Balanced diet

A balanced diet provides enough energy and the right amounts of nutrients, fibre and water to meet the body’s needs. Balance depends on both the range of substances eaten and how much of each is consumed.

The nutrients in a balanced diet include carbohydrates, fats, proteins, vitamins and mineral ions. Carbohydrates and fats provide energy, while protein is needed for growth and repair. Vitamins and mineral ions are needed in smaller amounts to keep the body functioning normally. Fibre helps food move through the digestive system, and water is needed for many processes in the body.

The amounts needed are not the same for everyone. They vary with factors such as age, activity level and growth. For example, a more active person generally needs more energy, while a growing person needs sufficient nutrients to support growth.

Dietary carbohydrates

Carbohydrates are a major source of energy in the diet. Starch and sugars are carbohydrates: foods such as bread, rice and potatoes contain starch, while fruit contains sugars.

During digestion, starch and other digestible carbohydrates are broken down into simple sugars, including glucose. Glucose is absorbed into the blood and carried to cells, where it can be used to release energy.

Dietary fats and oils

Fats and oils provide a concentrated source of energy in the diet. The body can store fat as an energy reserve, which can be used when energy is needed later.

Stored fat also helps insulate the body, reducing heat loss. Fat around organs helps protect them by cushioning them against impacts.

Dietary sources include oils, butter, nuts and fatty foods. Oils are generally liquid at room temperature, whereas fats are generally solid.

Dietary proteins

Proteins in food supply amino acids, the building blocks the body uses to make its own proteins. During digestion, dietary proteins are broken down into amino acids, which are absorbed and used to build body proteins.

These proteins are especially important for growth and the repair of tissues. The body also uses amino acids to make substances such as enzymes, which help chemical reactions take place in the body.

Principal dietary sources of protein include meat, fish, eggs, dairy products and beans.

Vitamin C in the diet

Vitamin C helps the body maintain healthy tissues, including the skin and gums. It is also needed for normal wound healing, so the body can repair damaged tissue.

Fresh fruit and vegetables provide vitamin C. Citrus fruits, such as oranges, are particularly good dietary sources.

A prolonged lack of vitamin C causes scurvy. Without enough vitamin C, tissues cannot be maintained and repaired normally. Symptoms of scurvy include bleeding gums and poor wound healing.

Vitamin D in the diet

Vitamin D helps the body absorb calcium from food. Calcium is needed to develop and maintain strong bones and teeth, so getting enough vitamin D supports their strength.

Dietary sources of vitamin D include oily fish, eggs and fortified foods. Fortified foods have vitamin D added to them.

Vitamin D deficiency and rickets

If a child has too little vitamin D for a prolonged period, the body cannot absorb enough calcium to develop strong bones. This can cause rickets, a condition in which bones become soft and may bend or become deformed.

Dietary calcium

Calcium is a mineral ion needed to form and maintain strong bones and teeth. It also helps muscles contract normally and is needed for blood to clot after an injury.

Dietary sources of calcium include dairy products, such as milk, cheese and yoghurt, and some green leafy vegetables, such as kale. Adequate vitamin D helps the body absorb calcium from food in the intestine, making it available for these functions.

Dietary iron

Iron is a mineral ion needed to make haemoglobin, the substance in red blood cells that carries oxygen around the body.

If the diet does not provide enough iron, the body cannot make enough haemoglobin. This reduces the blood’s capacity to carry oxygen and can cause anaemia.

Dietary sources of iron include red meat, beans and dark green leafy vegetables.

Dietary fibre

Dietary fibre, also called roughage, is material in plant foods that human digestive enzymes cannot digest. Sources include wholegrain foods, fruits and vegetables.

Fibre adds bulk to the contents of the intestines. This helps food waste move through the intestines, supporting regular bowel movements and helping to prevent constipation.

Water in the diet

Water enters the body through drinks and foods. It is needed inside cells and in fluids that carry substances around the body.

Water provides a medium for chemical reactions in cells: many substances dissolve in it, allowing them to take part in reactions. Water also transports dissolved substances around the body in fluids such as blood.

Water helps regulate body temperature. For example, when sweat evaporates from the skin, it removes heat from the body and helps cool it.

The body loses water in urine and sweat. This water must be replaced through food and drinks so that the body can continue to function.

Digestive system organs and their roles

The alimentary canal is the route food takes through the body, from the mouth to the anus. As food travels along it, digestion breaks food down, nutrients are absorbed, and undigested material is removed. Other organs, including the salivary glands, pancreas, liver and gall bladder, help digestion by adding substances to the canal.

From the mouth to the stomach

Food and drink enter the mouth during ingestion. Chewing breaks food into smaller pieces, while saliva begins chemical digestion. The salivary glands produce the saliva that enters the mouth.

After food is swallowed, the oesophagus moves it to the stomach. The stomach churns the food and begins further chemical digestion.

Digestion and absorption in the small intestine

The first part of the small intestine, the duodenum, receives digestive substances from other organs:

• The pancreas releases digestive enzymes into the duodenum.
• The liver produces bile.
• The gall bladder stores bile before releasing it into the duodenum.

Digestion continues in the small intestine. The resulting nutrients are absorbed through the wall of the small intestine, especially the ileum, into the blood. Body cells then take up and use the absorbed nutrients; this is called assimilation.

The large intestine and egestion

Material that remains after digestion passes into the large intestine. The colon absorbs water from this material. The rectum stores faeces, which are then released through the anus. The removal of undigested food from the body as faeces is called egestion.