What are the functions of macronutrients?

Macronutrients provide energy and help maintain and repair the body.

Oxygen

Cells burn fuel for energy by combining it with oxygen.

Water

Your cells and organs depend on the involvement of water in all body processes, including digestion, absorption, circulation and excretion. Water surrounds and fills cells and tissues, forms the basis of body fluids, acts as a lubricant, transports oxygen and nutrients, keeps food moving through your gut, helps to regulate your body temperature and keeps your skin moist. Without food you could probably survive for several weeks; without water you would die in a few days.

Water makes up around 60 per cent of body weight and the average person needs two to three quarts (liters) of water per day to replace what is lost through the skin, urine, bowels and lungs. You obtain water from food, as a by-product of metabolism, and from drinking. Making sure you drink enough water is vital to maintaining good health. Many people do not drink enough.

Protein

Proteins are organic molecules containing carbon, oxygen, hydrogen, nitrogen and in some cases, sulfur.

They are made of linked chains of smaller molecules known as amino acids. These amino acids can combine in an infinite number of ways, and thousands of different proteins have been identified. Each protein has a unique sequence of amino acids which gives it a specific structure, shape and chemical characteristics.

The proteins in your body are made up of 20 main naturally occurring amino acids and some other minor ones. Some of these amino acids are essential constituents of your diet as your body cannot make them, whereas others are nonessential as your body can make them. Some amino acids can be considered semi-essential as they are only essential in your diet at certain times in your life, such as during childhood and in high growth demand states such as pregnancy. The essential amino acids are tryptophan, lysine, methionine, phenylalanine, threonine, valine, leucine and isoleucine. Arginine and histidine are considered semi-essential. The nonessential amino acids are tyrosine, glycine, serine, glutamic acid, aspartic acid, taurine, cystine, proline, and alanine.

Proteins are involved in growth, repair and maintenance of body tissues. Enzymes are protein molecules which act as catalysts to stimulate biochemical reactions. There are literally thousands of different enzymes within a single cell that have many functions including the breaking down, joining together or separation of a wide variety of substances. Many hormones are proteins, including insulin, which regulates your blood sugar levels; and thyroid hormone, which controls your metabolic rate. Proteins also play a vital role in the functioning of your immune system, help to keep the correct amount of water in the cells, help to normalize the acid-base balance by acting as buffers, and can also be used as a source of energy for your body. One gram of protein provides 4 calories (17 kJ) of energy.

Carbohydrates

Carbohydrates are organic molecules which contain the elements carbon (C), hydrogen (H), and oxygen (O) in a ratio of 1:2:1. They come in several different types; including simple carbohydrates such as glucose, sucrose and fructose, which are found in refined sugar, fruits and honey; and complex carbohydrates, which are mainly starches and are found in grains, legumes and vegetables. Fiber is also a carbohydrate. Glucose is the end product of most carbohydrate metabolism in the body. The most important function of dietary carbohydrates is to provide you with energy for all your body functions, including heat production and muscle exertion. Glucose is converted to carbon dioxide and water and energy is released. Each gram of carbohydrate releases four calories of energy for use by the body. Carbohydrates also help in the digestion and assimilation of foods and in the regulation of protein and fat metabolism.

Glucose is the most common source of energy in the body and is particularly important for the brain and red cells as it is the sole source of energy for these body structures. Glucose is important in pregnancy for the formation of structural carbohydrates and lactose for lactation. Excess glucose is stored in the liver and muscles as glycogen, and when these reserves are filled to capacity, excess glucose is converted to, and stored as, fat.

Some carbohydrates cannot be digested by your body. These include cellulose, which therefore acts as a dietary fiber. Eating plenty of fiber helps to move food through your intestines, which contributes to the elimination of waste products.

Fats

Fats, which are also known as lipids, are compounds that are not soluble in water but are soluble in organic solvents. Like carbohydrates, they contain carbon, hydrogen and oxygen; and some contain nitrogen and phosphorus. The main type of dietary fat is triglycerides; others include phospholipids such as lecithin, and sterols such as cholesterol.

Fats have several vital functions including protecting internal organs from damage, providing insulation, helping to regulate body temperature, and transporting fat soluble substances such as vitamins A, D, E and K. Fats are also a source of energy. One gram of fat provides 9 calories (38 kJ) of energy after absorption. Fats also add flavor to food and help you feel full.

Fats are part of every cell membrane, organ and tissue. The type of fat you eat influences the characteristics of your cell membranes and organs, keeping them healthy or making them susceptible to disease. Fats are also important for the function of the nervous system and are involved in the manufacture of hormones and hormone-like compounds known as prostaglandins.

There are several different types of fats, which affect your health in varying ways. These include saturated, polyunsaturated and monounsaturated fats. All fats contain a mixture of these three types of fat, and one type predominates in a particular food.

Fats that are solid at room temperature are mostly saturated fat. Animal products, such as beef, pork, poultry, whole milk, cheese, sour cream, and yoghurt, as well as coconut, palm and palm kernel oils, contain mostly saturated fats. These fats can increase blood cholesterol levels and therefore raise the risk of heart disease. Eating foods high in saturated fats can also increase the risk of some cancers, including bowel cancer. Trans fats are produced from polyunsaturated fats, typically in margarine manufacture. Due to their adverse effects on health, they are usually grouped with saturated fats.

Fats and oils that are liquid at room temperature are mostly unsaturated, either monounsaturated or polyunsaturated. Canola, olive, and peanut oils are high in monounsaturated fats while corn, soybean, and sunflower oils are high in polyunsaturated fats. Two polyunsaturated fats are necessary for healthy body function. These are the essential fatty acids, linoleic and alpha linolenic acid.

For a few years, people were advised to eat polyunsaturated fats because they lower total blood cholesterol. However it is now known that polyunsaturated fats are susceptible to damage by free radicals and lower levels of beneficial HDL cholesterol. This increases damage to tissues and also the build-up of atherosclerotic plaque in the arteries. High intake of polyunsaturated fats has also been linked to cancer.

Monounsaturated fats can also help to lower levels of harmful LDL cholesterol and raise levels of beneficial HDL cholesterol. As they are less susceptible to oxidation, they do not increase the risk of disease. People who live in countries where the most commonly used type of fat is olive oil tend to have lower risk of diseases such as heart disease and cancer.