While we each choose a variety of foods and beverages for different reasons, the biological reason that we consume foods and drinks is to obtain the nutrients our bodies need to grow, maintain good health, repair damage, do work and reproduce. The essential nutrients, or ‘macronutrients,’ that are found in the largest amounts in foods and drinks are proteins, carbohydrates, and fats. Most foods and drinks contain relatively small amounts of other essential nutrients too, including vitamins, minerals, and dietary fibre.
While people talk about carbohydrate foods or protein foods, the reality is that most foods contain a mix of the macronutrients in varying proportions. For example, carbohydrate-rich foods like bread contain protein and fat, and protein-rich foods like meat contain fat. There are a few exceptions, such as refined fats, oils and sugar, but these are generally used as ingredients rather than eaten on their own.
The word energy brings to mind different concepts, from vitality, to have the ‘get up and go’ to do things, to the amount of kilojoules or calories in foods and drinks. Energy is all of these things.
The macronutrients protein, carbohydrate and fat, are made up of four basic building blocks: carbon, hydrogen, oxygen, and nitrogen. These are broken down through complex biological processes, and the energy from these elements is converted to the body’s universal energy currency, adenosine triphosphate (ATP). This is used to drive all the body’s processes — growth, maintenance, repair and reproduction.
In the metric system, energy in foods is measured in kilojoules (kJ), which is a mandatory component in nutrition information panels on food labels. Calories (cal) are still commonly used, and can also be found in many nutrition information panels. To convert kilojoules to calories, simply divide them by 4.2.
It’s worth knowing that the average estimated daily energy requirement for adult Australians and New Zealanders is 8700 kJ (2070 cal) a day. For the EU and many other parts of the world, it is 8400 kJ (2000 cal) a day. This slight difference is due to the way the requirements are estimated.
Most people know that muscles are made up of protein, and that meat is a good source of protein, but what is protein?
Proteins are chemical compounds made up of chains of amino acids, which themselves are composed of the elements carbon, hydrogen, oxygen, and nitrogen. Twenty amino acids are essential to humans. Nine of these amino acids (histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan and valine) are considered essential because they must be obtained from foods and drinks, whereas the non-important amino acids can be synthesized within the body. All of the proteins that are found in the body are made from these 20 amino acids.
Proteins are essential parts of the structure and function of every cell in the body. The body of an average 80 kg adult contains about 13 kg of protein, with 43% found in muscle, 16% in blood and 15% in skin. The protein in the cells is constantly being recycled and replenished, which is one reason why we need to obtain a range of proteins from foods and drinks each day. In addition to being used to make proteins, amino acids can be used for specific purposes within our cells, such as the formation of nerve transmitters and hormones.
Carbohydrates are made up of carbon, hydrogen, and oxygen molecules. They are primarily used as an energy source by our bodies, being the preferred fuel for our brains and nervous systems, and for our exercising muscles. They have many other roles, and they add taste, texture, and color to our foods and drinks. Carbohydrates are found in plant foods and some dairy products, including milk. The three most common kinds are sugars, starches, and dietary fibres.
The simplest form of carbohydrates are the monosaccharides (literally ‘one sugar’) fructose, glucose and galactose. As the name suggests, fructose is found in fruit, making up around half the carbohydrate in a typical piece, and is commonly called fruit sugar. It is also found in other foods like honey and in the sap of certain plants (for example, agave and maple trees). Glucose is also found in fruit and honey, as well as in grains and vegetables.
Two monosaccharides joined together are known as disaccharides (literally ‘two sugars’). The most common varieties are sucrose, maltose, and lactose. Sucrose is made up of fructose and glucose and is the most common form of sugar in foods and drinks. It occurs naturally in fruit, but two of the best sources are sugar cane and sugar beets, which are both used to make a variety of refined sugars and are added to foods and drinks for taste, texture, and color. Maltose is made up of two units of glucose. It is found naturally in grains, such as barley, and is commonly added to foods as an ingredient. Lactose is made up of glucose and galactose and is found in milk and yogurt. It is also added to some foods as an ingredient.
Starches are made up of long chains of glucose and are found naturally in a wide range of foods, including grains, legumes, starchy vegetables, nuts, and seeds. They are also added to many foods as an ingredient (for example, thickeners). There are two main kinds of starches, amylose, and amylopectin. Amylose is like a string of glucose molecules that tend to line up in rows and form tight, compact clumps, whereas amylopectin is a string of glucose molecules with lots of branching points — a bit like a tree.