A recent editorial in the British Journal of Sports Medicine dismissed the widely held belief that a diet rich in saturated fats increases our risk of heart disease as "just plain wrong".
The authors concluded we have been concentrating on reducing one type of fat in our diet when instead we should be promoting a Mediterranean diet, exercise and reducing stress to reduce our chances of heart disease.
So, is saturated fat really the killer it's made out to be. Or, as the editorial suggests, have we really got it all wrong?
The case for saturated fat's role in heart disease
David Sullivan, Clinical Associate Professor, University of Sydney
According to evidence collected over more than five decades, the level of saturated fat in our diet is one of the most powerful environmental risk factors for the inflammatory process in the artery wall that leads to heart attack and stroke.
Science and medicine have never suggested saturated fat itself, for instance from a diet rich in fatty meat or processed foods, blocks your arteries (vessels that carries blood from the heart to other organs).
What we know is eating too much saturated fat raises levels of blood cholesterol, in particular the type commonly called "bad" cholesterol, low-density lipoprotein cholesterol or LDL.
LDL and related particles enter the artery wall where they are chemically modified, triggering a vicious cycle of inflammation and cholesterol accumulation. It is this cycle of inflammation and cholesterol accumulation that leads to heart disease and stroke.
That's why doctors like to keep an eye on your blood cholesterol levels, as part of assessing your risk of heart disease.
But it's not just LDL particles that contribute to the inflammatory process; other related particles do so too. So doctors look beyond simple measures of LDL cholesterol to measure your risk. It's better to start with your total level of blood cholesterol, then take away levels of so-called "good" cholesterol - the high-density lipoprotein or HDL cholesterol. This gives you an idea of the level of damaging particles (or non-HDL cholesterol).
If we understand that raised blood cholesterol levels (in particular, raised LDL) increase your risk of heart disease and stroke, then it stands to reason that reducing their levels might decrease your chances.
This is exactly what two recentstudies showed. These provided the highest form of evidence in over 40,000 patients; they looked at how two cholesterol-lowering drugs significantly reduced cardiovascular events, like heart attacks and stroke.
Neither drug has anti-inflammatory effects. Instead, their success is attributed to reduced levels of harmful cholesterol-carrying particles, including LDL cholesterol.
So, what role does diet play in all this? Two landmarkstudies in people who ate Mediterranean-style diets show what happens when you eat less saturated fat. Replacing saturated fat in the diet with foods containing healthier unsaturated fat, like the fats in nuts, extra virgin olive oil, polyunsaturated margarine - but not processed carbohydrates - reduced levels of heart attacks and premature death.
Downplaying the role of dietary saturated fat in heart disease prevents health care workers from managing cardiovascular risk using diet. Any recommendation to not be so focused on saturated fat will therefore increase population levels of blood cholesterol, increasing the need for statins and other cholesterol-lowering drugs.
The implication, that one of the most thoroughly researched areas of medical science - that excess saturated fat puts you at risk of heart disease and stroke - is a hoax, misrepresents the evidence.
The case against labelling saturated fat 'bad'
Yutang Wang, Senior Lecture at Federation University Australia
Saturated fat (for instance high in fatty meat or full-fat dairy) is thought to clog the arteries and increase the risk of heart disease. But currently available evidence does not support these common beliefs.
First, let's look at whether saturated fat really clogs the heart's arteries leading to coronary atherosclerosis (when plaque builds up inside your arteries, in time hardening and narrowing them). In a surprise finding, one study in women who had been through the menopause found a diet richer in saturated fat was linked with less, not more, progression of coronary atherosclerosis.
Second, whether eating saturated fat increases your chances of dying from heart disease. When researchers combined the results from 41 research papers published from 1981 to 2014, eating saturated fat was not linked with dying from heart related diseases, like heart attack, stroke or type 2 diabetes.
Many of us think saturated fat is bad for us because it increases levels of low-density lipoprotein cholesterol or LDL in our blood. But is LDL-cholesterol really that bad?
When researchers studied all the research papers written in English that investigated the effects of LDL-cholesterol on the deaths in people over 60, they had some surprising results. In most of the papers (representing 92% of participants), LDL-cholesterol was linked with a lower death rate, and there was no link in the remaining 8% of participants.
It is not the level of LDL itself that predicts people's risk of heart disease, but the ratio of total cholesterol and another type of cholesterol, high-density lipoprotein (HDL) cholesterol, that's the key.
While LDL is largely responsible for delivering cholesterol to cells around the body, HDL cholesterol transports extra cholesterol back to the liver for recycling. So LDL and HDL work together to ensure cells in our body maintain the right levels of cholesterol.
A higher ratio of total cholesterol against HDL-cholesterol is commonly associated with higher incidence of heart disease because a higher ratio reflects that more cholesterol will be deposited into the blood vessel and less will be removed from it.
But when we eat saturated fat, both LDL and HDL cholesterol levels increase. So, eating normal amounts of saturated fat will not tip the balance. We need saturated fat in our diet to form the building blocks for the cells in our body and to help our cells communicate with each other.
The long-term effect of eating too much cholesterol on the ratio of total against HDL cholesterol is not clear. A short-term study suggests eating moderately high levels of cholesterol may not be bad. Researchers found that eating three eggs a day (containing 640 mg cholesterol) for 12 weeks did not increase LDL-cholesterol.
Instead it significantly increased HDL-cholesterol by 20% compared to those who ate an egg substitute without cholesterol.
So, we may need to stop thinking about "bad" saturated fat and "bad" cholesterol. Rather, we should enjoy our meals containing moderate amounts of saturated fat and be physically active. That will be more effective in keeping us healthy.
The author mentions the study of progression of artery disease in women who have gone through the menopause. This shows women with the highest saturated fat intake had the lowest LDL cholesterol levels (despite taking less lipid-lowering treatment). This might be explained, as some researchers suggest, by genetic factors that allow some people to tolerate saturated fat better, leading to a lower LDL and a reduced risk of heart disease.
And rather than eating moderate amounts of saturated fat, as the author suggests, there is ample evidence for avoiding saturated and trans fats, replacing them with healthier ones and reserving saturated fats as treats.
We also differ in our opinions of the best marker of heart-disease risk that your doctor might consider when analysing blood test results.
Use of the total cholesterol to HDL ratio, as the author proposes, has declined because levels of HDL cholesterol itself may not be a marker of heart disease protection. And raising HDL has not reduced the risk of heart disease. Instead, non-HDL cholesterol has been introduced as a superior measure to LDL, as I have mentioned.
Finally, there's confusion over how the terms cholesterol and saturated fat are used. Saturated fat is chain-shaped and consumed in much greater amounts than the ring-like cholesterol. And it's saturated fat in the diet that's the main determinant of cholesterol levels in the blood.
Despite decades of research on whether eating saturated fat increases our chances of dying from heart disease, the results are not consistent. Some show eating high saturated fat is bad, whereas others do not.
For instance, as recently as 2016, a study showed higher total saturated fat intake was linked to lower levels of heart disease.
Yes, LDL cholesterol can be chemically modified and involved in the inflammation process. However, this does not mean non-modified LDL cholesterol in the blood is bad.
There is no doubt that cholesterol-lowering drugs can lower your risk of a heart attack or stroke. These drugs can decrease LDL-cholesterol. However, the beneficial effect of cholesterol-lowering drugs may be largely because of the favourable change in the balance between total cholesterol and HDL-cholesterol, rather than lowering "harmful" LDL-cholesterol alone.