Time to lay off the egg

We may have gone too far in our headline there but we got so egg-cited about this egg-cellent topic.

This week we decided to look at a recent study that suggested that an egg a day may be an ideal food-based strategy to attenuate the fall in winter vitamin D status.
 
But before we even touch on whether eggs might provide a vitamin D boost, many may question the safety of an egg a day – what about blood cholesterol levels?

Eggs have caused a great deal of confusion as to their role in a healthy diet. And most of this confusion is because they’re rich in dietary cholesterol.

Eggs contain around 395 mg of cholesterol per 100 g. So, an average egg – weighing around 50 g – contains approximately 200 mg of cholesterol.

We have plenty of evidence on the link between blood cholesterol levels and heart disease, but misinterpretation of data from studies on the associations between dietary cholesterol and blood cholesterol triggered a backlash against high cholesterol foods, especially eggs.

To understand these dietary associations with blood cholesterol concentration, let’s take a step back in time.

The Research Studies

Work carried out in the early 1900s involved feeding rabbits high amounts of cholesterol – this induced atherosclerosis in these animal models.

But there are two things to note about this animal work.

Rabbits, who are herbivores, are hypersensitive to dietary cholesterol and the amount of cholesterol fed to these rabbits was high, and I mean high – 1000 mg to 10,000 mg per 1000 kcal.

In humans this is equal to about 10,000 mg to 15,000 mg cholesterol per day - equivalent to about 50 to 75 eggs per day

This reminds me of the iconic 1960s movie classic Cool Hand Luke – where Paul Newman’s character bets he can eat 50 boiled eggs in an hour. What follows is a rather unpleasant scene where he successfully knocks back 50 eggs just short of the one-hour deadline ­– a definite classic. Watch here.
 
While these experiments (those on rabbits, not Luke) were useful for providing important information on the link between blood cholesterol concentrations and the development of atherosclerosis, they were less helpful for exploring the link between dietary cholesterol and blood cholesterol in humans.
 
Data from epidemiological studies in the 1960s and 1970s further confused matters. These studies, which showed a correlation between dietary and blood cholesterol, didn’t account for the fact that many high cholesterol animal foods are also high in saturated fat. This collinearity of nutrients can lead to misinterpretation of associations.

In 1968, the American Heart Association (AHA) recommended we consume no more than three egg yolks per week. This simple to follow advice caught on – causing the egg industry to crack.

Since then we have seen twists and turns in guidelines - not always aligned with the best evidence - as we have refined our understanding of the effects of dietary cholesterol on blood cholesterol.

Some of the most important work to help us understand the relationship between dietary cholesterol and blood cholesterol was done by Ancel Keys – one of the most influential nutrition researchers – EVER.

For reasons I don’t fully understand, Ancel Keys’ work has been vilified by certain sectors of the 'nutrition 'community. I'm guessing lots of people enjoy a conspiracy. Check out this White Paper, which addresses some of his critic’s concerns and pulls Ancel out from under the bus. His work, which was done over 60 years ago, remains largely relevant today.
 
Keys conducted tightly controlled feeding trials in metabolic wards providing different levels and types of fat and dietary cholesterol.

From these he developed predictive equations that propose that a given amount of saturated fat will raise blood cholesterol levels about twice as much as a given amount of polyunsaturated fat will lower them. Dietary cholesterol has an impact, but it is small.

For example, in Keys' feeding studies, he showed a 100 mg increase in dietary cholesterol, increased blood cholesterol by only 0.06 mmol/L.

Keys also showed halving cholesterol intakes from 300 mg/1000 kcal reduced blood cholesterol by around 0.2 mmol/L, but you triple this effect by halving your saturated fat intake.
 
Subsequent research has corroborated these findings, further highlighting the importance of fat type, in particular the polyunsaturated:saturated (P:S) ratio.

A controlled feeding study in th early 1980s assessed the effects of different amounts of cholesterol (given as eggs) at different levels of P:S ratios ranging from 0.25 to 2.5. The diet with the lowest P:S ratio (i.e., high saturated fat, low PUFA) increased LDL-C when combined with the addition of 750 mg and 1500 mg cholesterol intake.

But adding 750 mg or 1500 mg of cholesterol to the highest P:S ratio diet did not result in significant changes in LDL-C. This suggests an additive effect of saturated fat and dietary cholesterol.
 
A meta-regression analysis published in 2019 also supports Key’s work. The researchers used various models to estimate changes in LDL cholesterol with a 100 mg of dietary cholesterol – increases ranged from 0.05 mmol/L to 0.12 mmol/L.
 
The Mechanisms

Why does dietary cholesterol only have a small effect on blood cholesterol in comparison to saturated fat?

Most of the cholesterol in our bodies is endogenously synthesised (we make our own). Our bodies make around 1 g per day, while the average intake in most diets today is less than 300 mg/d.

On top of this, cholesterol absorption is low to moderate at only around 25 to 30%. This means for most people, dietary sources only make up a small portion of cholesterol in our bodies.

We also have good mechanisms to regulate cholesterol homeostasis. An increase in dietary cholesterol will reduce endogenous synthesis.
 
Saturated fat on the other hand down-regulates LDL receptor activity. These receptors are important for clearing cholesterol from our circulation. Down regulation means less uptake of LDL-C by the cells – resulting in higher circulating blood cholesterol concentrations. When there’s too much LDL-C in our blood, these particles can form deposits in our arteries leading to the process of atherosclerosis.  
 
So, to put all this in practical terms – our worst-case scenario is diets high in saturated fat and cholesterol. This highlights the importance of looking at the diet as a whole and why we see a de-emphasis on limiting eggs.

Eggs are high in cholesterol, but most of their fat is unsaturated ­– they have a high P:S ratio. One egg has around 4 g of fat, but only 1 g is saturated fat.

We get more bang for our buck by reducing food sources in our diets that are high in both saturated fat and cholesterol.

The last Adult Nutrition survey in New Zealand showed that while eggs and egg-based dishes were the top contributors to dietary cholesterol (13%), other sources such as meat, poultry, processed meat, and dairy collectively contributed 40%. Choosing lower saturated fat versions of these foods is likely to have the largest impact on blood cholesterol.

The Dietary Guidelines

Today, there is strong evidence that consumption of eggs, even every day, is not associated with increased risk of heart disease of health risks yet the cholesterol hangover remains.

Despite the high quality evidence presented by Keys and others, the Heart Foundation of New Zealand in the 1990s agreed with the AHA earlier stance – recommending a limit of three eggs a week. But a 2016 update relaxed this advice, recommending eggs be included as part of a heart healthy eating pattern. It does caution those at risk of heart disease, including those with type 2 diabetes, to limit eggs to six per week. The latest New Zealand government dietary guidelines include no limit on eggs.
 
The US dietary guidelines have followed a similar winding path. In the 1980s Americans were advised to avoid too much cholesterol and to eat eggs in moderation (whatever moderation means?). In 1995, along with consuming eggs yolks in moderation, Americans were advised to consume 300 mg/d of cholesterol or lower.

This recommendation to limit cholesterol continued into the year 2000, along with the statement “Foods that are high in cholesterol also tend to raise blood cholesterol ” – eggs were listed in the foods high in cholesterol. In 2005, eggs were no longer highlighted.

In 2010 the recommendation to consume less than 300 mg/d cholesterol persisted, but the guidelines now suggested one egg per day was fine – acknowledging the potential negative effects of dietary cholesterol are relatively small compared to those of saturated and trans fatty acids.

In 2015-2020 the guideline of 300 mg/d or less of cholesterol disappeared – but Americans were cautioned “this change did not suggest that dietary cholesterol is no longer important to consider when building healthy eating patterns”.

In the latest 2020-25 guidelines, again they have no specific cholesterol quantity, but instead the statement “dietary cholesterol consumption to be as low as possible without compromising the nutritional adequacy of the diet ... individuals should eat as little dietary cholesterol as possible while consuming a healthy eating pattern.”

What a rollercoaster of a ride it's been. Nowhere in the history of nutrition, has one food been most maligned like the egg, and specifically singled out for restriction to reduce heart disease risk in the population.

Today, the toning down of dietary cholesterol guidelines reflect our understanding that although cholesterol does have an effect on blood cholesterol, it’s very small compared to other dietary components such as saturated fat.

Time to lay off the egg.