February 9, 2021

expert reaction to observational study on eggs, cholesterol and death

A study published in PLOS Medicine looks at egg and cholesterol consumption, and mortality from cardiovascular and different causes in the U.S.

Dr Ada Garcia, Senior Lecturer in Public Health Nutrition at the University of Glasgow, said:

“In this large study, half a million retired Americans were asked to complete a questionnaire to recall how many and how often they ate food items from a list of 124 foods. The questionnaire asked to recall what they ate over the past year back in 1995. The food list included egg and egg substitutes and also how the eggs were cooked. The participants were followed for about 16 years and it was recorded if they had developed chronic disease like cardiovascular disease or diabetes or if they died.

“This research makes it possible to look at relationships between what people ate in the past and whether that could be related to disease or death in the future. The authors took a very careful approach to analysed the data and consider many aspects that could distort the findings.

The study concluded that retired people who ate more whole eggs 7% higher risk to develop cardiovascular disease. The highest consumption of eggs was about half a small egg in  2000 Kcal per day.  

“The most important drawback of this study is the way egg consumption is collected. What and how much food people ate was asked just once and people were asked to recall what they ate in  the past, this is problematic because people tend to forget and also their diets might change a lot, in particular if they become ill with time.

“Studies that observe diet and lifestyle behaviours are useful to understand whether what we eat is linked to becoming ill or dying, but they are not conclusive. We eat a combination of foods and one specific part of the diet – in this case whole eggs – cannot be fully blamed.  The key thing to remember is to keep a balance in what we eat and what we do.  Eating a variety of healthful foods, avoiding overeating or indulging on foods that are too high in sugar and energy, keeping an eye on how much salt we eat. 

“Being overweight or obese, smoking or consuming too much alcohol and not being physically active are much more important factors to consider for prevention of chronic diseases and related death.

“The conclusions of this study are overblown.  Blaming eggs alone for an increased risk of cardiovascular disease is a simplistic and reductionist approach to the concept of diet and disease prevention.”

Professor Bruce Griffin, Professor of Nutritional Metabolism at the University of Surrey, said:

“The strength of evidence to link eggs and dietary cholesterol with cardiovascular disease and cancer in this study, and many others like it, have an absolute dependence on statistical adjustments for a wide spectrum of risk factors that will cause death from these diseases in egg-eaters. The findings of this study are inconsistent with what we understand about the relatively minor effects of eggs and dietary cholesterol on blood cholesterol and cardiovascular disease, in comparison to the myriad of other risk factors for this disease.  The findings of this study are also at odds with the totality of evidence to show that a high intake of eggs and dietary cholesterol are markers of an unhealthy diet and lifestyle, and not directly linked to disease mortality.  

“The paper produces clear evidence to show that high egg consumers were more likely to be obese or overweight smokers, have higher blood cholesterol and an unhealthy diet (higher intake of total fat, saturated fat, sugar sweetened beverages, lower intake of fruit and vegetables and fibre). They also had a lower income and were less well educated.  “There is incontrovertible evidence that these characteristics make a causative and cumulative contribution to death from all causes, cardiovascular disease and cancer. While statistical modelling can be effective in adjusting for continuous and categorical variables, such as raised blood cholesterol and smoking status, respectively, it is much less effective in adjusting for self-reported diet and lifestyle variables, which have been measured with much less accuracy and precision.”

Prof Riyaz Patel, Consultant Cardiologist at UCL, said:

“Zhuang and colleagues examined food survey data from over half a million people from the US and report that higher egg consumption is associated with greater risk of dying from all causes including cardiovascular disease and cancer.  They also conclude that this risk is driven mostly by the high cholesterol content of eggs and advocate reducing egg intake or replacing with egg whites or substitutes for improving cardiovascular health and longevity.

“Despite many years of research this question about eggs and health has not been answered, with multiple observational studies over the last few decades showing conflicting results – some suggesting moderate egg intake is good, while others suggesting it may be bad. This study, although well conducted, unfortunately only adds more noise to the discussion.

“The authors themselves acknowledge the biggest limitation of this study is that it is observational in nature showing associations only, which does not indicate eggs are the cause of the worse outcomes observed.

“For example, the study shows that people who ate more eggs were more likely to smoke, were more overweight, less active and less affluent, among other differences.  Although the authors tried to account for these differences in their analysis, it is more than likely that other differences, that were not captured in the standard surveys, contributed to the associations. For example, those eating fewer eggs may have eaten better nutritionally complete meals, perhaps under-reported dairy intake, took their pills more regularly, went more regularly for medical checks with their doctors and looked after themselves better. 

“Importantly, it is generally very difficult to assess the impact of any one type of food as we rarely eat foods in isolation, or eat them every day and capturing dietary patterns with single questionnaires is susceptible to many recall biases. We also vary enormously in how we cook food items (fried, boiled, grilled) or the way we combine them with other foods.  For example, a regular pattern of eating two eggs as part of a large fry up will likely have a different impact due to higher saturated fat content, compared to eating two boiled eggs on their own.

“The suggestion that the risk from eggs is mostly explained by the cholesterol content of eggs is also problematic as we know that dietary cholesterol does not have a clear link to levels of circulating cholesterol in the blood.  It is a highly variable process which means one individual may eat 6 eggs and not shift their blood cholesterol levels while another may eat just one and see an increase. 

“Overall, this is a well conducted and large association study, but in my view the recommendation made by the authors to replace whole eggs with egg whites/substitutes is not supported by the entirety of evidence available.  I do not think this study changes the general advice, that for most people, eggs can be eaten in moderation as part of a balanced diet, unless they have been advised not to for a specific medical or dietary reason.”

Victoria Taylor, Senior Dietitian at the British Heart Foundation, said:

“The role of eggs in relation to heart and circulatory diseases has been much debated. This study found that people who ate more whole eggs were at greater risk of dying from cardiovascular disease; it has limitations however, and only shows an association rather than proving cause and effect. There are also other factors that could have influenced the results, which specifically looks at a US diet, rather than UK-based eating habits. More research would be needed to confirm these findings.

“Reports suggesting that certain foods are bad for us can be worrying, but when it comes to eating for our heart and circulatory health it’s our overall diet that is most important. Keeping meals varied and balanced and rich in foods we know we should be eating more of – like fruits, vegetables, pulses, wholegrains, fish, nuts and seeds – is a healthy approach.”

Dr Duane Mellor, Senior Teaching Fellow in the Aston Medical School, Aston University, said:

“Although this study reported the association between egg intake and cholesterol intake and death from heart disease, cancer and all causes in over half a million people over a period of about 15 years, it is important that this observation should be treated with caution.

“’How many eggs’ or ‘how much cholesterol’ was based on a questionnaire, which asked people how often over the past year they ate eggs and other foods and which people only completed once. So it did not look at how peoples food intake changed over the following decade.

“It did not look at how the eggs were eaten, it just tried to estimate how many eggs were eaten – so it did not consider if the eggs were eaten as part of a fried breakfast or in a Niçoise salad. The effect of the foods eaten with the eggs was not fully considered, and it is clear that eggs can be eaten both as part of a high fat, high sugar diet as they can be eaten as part of a vegetable rich Mediterranean type diet.

“The authors tried to hypothesise the effect of swapping whole eggs for egg whites, egg substitutes, poultry, fish, dairy, nuts or legume; however the idea of replacing eggs in the diet this way was not tested in the study, and instead they compared people who reported eating eggs with those who did not.

“Although this association might have been found, the people in this study did not know they were making any substitutions, they simply reported what they ate and were followed up many years later. This approach is not uncommon in this type of nutritional epidemiology and it is important that these points are considered carefully to appreciate that the results may show an association, but ultimately may not show causality and further more might have further challenge based on the number of assumptions the research makes to arrive at its conclusion.

“Current advice in the UK and more recently in the USA considers cholesterol to be no longer a nutrient of interest – this is based on the analysis of a number of studies across a range of populations. As with a good meal, it is better to look at its overall balance, and not just focus on one ingredient, food or nutrient; it’s the overall pattern of a diet which influences its true and overall effect on our health.”

‘Egg and cholesterol consumption and mortality from cardiovascular and different causes in the United States: A population-based cohort study’ by P Zhuang et al. was published in PLOS Medicine at 7pm UK TIME on Tuesday 9 February 2021.

https://doi.org/10.1371/journal.pmed.1003508

Declared interests

Prof Patel: “No conflicts to declare.”

Dr Mellor: “I have no conflicts of interest to declare.”

Dr Garcia: “No conflict of interest.”

None others received.