“They couldn’t even boil an egg!” might be how you describe someone who couldn’t cook to save their life. But if cooking an egg is considered so simple then why are scientists making such a meal of working out whether eggs are good or bad for health?

It has now been over 50 years since the American Heart Foundation first suggested we should be limiting egg intake to help protect us from heart disease. In that time research has helped us further understand the links between egg consumption and heart health but confusion persists as to whether eggs are a ‘healthy’ food or not.

Much of the reason for this confusion is that the link between diet and health is not simple. Nutrition experts will often tell you there are no ‘good’ or ‘bad’ foods. The health of a diet is not determined by the intake of any one nutrient or food but rather how the balance of these matches the personal requirements of the individual.

Eggs and heart disease

The main reason behind the American Heart Foundation’s recommendation to limit egg intake is due to the link between the cholesterol found in egg yolks and the risk of heart disease. Cholesterol is a type of fat, which at high levels in the blood is a well-studied risk factor for heart disease.

However, what has been less clear is whether the amount of cholesterol in your diet is important in determining the amount of cholesterol in your blood. The current understanding is that although the amount of cholesterol in your diet does have a small impact on your blood cholesterol levels, the intake of foods that are rich in saturated or trans fat is much more important¹. Sources of saturated fat include butter, lard, palm oil and coconut oil as well as products made from these, such as biscuits, cakes and ice cream.

What is perhaps more important is considering whether eating eggs is associated with an increased risk of heart disease.

Although the research is still ongoing, the current consensus is that eggs have a neutral relationship with heart health, meaning they neither increase nor decrease the risk of heart disease.

The only exception to this is in people with type 2 Diabetes Mellitus where research has shown that consuming more than 7 eggs per week is associated with 69% more heart attacks or strokes¹. For this reason, the National Heart Foundation of Australia recommends that people with type 2 diabetes limit their egg intake to a maximum of 7 eggs per week.

Eggs and the microbiome

An emerging area of science is exploring the link between egg consumption and the microbes living within our gut. Some nutrients we eat are digested and absorbed in the small intestine while others are delivered to the colon where microbes convert them into compounds that impact human health.

An emerging risk factor for heart disease is a compound called trimethylamine-n-oxide or TMAO. Studies have shown that people with the highest blood levels of TMAO have a 62% increased risk of heart attack or stroke compared to those with the lowest levels of TMAO².

TMAO in the blood originates from the microbiome. Some microbes can convert choline and carnitine into a compound called TMA, or trimethylamine. This is then absorbed and converted in the liver to TMAO (Fig 1.).

Eggs are the richest dietary source of choline, so until recently, if someone’s microbiome contained lots of microbes with the capacity to produce TMA they were advised to limit their consumption of eggs. However, in recent years numerous research studies have confirmed that egg consumption does not lead to increased blood levels of TMAO³.

The reason for this is that eggs contain a lipid-soluble form of choline which is very well absorbed in the small intestine so does not reach the microbes in the colon and therefore does not lead to the microbial production of TMA⁴. As such, if someone has a high potential to make TMA in their microbiome we no longer recommend limiting egg intake and instead suggest they focus on the reduction of dietary carnitine, mostly found in red meat.

The link between red meat intake and heart disease is well established with the National Heart Foundation of Australia recommending that heart-healthy dietary patterns should include a maximum of 350g (cooked weight) of unprocessed red meat per week. We now understand that this recommendation is of particular importance for those with a high capacity to produce TMA in their microbiomes.

Although we no longer recommend reducing the intake of choline-rich foods (such as eggs) if someone has a high potential to make TMA in their microbiome, it may still be warranted to avoid choline supplements. Most choline supplements contain water-soluble choline which does reach the microbiome and can lead to increased blood TMAO levels⁴.

Unfortunately, this does not mean that eggs are totally out of the spotlight when it comes to personalising diet to the microbiome. Some microbes are able to convert the sulphur-containing amino acids, cysteine and methionine, into hydrogen sulphide gas⁵. Elevated levels of hydrogen sulphide can be toxic to the gut barrier leading to gut inflammation in susceptible individuals. If a person has a high potential to make hydrogen sulphide in their gut microbiome, they may wish to limit their intake of sulphur-containing amino acids which are found in high amounts in eggs, poultry and fish.

Summary

The role of eggs in a healthy diet has been debated for many decades. The reason for the confusion is that whether a person should be increasing or decreasing their egg intake depends on many individual factors including their current diet, nutritional needs, medical history and the composition of their microbiome.

The good news is that for most of us the latest research is saying that consuming eggs as part of a healthy dietary pattern is not linked to an increased risk of heart disease and does not lead to an increase in microbial TMA production.

Including eggs as part of a healthy diet can be an important source of nutrients including protein, vitamin B12, iron, and vitamin D.

Testing our microbiome can identify a high potential to produce trimethylamine (TMA) and hydrogen sulphide. Those with a high potential to produce hydrogen sulphide may benefit from avoiding excessive egg consumption.

For those with type 2 diabetes, the National Heart Foundation of Australia recommends limiting egg intake to a maximum of 7 eggs per week. Others may avoid eggs completely due to egg allergies or a vegan or lacto-vegetarian diet.

We always recommend testing the gut microbiome with a Microba Informed Practitioner who has received training on interpreting the Insight™ report and is uniquely placed to develop personalised dietary plans to meet individual health needs and goals.

About the Author

Dr Paula Smith-Brown (APD, PhD)

Paula is the Clinical Science Lead and Accredited Practising Dietitian for Insight at Microba. Paula was awarded her PhD from the University of Queensland in 2017 which explored the association between diet, gut microbiota and body composition in children and has over 10 years of clinical experience.

Follow them: