September 19, 2020
But there's a growing body of scientific evidence suggesting that the health effects of the fat in the food we eat are a lot more complicated than conventional wisdom suggests.
The US government first started suggesting that Americans should switch to low-fat food in 1977, and it hasn’t changed its mind since then. The current FDA guidelines state that people should switch to low-fat dairy products and lean meats, and ‘limit calories from saturated fats.’
But our own research shows that even genetically identical twins can have very different responses to meals with exactly the same amounts and type of fat.
So what’s the truth? Let’s take a deep dive into the fascinating world of fat.
Fat is not one single entity. There are several types of naturally occurring fat in foods, and they have different properties and effects on our health.
On a molecular level, fats are made up of long chains of carbon and hydrogen atoms. What makes the difference between the main types of fat we eat – saturated, unsaturated and trans-fats – are slight variations in how the carbon atoms are joined together.
Take a look at this handy diagram:
All the carbon atoms in saturated fats are joined together with single chemical bonds (those are the single black lines). This chemical structure means they’re relatively rigid, so saturated fats like butter and lard are solid at room temperature.
Unsaturated fats have double bonds between some of the carbons (highlighted by pink circles), changing their physical and chemical properties and giving them a bit more wiggle room on a molecular level. This means that they are liquid at room temperature, but can solidify at cooler temperature.
Mono-unsaturated fats, with just one double bond in every fat molecule, are the main components of olive and canola oil. Sunflower and fish oils are rich in poly-unsaturated fats, with two or more double bonds per molecule.
So, what difference does a few double bonds make when it comes to health?
For a long time it has been thought that eating a lot of saturated fats causes higher levels of low-density lipoprotein (LDL) or ‘bad’ cholesterol in your blood.
Increased LDL has been shown to increase risk of developing heart disease, so it makes sense that cutting down on saturated fat in your diet should be good for your heart – the idea underpinning the current nutritional advice.
Around a fifth of the saturated fat in our food comes from dairy products. As a result, nutritional guidelines often suggest that people should reduce their dairy consumption and switch to low-fat dairy options.
However, there’s a growing recognition that labelling all saturated dairy fat as ‘bad’ is overly simplistic and even misleading.
Low-fat dairy products frequently replace saturated fat with carbohydrates. For example, low-fat fruit yoghurts may contain extra sugars, either from natural sources like fruit and honey or from refined sugar.
But replacing fat with carbohydrates does not reduce the risk of heart disease, and may actually increase it. A recent study, known as the PURE study, showed that eating more saturated fats from dairy (except butter) was associated with a lower number of deaths compared with eating more carbohydrates. And a large analysis of 17 studies showed that drinking milk might be associated with a reduced risk of cardiovascular disease.
As well as saturated and unsaturated fats, there’s another type of fat that you might have heard of: trans-fats.
From a chemical perspective, trans-fats look very similar to unsaturated fats but are twisted a different way around their carbon double bonds (shown in blue in the image below).
These rearranged molecules melt at higher temperatures compared with other unsaturated fats, making them solid at room temperature. But it also makes them much more stable in their liquid form at high temperatures than unsaturated oils.
There are two types of trans-fats in foods: artificially- and naturally-produced.
First developed in around 1900, artificial trans-fats are created from industrially processed vegetable oils.
They have similar properties to animal fats like butter, giving foods a desirable taste and texture, but are much cheaper and have a longer shelf-life. It’s therefore not surprising that they quickly became ubiquitous in baked goods like cakes and pastries.
But in the 1990s, researchers started to link consuming artificial trans-fats with raised LDL cholesterol levels and an increased risk of heart disease, cancer, diabetes, and depression. As a result, their use is restricted in Europe and they have been banned by the US FDA.
However, the story is very different for naturally-occurring trans-fats, which are molecularly different from the trans-fats created through industrial manufacturing.
Certain trans-fats occur naturally in dairy and meat from sheep, cows and goats, and may even be beneficial rather than harmful to health.
The problem with blanket labeling some fats as ‘good’ and some as ‘bad’ is that it ignores the physical and chemical complexity of the foods we eat.
Many countries are now incorporating food-based recommendations into their nutritional guidelines, as mounting evidence suggests that eating high-quality food is much more important than focusing on simplistic nutrient ratios.
New research shows that the health effects of saturated fat depends on the specific chemical composition of the fat and where it comes from.
Food often has a complex structure that affects its digestion, nutrient absorption, physiological properties, and health properties. As a result, the health effects of food depend on the food itself, not the individual nutrients within the food.
We are all individuals, and it makes sense that we all respond to fats in different ways. In our PREDICT 1 study, we measured how blood fat levels change after eating in a thousand participants. The results showed that personal responses to eating fat are highly individual, and even identical twins can respond to fats in different ways. Each blue line in the graph represents the blood fat level of a healthy person after eating. In fact, there was a wide range of responses that fell outside of the 'average'.
So, are fats 'good' or 'bad'? It depends on which foods they're found in, your wider dietary patterns, and your unique biological response! Join ZOE today, discover the best foods for your body and let your biology do the work for you.