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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 type. The information may also apply to other eggs, but this is not ensured.
Components of the Yolk
The yolk of eggs contain approximately 50% oleic acid, an omega-9 monounsaturated fat commonly found in olive oil. The remaining fat content consists of saturated fats along with a small amount of polyunsaturated fats (PUFAs). The ratio of different PUFAs tends to depend how the chickens were raised. Since the majority of commercially-raised chickens are fed grains high in omega-6 fatty acids, the typical 'garden-variety' eggs found your local supermarket will tend to be high in omega-6 fatty acids relative to the omega-3 variety. On the other hand, chickens that are pasture-fed or fed with a special omega-3 enriched diet will tend to have a more balanced ratio of omega-3 to omega-6 fatty acids.
Egg yolks also have high levels of carotenoids (mostly lutein and zeaxanthin) that are capable of increasing carotenoid concentrations in both plasma and specific tissues such as the eyes. Perhaps more importantly, eggs yolks are among the richest sources of Choline, a nutrient associated with a number of health benefits.
Although yolks contain less protein compared to whites, yolks have higher levels of the essential amino acid Leucine.
Egg yolks contain mostly fatty acids, cholesterol, fat soluble nutrients. Although lower in protein, yolks contain high concentrations of leucine, an essential amino acid.
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.
The white also contains the protein avidin, which can bind and sequester certain B-vitamins such as biotin, preventing absorption. Luckily, a sizeable portion of avidin is destroyed by prolonged heating or pasteurization, so nutrient loss can be mitigated.
The majority of the albumin of eggs consists of water and protein. Albumin also contains the anti-nutrient avidin. Since avidin is destroyed by cooking, egg whites should be cooked before consumption to avoid possible nutrient loss.
Omega-3 is sometimes added to the diets of hens to increase the omega-3 fatty acid content of the yolks, since the fatty acid composition the yolk tends to depend on the diet of the hen. Omega-3 fatty acids given to hens are usually in the form of ALA (Alpha-linolenic acid), although the omega-3 fatty acids EPA and DHA from Fish Oil are also used. Eggs from hens with EPA/DHA-enriched diets tend to be associated with a 'fish-like' taste and scent, which tends to make these eggs less popular.
The omega-3 content of egg yolks can be increased by altering the hen's diet through supplemental omega-3 fats in the form of ALA or the fish oil omega-3s EPA and DHA. Since ALA tends to have a low conversion rate to EPA and DHA in adults, eggs enriched with EPA and DHA are seen as more beneficial. This may come at a cost, however, producing a minor fish-like taste and scent in eggs with high levels of EPA and DHA.
It should be noted that cholesterol (a steroid molecule) is wholly different from the lipoproteins such as LDL and HDL, which are 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.
It should also be noted that the body can synthesize its own cholesterol. Cholesterol levels in the blood that are measured during typical blood tests called lipid panels come from two sources: the diet, and that which is manufactured by the body itself. In most people, the amount of cholesterol made by the body is greater than the amount ingested in the diet.
Cholesterol is capable of forming small crystal aggregates that are found in atherosclerotic plaques. This can activate a specialized type of immune cell called macrophages, which take up the crystals, triggering activation of the NLRP3 inflammasome. Inflammasome activation, in turn, triggers the release of a number of pro-inflammatory cytokines including IL-1beta and IL-18, which appear to be critical for atherosclerotic progression. Inflammasome activitation may be due to the uptake or crystals by macrophages which cause intracellular damage, triggering the inflammasome. Supporting this idea, other crystals such such as silica and uric acid have been shown to trigger inflammasome activation.
It is mechanistically possible that cholesterol can form crystals which can trigger an inflammatory response that may promote atherosclerosis.
One observational study found no increased risk of stroke and coronary artery disease in people 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. Similar results were found in another observational study in people with diabetes when comparing consumption of a single egg a week to none. In contrast, one study failed to note any connections between egg consumption and cardiovascular disease, but did find associations between increased mortality and egg consumption that were stronger in diabetic patients.
Another observational study found that in persons following a Mediterranean diet, there was no association between eggs and risk for cardiovascular disease. Similar results were noted in an observational study in middle-aged Japanese persons.
Observational studies looking at egg consumption specifically (rather than dietary cholesterol overall), have not found egg consumption to be associated with any form of cardiovascular disease overall. Connections have been drawn between cardiovascular disease and egg consumption in diabetics, however.
A controlled trial of 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 with egg protein containing added nutrients was seen as beneficial.
Healthy college students were also not seen to have worse risk factors when eggs were added to their diet. Students were randomized to eat either a breakfast with 2 eggs or without eggs 5 times a week for 14 weeks. They were allowed to eat whatever they wanted otherwise, although the without-eggs group were encouraged not to eat any eggs. While both groups gained weight over the course of the study, and both had a worse blood lipid profile by the study's end, there was no significant difference between the groups.
Controlled trials of egg consumption have not found an increased risk of cardiovascular disease associated with egg consumption.
Although the observational evidence above may suggest a link between heart disease and egg consumption in diabetics, randomized controlled trials have found no such link.
In one study, 140 people with diabetes or prediabetes were randomized to eat either two eggs per day for six days out of the week, or 2 or fewer eggs for the entire week. The study went on for three months. At the end of the trial, there was no difference between the groups in HDL, LDL, or triglyceride levels. Glycemic control also was not different between the groups. Similar results were found in another trial in 37 people with type 2 diabetes and the metabolic syndrome, who were all put on a moderately carbohydrate-restricted diet and then randomized to one of two groups for 12 weeks: either eating 3 whole eggs per day or eating 3 yolk-free eggs per day. Both groups had improvements in their lipid profiles and insulin sensitivity and both groups lost weight, but the whole-egg group had an improved lipid profile in some aspects: they had raised HDL, less VLDL, and a better LDL and HDL diameter profile than the yolk-free group. A follow-up analysis of the same study also found improvements in markers of inflammation in the whole-egg group over and above those found in the yolk-free egg group.
Another randomized crossover study with 29 subjects with type 2 diabetes had the participants either consume one egg with vegetables and bread or a half cup of oatmeal every day for 5 weeks. No difference in plasma glucose, insulin sensitivity, blood pressure, inflammatory markers, or plasma lipids was found.
Randomized controlled trials in people with prediabetes or diabetes found that risk markers for cardiovascular disease and insulin sensitivity and blood glucose were unaffected by consuming eggs.
Some observational evidence suggests there may be a correlation between egg consumption and diabetes. In one such study, consumption of eggs in the diet, ranging from 'almost never' to 'almost daily', did 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). Two studies have noted a stronger association in women than in men.
If you look ONLY at observational evidence, there would appear to be some connection between egg consumption and diabetes risk.
Healthy college students randomized to eat a breakfast either with 2 eggs or without eggs 5 times a week for 14 weeks both gained weight over the course of one study, but there was no difference in weight gain between the egg and no-egg groups.
One study where two eggs daily were consumed by obese diabetic people in conjunction with a weight loss diet over 12 weeks found an increase in HDL cholesterol relative to a 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%).
Another randomized crossover trial in 29 people with diabetes found no weight gain or fat gain when one egg was consumed daily versus a half cup of oatmeal over the course of five weeks.
Limited evidence on diabetics losing weight have failed to note adverse effects on blood lipid profiles associated with egg consumption.