphingomyelin in egg yolk may partially explain these observations. Ingestion of these phospholipids influence intestinal lipid metabolism [32
] and decreased lymphatic absorption of cholesterol [33
]. Egg white protein may also help reduce cholesterol absorption by inhibiting micellar solubility of cholesterol in the intestine [35
]. Along with a limited dietary cholesterol absorption, plasma total cholesterol concentration was not affected by increasing whole egg intake in this study and previous study also found no effect of whole egg consumption (0-, 1-, 2-, and 4-whole egg diet) on postprandial total cholesterol concentration in healthy young men [36
]. In contrast, healthy men who consumed increasing amount of dietary cholesterol from egg yolk presented greater postprandial plasma cholesterol concentration [19
]. Research with rats showed that a whole egg-enriched diet lowered plasma LDL concentration and increased fecal bile acid content, compared to a high-cholesterol diet and egg yolk-enriched diet (dietary cholesterol content matched) [37
]. The rats fed the whole egg-enriched diet had higher mRNA levels of LDL-receptor and cholesterol 7a hydroxylase, consistent with whole egg activating LDL receptor–mediated catabolism, bile acid synthesis, and the excretion of fecal cholesterol. Egg white protein is suspected to have a favorable effect on blood lipoprotein profiles [38
] and limited research in humans observed that egg white protein ingestion reduced serum cholesterol in young women [39
]. In the current study, phosphatidylcholine and sphingomyelin from egg yolk and egg white protein may have contributed to limiting cholesterol absorption after subjects consumed whole eggs.
While results from both study 1 and study 2 indicate that consuming cholesterol-rich whole eggs did not acutely increase cholesterol absorption or plasma total cholesterol concentrations, higher triacylglycerol concentrations within the TRL fractions and plasma were observed. These results are consistent with research showing that higher dietary fat intake (0 g vs. 45 g) caused greater triacylglycerol within TRL fractions and plasma [19
], and that progressively higher dietary fat content of meals increased postprandial plasma triacylglycerol responses [31
]. Another previous human intervention study also assessed the impact of amount (3, 8, and 20 g) and source (canola oil, soybean oil, or butter) of dietary fat on postprandial triacylglycerol responses and regardless of source, consuming higher amount of dietary fat induced greater triacylglycerol content within TRL fractions [40
]. This greater absorption of triacylglycerol may explain the greater absorption of carotenoids and vitamin E from the mixed vegetable meals consumed by our participants [21
] since those nutrients are fat-soluble and co-consuming dietary fat enhances their absorption [40
]. Collectively, although cholesterol absorption may be limited with whole egg intake, they may not affect the absorption of triacylglycerol and fat-soluble nutrients.
Strengths of this research include using data from two investigator-blinded, randomized, crossover, diet-controlled studies and assessing cholesterol and triacylglycerol absorption based on results from TRL fractions versus plasma. Although assessments of cholesterol and triacylglycerol responses in TRL fractions were secondary measurements, retrospective power calculations support both studies having adequate sample size. It is important to note that these results come from acute feeding trials and postprandial changes in total cholesterol content in TRL fractions includes both dietary and biliary cholesterol. Estimates indicate that biliary cholesterol contributes about 75–85% of intestinal cholesterol content [4
]. The experimental design and methods we used preclude distinguishing between dietary and biliary cholesterol in TRL fractions and the greater quantity of biliary cholesterol in the intestinal lumen may affect the accuracy of dietary cholesterol absorption assessments. A priori, we chose to use the AUC results as the foundation for interpreting the study. The AUC-based result that cholesterol from whole eggs is not well absorbed is also shown at each postprandial time point.