Health effects of eggs: Where do we stand?

Are eggs good for you or not? The 2015 to 2020 Dietary Guidelines for Americans recommends the nutrient-dense food as a source of protein, but an article in The Journal of the American Medical Association (JAMA) this month made a stir when it reported an association between eating eggs and an increased risk of heart disease and early death.

Eating eggs in moderation may be beneficial to heart health, but recent research says excessive egg consumption is associated with increased risk of heart disease. The risk identified in the JAMA research was linked to eating an additional three to four eggs per week, or 300 milligrams of dietary cholesterol a day. Previous studies show decreased and no heart disease risk in those who ate up to one egg a day.

Though eggs provide protein, minerals, vitamins and other nutrients, the yolk is also a major source of cholesterol. According to the US Department of Agriculture, the yolk of one large raw egg contains 184 milligrams of cholesterol.

High cholesterol levels are a risk factor for cardiovascular events like a heart attack or stroke, according to the National Heart, Lung, and Blood Institute. However, this condition depends on many factors, like the levels of good versus bad cholesterol, genetics, lifestyle and diet.

If you've been confused by whether egg consumption is good for your health, you're not alone. Let's look at the historical journey of the egg and see how the research has shifted over the years.

5400 BC: Behold! A new, easy-to-hunt food source

The earliest fossils resembling chickens date to 5400 BC in Southeast Asia. Confirmation with genetic material shows that today's chickens descend from several prehistoric birds, one of those being the red junglefowl. Today's domesticated chicken has a gene that controls reproduction and allows them to lay hundreds of eggs. With domestication, chickens were introduced to the global market through trade routes an estimated 3,000 years ago.

Early 1900s: Egg farming becomes safer

Families used chickens and their eggs both as a source of income and for their own use. In the early 1920s, conditions such as seasonality and poor storage contributed to deteriorating conditions for chickens. As the animals were moved indoors in the 1930s, they were protected against environmental factors -- weather, larger animals, disease -- and their health improved.

1950-60s: A chicken in every pot

As production increased and more hens survived, farmers noticed that their egg production was increasing. The industry of chickens became more affordable, as opposed to their previous role as a luxury food.

1968: American Heart Association makes strict recommendation

The group's early recommendations included no more than 300 milligrams of cholesterol per day and no more than three eggs per week. This was based on debatable animal and clinical studies. These animals are often herbivores, less adapted to digest dietary cholesterol than omnivores, like humans. The clinical studies did not account for other sources of cholesterol found in a typical diet. With more research, guidelines were modified.

1970: Now hiring egg inspectors

As hens became more productive, food scientists and egg producers realised the need for new policy. Congress passed the Egg Products Inspection Act, which made sure eggs were safe for consumers.

1976: Boxer drinks raw eggs

Rocky, Sylvester Stallone's boxer, famously drank raw eggs in the hit film. But research has since shown that there is more protein available for digestion in cooked eggs than raw eggs. Consuming raw eggs increases the risk of contracting the bacteria salmonella and developing a deficiency of biotin, a vitamin important for skin, hair and nails. Eggs contain avidin, a protein partially destroyed when cooked. In raw eggs, avidin more readily binds and reduces biotin.

1984: Time magazine's cover features a disheartened breakfast

Time published a cover depicting the face of cholesterol: a plate with two fried eggs for eyes and a bacon frown. The same year, the Egg Nutrition Center was created with a goal of clarifying skepticism around cholesterol.

1995: Attempt to resolve confusion

To create standard dietary recommendations in the US, organisations like the American Heart Association, the National Centers for Environmental Prediction and the FDA set a unified goal for Americans: less than 300 milligrams per day from dietary cholesterol. One large boiled egg contains 186 milligrams of dietary cholesterol.

2002: American Heart Association loosens up

The organisation gave up its restriction on eating a certain number of eggs per week but kept the guideline of less than 300 milligrams of dietary cholesterol per day. While the US continued its internal battle of what to do about eggs, other countries began removing their national dietary guideline restrictions on eggs.

2013: No association found between egg consumption and cardiovascular disease

A large meta-analysis concluded that consuming one egg per day is not associated with increased heart disease risk. In a literature search from 1966 to 2012, researchers looked at patients followed for coronary heart disease and a history of stroke. They found no significant association between egg consumption and heart disease.

2016: Oldest person credits raw eggs

Italy's Emma Morano earned the title of oldest living person on her 117th birthday. Morano came from a lineage of long-living women: a mother who made it to 91 and sisters who lived to see a century. Although genes were most likely a factor, Morano partially attributed her longevity to a life of eating raw eggs. Her physician said she had great cholesterol levels.

2018: Egg a day keeps doctor away

A study of more than 400,000 Chinese adults found an association between daily egg consumption and an 18% decrease in death risk related to cardiovascular disease. The authors said the country has its own dietary and lifestyle characteristics, so there should be caution in generalisation. Heart disease is a leading cause of death in China and throughout the world, according to the World Health Organization.

The evolution, domestication and research of chickens and eggs led to our dinner tables. The most recent research states that excessive egg consumption is associated with increased risk of cardiovascular disease, but don't forget that the types of cholesterol, genetics and lifestyle factors play a role, too. While you keep that in mind, we'll be here awaiting the next study or guideline change.