Eggs are food products that have both been called numerous things ranging from "Amazing little nutrient sacs that are a vital component of any diet" all the way to "Bringer of cardiovascular death". While there are many different types of eggs, the most commonly consumed egg is the chicken egg and the following research refers to this particular species exclusively; the information may also apply to other eggs, but this is not ensured
What are in Eggs?
Components of the Yolk
The yolk of eggs contain approximately 50% oleic acid (an omega-9 monounsaturated fat commonly found in olive oil) with the majority of what is left being saturated fat with a small content of polyunsaturated fats (PUFAs); when looking at the ratio of PUFAs, there appears to be a preference of omega-6 fatty acids relative to omega-3.
The carotenoids (mostly lutein and zeaxanthin) in the egg yolk appear capable of increasing plasma carotenoid concentrations and even in certain tissues such as the eyes; due to this, they are likely to confer some benefits associated with eggs. Furthermore, a Choline content may also be relevant to egg consumption.
There is not a large amount of protein in the yolk (relative to the whites), but the protein in the yolk tends to be a high concentration of the amino acid Leucine.
The yolk contains most of the fatty acids, cholesterol, and any nutrients which happen to be fat soluble. It does not appear to have a high protein content, despite the relatively high leucine content
Protein and the Albumin
The white of the eggs, known as the albumin, contains the majority of the protein within the whole egg and some B-vitamins as well.
There are enzymes that can inhibit protein absorption to a degree, and proteins that can bind to (and sequester) B-vitamins, in the egg albumin that are destroyed with heat application or pasteurization. One of these binding proteins, known as avidin, is known to sequester the B-vitamin known as riboflavin.
The majority of the albumin is water and protein, and all of the components of eggs that are known to have negative effects when not cooked or pasteurized are located in the albumin; raw egg yolks do not contain a B-vitamin or protein absorption risk like the albumin may in high levels
Omega-3 is sometimes added to the diets of some hens to increase the omega-3 fatty acid content of the yolks, which is due to the fatty acid composition of a yolk being variable depending on the diet of the hen. The omega-3 fatty acid is usually in the form of ALA (Alpha-linolenic acid), as while the Fish Oil omega-3 fatty acids can be added to the eggs they are also known to sometimes confer a fish-like taste and scent to the eggs (reducing consumer compliance).
The omega-3 content of egg yolks can be increased by altering the hen's diet, and the omega-3 can be either ALA or the fish oil omega-3s EPA and DHA depending on what the hen is fed. EPA and DHA are seen as more beneficial, but may produce a minor fish-like taste and scent to the eggs
It should be noted that dietary cholesterol (a steroid molecule) is wholly different from the lipoproteins, large molecules containing both fatty acids (lipo-) and amino acids (-proteins) that serve as transportation molecules in the body.
While there may be interaction between dietary cholesterol and serum lipoproteins (since dietary cholesterol is indeed a component of the lipoproteins, among many other things) they are by no means synonymous.
Dietary cholesterol itself appears to be capable of forming small crystal aggregates (normally found in atherosclerotic plaque) and activating the NLRP3 inflammasome in macrophages (immune cells) which appears to be critical for atherosclerotic progression. This may simply be due to crystal formation (as it has been seen with other crystals such as silica and uric acid).
It is theoretically possible that cholesterol itself can form crystals, and the formation of crystals appears to cause an inflammatory response that may promote atherosclerosis
There appears to be no increased risk (stroke and coronary artery disease) in persons consuming 1-6 eggs a week or 'greater than six eggs a week' relative to no egg consumption overall, but there appeared to be an increased risk when looking at diabetics consuming more than six eggs a week and this has been found elsewhere in diabetic persons with a lone egg a week relative to none. When there are associations with increased mortality and egg consumption, they are stronger in diabetic patients.
Hyperlipidemic patients consuming eggs (relative to other dietary sources of cholesterol and fats) do not experience any negative effects associated with eggs, although substituting the eggs (for egg protein with added nutrients) was seen as beneficial.
When looking at egg consumption specifically (rather than dietary cholesterol overall), egg consumption does not appear to be associated with any form of cardiovascular disease overall. However, when controlling for diabetic persons only it does appear to be associated with increased progression of coronary artery disease
Consumption of eggs in the diet, ranging from 'almost never' to 'almost daily', does not appear to be associated with increased risk for developing diabetes although elsewhere a positive association has been noted with the controls in place being education, family history of diabetes, and baseline biomarkers for disease states (such as plasma triglycerides) with one study controlling for total caloric intake, although the magnitude was still highly variable with an OR of 4.36 (95% CI of 1.00-19.00). Two studies have noted a stronger association in women than in men.
There appears to be some connection between egg consumption and diabetes risk that is not well elucidated at this moment in time. It isn't really elucidated which populations may be at higher risk (except perhaps women) nor is the evidence yet causative
Some studies that have failed to find an increased risk in coronary artery disease associated with egg consumption overall have noted increased risk in persons with diabetes consuming more than six eggs a week; specifically, the nonsignificant relative risk (RR) and confidence interval (95% CI) went from 1.1 (RR) and 0.9-1.3 (95% CI) to RR 2.0 (RR) and 1.0-3.8 (95% CI) with diabetics not being at increased risk for stroke.
High egg consumption, assuming we are only looking at type II diabetic persons, appears to be associated with a slight but significant increase in the risk for coronary artery disease; this does not extend to stroke, and this risk is not seen in persons without diabetes
Despite the aforementioned possible risk associated with high egg consumption in diabetics, it appears that when eggs (two eggs daily) are consumed by obese diabetic persons in conjunction with a weight loss diet over 12 weeks is able to increase HDL cholesterol (relative to low egg control) with no significant difference between groups on LDL or total cholesterol which decreased in both groups equally. There were no differences in blood glucose or pressure over time between groups, although the reduction in fasting insulin seen with weight loss was lesser with eggs (6%) relative to low egg control (18%).
Limited evidence on diabetics losing weight have failed to note adverse effects on blood lipid profiles associated with egg consumption
Scientific Support & Reference Citations
- Goodrow EF, et al. Consumption of one egg per day increases serum lutein and zeaxanthin concentrations in older adults without altering serum lipid and lipoprotein cholesterol concentrations. J Nutr. (2006)
- Vishwanathan R, et al. Consumption of 2 and 4 egg yolks/d for 5 wk increases macular pigment concentrations in older adults with low macular pigment taking cholesterol-lowering statins. Am J Clin Nutr. (2009)
- Bovet P, et al. Decrease in blood triglycerides associated with the consumption of eggs of hens fed with food supplemented with fish oil. Nutr Metab Cardiovasc Dis. (2007)
- Ohman M, et al. Biochemical effects of consumption of eggs containing omega-3 polyunsaturated fatty acids. Ups J Med Sci. (2008)
- Small DM. George Lyman Duff memorial lecture. Progression and regression of atherosclerotic lesions. Insights from lipid physical biochemistry. Arteriosclerosis. (1988)
- Rajamäki K, et al. Cholesterol crystals activate the NLRP3 inflammasome in human macrophages: a novel link between cholesterol metabolism and inflammation. PLoS One. (2010)
- Duewell P, et al. NLRP3 inflammasomes are required for atherogenesis and activated by cholesterol crystals. Nature. (2010)
- Hornung V, et al. Silica crystals and aluminum salts activate the NALP3 inflammasome through phagosomal destabilization. Nat Immunol. (2008)
- Martinon F, et al. Gout-associated uric acid crystals activate the NALP3 inflammasome. Nature. (2006)
- Qureshi AI, et al. Regular egg consumption does not increase the risk of stroke and cardiovascular diseases. Med Sci Monit. (2007)
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