The headline of a recent Los Angeles Times article is, “No yolk: eating the whole egg as dangerous as smoking?” The question mark is meant to convey uncertainty or surprise, or perhaps both. The article represents much of what is wrong with science and health reporting by mainstream media.
The news report is based upon an article published in Atherosclerosis – Egg yolk consumption and carotid plaque. The study is highly problematic in ways not explored at all in the LA Times article. The researchers surveyed 1,262 patients seen in a vascular prevention clinic, asking them about their egg consumption, smoking history, and other lifestyle factors. They created a measure known as “egg yolk years” – the number of egg yolks consumed on average per week times the number of years of consumption. They compared this to pack years of smoking – packs per day times number of years, a routine method of reporting smoking history.
“Our findings suggest that regular consumption of egg yolk should be avoided by persons at risk of cardiovascular disease. This hypothesis should be tested in a prospective study with more detailed information about diet, and other possible confounders such as exercise and waist circumference.”
The LA times times used this as their money quote:
“We believe our study makes it imperative to reassess the role of egg yolks, and dietary cholesterol in general, as a risk factor for coronary heart disease,” the study authors write.
The authors feel the role of dietary cholesterol needs to be reassessed because current evidence does not support a significant role for dietary cholesterol in heart disease, but more on that below.
The weaknesses of this study include the fact that it is retrospective and based on survey data, which is notoriously inaccurate. Further, it is an observational study and therefore there are many confounding factors that are not controlled for. Perhaps people who eat more egg yolks also eat more bacon, or have a generally poorer diet, or don’t exercise as much. The authors acknowledge this in their last line about needed a prospective study that controls for possible confounding factors.
But the data in the study is even more problematic, in my opinion. The article itself is behind a paywall, but here is table two containing the key data.
|Egg yolk years||<50||50-110||110-150||150-200||>200|
|Age at first visit||55.70||57.97||56.82||64.55||69.77|
|Eggs per week||0.41||1.37||2.30||2.76||4.68|
|Smoking (pack years)||14.14||14.37||16.57||13.88||17.00|
|Plaque area (mm2)||101.45||110.35||113.58||135.76||175.77|
The table does indeed show a significant increase in carotid plague, the build up of cholesterol on the inner lining of the main arteries that feed the brain, with increase in egg yolk years. There are significant confounders and contradictions in the data as well, however. The most glaring to me is that total cholesterol, triglycerides, HDL and LDL do not vary significantly across the egg yolk years columns. Apparently what the authors have shown (which is consistent with previous data) is that eating lots of eggs does not increase total cholesterol or bad cholesterol (LDL) nor does it decrease good cholesterol (HDL). In my mind this leaves the authors completely without a mechanism to explain a causal relationship between egg consumption and carotid plaque. This strongly suggests the association is not causal but is incidental or spurious – unless an alternate mechanism can be proposed and supported by evidence.
Further, egg yolk consumption was associated with increased age at first presentation (a known significant risk factor) and pack years of smoking (although inconsistently). The authors say that the association between egg yolks and carotid plaques remained significant even after adjusting for risk factors and for age. However, these associations suggest the probable existence of other factors not controlled for.
Overall the data are not very compelling. The lack of correlation with cholesterol is most damning, in my opinion.
The story of eggs, cholesterol, and vascular disease is actually a bit complex. Recent studies are all over the place in terms of correlations. This recent study, for example, shows that eating eggs increases HDL and improves the cholesterol profile. A recent review also concludes:
Egg intake has been shown to promote the formation of large LDL and HDL subclasses in addition to shifting individuals from the LDL pattern B to pattern A, which is less atherogenic. For these reasons, dietary recommendations aimed at restricting egg consumption should be taken with caution and not include all individuals. We need to acknowledge that diverse healthy populations experience no risk in developing coronary heart disease by increasing their intake of cholesterol but in contrast, they may have multiple beneficial effects by the inclusion of eggs in their regular diet.
Another study found no association between egg consumption and cholesterol or mortality in men but did find an increase for women. And yet another study finds an increased risk for vascular disease from high cholesterol and lipid diets in the older population, especially diabetics.
A 2001 BMJ systematic review of fat and cardiovascular disease concluded:
There is a small but potentially important reduction in cardiovascular risk with reduction or modification of dietary fat intake, seen particularly in trials of longer duration.
The findings are suggestive of a small but potentially important reduction in cardiovascular risk on modification of dietary fat, but not reduction of total fat, in longer trials. Lifestyle advice to all those at risk of cardiovascular disease and to lower risk population groups, should continue to include permanent reduction of dietary saturated fat and partial replacement by unsaturates. The ideal type of unsaturated fat is unclear.
My conclusion is that the topic is very complicated and the current data is a bit unclear. There does seem to be a consistent signal in the research data that high LDL and triglycerides and low HDL are risk factors for vascular disease, especially when combined with other risk factors. Improving the lipid profile through diet, exercise, and medication is beneficial to vascular risk.
The dietary recommendations are a bit complex, though. Reducing total fat 1980s style is probably not a good idea, as the reduction in good cholesterol probably more than outweighs the reduction in bad cholesterol. Further, the dietary contribution to lipid profile for most people is probable mild (unless you have an extreme diet). Avoiding trans fats and minimizing saturated fats seems to be supported by the evidence.
Overall, if you have a balanced diet and avoid extremes and exercise regularly you are probably OK. Further tweaking your diet is unlikely to produce large health benefits. If you are in a high risk group, such as diabetics, you need to be more specific about your eating habits, and should consult with your physician.
With respect to eggs specifically, it seems that moderate consumption of eggs are not a health risk and may even be beneficial. It is reasonable, based on the data, to avoid extreme high egg consumption – but you can probably say that about most things.
When it comes to diet the rule of thumb, everything in moderation, seems to be a good first approximation of the scientific data.