Nutrition: How Food Becomes Energy, Growth and Health

What Nutrition Really Means

Nutrition is the set of biochemical and physiological processes by which a living organism uses food and water to stay alive. Food supplies nutrients: substances that the body breaks down to release energy, uses as raw material to build and repair its tissues, or needs in small amounts to keep its chemical reactions running. Every heartbeat, every thought and every growing hair depends on molecules that once arrived as part of a meal.

Nutrition is more than eating. It includes the whole journey of food: choosing and taking it in, digesting it into smaller pieces, absorbing those pieces into the blood, transporting them to the cells and finally using them in metabolism, the network of chemical reactions that keeps a cell alive. The science that studies all of this is called nutritional science. It draws on chemistry, biology and medicine, and while it covers every kind of organism, most research focuses on human nutrition.

A key idea is balance. A nutrient is called essential when the body cannot make it, or cannot make enough of it, and must get it from food. Having too little of an essential nutrient causes problems, but so can having too much. Both situations are forms of malnutrition, a word that literally means bad nourishment rather than simply not enough food.

The Main Classes of Nutrients

Nutrients are usually divided into two large groups. Macronutrients are needed in large amounts, measured in grams, and most of them provide energy. Micronutrients are needed in tiny amounts, measured in milligrams or even micrograms, but they are no less important.

Carbohydrates

Carbohydrates are made of carbon, hydrogen and oxygen. The simplest are sugars such as glucose and fructose; longer chains form starches, found in bread, rice, potatoes and pasta. The body breaks starches down into glucose, which is the main fuel for the brain and an important fuel for muscles. Extra glucose can be stored as glycogen in the liver and muscles, or converted into fat.

Proteins

Proteins are long chains of smaller units called amino acids. Human proteins are built from twenty standard amino acids, and nine of them are essential for adults, meaning they must come from the diet. Proteins form muscles, skin, hair and enzymes, the molecular machines that speed up chemical reactions. Meat, fish, eggs, dairy products, beans, lentils and nuts are rich sources. Animal proteins usually contain all the essential amino acids in good proportions, while plant foods can be combined, for example grains with legumes, to provide the full set.

Fats

Fats, or lipids, are the most concentrated source of energy in food. They also form the membranes around every cell, help the body absorb certain vitamins, and act as insulation and protective padding. Two fatty acids, linoleic acid (an omega-6) and alpha-linolenic acid (an omega-3), are essential for humans. They are found in plant oils, nuts, seeds and fish.

Vitamins

Vitamins are organic compounds that the body needs in small amounts. Humans need thirteen of them. Four are fat-soluble (A, D, E and K) and can be stored in body fat and the liver. The rest are water-soluble: vitamin C and the eight B vitamins. Most of these are not stored for long, so they need to be eaten regularly. Many vitamins help enzymes do their work; vitamin A is needed for vision, vitamin D for strong bones, and vitamin K for blood clotting.

Minerals

Minerals are chemical elements that the body needs, such as calcium, phosphorus, potassium, sodium, magnesium, iron, zinc and iodine. Calcium and phosphorus build bones and teeth. Iron sits at the heart of haemoglobin, the protein in red blood cells that carries oxygen. Iodine is used by the thyroid gland to make hormones that control how fast the body uses energy.

Water and Fibre

Water is sometimes left off lists of nutrients, yet it is the one we can survive without for the shortest time. It makes up roughly 60 percent of an adult’s body weight, carries nutrients and wastes, and helps control body temperature through sweating. Dietary fibre is a group of plant carbohydrates that human digestive enzymes cannot break down. Fibre adds bulk that keeps the gut moving, and bacteria in the large intestine ferment part of it, producing substances that nourish the cells of the gut wall.

From Plate to Cell: Digestion and Metabolism

Digestion begins in the mouth, where teeth crush food and saliva starts breaking down starch. In the stomach, strong acid and enzymes begin to unravel proteins. Most of the real work happens in the small intestine, where enzymes from the pancreas and bile from the liver finish splitting carbohydrates into simple sugars, proteins into amino acids and fats into fatty acids. The inner wall of the small intestine is covered in millions of tiny finger-like folds called villi, which give it an enormous surface for absorbing these small molecules into the blood and lymph.

Once inside the body, nutrients take part in two kinds of metabolism. Catabolism breaks molecules down to release energy. In the cells, glucose and fatty acids are gradually oxidised, mostly inside structures called mitochondria, and the energy is captured in a molecule called ATP, the cell’s energy currency. Anabolism does the opposite: it uses energy to build large molecules, such as new proteins for growing muscles or new DNA for dividing cells.

The body is also home to trillions of microbes, especially in the large intestine. This community, known as the gut microbiota, helps break down fibre and produces some vitamins, including vitamin K and certain B vitamins. Researchers are still discovering how strongly these microbes influence digestion and health.

Energy: Counting Calories

The energy in food is measured in kilocalories or in kilojoules. One kilocalorie is the energy needed to warm one kilogram of water by one degree Celsius. On food labels this unit is often written as a Calorie with a capital C, which can be confusing, because one food Calorie equals one thousand small calories.

Scientists first measured food energy by burning food in a sealed, water-surrounded container called a bomb calorimeter and recording how much the water warmed up. In the late nineteenth century the American chemist Wilbur Atwater worked out how much of this energy the human body can actually use. His simple rule is still applied today: about 4 kilocalories per gram of carbohydrate, about 4 per gram of protein and about 9 per gram of fat.

Even at complete rest, the body spends a large share of its energy just staying alive: keeping the heart beating, the lungs breathing and the body warm. This is called the basal metabolic rate. The brain alone uses about a fifth of the body’s resting energy, even though it makes up only around 2 percent of body weight. Children need extra energy and nutrients for growth, which is why good nutrition in childhood is so important.

When Nutrition Goes Wrong: Malnutrition

Undernutrition

When a person does not get enough energy or protein over a long time, the body begins to use its own tissues as fuel. Severe forms include marasmus, in which the body becomes extremely thin and wasted, and kwashiorkor, which is linked to a serious lack of protein and causes swelling, especially of the legs and belly. Undernutrition weakens the immune system, so infections become more frequent and more dangerous, and in children it can lead to stunting, meaning they grow less tall than they should.

Deficiency Diseases

A lack of a single micronutrient can cause a specific illness. Without vitamin C, collagen cannot be made properly and scurvy develops, with bleeding gums and slow-healing wounds. A shortage of vitamin D, or of calcium, in childhood causes rickets, in which bones become soft and bend. Too little thiamine (vitamin B1) causes beriberi, which damages nerves and the heart, while a lack of niacin causes pellagra, affecting the skin, gut and brain. Lack of iodine can enlarge the thyroid gland into a goitre and, during pregnancy, harm a baby’s brain development. According to the World Health Organization, iron deficiency is the most common nutritional deficiency in the world, and vitamin A deficiency is a leading preventable cause of blindness in children. Because these shortages can exist even when people have enough calories, they are sometimes called hidden hunger.

Overnutrition

Malnutrition also includes eating more energy than the body uses. The extra energy is stored as fat, and over years this can lead to obesity, which raises the risk of type 2 diabetes, heart disease and some other conditions. Too much of a single nutrient can also be harmful. Fat-soluble vitamins in particular can build up to toxic levels: Arctic explorers who ate polar bear liver, which is extraordinarily rich in vitamin A, became seriously ill. Today many countries face a double burden of malnutrition, with undernutrition and obesity existing side by side, sometimes even within the same family.

How Scientists Discovered Nutrients

For most of history, people knew that some foods kept them healthy without knowing why. One of the first controlled experiments in medicine was about nutrition. In 1747 the Scottish naval surgeon James Lind divided sailors suffering from scurvy into small groups and gave each group a different treatment. Those who received oranges and lemons improved strikingly fast. It still took decades before the British navy regularly supplied citrus juice to its sailors.

In the late eighteenth century the French chemist Antoine Lavoisier showed that breathing is a kind of slow burning: the body uses oxygen to release energy from food and gives off carbon dioxide, just as a candle does. This idea laid the foundations of the study of metabolism. During the nineteenth century, chemists identified carbohydrates, fats and proteins, and many believed that these, together with minerals and water, were all a person needed.

That belief collapsed thanks to a puzzle about beriberi. In the 1890s the Dutch doctor Christiaan Eijkman, working in the Dutch East Indies (today Indonesia), noticed that chickens fed polished white rice developed a beriberi-like illness, while chickens fed unpolished rice did not. Something in the outer layers of the grain protected them. In 1912 the Polish biochemist Casimir Funk proposed that such diseases were caused by the lack of special substances he called vitamines, from vital amines. When it turned out that not all of them were amines, the final e was dropped, giving the modern word vitamin. Eijkman shared the 1929 Nobel Prize in Physiology or Medicine with the English biochemist Frederick Gowland Hopkins, who had shown that animals need extra accessory food factors beyond the known nutrients.

In the early twentieth century the American doctor Joseph Goldberger showed that pellagra, then widespread in the southern United States, was caused by a poor diet rather than by germs. In the 1920s, Switzerland and the United States began adding iodine to table salt, one of the simplest and most successful public health measures ever, which sharply reduced goitre.

Nutrition Beyond Humans

All living things need nutrition, but they obtain it in very different ways. Autotrophs, such as plants, algae and some bacteria, make their own organic molecules. Plants use sunlight, carbon dioxide and water in photosynthesis, then take up minerals from the soil through their roots. The three minerals they need in the largest amounts are nitrogen, phosphorus and potassium, which is why these are the main ingredients of garden fertilisers. Heterotrophs, including animals and fungi, must get their organic molecules by eating or absorbing other organisms or their remains.

Nature has produced some surprising solutions. Carnivorous plants such as the Venus flytrap grow in poor, boggy soils and catch insects mainly to obtain nitrogen and other minerals, not energy. Cows and other ruminants cannot digest the cellulose in grass on their own; microbes in their large, multi-chambered stomachs do it for them. Termites, which feed on wood, likewise depend on microbes in their gut to break it down.

Building a Balanced Diet

No single food contains every nutrient in the right amounts, so health experts recommend eating a variety of foods. Many countries publish dietary guidelines shown as simple pictures. The United States replaced its well-known food pyramid with a plate diagram called MyPlate in 2011, showing that vegetables and fruits should fill about half of a meal, with grains and protein foods sharing the rest.

Common advice found in most national guidelines includes:

• Eat plenty of vegetables, fruits, whole grains, beans and nuts.
• Choose water as the main drink.
• Limit sugary drinks and foods high in added sugar. The World Health Organization advises keeping free sugars below 10 percent of daily energy intake.
• Use less salt. For adults, the World Health Organization recommends less than 5 grams a day.
• Prefer unsaturated fats, found in plant oils, nuts and fish, over large amounts of saturated fat.

Needs also change through life. Babies, growing teenagers, pregnant women and older adults all have particular requirements, which is why nutrition advice is often tailored to age and life stage.

Surprising Facts About Nutrition

• Most mammals can make their own vitamin C, but humans cannot. We share this inability with apes and monkeys, guinea pigs and some bats, which is why scurvy can affect people but not dogs or cats.
• Vitamin D is unusual: the skin can make it when exposed to sunlight, so in a sense it is part vitamin and part hormone.
• Vitamin B12 is made only by microbes. Animals get it from their food or from bacteria, which is why people who eat no animal products are advised to use fortified foods or supplements.
• Fibre was long seen as useless roughage. Today it is valued as food for the gut microbiota and for its link with a healthy digestive system.
• The idea that you are what you eat is literally true in one sense: many of the atoms in your body are steadily replaced by atoms that came from your food and drink.