Eggs

Over the past 4 decades or so, the egg has been both reviled as a source of bad cholesterol and rehabilitated as an inexpensive source of protein for an increasingly hungry world.

Females of every species produce eggs, however, eggs are laid by birds, reptiles, amphibians, and fish have been eaten by humans for thousands of years. Eggs consist of a protective shell of varying colors, albumen (egg white), and vitellus (egg yolk).

Egg yolks and whole eggs have significant amounts of protein and choline, and are widely used in cooking. The USDA categorizes eggs as Meats within the Food Guide Pyramid due to their high protein content.

The most commonly used bird eggs are those from the chicken. Duck and goose eggs, and smaller eggs, such as quail eggs, are sometimes used as a gourmet ingredient in western countries. Other popular choices for egg consumption are roe, and caviar (fish).

Most bird, amphibian and reptile eggs are the shape of a spheroid with one end larger than the other.

The larger end of the egg contains an air cell that forms when the contents of the egg contract after it is laid. Chicken eggs are graded according to the size of this air cell, measured during a process called candling. A newly laid egg has a small air cell and receives a grade of AA. As the size of the air cell increases the quality of the egg decreases and the grade changes from AA to A to B. This provides a way of testing the age of an egg. You can also place an egg in water, if it floats, don’t eat it.

Egg shell color is caused by pigment deposition during egg formation and can vary according to species and breed, from the more common white or brown to pink or speckled blue-green. Generally, chickens with white ear lobes lay white eggs, and chickens with red ear lobes lay brown eggs. There is no link between shell color and nutritional value.

The albumen is also known as the white or the clear liquid within an egg. The purpose of egg white is to protect the yolk and provide nutrition for the growth of the embryo if the egg is fertilized.

The egg white consists of 90% water with 10% dissolved proteins. Unlike the yolk, which is high in fats, the egg white contains almost no fat and the carbohydrate content is less than 1%.

The yolk in a newly laid egg is round and firm. As the yolk ages, it absorbs water from the albumen, which increases its size and causes it to stretch and weaken the membrane enclosing the yolk. Yolk color is dependent on the diet of the hen.

Storage

Careful storage of edible eggs is extremely important, as an improperly handled egg can contain elevated levels of Salmonella bacteria that can cause severe food poisoning. In the US, eggs are washed, and this cleans the shell, but erodes the cuticle. The USDA thus recommends refrigerating eggs to prevent the growth of Salmonella.

Refrigeration also preserves the taste and texture. However, intact eggs can be left un-refrigerated for several months without spoiling.

Nutrition

Eggs are among the most nutritious foods on the planet. A single large boiled egg contains:

Vitamin A: 6% of the RDA.
Folate: 5% of the RDA.
Vitamin B5: 7% of the RDA.
Vitamin B12: 9% of the RDA.
Vitamin B2: 15% of the RDA.
Phosphorus: 9% of the RDA.
Selenium: 22% of the RDA.
Eggs also contain Vitamin D, Vitamin E, Vitamin K, Vitamin B6, Calcium and Zinc.
They have an average of 77 calories, 6 grams of protein and 5 grams of healthy fats.

Eggs also contain various other trace nutrients that are important for health.

Eggs are high in cholesterol. In fact, a single egg contains 212 mg, which is over half of the recommended daily intake of 300 mg.

However, it’s important to know that cholesterol in the diet doesn’t necessarily raise cholesterol in the blood. The liver actually produces large amounts of cholesterol every single day. When we eat more eggs, the liver just produces less cholesterol instead, so it tends to even out. In 70% of people, eggs don’t raise cholesterol at all.

In the other 30% (termed “hyper responders”), eggs can mildly raise total and LDL cholesterol.

Eggs raise HDL (The “Good”) cholesterol. People who have higher levels of HDL usually have a lower risk of heart disease, stroke and various health problems. Eating eggs is a great way to increase HDL. Egg consumption consistently leads to elevated levels of HDL (the “good”) cholesterol, which is linked to a reduced risk of many diseases.

Eggs contain choline, an important nutrient most people don’t get enough of or even know exists. Yet, it is an incredibly important substance and is often grouped with the B vitamins.

Choline is used to build cell membranes and has a role in producing signaling molecules in the brain, along with other functions

Dietary surveys have shown that about 90% of people in the U.S. are getting less than the recommended amount of choline. Whole eggs are an excellent source of choline. A single egg contains more than 100 mg of this nutrient.

LDL cholesterol is generally known as the “bad” cholesterol. It is well known that having high levels of LDL is linked to an increased risk of heart disease

But what most people don’t know, is that there are subtypes of LDL that relate to the size of the LDL particles. There are small, dense LDL particles and then there are large LDL particles.

Many studies have shown that people who have predominantly small, dense LDL particles have a higher risk of heart disease than people who have mostly large LDL particles Even if eggs tend to mildly raise LDL cholesterol in some people, studies show that the particles change from small, dense to large LDL

Egg consumption appears to change the pattern of LDL particles from small, dense LDL (bad) to large LDL, which is linked to a reduced heart disease risk.

Eggs contain lutein and zeaxanthin and antioxidants that have major benefits for eye health which is an important consideration for people with diabetes who are at risk of blindness because of their disease.

Lutein and Zeaxanthin are powerful antioxidants that build up in the retina of the eye. Studies show consuming adequate amounts of these nutrients can significantly reduce the risk of cataracts and macular degeneration, two very common eye disorders.

Not all eggs are created equal. Their nutrient composition varies depending on how the hens were fed and raised. Eggs from hens that are raised on pasture and/or fed Omega-3 enriched feeds are higher in Omega-3 fatty acids.

Omega-3 fatty acids reduce blood levels of triglycerides, a well-known risk factor for heart disease. Studies show that consuming Omega-3 enriched eggs is an effective way to reduce triglycerides in the blood. In one study, consumption of just five omega-3 enriched eggs per week for 3 weeks reduced triglycerides by 16-18%

Cardiovascular risk

Many studies published in the past two decades have examined the relationship between egg consumption and the risk of heart disease. Eggs do NOT raise your risk of heart disease and may reduce the risk of stroke

A 1999 Harvard School of Public Health study found “that consumption of up to 1 egg per day is unlikely to have substantial overall impact on the risk of CHD or stroke among healthy men and women”.

A 2007 study of nearly 10,000 adults demonstrated no correlation between moderate (six per week) egg consumption and cardiovascular disease or strokes, except in the small sub-population of diabetic patients who presented an increased risk of coronary artery disease. One explanation for this trend is that dietary cholesterol may be so high in the typical Western diet that adding somewhat more has little additional effect on blood cholesterol.

A 2009 study of over 21,000 individuals suggests that “egg consumption up to 6 eggs per week has no major effect on the risk of cardiovascular disease and mortality and that consumption of 7+/week is associated with a modest increased risk of total mortality” in males, whereas among males with diabetes, “any egg consumption is associated with an increased risk of all-cause mortality and there was suggestive evidence for an increased risk of myocardial infarction and stroke”. However, a 2013 meta-analysis found no association between egg consumption and heart disease or stroke. A 2013 systematic review and meta-analysis found no association between egg consumption and cardiovascular disease or cardiovascular disease mortality, but did find egg consumption more than once daily increased cardiovascular disease risk 1.69-fold in those with type 2 diabetes compared to type 2 diabetics who ate less than 1 egg per week. Another 2013 meta-analysis found that eating 4 eggs per week increased the risk of cardiovascular disease by 6%.

In one review of 17 studies with a total of 263,938 participants, no association was found between egg consumption and heart disease or stroke. Whether the eggs are actually causing the increased risk isn’t known, because these types of studies can only show statistical association. They cannot prove that eggs caused anything.

Type 2 diabetes and eggs

Studies have shown conflicting results about a possible connection between egg consumption and type two diabetes. A 2008 study by the Physicians’ Health Study I (1982–2007) and the Women’s Health Study (1992–2007) determined “high levels of egg consumption (daily) are associated with an increased risk of type 2 diabetes.” However, a study published in 2010 found no link between egg consumption and type 2 diabetes. A meta-analysis from 2013 found that eating 4 eggs per week increased the risk of diabetes by 29%. Another meta-analysis from 2013 also supported the idea that egg consumption may lead to an increased incidence of type two diabetes.

As with most foods, moderation is key.

Eggs are incredibly fulfilling and score high on a scale called the Satiety Index, which measures the ability of foods to induce feelings of fullness. In one study of 30 overweight women, eating eggs instead of bagels for breakfast increased feelings of fullness and made them automatically eat fewer calories for the next 36 hours

They are inexpensive, easy to prepare and versatile.

Sources

Agricultural Marketing Service. “How to Buy Eggs”. Home and Garden Bulletin. United States Department of Agriculture (USDA) (264): 1.

Howe, Juliette C.; Williams, Juhi R.; Holden, Joanne M. (March 2004). “USDA Database for the Choline Content of Common Foods” (PDF). United States Department of Agriculture (USDA): 10.

Marian Burros, “What the Egg Was First”, The New York Times, 7 February 2007

How to Store Fresh Eggs Mother Earth News, November/December (1977)

“Eggs – No Yolking Matter.” Nutrition Action Health Letter, July/August 1997.

U.S. Department of Agriculture, Agricultural Research Service. 2007. USDA National Nutrient Database for Standard Reference, Release 20.

Weggemans RM, Zock PL, Katan MB (2001). “Dietary cholesterol from eggs increases the ratio of total cholesterol to high-density lipoprotein cholesterol in humans: a meta-analysis”. Am. J. Clin. Nutr. 73 (5): 885–91.

Djoussé, L; Gaziano, JM; Buring, JE; Lee, IM (2009). “Egg Consumption and Risk of Type 2 Diabetes in Men and Women”. Diabetes Care. Biowizard.com. 32 (2): 295–300. doi:10.2337/dc08-1271. PMC 2628696.

“Egg consumption and risk of type 2 diabetes in older adults”. American Society for Nutrition. Retrieved 12 December 2010.

Willyard, Cassandra (30 January 2013). “Pathology: At the heart of the problem” (PDF). Nature. 493: S10–S11. doi:10.1038/493s10a. Retrieved 20 May 2013.

Qureshi AI, Suri FK, Ahmed S, Nasar A, Divani AA, Kirmani JF (2007). “Regular egg consumption does not increase the risk of stroke and cardiovascular diseases”. Med. Sci. Monit. 13 (1): CR1–8.

Nathan Gray. “No link between eggs and heart disease or stroke, says BMJ meta-analysis.” January 25, 2013.

“Egg Labels”. EggIndustry.com. Retrieved 10 January 2010.