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expert reaction to study looking at eggs, cholesterol and heart disease

Reactions to a study published in JAMA which demonstrates that higher consumption of dietary cholesterol or eggs was significantly associated with higher risk of cardiovascular disease.

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

“There has been much debate about the role of eggs in relation to heart and circulatory disease.  This study suggests that people who eat more eggs are at a greater risk of heart disease because of the cholesterol that’s in them.  But this type of study can only show an association, rather than cause and effect, and more research is needed for us to understand the reasons behind this link.

“Eating healthily is all about balance.  If you’re eating too much of one thing it leaves less room in the diet for other foods that may have more health benefit.

“Eggs are a nutritious food and, while this study focuses on the amount we’re eating, it’s just as important to pay attention to how the eggs are cooked and to the trimmings that come with them.  For example, poached eggs on wholegrain toast is a much healthier meal than a traditional fry up.”

Prof Kevin McConway, Emeritus Professor of Applied Statistics, The Open University, said:

“This is a good, careful, study that uses appropriate statistical methods.  The results are based on a large number of people.  But it still can’t come close to answering all the questions about links between eggs, cholesterol, and death rates.

“A fundamental issue with any study like this is that it can’t determine what causes what, only give hints and suggestions.  That’s because, for instance, there will be many other differences between people that eat many eggs and people that eat few other than their egg consumption.  These other differences might be what’s causing higher death rates in people who eat a lot of eggs, rather than anything to do with the eggs themselves.  The researchers point out that this has been a particular problem in some previous studies, and that this may have been a reason for inconsistency in the results of those studies.  They have made considerable efforts to allow statistically for other differences in the new study.  But they, correctly, point out that their own study is still not immune from this problem (known as residual confounding), and that therefore it’s impossible to conclude from this new study that eating eggs, or consuming more cholesterol in the diet, is the cause of the differences in cardiovascular disease rates and overall death rates that they observed.

“It’s also important to point out that the associations that they discovered are generally not very strong – the researchers themselves describe them as “modest”.  As one example, they report that each additional half an egg eaten (on average) per day increases the risk of death from any cause by just under two percentage points.  This needs unpicking a bit.  First, that risk increase is in the risk of death over the maximum length of follow-up, which was about 30 years (31.3 to be exact).  Imagine 100 people like those in the cohorts they studied.  They weren’t all followed up for 30 years, but a very rough ballpark figure, calculated from numbers given in the research paper, indicates that somewhere around 38 of them would have died if they had been followed up for 30 years.  (Indeed that is likely to be an underestimate – 30 years is a long time and in some of the cohorts they studied, the people involved were not young to begin with.)  If all these people ate an extra half an egg a day, then the number who would die in 30 years would go up from about 38 to around 40.  Yes, an increase, but not a large one, and we can’t be sure that it’s the eggs, or the cholesterol, causing it.  Also, though an extra half an egg a day perhaps doesn’t sound like much, the current average egg consumption in the US is about half an egg per person per day, so if everyone ate half an egg a day more on average, that would double egg consumption in the USA.

“A couple more reasons for caution – both of them stated clearly by the researchers in their report.  First, consumption of eggs and of cholesterol was measured in different ways in different ones of the cohorts from which the researchers obtained their data, and, importantly, it was measured only once.  People might have changed their diets in all kinds of ways over the period of follow-up, which could have been as much as 30 years and averaged 17.5 years.  (You have to bear in mind that US dietary guidelines on eggs and on cholesterol-containing foods did change over that period, and perhaps this changed what some participants ate.)  Then the results are informative, in certain ways, about the USA, but things are different in different countries because of different patterns of nutrition and of disease, so they can’t say much directly about the position in the UK.

“None of these issues are intended to say that this study isn’t good – it is very good, in statistical terms at least, in my view.  But they reinforce that this is a difficult area to research, and no single study is going to settle things.  The way to proceed is to look systematically across many different studies in different contexts (countries, timescales, and so on).  Doubtless that will eventually be done, but obviously a review of that kind that includes this new study hasn’t yet been done.”

Prof Tom Sanders, Professor emeritus of Nutrition and Dietetics, King’s College London, said:

“Dietary cholesterol has a small effect on blood cholesterol concentrations.  Cholesterol in the diet is provided mainly by eggs and meat.  A large British egg weighs 68 g and supplies 265 mg of cholesterol.  The average intake of eggs in the UK is about 3 eggs a week and average cholesterol intakes are the range 200-250 mg per day.  Feeding trials (previous studies) show that an intake of 300 mg of cholesterol from egg increases low density lipoprotein (LDL) cholesterol by 0.15 mmol/L (about a 4% increase) which would be predicted to increase the relative risk of cardiovascular disease (CVD) by 3%.  For a non-smoking 55 year-old man with a typical risk of CVD (10% over the next 10 years) this would result in an additional 3 cases of CVD for every 1000 people (i.e. a 0.3 increase in absolute risk).

“This new report is concerned with egg and cholesterol intake and risk of cardiovascular disease (CVD).  It pools data from six community-based cohort studies in the USA with a total of 29,615 participants.  Its strength is that it was more representative of the ethnic diversity of the USA population and the diets consumed by ordinary Americans.  Its limitations are the dependence on a single measure of dietary intake as baseline when the average follow-up period was 17 years, and the strong correlation of egg intake with unhealthy aspects of lifestyle, including smoking, obesity, high red and processed meat intake and low intake of fruit and vegetable intake.  It is impossible to rule out confounding factors when habits are strongly correlated with each other.

“The new report concludes that an additional 300 mg per day of dietary cholesterol is associated a 3.24% greater absolute of risk CVD over the follow-up period (i.e. 32 addition case of CVD per 1000 participants).  This is very much greater than would be predicted from the effect of dietary cholesterol on LDL cholesterol.  These finding are in contrast to the null effects reported in the Nurses’ Health and Health Professionals’ Studies (117,953 participants), also conducted in the USA, which were less is likely to be confounded by unhealthy lifestyles given the occupations of the participants1.  Furthermore, a previous meta-analysis on over 3 million adults published in the BMJ2 also found a null effect of egg consumption on CVD risk.  More recently, a study3 of 0.5 million people in China found that egg consumption was associated with a lower risk of CVD!

“Prospective studies such as these can only show associations and do not prove causation.  However, the take home message supported by the accompanying editorial would support the view that a typical USA diet which contains lots of meat and plenty of eggs is associated with poor cardiovascular health and that the USA dietary guidelines should reinstate its recommendation that cholesterol intake should not exceed 300 mg per day.

“However, importantly, I think this finding is only relevant to the USA diet, and not to the UK diet.  The average US intake of cholesterol can be up to around 600 mg per day – this is much higher than the average UK diet, which is around 225 mg per day.  Eggs in moderation – around 3 to 4 per week – is fine, and that is what current UK dietary guidelines say.”

1 Hu FB, Stampfer MJ, Rimm EB, et al. A Prospective Study of Egg Consumption and Risk of Cardiovascular Disease in Men and Women. JAMA. 1999;281(15):1387–1394. doi:10.1001/jama.281.15.1387

2 Rong Y, Chen L, Zhu T, Song Y, Yu M, Shan Z, Sands A, Hu FB, Liu L. Egg consumption and risk of coronary heart disease and stroke: dose-response meta-analysis of prospective cohort studies. BMJ. 2013 Jan 7;346:e8539. doi:10.1136/bmj.e8539

3 Qin C, Lv J, Guo Y, Bian Z, Si J, Yang L, Chen Y, Zhou Y, Zhang H, Liu J, Chen J, Chen Z, Yu C, Li L; China Kadoorie Biobank Collaborative Group. Associations of egg consumption with cardiovascular disease in a cohort study of 0.5 million Chinese adults. Heart. 2018 Nov;104(21):1756-1763.doi: 10.1136/heartjnl-2017-312651

‘Associations of dietary cholesterol or egg consumption with incident cardiovascular disease and mortality’ by VictorW. Zhong et al. was published in JAMA at 15:00 UK time on Friday 15 March 2019.

Declared interests

Prof Kevin McConway: “Kevin McConway is a Trustee of the Science Media Centre.”

Prof Tom Sanders: “Honorary Nutritional Director of HEART UK.  Scientific Governor of the British Nutrition Foundation.  He is now emeritus but when he was doing research at King’s College London, the following applied: Tom does not hold any grants or have any consultancies with companies involved in the production or marketing of sugar-sweetened drinks.  In reference to previous funding to Tom’s institution: £4.5 million was donated to King’s College London by Tate & Lyle in 2006; this funding finished in 2011.  This money was given to the College and was in recognition of the discovery of the artificial sweetener sucralose by Prof Hough at the Queen Elizabeth College (QEC), which merged with King’s College London.  The Tate & Lyle grant paid for the Clinical Research Centre at St Thomas’ that is run by the Guy’s & St Thomas’ Trust, it was not used to fund research on sugar.  Tate & Lyle sold their sugar interests to American Sugar so the brand Tate & Lyle still exists but it is no longer linked to the company Tate & Lyle PLC, which gave the money to King’s College London in 2006.  Tom also used to work for Ajinomoto on aspartame about 8 years ago.  Tom was a member of the FAO/WHO Joint Expert Committee that recommended that trans fatty acids be removed from the human food chain.  Tom has previously acted as a member of the Global Dairy Platform Scientific Advisory Panel and Tom is a member of the Programme Advisory Committee of the Malaysian Palm Oil Board.  In the past Tom has acted as a consultant to Archer Daniel Midland Company and received honoraria for meetings sponsored by Unilever PLC.  Tom’s research on fats was funded by Public Health England/Food Standards Agency.”

None others received.

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