What do you think the media touted as the next food ready to knock years off your life? This time it’s not cakes and soda, it's not red meat, it's not alcohol...it’s the humble egg.
Dr. Brianna Stubbs unpacks the study behind the batch of fear-mongering headlines. The study in question was published in a well respected journal called JAMA. The study is titled: Associations of Dietary Cholesterol or Egg Consumption With Incident Cardiovascular Disease and Mortality.
After discussing the controversial JAMA study, Dr. Brianna references two additional papers to help paint a more balanced picture around egg consumption:
What's the truth about eggs? Tune in below and find out.
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It’s probably best to get context by sharing some of the headlines that the media ran amuck with.. Some give you a 1000ft view of the results of a super complex study “Three or more eggs a week increases your risk of heart disease and stroke’ Another tends towards the hysterical “New study confirms that eggs are stroke in a shell” and another sums up the confusion that we are all probably feeling right now “Superfood or stroke risk.. Whats the truth about eggs?” Reading the outpouring of sensationalism really “cracked” me up (ba dum chhhh!)
For years, eggs have been held up as a powerhouse of nutrition. This reputation has been due to eggs’ exceptional nutrition profile as a nutrient-dense food containing high quality protein and a substantial amount of many essential vitamins and minerals. Eggs have a high nutrient density in proportion to their energy content, and they are very economical. Eggs contribute only a small amount (under 2%) of the total calories in the American diet but substantial amounts of high quality protein, folate and riboflavin as well as a number of other nutrients. Eggs are a good source (10 to 19 percent of the Daily Value of six nutrients and an excellent source (20+% of the Daily value) of five others. Few foods are as nutrient-dense, and eggs are among the few food sources of vitamins D and K. Other nutrients in eggs include vitamins A, D, E, B12, folate, zinc, iron, luetin and choline. The fact that non-consumers of eggs were more likely to fall short of the RDA for vitamins A, E and B-12 demonstrates the important role eggs can play in ensuring nutrient adequacy. Lutein is a caroteniod, which is an antioxidant-like compound that has been shown to help in the prevention of macular degeneration, a leading cause of blindness in the elderly, and has been associated with lower risk of cataracts. Another less familiar nutrient in eggs is choline, a compound which is critical for brain and memory development in utero and early in life.
The egg is the latest in a long line of foods to evoke consumer fear, with the discovery that they were also a source of dietary cholesterol, containing around 300mg per egg. The 1960s was the first time that consumers were told to limit or avoid eggs, despite a lack of research at that time on whether eggs themselves elevated blood cholesterol. Eggs have become a symbol of the negative aspects of the American diet, and fear among the American population of eating eggs may be resulting from a lack of consensus in the scientific community and the widespread marketing of low cholesterol foods.
We now know that dietary cholesterol doesn’t really affect blood cholesterol except in the few people we call hyper responders. To find out more, listen to the HVMN Podcast Episode featuring Dave Feldman - we reference this episode a bunch because Dave truly encyclopedic on this topic! Most of us can tolerate eggs and dietary cholesterol just fine. Genetics is actually one of the key factors that determines our blood cholesterol levels. Of course, if you have a diet that’s high in certain types of fat, that can affect your cholesterol. But most people who eat well and have elevated blood cholesterol anyhow have livers that just make too much cholesterol. This is hereditary, so if your parents had high cholesterol, you’re more likely to have it too.
OK, so I don’t want to get too deep into the weeds of dietary cholesterol and how it affects our health- we could talk for hours about that. Lets get on to unpacking the study behind the latest batch of in-egg-scusable headlines. The study in question was published in a well respected journal called JAMA.
The key reported findings of this paper that led to all those headlines were, that each additional 300 milligrams of dietary cholesterol a day – or less than that of two egg yolks – was associated with a 3.2 % higher risk of cardiovascular disease and a 4.43% increase in risk of death (all cause mortality). When egg consumption specifically was looked at, each half an egg extra per day consumed increased cardiovascular disease risk by 1.1% and all cause mortality by 1.9%. 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.
Let’s backtrack a little and look at how dietary intake was measured. The study was a meta-analysis, which pooled the results of six prospective cohort studies from the US. The combined 6 studies involved just under 30,000 adults (45% men), with an average age of 51.6. During the average follow-up of 17.5 years, there were 5,400 CVD events and 6,132 all-cause deaths. If the deaths seem high, the longest period of follow-up was 31 years and many people would have been pensioners at the start of the study. People were interviewed about their diets once at the start of the studies, but not after that.
This is the genesis of many of the criticisms being raised against this study.
The questionnaire format for determining dietary intake is far from ideal. Try and imagine now you get a question that asks you how many apples have you eaten in the last 12 months? And it's a multiple choice. Who actually knows how many apples they’ve eaten in the last year, unless you're a carnivore, which would mean it's been zero. None of the possible choices are, I don't know or I don't remember so people tend to guess.
The ‘guess-work’ nature of the questionnaire is reflected in the estimated energy intake of the study, which in the lowest dietary cholesterol group was one third of that in the highest cholesterol group. Maybe people weren’t eating more eggs or dietary cholesterol – they were just more honest about, or better at recalling their food intake!
Not only all this, but this questionnaire was only completed once, at the start of the study, and therefore doesn’t account for the real chance that people’s diets changed over the follow up period. I imagine you eat a little differently now than 17 years ago. To be fair to the researchers, they acknowledge the questionnaire format as a limitation in their discussion, but hopefully you can see that this certainly influences the strength of the conclusions that can be drawn.
What about other issues? Firstly, the study population is based in America, so non-US citizens should ignore any findings, as you haven’t been studied, and results from specific population studies are non-generalisable.
Next up, while the study controlled for some lifestyle factors like activity level, smoking, and alcohol use, it didn’t look at others, such as socioeconomic status, and stress levels. Again the authors acknowledge this saying that ‘residual confounding was likely.’ These other factors can clearly also have an impact on someone’s risk of a heart attack.
Ultimately correlation does not equal causation, but the popular media prefer to ignore this nuance..
Now, a similar study that found the opposite result was published in The BMJ in 2013. The title was
In this study a group of researchers from China and Boston performed a meta-analysis of 8 studies that included over 250,000 participants studied for the occurrence of coronary heart disease (CHD) and over 210 000 participants studied for stroke and followed them for 8 to 22 years. In this large analysis the authors found no evidence for an association between egg consumption and either coronary heart disease or stroke. This study shares many of the similar problems to the first one we discussed, the cohorts used in the comparison coming from the USA and Japan limiting generalisability and also the fact that questionnaires were used to measure intake. However, at least in four of the 8 studies here, the researchers used repeated measurements of diet, which is a significant strength over the previous study.
The relative risk of coronary heart disease for adding 1 egg per day was almost exactly 1, which means there was no difference and the relative risk of stroke for adding 1 egg per day was 0.91, so nearly 1 again, meaning no difference.. However, an increased risk for coronary heart disease was observed in the subgroup of patients with diabetes in the group of participants with the highest egg consumption compared with those with the lowest consumption (relative risk 1.54, meaning 50% higher ). No similar increase in the risk of stroke was observed in this group, in fact a protective effect against hemorrhagic stroke was observed in those with the highest egg consumption (relative risk 0.75, meaning 25% less). So all in all a less alarming picture.
The authors concluded that the findings of their meta-analysis "do not support a positive association between egg consumption and cardiovascular disease outcomes in the general population."
OK, so its clear that its hard to draw firm conclusions on health risks from this type of study. You can see that the interaction between nutrition and disease 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.” It makes sense why some people might be concerned about eating eggs, but getting worried about an extra half an egg a day are verging on egg-treme (pardon the pun, it’s almost too easy at this point).
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 the trimmings that come with them. For example, poached eggs could be better than egg dipped french toast covered in syrup…
Another factor you might want to consider is that there are different types of eggs, with the metabolic effects of different types being fairly material. This is the subject of the last paper we are going to discuss today. This paper was published in 1991 in the American Journal of Clinical Nutrition and is called
For context, you can buy eggs from hens that have been raised in different ways.. Conventional eggs are your standard supermarket eggs. The hens that lay these eggs are usually fed grain, supplemented with vitamins and minerals. Another type of eggs are omega-3-enriched eggs: the hens laying here are like conventional chickens except that their feed is supplemented with an omega-3 source like flax seeds.
In this study the effects of dietary eggs enriched with ω-3 fatty acids on lipid concentrations in plasma and lipoproteins and blood pressure were determined in 11 men and women in two groups. Group 1 consumed four ω-3 fatty acid eggs per day during the first 4-wk period and four control eggs for the second 4-wk period. Group 2 ate the same number of eggs in the reverse order. These eggs were consumed in addition to their normal diet.
What were the results? Mean plasma cholesterol concentration was significantly increased by adding in control eggs to the diet but unchanged by ω-3 eggs. Similarly, mean plasma triglyceride concentration was decreased by ω-3 eggs but increased by control eggs. The authors suggest that eggs enriched in omega 3 fatty acids could be a good substitute for oily fish in people’s diets. Ultimately, they conclude that the ω-3 eggs may be more healthful than the control eggs.
We can see that there is plenty of nuance to consider before cutting out eggs. What is the rest of your diet like? What are other risk factors to your health you might be able to address and make more of a difference? What type of eggs are you eating? How many are you eating? The list goes on and on and on.
My take-away messages are:
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