Are eggs helpful to our health… or a cause of heart disease?

* This story is featured in BBC Future’s “Best of 2019” collection. Discover more of our picks.

If there was such a thing as a perfect food, eggs would be a contender. They’re readily available, easy to cook, affordable and packed with protein.

“The egg is meant to be something that has all the right ingredients to grow an organism, so obviously it’s very nutrient dense,” says Christopher Blesso, associate professor of nutritional science at the University of Connecticut in the US.

Eating eggs alongside other food can help our bodies absorb more vitamins, too. For example, one study found that adding an egg to salad can increase how much vitamin E we get from the salad.

But for decades, eating eggs has also been controversial due to their high cholesterol content – which some studies have linked to an increased risk of heart disease. One egg yolk contains around 185 milligrams of cholesterol, which is more than half of the 300mg daily amount of cholesterol that the US dietary guidelines recommended until recently.  

Does that mean eggs, rather than being an ideal food, might actually be doing us harm?

You might also like:
• Can chicken soup cure the common cold?
• Are you eating enough salt?
• How much water should you drink a day?

Cholesterol, a yellowish fat produced in our liver and intestines, can be found in every one of our body’s cells. We normally think of it as “bad”. But cholesterol is a crucial building block in our cell membranes. It also is needed for the body to make vitamin D, and the hormones testosterone and oestrogen. 

We produce all the cholesterol we need on our own, but it’s also found in animal produce we consume, including beef, prawns and eggs, as well as cheese and butter.

Cholesterol is transported around our body by lipoprotein molecules in the blood. Every person has a different combination of various types of lipoproteins, and our individual make-up plays a role in determining our risk of developing heart disease.

Low-density lipoprotein (LDL) cholesterol – referred to as “bad” cholesterol – is transported from the liver to arteries and body tissues. Researchers say that this can result in a build-up of cholesterol in the blood vessels and increase the risk of cardiovascular disease.

But researchers haven’t definitively linked consumption of cholesterol to an increased risk of cardiovascular disease. As a result, US dietary guidelines no longer have a cholesterol restriction; nor does the UK. Instead, emphasis is placed on limiting how much saturated fat we consume, which can increase the risk of developing cardiovascular disease. Foods containing trans fats, in particular, increase our LDL levels. Although some trans fats occur naturally in animal products, most are made artificially and are found in highest levels in margarines, snacks, and some deep-fried and baked foods, such as pastry, doughnuts and cake. (Read more about whether diets encouraging people to eat more saturated fat are good for you.)

Meanwhile, along with prawns, eggs are the only food high in cholesterol that are low in saturated fat. 

“While the cholesterol in eggs is much higher than in meat and other animal products, saturated fat increases blood cholesterol. This has been demonstrated by lots of studies for many years,” says Maria Luz Fernandez, professor of nutritional sciences at the University of Connecticut in the US, whose latest research found no relationship between eating eggs and an increased risk of cardiovascular disease.

The discussion on the health effects of eggs has shifted partly because our bodies can compensate for the cholesterol we consume.

“There are systems in place so that, for most people, dietary cholesterol isn’t a problem,” says Elizabeth Johnson, research associate professor of nutritional sciences at Tufts University in Boston, US.

In a 2015 review of 40 studies, Johnson and a team of researchers couldn’t find any conclusive evidence on the relationship between dietary cholesterol and heart disease.

“Humans have good regulation when consuming dietary cholesterol, and will make less cholesterol themselves,” she says.

And when it comes to eggs, cholesterol may pose even less of a health risk. Cholesterol is more harmful when oxidised in our arteries, but oxidisation doesn’t happen to the cholesterol in eggs, says Blesso.

“When cholesterol is oxidised, it may be more inflammatory, and there are all kinds of antioxidants in eggs that protect it from being oxidised,” he says.

Also, some cholesterol may actually be good for us. High-density lipoprotein (HDL) cholesterol travels to the liver, where it’s broken down and removed from the body. HDL is thought to have a protective effect against cardiovascular disease by preventing cholesterol from building up in the blood.

“People should be concerned about cholesterol that circulates in their blood, which is the one that leads to heart disease,” says Fernandez.

What matters is the ratio of HDL to LDL in our bodies, as elevated HDL counteracts the effects of LDL.

However, while most of us are able to buffer the cholesterol we consume with the cholesterol we synthesise in our livers, Blesso says around a third of us will experience an increase in blood cholesterol by 10% to 15% after consuming it.

Trials have found that lean and healthy people are more likely to see an increase in LDL after eating eggs. Those who are overweight, obese or diabetic will see a smaller increase in LDL and more HDL molecules, Blesso says. So, if you’re healthier to begin with, eggs potentially could have a more negative effect than if you’re overweight – but if you’re healthier, you’re also more likely to have good HDL levels, so an increase in LDL probably isn’t very harmful.

Research published earlier this year, though, challenged the recent consensus that eggs pose no harm to our health. Researchers looked at data from 30,000 adults followed for an average of 17 years and found that each additional half an egg per day was significantly linked to a higher risk of heart disease and death. (They controlled for the subjects’ diet patterns, overall health and physical activity to try to isolate the effects of eggs.)

“We found that, for every additional 300mg cholesterol person consumed, regardless of the food it came from, they had a 17% increased risk of cardiovascular disease, and 18% increased risk of all-cause mortality,” says Norrina Allen, one of the study’s authors and associate professor of preventive medicine at Northwestern University in Illinois, US.

You might also like:

• How meal timings affect your waisteline
• Is breakfast the most important meal of the day?
• Why humans have evolved to drink milk

“We also found that each half egg per day led to a 6% increased risk of heart disease and 8% increased risk of mortality.”

Despite the study being one of the largest of its kind to address this specific relationship between eggs and heart disease, it was observational, giving no indication of cause and effect. It also relied upon a single set of self-reported data – participants were asked what they ate over the previous month or year, then followed up their health outcomes for up to 31 years. This means the researchers only got a single snapshot of what the participants were eating, even though our diets can change over time.

And the study conflicts with past results. Numerous studies suggest eggs are good for heart health. One previous analysis of half a million adults in China, published in 2018, even found the exact opposite: egg consumption was associated with lower risk of heart disease. Those who ate eggs every day had an 18% lower risk of death from heart disease and 28% lower risk of stroke death compared to those who didn’t eat eggs.

Like the previous study, it too was observational – meaning it’s impossible to tease out cause and effect. (Do healthier adults in China simply eat more eggs, or do the eggs make them healthier?). That, of course, may be a big part of the confusion.

Good egg

While these studies have reignited the debate on the impact of cholesterol in eggs on our health, we do know some ways in which eggs could affect our risk of disease.

One way is through a compound in eggs called choline, which may help protect us against Alzheimer’s disease. It also protects the liver. (Find out if eggs are a good way of stopping a hangover.)

But it may have negative effects, too. Choline is metabolised by gut microbiota into a molecule called TMO, which is then absorbed into people’s livers and converted to TMAO, a molecule associated with an increased risk of cardiovascular disease. Blasso has wondered if eating a lot of choline from eggs could lead to elevations of TMAO: he found studies where people were observed to have elevated TMAO levels up to 12 hours after eating eggs.

Research measuring egg consumption and TMAO has so far only found transient increases in TMAO. However, TMAO is measured as a marker for heart disease only at a baseline level, which can be detected when people are fasting. Blasso likens this to how our blood sugar levels increase temporarily after eating carbohydrates, but elevated blood sugar levels are only associated with diabetes when these levels are continuous.

This may be because when we eat eggs, we might only get choline’s beneficial effects, he says.

“The problem is when, instead of being absorbed into the blood, choline continues to the large intestine, where it can become TMA and then TMAO,” says Fernandez.

“But in eggs, choline is absorbed and doesn’t go to the large intestine, so it doesn’t increase the risk of heart disease.”

Meanwhile, scientists are beginning to understand other health benefits of eggs. Egg yolks are one of the best sources of lutein, a pigment that has been linked to better eyesight and lower risk of eye disease, for example.

“There are two types of lutein found the retina of the eye, where it can protect the retina from light damage by working as a blue light filter, as exposure to light makes the eye deteriorate,” says Johnson.

While researchers are a long way from understanding why eggs affect us differently, the vast majority of recent research suggests they pose no risk to our health, and are much more likely to provide health benefits.

Even so, having eggs for breakfast every day probably isn’t healthiest option, either – at least as it’s recommended we have a varied diet… rather than put all our eggs in one basket.