Eggs have been called numerous things, ranging from “amazing little nutrient sacs that are a vital component of any diet” all the way down to “bringer of cardiovascular death”. There are many different types of eggs, but the most commonly consumed and studied is the chicken egg. Conclusions from a study would probably apply to other bird eggs, but we can’t be certain.
The albumen (the white of the egg) is mostly made of protein. It contains B-vitamins but also avidin, a protein that can bind certain B-vitamins, such as biotin, and thus prevent their absorption. Luckily, a sizeable portion of avidin is destroyed by prolonged heating (including pasteurization), so nutrient loss can be mitigated.
The albumen is mostly water and protein. Alas, it contains the anti-nutrient avidin. Since heat can destroy avidin, egg whites should be cooked to avoid possible nutrient loss.
Egg yolk fat is about 46% oleic acid, an omega-9 monounsaturated fat commonly found in olive oil, 38% saturated fat, and 16% polyunsaturated fats (PUFAs).
The PUFA ratio depends on how the chickens were raised. Since most chickens are fed grains high in omega-6 PUFAs, eggs at your local supermarket will usually be much higher in omega-6 than in omega-3 PUFAs, whereas chickens that are pasture-fed or fed with a special omega-3 diet will have a more balanced PUFA ratio.
Alpha-linolenic acid (ALA), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA) are omega-3 PUFAs. In humans, ALA converts to the more active EPA and DHA, but this conversion isn’t very efficient and further diminishes as we age, so adding EPA and DHA to our diets is seen as more beneficial than adding ALA.
Most hens fed an omega-3 diet are given ALA. Fewer are given fish oil, which contain EPA and DHA; the “fish-like” taste and smell of their eggs tend to make those less popular.
The omega-3 content of yolks can be increased by altering the hens’ diet through supplemental omega-3 fats in the form of ALA or EPA+DHA. Eggs enriched with EPA+DHA (through fish oil) are considered more beneficial, but their minor fish-like taste and smell tend to make them less popular.
Yolks also have high levels of carotenoids (mostly lutein and zeaxanthin) that are capable of increasing carotenoid concentrations both in plasma and in specific tissues such as the eyes. Perhaps more importantly, yolks are among the richest sources of choline, a nutrient associated with a number of health benefits.
Finally, although the yolk contains less protein than the albumen, it has higher concentrations of the essential amino acid leucine.
The yolk is mostly made of fatty acids, cholesterol, and fat-soluble nutrients. Although lower in protein than the albumen, it contains higher concentrations of leucine, an essential amino acid.
Cholesterol is a fat-like substance present in all our cells. It serves many functions, such as providing the raw material for pregnenolone, from which are derived many other hormones: cortisol, DHEA, testosterone …
Cholesterol is shuttled throughout the body by two kinds of carriers made of fat on the inside and protein on the outside: low-density lipoproteins (LDL, often called the “bad cholesterol”) and high-density lipoproteins (HDL, often called the “good cholesterol”).
Cholesterol levels as measured by typical blood tests reflect both the cholesterol than we produce and the cholesterol that we ingest. (Most people produce more cholesterol than they ingest.)
Cholesterol can form small crystal aggregates, found in atherosclerotic plaques. Immune cells called macrophages can take up those crystals, thus activating the NLRP3 inflammasome. Supporting this idea, other crystals such such as silica and uric acid have been shown to trigger inflammasome activation.
Inflammasome activation triggers in turn the release of a number of pro-inflammatory cytokines, including IL-1beta and IL-18, which appear to be critical to atherosclerotic progression.
It is mechanistically possible for cholesterol to form crystals that can trigger an inflammatory response that may promote atherosclerosis.
Observational studies in middle-aged Japanese people and in people on a Mediterranean diet found no association between egg consumption and risk of cardiovascular disease. Another observational study found no increase in the risk of stroke or coronary artery disease in people consuming 1–6 eggs per week, whereas “greater than 6 eggs per week” appeared to increase the risk of coronary artery disease only in diabetics.
Similar results were found in an observational study in diabetics that compared one egg per week with no egg. Another study noted no connection between egg consumption and cardiovascular disease, but did find stronger associations in diabetics between egg consumption and increased mortality.
Observational studies looking at egg consumption specifically (rather than at dietary cholesterol overall) have not found it to be associated with any form of cardiovascular disease, except maybe in diabetics.
Although observational evidence may suggest a link between egg consumption and heart disease in diabetics, randomized controlled trials have found no such link.
In a 3-month study, 140 people with diabetes or prediabetes were randomized to eat either 2 eggs six times a week or 2 or fewer eggs for the entire week. No difference in HDL, LDL, triglyceride levels, or glycemic control was found.
In a 5-week randomized crossover study, 29 people with type-2 diabetes consumed at breakfast either 1 egg with vegetables and bread or half a cup of oatmeal with milk. No difference in plasma glucose, insulin sensitivity, blood pressure, inflammatory markers, or plasma lipids was found between the egg and oatmeal periods.
In a 12-week study, 37 people with type-2 diabetes and the metabolic syndrome were put on a moderately carbohydrate-restricted diet and then randomized between two groups: one eating 3 whole eggs per day; the other 3 albumens per day. Both groups lost weight and saw improvements in insulin sensitivity and their lipid profiles, but the whole-egg group saw a greater improvement in their lipid profiles in some respects: They had more HDL, less VLDL, and a better LDL and HDL diameter profile than the albumen group. A follow-up analysis of the same study also found more improvements in markers of inflammation in the whole-egg group than in the albumen group.
A controlled trial of hyperlipidemic patients consuming eggs (relative to other dietary sources of cholesterol and fat) associated no negative effects with egg consumption, although substituting the eggs with egg protein containing added nutrients was seen as beneficial.
Similarly, risk factors didn’t worsen in healthy college students when eggs were added to their diet. These students were randomized to eat either a breakfast with 2 eggs or without eggs five times a week for 14 weeks. They were allowed to eat whatever they wanted otherwise, although people in the “without eggs” group were encouraged not to eat any eggs at all. By the end of the study, both groups had gained weight and had worse blood lipid profiles, with no significant difference between groups.
In controlled trials, whether in healthy people or in people suffering from diabetes or hyperlipidemia, egg consumption was not associated with an increase in risk markers for cardiovascular health, insulin sensitivity, or blood glucose.
As we’ve seen, observational evidence may suggest a link between egg consumption and heart disease in diabetics. Observational evidence may also suggest a link between egg consumption and the risk of developing diabetes. In one study, eggs in the diet (from “almost never” to “almost daily”) didn’t appear to be associated with an increased risk of developing diabetes, but other studies noted a positive association, with the controls being education, family history of diabetes, and baseline biomarkers for disease states (such as plasma triglycerides). Finally, two studies reported 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 the risk of developing diabetes.
In a 14-week study, healthy college students randomized to eat either 2 eggs five times a week or no eggs gained weight in an equal measure.
In a 12-week study, two groups of diabetics suffering from obesity started a weight-loss diet. One group ate 2 eggs per day; the other, none. Both groups saw an equal decrease in LDL and total cholesterol, but the egg group enjoyed a greater increase in HDL. There was no difference in blood pressure or blood glucose between groups, but the reduction in fasting insulin seen with weight loss was lesser in the egg group.
In a 5-week randomized crossover study, 29 people with type-2 diabetes consumed at breakfast either 1 egg with vegetables and bread or half a cup of oatmeal with milk. As we have previously noted, no difference in plasma glucose, insulin sensitivity, blood pressure, inflammatory markers, or plasma lipids was found between the egg and oatmeal periods. No difference in body weight, body fat, or BMI was found either.
Studies on diabetics have noted no adverse effects of egg consumption on different health markers.