If there was such a thing as perfect food, eggs would be a contender. They are readily available, easy to prepare, affordable and full of protein.
"An egg is meant to be something that has all the right ingredients for growing an organism, so it's obviously a very dense nutrient," says Christopher Bleso,
Eating eggs with other foods can help our bodies to absorb and more vitamins. For example, one study found that adding an egg to a salad can increase how much vitamin E we get from a salad.
But, for decades, eating eggs has also been controversial because of their high cholesterol content – which some studies have linked to an increased risk of heart disease. An egg yolk contains about 1
Does this mean that eggs, not the perfect food, can actually harm us?
You may also like:
• Can chicken soup cure the common cold?
• Do you eat enough salt?
• How much water should you drink per day?
Cholesterol, a yellowish fat produced in our liver and intestines, can be found in any of the cells of our body. We usually think of it as "bad". But cholesterol is a crucial building block in our cell membranes. It is also necessary for the body to produce vitamin D and the hormones testosterone and estrogen.
We ourselves produce all the cholesterol we need, but it is also found in the animal products we consume, including beef, shrimp and eggs, as well as cheese and butter.
Cholesterol is transported around our body via lipoprotein molecules in the blood. Everyone has a different combination of different types of lipoproteins and our individual makeup plays a role in determining the risk of developing a heart disease.
Low-density lipoprotein (LDL) cholesterol – called "bad" cholesterol – is transported from the liver to the arteries and tissues of the body. Researchers say it can lead to cholesterol buildup in blood vessels and increase the risk of cardiovascular disease.
But researchers have not definitively linked cholesterol consumption to an increased risk of cardiovascular disease. As a result, dietary guidelines in the United States no longer have cholesterol restriction; nor the United Kingdom. Instead, the emphasis is on limiting how much saturated fat we consume, which can increase the risk of developing cardiovascular disease. Foods containing trans fats increase our LDL levels. Although some trans fats occur naturally in animal products, most are artificially produced and are found in the highest levels in margarines, snacks and some foods fried and baked, such as pasta, donuts and cake.
Meanwhile, together with shrimp, eggs are the only high-cholesterol diet that is low in saturated fat.
"While cholesterol in eggs is much higher than in meat and other animal products, saturated fat raises cholesterol in the blood. This has been proven by many studies over the years, "says Maria Luz Fernandez, a professor of nutritional science at the University of Connecticut in the United States whose recent studies find no link between eating eggs and an increased risk of cardiovascular disease.  The discussion about the health effects of eggs has shifted in part because our bodies can compensate for the cholesterol we consume.
"There are systems in place that dietary cholesterol is not a problem for most people," says Elizabeth Johnson, Doc. Food Science Research at Tufts University in Boston, USA.
In a 2015 review of 40 studies, Johnson and a team of researchers found no clear evidence of a link between dietary cholesterol and heart disease.
"People have good regulation when they consume dietary cholesterol and make less cholesterol themselves," she says.
And when it comes to eggs, cholesterol can be an even lower health risk. Cholesterol is more harmful when it is oxidized in our arteries, but oxidation does not happen with cholesterol in eggs, Bleso says.
"When cholesterol is oxidized, it can be more inflammatory, and there are all kinds of antioxidants in eggs that
Also, some cholesterol may actually be helpful to us. High density lipoprotein (HDL) cholesterol travels to the liver where it is broken down and removed from the body. HDL is thought to have a protective effect against cardiovascular disease by preventing blood cholesterol from accumulating.
"People need to be concerned about the cholesterol that circulates in their blood, which is the one that causes heart disease," Fernandez says.
The important is the ratio of HDL to LDL in our bodies, since elevated HDL counteracts the effects of LDL.
However, while most of us are able to buffer the cholesterol we consume with the cholesterol we synthesize in our liver, Bleso says that about a third of us will experience a 10% to 15 increase in blood cholesterol % after consuming it.
Studies have found that lean and healthy people are more likely to observe an increase in LDL after egg consumption. Those who are overweight, obese or diabetic will see less increase in LDL and more HDL molecules, Bleso says. So if you are healthier to begin with, eggs could potentially have a more negative effect than if you are overweight – but if you are healthier, you are also more likely to have good HDL levels, so that increasing LDL is probably not very harmful.
Research published earlier this year, however, has challenged the recent consensus that eggs are not a detriment to our health. The researchers looked at data from 30,000 adults who followed an average of 17 years and found that each additional half egg a day was significantly associated with a higher risk of heart disease and death. (They controlled the subjects' diet, overall health and physical activity to try and isolate the effects of the eggs.)
"We found that for each additional 300 mg of cholesterol consumed, there is a 17% increased risk of cardiovascular disease and an 18% increased risk of all-cause mortality, "says Nora Allen, one of the authors of the study and an associate professor of preventive medicine at Northwestern University in Illinois, USA.
found that every half egg a day resulted in 6% pov ginseng risk of heart disease and 8% increased risk of mortality. "
Although the study is one of the largest of its kind to address this specific link between egg and heart disease, it was observational, giving no cause and effect. She also relied on one set of self-report data – participants were asked what they were eating in the previous month or year, followed by their health outcomes for up to 31 years. This means that the researchers received only a brief snapshot of what the participants ate, even though our diets may change over time.
And the study contradicts previous results. Numerous studies have shown that eggs are good for heart health. A previous analysis of half a million adults in China, published in 2018, even found the opposite: egg consumption is associated with a lower risk of heart disease. Those who ate eggs daily had an 18% lower risk of death from heart disease and a 28% lower risk of stroke death than those who did not eat eggs.
Like the previous study, it was also observational – i.e. impossible to find cause and effect. (Healthier adults in China just eat more eggs, or do eggs make them healthier?). This, of course, can be a big part of the confusion.
The Good Egg
While these studies have resumed the debate about the effect of cholesterol in eggs on our health, we know some ways in which eggs can influence our risk of disease.
One way is through a compound in the egg called choline, which can help protect us against Alzheimer's disease. It also protects the liver.
But this can also have negative effects. Choline is metabolized by the gut microbiota into a molecule called TMO, which is then absorbed into the liver of humans and converted to TMAO, a molecule associated with an increased risk of cardiovascular disease. Blasso wondered if eating lots of choline from eggs could lead to an increase in TMAO: he found studies in which people were observed to have elevated TMAO levels up to 12 hours after eating eggs.
Studies to measure the consumption of eggs and TMAO have so far only found a transient increase in TMAO. However, TMAO is measured as a marker of heart disease only at baseline, which can be detected when people are starving. Blasso likens this to how our blood sugar levels rise temporarily after consuming carbohydrates, but elevated blood sugar levels are only associated with diabetes when those levels are continuous.
That may be because when we eat eggs, we can only get the beneficial effects of choline, he says.
"The problem is that instead of being absorbed in the blood, choline goes on to the large intestine, where it can become TMA and then TMAO," says Fernandez.
"But in eggs, choline is absorbed and does not go into the colon, so it does not increase the risk of heart disease. ”
Meanwhile, scientists are beginning to understand the other health benefits of eggs. Egg yolks are one of the best sources of lutein, a pigment that is associated with, for example, better eyesight and less risk of eye diseases.
"There are two types of lutein found in the retina of the eye where you can protect the retina from damaging the light by working as a filter for blue light because exposure to light deteriorates the eyes," Johnson says.
While researchers are far away From understanding why eggs influence us differently, the vast majority of recent studies suggest that they do not pose a risk to our health and are much more likely to provide health benefits.
However, eating eggs for breakfast every day probably also n is the healthiest option – at least as it is recommended to have a varied diet … Instead of putting all your eggs in one basket.
Join one million future fans by liking us on Facebook ] or follow us on Twitter or Instagram .
If you liked this story on yo u, subscribe to bbc's weekly site. com for newsletter called "The Essential List". Selected selection of BBC Future, Culture, Capital and Travel stories delivered to your inbox every Friday.