Given that nutrition is so infernally complicated, let’s ask a simple question: Why must we eat?

Certainly, food today has become the source of a great deal of confusion and anxiety, which is a terrible pity because food is also the source of so much that is good — excellent, in fact. Food binds us together socially and culturally. It gives a great deal of pleasure while also providing the stuff that fuels life itself.

Energy is the most familiar of the things we need from food. Not a day goes by without seeing numbers written all over our foods, meals, and now also menus — like mathematical graffiti saying how much energy they contain and stern dietary guidelines warning how much of it we should eat. The labels don’t use the word energy, of course. You’re probably more familiar with the term Calories.

But what exactly is a Calorie?

It’s simply a unit of energy — a single Calorie is the amount of energy needed to raise the temperature of 1 kilogram of water (which is the same as a litre) by 1 degree Celsius, from 14.5 to 15.5°C.

Yes, this is a bizarre currency indeed, except, perhaps, if you have ever wondered how much food it takes to heat a bathtub. But it is rigorously exact, which we scientists tend to like. And so, everyone is stuck thinking in calories, even though they are hard to picture.

To confuse things further, you may also see calories written as kilocalories (kcal). This is because 1 Calorie (with a big “C”) equals 1,000 calories (with a small “c”). You may also have seen the energy content of a food presented as kJ — kilojoules, which is what we scientists mainly use, along with kcal.

These units have an even more unlikely-sounding definition: 1 kJ is the energy needed to move a 1-kilogram weight by a force of 1 Newton (which is itself a measure of gravitational pull), over a distance of 1 metre. One kilojoule equals 0.239006 Calorie (to be exact!).

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In this book we will mainly use “kcal” when presenting units of energy, but on occasions when presenting scientific results, we will use “kJ.” Throughout, we will use “calories” with a small “c” as a generic term for “energy.”

This means we determine the energy contained in food based on its theoretical power to fuel action — to heat water or move weights.

All food contains calories, with the exception of water, which is just as well, because without energy, our bodies wouldn’t be able to do anything, including make use of the other important thing we take from food: nutrients.

Energy comes from the main nutrients in our diet — the macronutrients, as they are known — each of which is chemically different. Once we consume these nutritional fuels — proteins, carbohydrates, and fats — they are broken down into smaller molecules that are burned within our cells.

Macronutrients deliver more than just energy, however. Proteins and their building blocks, the amino acids, also supply nitrogen, with which we make all sorts of other important things, including hormones, enzymes, and the information-storing molecules DNA and RNA. If we don’t ingest protein, we don’t live.

In the popular mind (and in many diet books), fats and carbohydrates (carbs) have almost become just another way of saying “Calories,” but there is a lot more to them than that. Fats insulate us from the cold, store vitamins, lubricate the skin, and cushion our eyeballs and joints.

Their fatty acid building blocks make up the membrane that surrounds every cell in our bodies, and special fats called sterols serve as messengers that help coordinate the complex chemistry that keeps us alive.

We can’t do without fats.

Carbohydrates include sugars, starches, and fibres. Like proteins and fats, most carbs are built of smaller units, in this case, simple sugars such as glucose and fructose. The nutritional properties of different carbs depend on which simple sugars they consist of and how they are strung together. The most abundant carbohydrate on our planet — the plant fibre cellulose — has its glucose units strung together so tightly that we can’t digest it.

Glucose is particularly important because it’s the main carbohydrate on which our bodies depend. In addition to providing energy, glucose partners with the nitrogen from protein to build DNA and RNA.

Our bodies can create glucose by breaking down proteins and fats, so, strictly speaking, we don’t have to eat any carbs to get glucose. But that is not the same thing as saying we don’t need to eat carbs at all — as we will show later.

And those are just the macronutrients. Vitamins and minerals are needed, too, though in tiny amounts compared with the big three, which is why they are called micronutrients.

These are used by the body for too many purposes to mention here. But keep in mind that sodium, calcium, magnesium, chloride, and potassium all generate electrical currents that quite literally make us tick — our hearts beat and our nerve cells crackle with electrical impulses.

The diagram above provides a summary of what’s in food.

As we can see from the diagram, foods are complex mixtures of many nutrients; not to mention diets, which are themselves complex mixtures of foods. To understand nutrition, we need to think not in terms of single nutrients but rather the balance of nutrients in these mixtures.

If an animal is to thrive, it needs to eat macro- and micronutrients in the right amounts — like Goldilocks, not too little, not too much. Some animals, such as parasites living within the body of their host, get all the nutrients they need, and in the right balance, from a single food source. For them, choosing the proper diet is easy.

All mammals, including ourselves, are lucky to start life in such ideal circumstances — because mother’s milk is as close as we’ll ever come to a perfectly balanced diet. It contains, in proper proportions, everything a newborn needs to grow. But after a mammal is weaned, nutrition becomes a much trickier undertaking.

It’s easy to see why. The things we eat are made up of nutrients in almost endless combinations. Some foods are richer in protein, others in fats or carbs — but all are mixtures. There are no single-nutrient foods.

Pasta and bread certainly live up to their carb-rich reputations, but about 10 per cent of the energy they contain comes from protein. A steak is a protein powerhouse but more than half of it is water, along with lots of fat and minerals, too.

We humans complicate matters even further because, unlike other animals, we tend not to eat single foods; rather, we assemble them into recipes and meals. Then we combine meals into varied diets and dietary patterns, sending complex mixtures of nutrients and other substances into our bodies, where they interact with our physiology.

Now, imagine having to consciously navigate and balance the proper mixture of all that on a thrice-daily basis. We would all need PhDs in mathematics and computing, and even then the calculations wouldn’t leave us time to do much else.