As well as being one of life’s great pleasures, food is vital to our survival. It serves as a fuel, enabling the body to create heat and energy, and it provides essential nutrients – all the substances that the body needs to function, but cannot manufacture by itself. Without an adequate food supply, the body would start to burn anything, including muscle, to keep itself alive.
Scientists have identified more than 40 different nutrients that are needed in the diet every day – from a wide variety of foods – in quite precise amounts. This means that we now have an excellent understanding of what constitutes a balanced diet and how to achieve health and prevent disease through the food we eat.
Nutrients can be split up into those that we need a lot of (macronutrients) and those that we need only in tiny amounts (micronutrients). Macronutrients include carbohydrates, proteins, fats and water. These provide us with our day-to-day energy, and enable the body to generate heat, grow, repair and maintain itself. Micro-nutrients include the vitamins, minerals and trace elements needed for the chemical processes that take place inside our bodies. Each nutrient is broken down into a usable form by the digestive system.
Macronutrients make up the bulk of our diet and are required in large amounts to fuel and repair the body.
The term ‘carbohydrates’ is a collective name for the sugars, starches and cellulose (the main component of fibre) that we get from food; they are found almost exclusively in plants. Chemically speaking, carbohydrates consist of various combinations of carbon, hydrogen and oxygen atoms; they are made by green plants from carbon dioxide, water and energy that is provided by sunlight, and stored as the plant’s main source of energy.
Carbohydrates are also our main source of energy – they are converted into glucose (the fuel we need for both physical and mental exertion) – in the small intestine. Glucose is the smallest and simplest breakdown product of starch and the form that sugar takes in the bloodstream. It is the body’s primary source of fuel. Cells use glucose as their basic source of energy, muscles burn it for the body’s activities, and it is the only fuel that the brain uses in normal situations. Glucose can either be used as an immediate source of energy or stored for a short time in the muscles and liver as a substance called glycogen.
Carbohydrates are divided into two broad categories – simple and complex.
The sugars found in foods such as table sugar (sucrose) and honey are called simple carbohydrates. They are divided into monosaccharides (simple sugars) and disaccharides (two linked simple sugars). The most common monosaccharides are glucose, found in fruit and vegetables, and fructose, found in honey and fruit. The main disaccharides are sucrose (found in fruits, vegetables, honey, cane sugar and beet), lactose (in milk), and maltose (in malt). Some simple carbohydrates can give us a fast ‘fix’ of energy because they are already broken down into their component parts, and are quickly absorbed into the bloodstream.
Groups of simple sugars linked together in long molecular chains are known as complex carbohydrates (or polysaccharides). The main complex carbohydrates are starch (found in cereals, legumes, and potatoes and some other root vegetables), fibre and glycogen (the form in which carbohydrates are stored in the body). Complex carbohydrates supply the body with glucose, but unlike simple carbohydrates, they provide a steady supply of energy rather than a quick boost, because the digestive system has to break them down into simple sugars first.
Foods derived from plants, including fruit, vegetables and legumes, contain indigestible polysaccharides that are more commonly known as dietary fibre. The two types of dietary fibre are soluble and insoluble fibre, and all plant foods contain variable amounts of each type. Although dietary fibre has no nutritional value (it passes virtually unchanged through the intestines and is excreted as waste matter), it serves a valuable purpose by acting as ‘nature’s broom’. Fibre adds bulk and moisture to foods passing through the digestive system, so that waste products are swept along quickly and eliminated regularly and comfortably. Foods derived from animals do not contain dietary fibre.
Essential components of all living matter, proteins are involved in thousands of the body’s vital functions, including the growth, maintenance and repair of cells. Protein also helps to create enzymes that enable us to digest food, as well as producing antibodies that fight off infection, hormones that keep the body working efficiently, and the brain’s messenger molecules (neuro-transmitters).
Protein is composed of different combinations of more than 20 amino acids. These are compounds that contain the four elements necessary for life: carbon, hydrogen, oxygen and nitrogen. Some also contain sulphur. Dietary proteins are the most complex of all the food compounds, and, after water, the most abundant substance in the body.
Most amino acids can be manufactured by the body – these are known as ‘non-essential’ because they do not need to be supplied by food. Nine ‘essential’ ones can only be obtained from the foods we eat. This is not a problem because animal foods, such as meat, poultry, fish, dairy products and eggs, contain all the essential amino acids and, as such, are referred to as high quality sources of protein. Some plant foods are also considered to be high quality; soya beans, for example, are an excellent source of protein. However, most plant sources of protein, such as beans, nuts, grains and vegetables, are generally referred to as low quality sources of protein because they do not contain all the essential amino acids.
Eating plant proteins in specific combinations, however – rice with lentils, for example – can provide the full complement of amino acids.
The body also uses amino acids to make its own proteins. Any excess protein is returned to the liver to be used as energy, stored or eliminated.
One of the most abundant nutrients in foods and a major fuel for the body, dietary fats are a concentrated source of calories; weight for weight they contain more than double the calories of carbohydrates or proteins. However, fats are not as easily converted into energy as glucose. Fats are made up of compounds called fatty acids that are bound together in molecular chains by a substance called glycerol. All fatty acids contain carbon, oxygen and varying amounts of hydrogen. Fatty acids are described as being ‘saturated’ or ‘unsaturated’, depending on how much hydrogen they contain. A third type of fat, called trans fatty acids, also exists.
Saturated fatty acids
These are fatty acids that are saturated with hydrogen; they come mainly from animal fats and are solid at room temperature. Examples include butter, cheese and lard. These fats should be closely monitored in the daily diet – a high intake contributes to increased levels of cholesterol in the blood, leading to an increased risk of heart disease.
Unsaturated fatty acids
These come from vegetable and fish oils, and are generally liquid at room temperature. There are two types of unsaturated fatty acids – mono-unsaturated and polyunsaturated. The mono-unsaturates, found in olive oil and avocados, are believed to reduce the body’s cholesterol levels. Polyunsaturates, found in most vegetable oils and oily fish, contain the least hydrogen of all fatty acids, and are thought to be good cholesterol fighters.
Although the body can make its own saturated and mono-unsaturated fatty acids from carbohydrates and proteins, it cannot make certain ‘essential’ polyunsaturated fatty acids – these have to be supplied by the foods that contain them. There are two families of essential polyunsaturates: the omega-3 family, derived from alpha-linolenic acid and found in flax (linseed), rape seed, pumpkin seeds, walnuts and their oils, and in carnivorous fish such as mackerel, herring, tuna and salmon; and the omega-6 family, derived from linoleic acid and found exclusively in seeds and their oils, especially sunflower, sesame and corn oil.
Trans fatty acids
A third type of fats, called trans fats, are thought to affect blood cholesterol in a similar way to saturated fats. When unsaturated fats are hardened during a food process called hydrogenation (used for many commercial spreads, such as margarine) they are transformed into trans fats. Trans fats also occur naturally in some foods of animal origin, but not in the potentially harmful amounts that are produced by food processing.
Once digested, fats are distributed to special fat storage (adipose) cells throughout the body – between muscle fibres and around all of the internal organs, for example. Body fat is an active tissue; it is continually storing, breaking down and releasing energy for the body’s functions. Any surplus calories in the body end up as fat – even if they started life as protein and carbohydrates. Fat serves as a long-term energy reserve; it is ideal for keeping extra reserves of calories (energy) because it stores them in such a concentrated form.
Although water is not strictly a nutrient, it is our prime nutritional need, and is fundamental for good health. Up to 60 per cent of the human body is composed of this vital fluid, which it uses in all its functions. The body needs water for digestion and the elimination of waste products. It is also essential for the regulation of body temperature and as cell and tissue fluid.
Around one-third of our daily fluid intake comes from solid foods. Fruit and vegetables supply the most water, but meat, fish and bread provide a fair amount, too. It is recommended that a moderately active adult consumes approximately 2 litres (3-1/2 pints) of water daily. A diet that is high in fruit and vegetables – about 0.9kg (2lb) of fruit and vegetables daily – can supply the individual with around 1 litre (2 pints) of water. The other litre required by the body is best consumed in the form of plain water, diluted fruit juices or fruit or herbal teas and infusions.