Saturated fats, unsaturated fats, cholesterol and heart health — what do we really know?
Modern life survival guide #2 — myth busting!
The conversation around fats in our diets has become a series of scare stories in which cholesterol is invariably the villain and our hearts and waistlines are the victims. And yet, versions of low carb / high fat and protein diets such as the Atkins or Keto diet appear to be astonishingly successful for many people, both in terms of weight loss and some health markers.
Confused? I certainly was, and that was before I admitted to myself that I wasn’t actually all that clear on the whole cholesterol — saturated — unsaturated fat thing anyway.
So what is the current research saying?
The difference between saturated and unsaturated fats is simply a tweak to their molecular design and yet, once they have been digested and absorbed, they have significantly differing effects on our bodies. Both types of fat are examples of lipids, as are hormones, much of our cell membranes, some vitamins such as A, D, E and K, and cholesterol.
The main sources of saturated fats are butter, cheese, coconut oil and palm oil, all of which comprise almost twice as much saturated fat as red meat. Unsaturated fats come in two options — monounsaturated fats, as found in olive oil, high oleic safflower oil, nuts and avocado; and polyunsaturated fats, as found in oily fish, sunflower oil and seeds, flaxseed, linoleic safflower oil and soybean oil.
Almost all medical, heart-health, government and WHO advice agree that a diet high in saturated fat is a significant risk factor for cardiovascular disease and stroke. This has been the case since the 1960s and is based on the work of Ancel Keys, an American nutritional scientist. The thing is though, his conclusions were drawn entirely from flawed observational studies.
Ancel Keys was looking to prove his own hypothesis that high levels of saturated fat in the diet caused high levels of heart disease in a population, and that is exactly what he found, comparing countries like Japan (little saturated fat intake, low heart disease) and the USA (high saturated fat intake, lots of heart disease). This study became the cornerstone of public health advice, in that we should all be following a high carb / low fat diet. However, as it turns out, he only included data from countries that proved his hypothesis, conveniently ignoring the data from Holland, for example, where people were eating a high fat diet but had little heart disease, and Chile, where he found that people followed a low fat diet but there were high levels of heart disease.
Essentially, the conclusion at the time boiled down to the assumption that as saturated fat raises cholesterol levels in the blood, and that cholesterol blocks arteries and causes heart disease, then it must follow that saturated fats in the diet cause heart disease.
The trouble is that once trials that have not adequately accounted for other lifestyle factors are discounted, reducing saturated fat in the diet is not found to lower cardiovascular disease or total mortality. Additional analysis of human trials has found that replacing saturated fats with unsaturated fats does reduce the risk factors for heart disease, high blood pressure, cancer, insulin resistance and chronic inflammation.
Hmmm — this doesn't quite add up . . .
The important point here is that saturated fats are essentially neutral, but unsaturated fats are actively protective.
Omega-3 and omega-6 are polyunsaturated fats, both of which are not merely an energy source but are also biologically active, playing a role in blood clotting and inflammation in the body. Omega-3 is anti-inflammatory and omega-6 is pro-inflammatory. One of the health concerns of the 21st century is that we typically have far too much omega-6 in our diets compared with omega-3. Omega-6 is needed for our immune response, to infections for example, but chronically high levels in our system may drive inflammatory conditions such as heart disease, diabetes, arthritis, Alzheimers disease and cancer.
In order to complete the story, we need to look more closely at exactly what cholesterol is, what it does and how it does it.
80% of our cholesterol is made by us, mostly in the liver and intestines (although every cell in our body has the ability to make it), only 20% comes from animal based foods in our diet. It is used to make steroid hormones such as cortisol, progesterone, oestrogen, testosterone; it is a vital component of every cell membrane and also bile salts, which are released by the gall bladder into the small intestine to help aid the absorption of fat and fat soluble vitamins A, D, E and K.
Since cholesterol is not soluble in water, it has to be transported around the body by lipoproteins. Essentially, there are two types of lipoprotein involved — high density (HDL) and low density (LDL). Oh, and then the LDL also comes as a small, dense variety and as larger, fluffy varieties. If you have had blood tests recently, you may recognise the acronyms.
LDL transports cholesterol to cells, but when cells have enough for the job in hand, they close their cholesterol receptors and LDL accumulates in the blood. If you have damaged arteries, then LDL (especially the small, dense LDL) can accumulate in the artery walls, contributing to the formation of plaques and atherosclerosis — hence the link to cardiovascular disease. HDL picks up excess cholesterol from cells and the bloodstream and returns it to the liver for excretion as bile. Clearly, measuring total cholesterol in your blood is meaningless, what we really want to know is our HDL:LDL ratio because the higher it is, the better. There is definitely a genetic element to this ratio and we know that high native HDL correlates with better cardiovascular health.
HDL levels are also affected by your lifestyle. Aerobic exercise, weight loss, reducing the amount of refined carbs and sugars in your diet, increasing omega-3 and other unsaturated fats will all be of benefit. Saturated fats seem to raise both HDL and LDL levels.
Plant cells synthesise cholesterol as well, but it is almost all used to make other compounds such as phytosterols which compete with cholesterol for absorption in the gut — snacking on avocado, flax seed and peanuts is a good way to reduce the amount of cholesterol you obtain from your diet.
The picture is complicated further when we consider how our gut microbes interact with fats, though it should be stressed that the research here is limited and drawing conclusions relies on putting two and two together … we know from twin studies that obese and lean twins have markedly different gut microbiomes with obesity being associated with a lower diversity. If you take the gut microbes from an obese human and transfer them into a germ free mouse, then the mouse will gain weight, even when its diet remains unchanged. Our gut microbes strongly influence how dietary fats (and other macronutrients) are absorbed and stored in the body.
When diets rich in saturated fat are followed over a period of time we find an increase in abundance of gut microbe species that are associated with increased insulin resistance, body mass index and chronic inflammation. Addition of polyunsaturated fats to the diet, especially omega-3, favours species of microbe that produce butyrate, a key short chain fatty acid that is the preferred energy source for the cells lining the colon, encouraging colon cells to produce a protective mucus layer and also to use up oxygen in the gut — an anaerobic (without oxygen) environment protects against pathogens such as Salmonella. Butyrate signals our colon cells to knit tightly together, preventing a leaky gut which is implicated in a host of autoimmune diseases including inflammatory bowel disease and some neuropsychiatric illnesses.
But its beneficial effects are not just localised to the gut — butyrate is also known to be involved in fortifying the blood brain barrier and in maintaining the normal functioning of microglia, our first responders to viral and bacterial infection in the brain. Butyrate has even been shown to enhance cognition in mice, probably because it influences how much brain derived neurotrophic factor (BDNF) is expressed. BDNF plays a pivotal role in supporting existing neurons and in encouraging the growth of new neurons and connecting synapses, especially in areas of the brain which are involved in learning and memory.
All of which brings me back to a theme that runs throughout my writing — as I read more and dig deeper into the research around the gut and nutrition, health and wellbeing, the more I come to appreciate that our bodies truly are a miracle. They have evolved to work with nutrient dense food in all its complexity and variety, and a reductionist, pill-popping approach to fuelling ourselves is never going to be the answer to specific physical or mental health concerns.
Let’s take olive oil as a case study.
Extra-virgin olive oil is another fat that we know is actively protective. It is a source of saturated fats, unsaturated fats and polyphenols, and evidence would suggest that we should be consuming a lot more of it, despite its high calorie content. Polyphenols are antioxidants, mopping up dangerous free radicals in our cells, anti-inflammatory (they have beneficial effects on the heart and blood vessels), protect against some cancers, they may prevent build up of fat tissues in the body, help prevent diabetes and they protect the neurons in our brains from degenerating. They act as a prebiotic for our gut microbes, encouraging growth of good microbes and inhibiting pathogens. There are many sources of polyphenols but olive oil, in all its complexity, seems to have the right mix of ingredients to work wonders. Taking a polyphenol supplement just does not have the same, if any, effect on our health, as would appear to be the case for the vast majority of food supplements.
So where do I stand on the fat issue? Personally, I don’t worry especially about the amount of saturated fat, regardless of source, in my diet but I am proactive in swapping saturated fat for unsaturated fat whenever possible, as much for the sake of my gut microbes as for cardiovascular health. Following a traditional Mediterranean style diet pretty much takes care of that — oily fish, olive oil, nuts, seeds and so forth.
In fact, the advice to switch to low fat, carbohydrate heavy diets (think low fat yoghurts, low fat cheese, margarine, pasta, breakfast cereals) from the 1960s to the present day is, in my view, perhaps the biggest single public health disaster of our times, because with it came the terrifying rise in processed and ultra-processed foods in the Western diet, and then followed an epidemic of 21st century illnesses.
To be continued.