Eggs and cholesterol: what the feeding trials actually found

They do raise it. Across 66 feeding trials in 3,185 people, eating more eggs raised total cholesterol, the LDL kind and the HDL kind.¹ Whether that shortens a life is where the long studies split. One puts an extra egg a day at 4% higher heart risk, another at none at all.^4,3

What a feeding trial can and cannot answer

A feeding trial hands people eggs. Participants are assigned to a higher intake or to a control diet, eat that way for weeks or months, and have their blood fats measured before and after. The outcome is a line on a lab report, not a heart attack.

That narrowness is the whole value. Out in the population, people who eat more eggs differ in other ways, and the differences lean toward risk. In three large US studies, the heavier egg eaters carried a higher body mass index, were less likely to be taking a statin, and ate more red meat.³ A feeding trial strips all of that out and asks one question: what does the cholesterol in a yolk do to the cholesterol in the blood?

The lipid answer, and its size

The largest feeding analysis covered 66 trials with 3,185 participants.¹ More eggs significantly raised total cholesterol, LDL cholesterol, HDL cholesterol, the ratio of total to HDL, and both of the proteins that ferry these particles around the bloodstream.¹ The remaining blood fats did not move.¹ Total cholesterol, HDL and the two proteins rose in a straight line with dose, while LDL and the total-to-HDL ratio bent.¹

A second analysis, restricted to healthy people and to 17 trials, put a size on the direction. The higher-egg groups finished with an LDL-to-HDL ratio 0.14 above the control groups.² Their LDL ran higher by 8.14 in the units the trials reported.² Trials running longer than two months produced a bigger effect than short ones, which argues against reading the rise as a temporary adjustment.²

The two analyses part company on the ratio, which is the measure that matters most. The 66-trial analysis concluded that eating more than one egg a day for under 12 weeks may lift some blood fats while leaving the LDL-to-HDL ratio alone.¹ The 17-trial analysis found that the ratio itself moved.² Two analyses of the same question reached different answers.

Two readings of the same long-term studies

The BMJ analysis combined 33 risk estimates covering 1,720,108 participants and 139,195 heart events.³ An extra egg a day came out unrelated to heart disease, at 0.98, with a range running from 0.93 to 1.03.³ Its own three study groups agreed, at 0.93 for people eating at least one egg a day against those eating fewer than one a month.³

Two years later a Circulation analysis combined 49 risk estimates covering 3,601,401 participants and 255,479 events.⁴ One additional 50 g egg a day, meaning one ordinary large egg, came out associated with a 4% higher heart risk, ranging from no effect at all up to 8%.⁴ In its own group of 27,078 men, each extra 300 mg of dietary cholesterol a day carried 10% higher overall mortality and 13% higher heart mortality.⁴

The same paper measured the middle link in the chain. More cholesterol in the blood went with 14% higher heart mortality, per standard step up the scale.⁴ Eggs move blood cholesterol, and blood cholesterol tracks risk. The feeding trials only ever cover the first of those two steps.

Both analyses are enormous. Both point different ways, with ranges that touch the no-effect line from opposite sides. Heterogeneity, the measure of how much the underlying studies disagree with each other, stood at 62% in the first and 80% in the second.^3,4 The studies argue among themselves as much as the reviews do.

Geography explains part of it. In the BMJ work, no link appeared in US or European studies, and an inverse link appeared in Asian ones.³ In the Circulation work, the risk sat in the US studies at 1.08 and was absent in the Asian ones.⁴ Eggs arrive on a plate alongside other food, and this kind of study cannot separate the egg from the breakfast.

Where the dose thresholds fall

The dose-response work is more useful than the yes-or-no headlines. An analysis of 55 studies in 2,772,486 people found each additional daily egg associated with 7% higher all-cause mortality and 13% higher cancer mortality, and with no change in heart mortality.⁵ Its curve showed little sign of raised risk below half an egg a day, or below 250 mg of dietary cholesterol a day counting every food.⁵ Across the whole range, each extra 100 mg a day of dietary cholesterol carried 6% higher all-cause mortality.⁵

A separate analysis of 39 studies in nearly two million people ran the other way. Intake up to six eggs a week went with fewer heart events than eating none at all: 0.95 at four eggs a week, and 0.94 at up to one a day.⁶ Stroke showed no link in either direction.⁶

Heart failure, the signal that keeps reappearing

One result recurs across analyses that otherwise contradict each other. In the dose-response work, one egg a day was associated with 15% higher heart failure risk, climbing further at higher intakes.⁶

A review of the reviews, covering seven of them and fifteen analyses, found that heart failure signal in two places.⁷ It then pointed out that both had drawn on the same three underlying studies.⁷ One dataset counted twice is not two findings.

Higher risk per additional egg a day, across long-term studies% higher risk

Cardiovascular disease (Circulation, 49 estimates)4%

All-cause mortality (Darooghegi Mofrad 2022)7%

Cancer mortality (Darooghegi Mofrad 2022)13%

Heart failure (Godos 2021)15%

What remains unknown

Who responds. Egg hyper-responders are discussed constantly, and the 17-trial analysis listed responder classification among the questions it planned to ask, but its abstract does not report what came back.² The size of that difference between people cannot be taken from this evidence. Nothing here compares egg whites with whole eggs either.

The review of the reviews states the structural problem plainly. Among the reviews it gathered, only one meta-analysis covered feeding trials on eggs and blood fats, and its results contradicted the observational studies.⁷ In the dose-response work, confidence in every result was rated low, except for stroke, where it was moderate and the finding was no risk.⁶ Disagreement between studies ran high nearly everywhere, and those authors ask that the small risk estimates be read with caution.⁵

No trial has fed people eggs for long enough to count heart attacks. That is why a question this old is still open.