What Exactly Is Adipose (Fat) Tissue, And How Does It Matter In Ageing? Part 1: Fat Molecules

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Fat isn’t a bad thing in and of itself. Having too much fat was rarely a problem for our evolutionary ancestors. Thanks to the power of technology, however, modern humans have found increasingly innovative ways to modify aspects of our environment that had remained mostly unchanged for millions of years. Through chemistry, we learnt to copy soil microorganisms and extract or ‘fix’ nitrogen from the air, enriching our fields with nutrients and allowing us to sustain unprecedented levels of agriculture. Alongside other technologies like mechanisation, this allowed some countries to produce significantly more food than they needed. Now, there are more people struggling to stay thin than there are who are undernourished, with around 2 billion overweight adults worldwide according to the WHO.

As an increasing proportion of the population become obese and overweight, the incidence of deadly and debilitating age-related diseases (such as heart disease, diabetes, and cancer) also rises. This problem has become so severe that the term ‘epidemic’ is being used to describe it. All this, it’s fair to say, has given fat a bad name. Yet fat is not the problem – fat is a type of molecule, and one that is essential to complex life. Fats are the body’s best means of storing energy long-term as a precaution against famine, and they also play many other important roles within our cells.

We would all benefit from understanding fat a little better, but trying to do so can be confusing. When it comes to both body fat and the fat that we eat, some fat is healthy and some fat is unhealthy. You don’t need to eat any fat to get fat, and some weight loss diets involve eating little else but fat. Fat isn’t a straightforward topic. Indeed, fat isn’t even a particularly good word for fat, as we’re about to see. So let’s explore the world of fat in more detail. In this first article, we will explore the fundamentals: what is fat, chemically speaking, and why do we need it? Future instalments will cover why some fat is considered ‘bad’ or ‘good’, what happens to fat tissue as we age and, in turn, why having too much of it seems to promote diseases of ageing.

What Are Fats?

The term fat is variably used to refer to a molecule or a tissue where said molecule is stored, also known as adipose tissue. We are interested firstly in the fat molecule, which is a type of lipid. Lipids that are solid at room temperature are referred to as fats, while lipids that are liquid at room temperature are referred to as oils. This is also confusing, because the flammable stuff that we extract from the ground is also called oil despite being chemically very different from lipids. So for the sake of clarity, let’s stop using the terms fat and oil for a moment.

What Are Lipids?

Lipids are a family of biological molecules that are mostly insoluble in water. For the purpose of understanding human health, the most important types of lipids are:

• Cholesterol: An organic molecule that is an important structural component of our cells. Cholesterol is also used to make vitamin D and some hormones.
• Fatty acids: Acidic molecules that can be transported via the blood. Such fatty acids are referred to as ‘free’ fatty acids, though they aren’t really free as they can only be transported this way while bound to proteins.
• Triacylglycerol (aka fat): A molecule made up of three fatty acids, each joined to a ‘backbone’ of glycerol, which is an alcohol three carbon atoms long. A triglyceride can have any combination of fatty acids, making them quite diverse.
• Phospholipids: A pair of fatty acids joined to a phosphate group. Phospholipids are an essential component of cell membranes.

When it comes to storing excess energy, triacylglycerols act as the primary long term storage molecule. Triacylglycerols can be broken down (this is called lipolysis) into their constituent components and fed to the mitochondria, the power plants of the cell, in order to produce the ‘cellular fuel’ known as ATP. Triacylglycerols (and lipids in general) are suited to their role as a storage molecule due to their inability to mix with water, which allows them to be stored inside cells without drawing water into those cells. This means that the space used to store triacylglycerols is used efficiently. This is in contrast with the body’s short-term energy store, glycogen, which is soluble in water. When a person stops eating, most of their energy comes from glycogen initially, and so most of the weight they lose over the first 24 hours or so of fasting is actually the result of lost water.

Thus, we need lipids to store any significant amount of energy in the long term, and the liver is capable of converting other nutrients into lipids for storage should nutrient intake exceed the demands of the body.

Cholesterol can be absorbed from food, but is also made by the liver. Like triacylglycerol, cholesterol is insoluble in water and is stored inside adipocytes. Cholesterol is also an important component of the membranes of our cells, composing about 30% of all animal cell membranes. Cholesterol helps keep cell membranes fluid across a range of temperatures, and also helps cells to communicate with each other. Cholesterol is a major component of the the insulating layers that surround our fast nerve fibres. This insulation, called the myelin sheath, protects nerve fibres and greatly increases the speed of their electrical signals. Cholesterol is also used to synthesise all steroid hormones, including the sex hormones oestrogen, progesterone and testosterone. These hormones cannot be made without cholesterol: if we don’t consume enough cholesterol in our food, the liver will step up its own cholesterol production to compensate.

Phospholipids are not often discussed when it comes to health, but they are nonetheless essential to our survival. Phospholipids have a lipid component, which is miscible with other lipids, and a phosphate component, which is miscible in water. This means phospholipids can form an interface that allows lipids to ‘dissolve’ in water. They make up the majority of membranes of cells and are also a major component of lipoproteins, structures that transport lipids via the blood.

How Are Lipids Moved Around The Body?

The body needs a way of moving lipids around via the blood. Lipids need to be carried between the intestines (where they are absorbed), the adipose tissue (where they are stored), the liver (where they can be processed, converted into other forms or removed from the body) and the tissues that can use them for energy. Transporting lipids in the blood isn’t as straightforward as it is for other nutrients because blood plasma is 90% water, in which lipids do not dissolve. Instead, triacylglycerols and cholesterol are transported inside special particles called lipoproteins.

You have probably heard of lipoproteins being referred to as low or high density (LDL and HDL), or even as ‘bad’ vs ‘good’ cholesterol – a confusing term since lipoproteins contain more than just cholesterol. We’ll cover these in more detail when we talk about what makes lipids ‘good’ or ‘bad’. For now, you can remember that LDL carries lipids to tissues throughout the body, while HDL collects lipids from most tissues and takes them to the liver, with the latter process being generally more desirable.

In addition to being carried inside lipoproteins as part of triacylglycerols, fatty acids that have been separated from their glycerol backbone can also be transported in the blood plasma as ‘free’ fatty acids. The term ‘free’ is somewhat of a misnomer, though, as these fatty acids must still be bound to proteins such as albumin, the most abundant protein in the blood plasma.

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That concludes our introduction to lipids. We hope it helped you understand what fat actually is at the molecular level, and why it’s essential for our survival. Now that we’ve established the fundamentals, we can start talking about that stuff that most people are trying to get rid of: the fat tissue itself. Yet aside from the inconvenience of having to buy a new pair of jeans, why is having too much fat a problem in the first place when fats are such a necessary part of our biology? Why are some fats considered ‘good’ and others ‘bad’? These questions will be explored in the subsequent parts.