In a world-first study, University of South Australia (UniSA) researchers investigated the independent impact of dietary cholesterol and saturated fat on "bad" cholesterol – low-density lipoprotein, or LDL – levels. What they found was that even eating two eggs a day, in an overall high-cholesterol but low-saturated-fat diet, lowered LDL levels and cardiovascular disease risk.

"Eggs have long been unfairly cracked by outdated dietary advice," said lead researcher Jon Buckley, a professor at UniSA. "They're unique – high in cholesterol, yes, but low in saturated fat. Yet it's their cholesterol level that has often caused people to question their place in a healthy diet.

In the randomized, controlled, cross-over trial, 61 healthy adults each tried three different diets for five weeks, with breaks in between to allow for a reset. While all diets contained the same amount of daily calories, the cholesterol and saturated fat content differed. An egg diet (two eggs a day) was high in cholesterol but low in saturated fat; an egg-free diet was low in cholesterol (no eggs) but high in saturated fat; and a control was high in both cholesterol and saturated fat, but included only one egg per week.

The egg diet featured 600 mg/day of cholesterol (6% saturated fat), the egg-free diet contained 300 mg/day (12% saturated fat), and the control plan included 600 mg/day (12% saturated fat).

They found that the egg diet significantly lowered LDL cholesterol, compared to the control (103.6 µg/dL compared to 109.3 µg/dL), while the egg-free plan saw LDL levels stay roughly the same as the control, even though it was lower much lower in dietary cholesterol. Across the board, saturated fat appeared to be the common denominator when it came to LDL shifts.

Investigating the mechanisms involved in this change, the scientists found that things got a little more complex. LDL cholesterol is made up of distinctly sized particles: large, fluffy LDL particles, which are generally considered less risky, and small, dense ones, which are more likely to sneak into artery walls and cause plaque buildup. The egg diet reduced overall LDL but changed the particle makeup – fewer large ones and a slight increase in the small ones. While this isn't necessarily negative, looking at the LDL drop overall compared to not eating any eggs, it's certainly something that isn't well understood and needs more research.

It's also worth noting that the no-eggs diet also saw a rise in small particles and a slight drop in large ones, but overall did nothing impactful in lowering LDL cholesterol levels.

However, the results revealed that the real culprit for raising LDL levels appears to be saturated fat, not the cholesterol in eggs as has long been believed. Cutting eggs out of a diet for health reasons, without also reining in saturated fat intake, is unlikely to shift the needle when it comes to LDL cholesterol.