Food and fuel

The energy in food is measured in calories. The calorie on a food label is the kilocalorie, written kcal, the energy needed to warm a kilogram of water by one degree Celsius, and many countries print kilojoules beside it. An adult uses roughly 2,000 to 2,500 a day, and most of that goes on simply staying alive: keeping warm, pumping blood, running the brain.

The nutrients do not carry equal amounts. A gram of carbohydrate supplies about 4 calories, and so does a gram of protein. A gram of fat supplies about 9, more than twice as much. That is why fat is the body’s choice for storing energy: it packs the most fuel into the least weight. It is also why fatty foods add up so quickly on a label.

Some nutrients are needed only in tiny amounts, yet the body fails without them, and most of them it cannot make for itself. The vitamins are the best known. On long sea voyages sailors used to sicken with scurvy. Their gums bled, old wounds opened and many died. In 1747 the naval surgeon James Lind gave pairs of sick sailors different remedies. Within a week the pair given oranges and lemons were nearly well, and the others were not. The missing substance was later named vitamin C. Minerals such as iron and calcium are needed in the same small way.

One more part of food matters although the body cannot digest it. Fibre is the tough material of plants. It passes along the gut almost unchanged, gives the gut’s muscles something to push against and keeps food moving through. Wholegrain bread, beans, fruit and vegetables supply it.

You are not made of bread or rice, yet you are built from what you eat. The reason is that food is first taken apart. The molecules of starch and protein are long chains of smaller units, and the molecules of fat are bulky as well. They are too large to pass through the wall of the gut into the blood. Breaking them down into molecules small enough to be absorbed is called digestion. Starch is cut into glucose, protein into amino acids, and fat mostly into fatty acids. These small pieces are absorbed, and the body builds its own substance out of them.

The work is done along one tube, several times as long as you are tall. Food is chewed in the mouth, churned in the stomach, broken down and absorbed in the small intestine, and what is left is dried out in the large intestine.

Digestion starts before you swallow. Teeth grind food into a paste, and saliva wets it and begins to cut the starch in it into sugar. Chew a piece of bread for a minute and it starts to taste sweet.

The stomach is a muscular bag that churns each meal for two to four hours. Its wall releases hydrochloric acid, strong enough to kill most of the microbes swallowed with food, and with it a substance that starts taking protein apart and works best in that acid.

Why does the stomach not digest itself? Its lining is coated with a thick layer of mucus, a slippery gel that the acid cannot easily cross, and the cells beneath are replaced every few days. Where that barrier is weakened, most often by a bacterium that lives in the stomach or by certain painkillers, the acid reaches the wall and an ulcer forms.

Left to itself in warm water, a lump of starch would stay starch for longer than a lifetime. In the gut it is gone in hours, because of enzymes. An enzyme is a protein that speeds up one particular chemical reaction without being used up, so the same enzyme molecule can cut thousands of food molecules one after another.

Each enzyme does one job only. The one in saliva cuts starch and ignores protein. The one in the stomach cuts protein and ignores starch. The reason is shape. Part of the enzyme is a pocket shaped to fit one kind of molecule exactly, as a lock fits one key. A molecule of the right shape slots in, is cut and is released. Any other molecule does not fit, and nothing happens to it.

Because an enzyme works by its shape, anything that changes the shape stops the work. Temperature is the main thing. Warm an enzyme and it works faster, because molecules move faster and meet more often. The enzymes of the human body work fastest at about 37 °C, which is the temperature the body keeps.

Heat it much further and the delicate folding of the protein comes apart. The pocket loses its shape, nothing fits it, and the enzyme stops working. It is then said to be denatured, and the change usually cannot be undone. You have watched it happen. The clear white of an egg turns solid and opaque in the pan because its proteins are denatured, and no amount of cooling turns it clear again. This is one reason a very high fever is dangerous.

Fat sets the gut a special problem. The enzymes that digest it are dissolved in water, and fat and water do not mix. The fat of a meal gathers into large greasy globules, and an enzyme can work only on the outside of a globule.

The liver supplies the answer. It makes a greenish liquid called bile, which is stored in the gall bladder and squirted into the small intestine when fat arrives. Bile is not an enzyme and digests nothing. It works as washing-up liquid works on a greasy pan: it breaks the fat into droplets, thousands of tiny ones in place of each large globule. The same amount of fat now has a vastly greater surface, and the enzymes can reach it and finish the job quickly.

Most digestion, and almost all absorption, happens in the small intestine. It is the longest part of the gut, five or six metres of narrow tube, and its inner wall is not smooth. It is covered with millions of finger-like projections called villi, each about a millimetre long, like the pile of a thick towel. Inside each one is a net of fine blood vessels, and the glucose and amino acids pass through its thin wall into the blood. With the villi, and with still finer folds on their cells, the wall has an absorbing surface of about thirty square metres, the floor of a small flat.

Most of the water in a meal goes the same way. What remains, chiefly fibre and water, moves on into the large intestine, which takes back most of the water that is left. The leftovers, now solid, leave the body.

Absorbed glucose is carried by the blood to every cell, and there its energy is finally released. The process is called respiration. Glucose is combined with oxygen, and what comes out is carbon dioxide, water and energy.

It is the same overall change as burning sugar in a flame, but the cell does it in many small, controlled steps at body temperature, so the energy comes out a little at a time. Part of it is put to work, and the rest keeps you warm.

This is what breathing is for. The oxygen you breathe in is carried by the blood to the cells for respiration, and the carbon dioxide they make is carried back to the lungs and breathed out. A cell working hard uses more oxygen and makes more carbon dioxide, which is why running makes you breathe faster.

Where in the cell does this happen? Mostly in the mitochondria, small rod-shaped compartments found in almost every cell. A cell that uses a great deal of energy, such as a muscle cell, has thousands of them.

The mitochondria do not hand the energy out raw. They use it to make a small molecule called ATP, which works like a charged battery. Wherever the cell needs energy, to contract a muscle or build a protein, a molecule of ATP is spent there and is then sent back to be recharged.

And the glucose itself? It was made by a plant. In photosynthesis a plant uses sunlight to join carbon dioxide and water into glucose, releasing oxygen. Respiration runs the other way. The energy your cells release from a meal arrived on Earth as sunlight.

Blood leaving the small intestine does not go straight round the body. It flows first to the liver, the largest organ inside you, which deals with what the meal has delivered.

After a meal the blood holds more glucose than the body needs at once. The liver takes up the spare and stores it until it is needed. Hours later, when the glucose in the blood begins to fall, the liver releases it again. In this way the brain and the muscles receive a steady supply, although meals come only a few times a day.

The liver also breaks down harmful substances that were absorbed with the food. And it deals with spare amino acids, which cannot be stored. It converts the part that cannot be used into a waste called urea and sends it out in the blood.

The body’s own chemistry makes waste, and getting rid of that waste is called excretion. Carbon dioxide leaves through the lungs. Urea leaves through the kidneys, two bean-shaped organs at the back of the abdomen. They filter the blood all day long, and about 180 litres of fluid pass through their filters every day. Almost all of it, with the glucose and salts it carries, is taken straight back into the blood. About a litre and a half, holding the urea, is passed out as urine.

The kidneys also decide how much water to let go. Drink a lot and they pass more. Go thirsty and they keep it back. This is one case of something the whole body does. Keeping conditions inside the body steady, such as its water, its temperature and the glucose in its blood, is called homeostasis.