Just when you thought you had solved the problem of how to eat to avoid or lower your risk of heart disease by staying away from saturated fat – the "bad" kind in cheese, eggs, full-fat dairy, and red meat – a study comes along that adds a new villain to the heart-healthy diet war games: Added sugar.

Now to be clear: No one is saying saturated fat is fine to eat if cardiovascular disease (CVD) runs in your family, or worse, your doctors have told you that you need to lower your cholesterol. But this latest research pinpoints sugar as a menace, because it drives up inflammation, makes your cells act in unhealthy ways, and potentially raises your cholesterol.

The study, published earlier this year in The American Journal of Clinical Nutrition, specifically looked at the benefits of restricting added sugar found in chips, crackers, bread, pasta, and sodas, and processed or packaged foods, and not pointing any fingers at the naturally occurring sugar in fruit or starchy vegetables or legumes. A low-carb diet appears to be better for your heart even if it means you eat more fat, prior studies have found, but this new research specifically says sugar is a culprit in the war on heart disease.

“I think we have been grossly wrong about saturated fats,” says Marit Kolby, first author of the AJCN study, and a nutritional biologist at Oslo New University College in Norway. “In my opinion, saturated fat has been blamed for what refined carbohydrates do.”

How is sugar intake related to cholesterol?

The first sign that the saturated fat theory might not be telling researchers the full story was that people who ate foods high in sat fat didn't develop cardiovascular disease at rates higher than those who didn't. Areview in the Oxford Academic Journal looked at observational and randomized controlled studies and found no consistent association between dietary intakes of saturated fat and heart disease.

However, this new paper has found that those who ate high levels of sugar began to show up with higher LDL cholesterol, as well as a host of other markers for cardiovascular disease, including high blood pressure and obesity.

According to the authors, it's long been known that inflammation is related to cardiovascular disease. "A large and growing body of evidence has pointed to low-grade chronic inflammation as a causal factor in the development of ASCVD [atherosclerotic cardiovascular disease, or the most common form of heart disease], and several mechanisms for the pathological changes have been demonstrated to result from inflammatory responses."

Half of your cell's membranes are made from the cholesterol you eat, and these permeable membranes are vital to the normal operation of your cells as they take up fuels and oxygen from the bloodstream and exchange it for waste, allowing it to get carted away by the blood.

When you eat healthy foods that contain polyunsaturated fatty acids, found in avocado, vegetable oil such as olive oil, as well as nuts and seeds, the cell membrane grows more permeable and takes up more cholesterol to help it stabilize, according to Kolby's theory. That's why polyunsaturated fatty acids (PUFAs) help lower LDL or so-called "bad" cholesterol.

But when we eat foods high in saturated fatty acids (SFAs), those same cell membranes become less permeable and need less cholesterol, so more of it remains circulating in the blood. Specifically, if the body is consistently exposed to saturated fat, in cheese, red meat, and full-fat dairy, it has nowhere to go but to cause tiny calcified deposits in the blood vessels, which then leads to plaque, blockages, and the blood vessels also narrow, meaning your heart has to work harder to pump the blood through your arteries to all of your cells, raising your blood pressure and also your heart disease risk.

This is where sugar comes in.

Refined sugar causes inflammation in the body, which in turn interferes with normal cell functioning, Kolby explains. Chronic inflammation leads to the development of cardiovascular disease. The amount of ultra-processed food a person eats is directly related to their risk for heart disease, studies have found. In one large observational study that followed more than 105,000 people for over five years, taking their food diaries daily, higher consumption of ultra-processed foods was associated with higher risks of cardiovascular, coronary heart, and cerebrovascular diseases while those who ate the least processed food had the lowest risk.

"Other components in the diets, [other than saturated fat] such as sugar, starch, and other refined ingredients" have been studied, according to Kolby and her co-authors, which have "the potential to greatly affect diet–microbiome interactions." The authors urge that elements like sat fat be seen in the context of what else someone eats, especially processed foods.

They concluded that a diet of both polyunsaturated fats from vegetable oils, nuts, seeds, and plant foods helps offset inflammation and excess saturated fat and lipids in the bloodstream.

They added that polyunsaturated fat intake appears to protect against heart disease, as part of a varied diet. "If verified, our model speaks for a different approach to dietary recommendations for the prevention of ASCVD, and for the discontinuation of simplified expressions such as “good HDL cholesterol” and “bad LDL cholesterol.”

A well-known cardiologist weighs in

"The is universal agreement that diets high in added sugars and refined carbohydrates do no favor to metabolic or cardiac health. There is also uniform agreement that inflammation favors disease," says Dr. Joel Kahn, author of The Plant-Based Solution and founder of the Kahn Center for Cardiac Longevity in Bingham Farms, Michigan. "The data that diets high in both saturated fats and sugar are of risk goes back to Ancel Keys, Ph.D., lead researcher of the Seven Countries Study, who demonstrated that pastries may be as dangerous as fatty meats due to the combination of sugar and fat."

Bottom line: To lower your risk of heart disease stay away from added sugar

Avoid added sugar and make sure to eat a variety of high-fiber plant-based foods that contain healthy fats such as polyunsaturated fat, omega 3s from nuts and seeds, and plant-based oils such as avocado or olive oil. You should still avoid saturated fat in full-fat dairy, eggs, cheese, and red meat, but when lowering your cholesterol you also need to stay away from added sugar.