Why do we eat food?

You may well be thinking that the question posed in the title of this blog has an all-too-obvious answer. We all know that we eat food to keep ourselves alive. But why do we find ourselves slaves to our appetites and rumbling stomachs? What is actually happening inside each of us that couldn’t happen without another slice of toast, or piece of fruit, or that most vaunted of mid-afternoon pick-me-ups, the sneakily-consumed bar of chocolate?

We’re all familiar with the concept of something needing fuel to keep it going. Just as a power station requires gas or coal to power its turbines and generate energy, so we need fuel – in the form of food – to power our continued existence.

The foods we eat provide us with a range of nutrients: vitamins, minerals, water, fat, carbohydrates, fibre, and protein. These nutrients are put to different uses – as building materials to construct the tissues and organs from which our bodies are made; as the components of the molecular machinery that keeps our cells running as they should. All of these uses are unified by a common theme: a requirement for energy to make them happen. And this is where one particular type of nutrient comes into its own. Step forward the carbohydrates.

Carbohydrates are better known to us as sugars – but in fact the sweet crystals we know as sugar are only part of this group. Carbohydrates come in very different shapes and sizes. One of the smallest is glucose, which acts as a chemical building block: multiple copies of glucose can join together to form a range of much larger molecules. For example, starch – found in potatoes and flour – is a carbohydrate formed from many individual molecules of glucose joined together in long chains. (Based on taste alone, you wouldn’t think that starch was made of glucose: even though individual molecules of glucose taste sweet to us, once they are linked together to form starch the sweetness is lost.)

To understand how the sugar in our food can power the processes occurring in our cells every minute of every day, let’s follow some starch on its journey through the body. Many of the foods we consume aren’t in a form our bodies can do anything useful with. Instead, they need to be digested. And so it is with carbohydrates such as starch. This process of digestion starts as soon as the food enters our mouth: our saliva contains special substances (called enzymes) that start attacking the long chains of starch, breaking it into smaller fragments.

Digestion continues as our food is swallowed and slides down into our stomach, where an arsenal of other chemical weapons set to work on the mouthful we’ve just consumed. Before long, what were initially mouth-watering morsels are reduced to something rather less appetising and leave the stomach to enter the long, snaking tunnel of our intestines. By now, the long chains of starch have been broken down into glucose, which is small enough to pass through the lining of our intestine and into our bloodstream. Our bloodstream acts as a short- and long-distance transport network, carrying the newly-arrived sugar molecules to cells all over the body.

When glucose arrives at its destination and first enters the cell, it undergoes a chemical make-over to transform it into a new substance called pyruvate. And this is where the real fun begins.

At this point, let me introduce you to a special inhabitant of our cells, the capsule-shaped mitochondrion (or mitochondria, if you’re referring to more than one). In essence, mitochondria provide each cell with its own power supply. The more active a cell is – and so the more energy it needs – the more mitochondria it contains. Muscle cells, which require a lot of energy to power their movement during muscle contraction, may contains thousands of mitochondria; by contrast, skin cells, which only require a modest energy supply, may contain only a few hundred.

But how do mitochondria actually power a cell? Well, mitochondria act as factories for a special chemical called ATP. ATP is like a portable mini-battery: it stores energy, and can be shuttled off to wherever in the cell that energy is needed (at which point the stored energy can be released).

So what has the production of ATP by mitochondria got to do with us eating carbohydrates? I mentioned earlier how glucose is converted into pyruvate when it enters the cell. This pyruvate is then shipped into the mitochondrion. Once inside the mitochondrion, pyruvate enters a chemical production line, a series of linked chemical reactions and molecular processes that use the pyruvate ultimately to produce ATP. (I won’t go into details, despite the fact that, to a biochemist like me, the process is ingenious. Just take my word for it if you will.)

This process of mini-battery production relies on more than just glucose to keep it going: it also needs a constant supply of oxygen. Indeed, this reliance upon oxygen is the whole reason why we need to breathe every minute of our lives. If we stop breathing, we stop supplying oxygen to the mitochondria in our cells – and they can no longer produce ATP. Without ATP, there is no energy to power the processes needed to keep a cell alive. Without energy, cells die.

The importance of ATP to our very existence is also highlighted in surprising ways: Agatha Christie’s Sparkling Cyanide was first published in 1945, and features two characters whose meals at a restaurant prove to be their last: they are both poisoned with cyanide. Cyanide has its lethal effect by blocking the chemical production line taking place in our mitochondria. If cells can’t produce ATP, they lose their energy source and quickly die (just like in the absence of oxygen). And if this happens in cells throughout the body simultaneously, it’s not long before the body as a whole can no longer function, as Agatha Christie’s characters had the misfortune to discover.