Because of their cholesterol content, eggs have long been touted as a driver of heart disease. As a SuppVersity reader, you know that there are multiple reasons why the notion that the consumption of eggs, or rather egg yolks, would increase your cardiovascular disease risk: (a) there’s no mechanistic “if your cholesterol is high, your CVD risk is also high”-link; (b) a causative link between the consumption of dietary cholesterol and serum cholesterol does not exist – at least not in the majority of people; (c) substances in egg yolks, in particular, have been shown to modulate the physical characteristics of your lipoproteins and will thus lower, not increase your CVD risk.
Since you know all that, it may seem less important for you than your doctor and other people who may still believe that cholesterol was a dietary villain you’d better avoid altogether to read about egg yolks recently discovered ability to significantly decrease your blood lipid levels.
|Table 1: Fatty acid profile and cholesterol content of pork belly oil and egg yolk (% of total fatty acid | Park. 2016)|
Today, people (including a handful of doctors and scientists 😉 acknowledge that the ingestion of eggs has been reported to lower, not increase blood cholesterol levels. In the absence of a mechanistic explanation for this phenomenon, however, large parts of the medical establishment still doubt that the anti-CVD and anti-cancer effects of eggs are driven by anything but their relatively high content of anti-oxidants. To identify the actual biochemical metabolic mechanisms by which the oral administration of egg yolk affects blood lipid reduction scientists from the Kangwon National University conducted a preliminary rodent study in which they observed similar reductions in ‘bad’ blood triglycerides and total cholesterol, as well as increases in ‘good’ high-density lipoprotein cholesterol as they were observed in the egg eating minority of human subjects in epidemiological studies.
This alone may not sound exciting enough for yet another rodent study to make it into theSuppVersity news. What is exciting enough, though, is the fact that the study is the first study to include another high cholesterol product in a control diet: pork belly oil – an “egg yolk analogue” that has a similar, but not identical fatty acid content as egg yolk (see Table 1); and here’s how the study that obviously had a saline control group, as well, worked:
- rats were fed, ad libitum, a purified pellet diet and drinking water for 6 weeks, and egg yolk or other treatments were orally administered every day at a fixed time;
- the amount of egg yolk / pork belly oil, namely 5mg/kg, i.e. the human equivalent of approximately 3-4 egg yolks, had been determined based on data from preliminary experiments in which the authors found that this was the amount of egg yolk where the blood lipid lowering effects plateaued (i.e. didn’t increase further, when more egg yolk was fed);
- feces, blood, weight and food intake were measured and/or analyzed daily / weekly;
- liver and abdominal fat were determined at the end of the 6-week study
The rats’ food intake was examined every 3 days. Their body weight was measured once per week. Based on this data the so-called “diet efficiency” was calculated as the ratio of the diet intake to the daily average body weight gain (diet intake/body weight gain).
|Figure 1: Body weight gain and diet intake of rats after oral dosing of egg yolk (Park. 2016).|
As you can see in Figure 1, the latter, i.e. the “diet efficacy”, was significantly higher for the pork-belly oil group (T2) in which the rodents gained significantly more body weight, even though they had a (likewise significantly) reduced food intake.
Beyond cholesterol: weight and body composition improve as well
Against that background, it is not really surprising that the animals “pork belly group” had also accumulated more abdominal and liver fat than those who were fed with egg yolk and ended up being a non-significant 10% leaner (as in having 10% lower abdominal fat masses) than their peers who received nothing but slightly salted water (saline placebo) on top of their std. rodent diet.
|Figure 2: Liver weight and abdominal fat weight in gram per 100g body weight (Park. 2016).|
The actual news, however, are the health-relevant improvements in triglycerides, HDL-C and LDL-C as well as the reductions in the predictors of selected heart and liver disease, namely the atherogenic index (AI) and the levels of the transaminase enzymes ALT and AST you can see in Figure 3 – changes of which scientists have concluded in other studies that they are indicative of significant metabolic improvements with downstream beneficial effects on your heart and metabolic health; effects for which the study at hand is the first to provide a mechanistic explanation.
|Figure 3: Atherogenic index (top, left), liver enzymes (transaminases ALT and AST | top, right) and lipid levels (triglycerides – TG; total cholesterol – TC; HDL; LDL | bottom | Park. 2016).|
The explanation revolves around the significant reduction in (compared to pork belly oil) or rather normalization (compared to control and pork belly oil) of HMG-CoA, the enzyme that’s responsible for the endogenous production of cholesterol and the promotion of cholesterol excretion – two potential mechanism, of which Park and Park point out that they are “supported by previous reports that investigated blood lipid reduction in rats that ingested boiled egg and found an increase in LDL-C in the group given pork belly oil (Houston et al., 2011 )” and in line with the fact that “many studies have emphasised that there is no correlation between the amount of egg ingestion and blood cholesterol in humans (Herron et al., 2004; Greene et al., 2005)” as well as studies showing that “lecithin in the egg yolk lowers the level of blood cholesterol, as it is used for the formation of micelles in the small intestine or increases excretion through the reabsorption of cholesterol as bile acids (Yang et al., 2007; Alqasoumi, 2014)”.
- Alqasoumi, Saleh I. “Evaluation of the hepatroprotective and nephroprotective activities of Scrophularia hypericifolia growing in Saudi Arabia.” Saudi Pharmaceutical Journal 22.3 (2014): 258-263.
- Cohn, Jeffrey S., et al. “Dietary phospholipids, hepatic lipid metabolism and cardiovascular disease.” Current opinion in lipidology 19.3 (2008): 257-262.
- Erami, Kazuo, et al. “Dietary Egg Yolk Supplementation Improves Low-Protein-Diet-Induced Fatty Liver in Rats.” Journal of Nutritional Science and Vitaminology 62.4 (2016): 240-248.
- Fernandez, Maria Luz. “Dietary cholesterol provided by eggs and plasma lipoproteins in healthy populations.” Current Opinion in Clinical Nutrition & Metabolic Care 9.1 (2006): 8-12.
- Greene, Christine M., et al. “Maintenance of the LDL cholesterol: HDL cholesterol ratio in an elderly population given a dietary cholesterol challenge.” The Journal of nutrition 135.12 (2005): 2793-2798.
- Herron, Kristin L., et al. “High intake of cholesterol results in less atherogenic low-density lipoprotein particles in men and women independent of response classification.” Metabolism 53.6 (2004): 823-830.
- Houston, D. K., et al. “Dietary fat and cholesterol and risk of cardiovascular disease in older adults: the Health ABC Study.” Nutrition, Metabolism and Cardiovascular Diseases 21.6 (2011): 430-437.
- Kern Jr, Fred. “Normal plasma cholesterol in an 88-year-old man who eats 25 eggs a day: mechanisms of adaptation.” New England Journal of Medicine 324.13 (1991): 896-899.
- Park, Byung-Sung and Park, Sang-O. “Lipid-lowering mechanism of egg yolk in normal rats.” International Journal of Food Science and Technology (2016) – doi: 10.1111/ijfs.13216
- Yang, Su Young, et al. “Effect of lecithin intake on lipid metabolism and antioxidative capacity in rats fed high fat diet.” Korean Journal of Nutrition 40.4 (2007): 312-319.
- Zdrojewicz, Zygmunt, Marta Herman, and Ewa Starostecka. “Hen’s egg as a source of valuable biologically active substances.” Postȩpy higieny i medycyny doświadczalnej (Online) 70 (2016): 751.
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