There was, and continues to be, fierce disagreements in nutrition science in regards to what constitutes a wholesome diet. A vital controversy is the role of saturated fats in health and disease. Saturated fats are proven to increase blood glucose levels, and increased blood cholesterol can be observed in people who develop cardiovascular disease.
In science, sometimes a new outlook can turn our interpretation of the information upside-down, and necessitate a paradigm change.
It’s been believed for more than half a century that saturated fats in the diet promote heart disease by raising blood cholesterol. However, a new model explains why this so-called “diet-heart hypothesis”, which has had a major influence on dietary guidelines, may have an alternative explanation.
In an article published in the American Journal of Clinical Nutrition, three scientists have raised a question which challenges the diet-heart-hypothesis: Why would saturated fats raise blood cholesterol, and should this be harmful? All things considered, saturated fats occur naturally in a wide variety of foods, such as breast milk.
“Cholesterol is a critically important molecule for all cells in the body,” explains associate professor Marit Zinöcker, the lead author at Bjørknes University College, Oslo, Norway. “A cell is surrounded by a fluid membrane that controls cell function, and the cells depend on the ability to incorporate a certain amount of cholesterol molecules, so that their membranes don’t become too stiff or too fluid.”
“The basis of the model is that when saturated fats replace polyunsaturated fats in the diet, less cholesterol is needed in the cell membranes,” she explains. The opposite is true when eating more polyunsaturated fatty acids, which include omega-3 and omega-6 fatty acids. This is only because polyunsaturated fats in the diet enter our cell membranes and also make them fluid. The cells adapt the fluidity of their membranes by incorporating cholesterol excreted in the blood. According to the model introduced by the researchers, this may explain why blood glucose levels fall when we consume more polyunsaturated fats.
The authors have named the model that the “Homeoviscous Adaptation to Dietary Lipids” (HADL) model.
“Cells need to adjust their membrane fluidity according to changes in their environment, such as the access to different types of fat”, says co-author Simon N. Dankel, researcher at the Department of Clinical Science, University of Bergen, Norway.
“This phenomenon is called homeoviscous adaptation, and has been described in both microorganisms, vertebrates and in human skin cells. We argue that this is a critical principle in human physiology. Our cells are normally capable of adjusting their cholesterol content according to changes in dietary fats.””Nutrition research often focuses on what changes in the body, but the question of why something, such as the blood cholesterol, changes, is of equal importance”
“Nutrition research often focuses on what changes in the body, but the question of why something, such as the blood cholesterol, changes, is of equal importance”, states co-author Karianne Svendsen, postdoctoral fellow in the Department of Nutrition, University of Oslo, Norway.
This is where the newest HADL version comes into play, providing an explanation based on adaptive human anatomy. “From the perspective of the HADL model, we find logical explanations for why cells need to change their cholesterol content, and thereby the blood cholesterol, when fats in the diet change,” states Zinöcker.
From the paper, other reasons for elevated LDL-cholesterol in people with cardiovascular disease are discussed, such as low-grade inflammation and insulin resistance. This implies that elevated blood cholesterol brought on by metabolic disruptions must be uncoupled from elevated blood cholesterol caused by a significant change in intake of dietary saturated fatty acids. It also questions the benefit of lowering blood cholesterol by adding polyunsaturated fatty acids into the diet, rather than fixing the root cause.
“There is at best weak evidence that a high intake of saturated fat causes heart disease,” says Dankel. “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.”
“The research and reasoning that the HADL model is based on indicates that the effect of dietary fats on blood cholesterol is not a pathogenic response, but rather a completely normal and even healthy adaptation to changes in diet.” Zinöcker concludes.
The authors state that although the model relies on existing understanding of cellular mechanisms, the model still needs to be verified. The authors consequently urge researchers to discuss the HADL version using #HADLmodel and also to test the model.
Related Journal Article: https://academic.oup.com/ajcn/article-abstract/113/2/277/6104795?redirectedFrom=fulltext