A key controversy is the role of saturated fats in health and disease.
Saturated fats are known to increase blood cholesterol levels, and increased blood cholesterol is often observed in people who develop heart disease.
It has been thought for more than half a century that saturated fats in the diet promote heart disease by increasing blood cholesterol.
But in a new study, researchers found that this so-called ‘diet-heart hypothesis,’ which has had a major influence on dietary guidelines, may be wrong.
They raised a question that challenges the diet-heart-hypothesis: Why do saturated fats increase blood cholesterol, and why should this be dangerous?
After all, saturated fats occur naturally in a wide variety of foods, including breast milk.
The research was conducted by a team at Bjørknes University College, Oslo, Norway.
Cholesterol is a critically important molecule for all cells in the body.
The basis of the model is that when saturated fats replace polyunsaturated fats in the diet, less cholesterol is needed in the cell membranes.
The opposite is true when eating more polyunsaturated fatty acids, which include omega-3 and omega-6 fatty acids.
This is because polyunsaturated fats from the diet enter our cell membranes and make them more fluid.
The cells adjust the fluidity of their membranes by incorporating cholesterol recruited from the bloodstream.
According to the model, this can explain why blood cholesterol levels decrease when we eat more polyunsaturated fats.
The team says cells need to adjust their membrane fluidity according to changes in their environment, such as access to different types of fat.
They think that this is a critical principle in human physiology. The cells are normally capable of adjusting their cholesterol content according to changes in dietary fats.
The causes of atherosclerosis and heart disease are multifactorial.
With this model, they propose to disconnect the blood-cholesterol raising effect of diet from the elevated blood cholesterol that is causally linked to heart disease.
In the paper, other reasons for elevated LDL-cholesterol in people with heart disease are discussed, such as low-grade inflammation and insulin resistance.
This shows that elevated blood cholesterol caused by metabolic disruptions must be uncoupled from elevated blood cholesterol caused by a major change in intake of dietary saturated fatty acids.
It also questions the benefit of lowering blood cholesterol by adding polyunsaturated fatty acids to the diet, and not addressing the root cause.
The team says there is at best weak evidence that a high intake of saturated fat causes heart disease.
The overall data are inconsistent and unconvincing, not to mention the lack of a logical biological and evolutionary explanation.
Also, people with metabolic disorders often do not show the expected changes in blood cholesterol when changing their fat intake, suggesting loss of the normal response.
One author of the study is associate professor Marit Zinöcker.
The study is published in the American Journal of Clinical Nutrition.
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