2.24-2.26 - Balanced diet and energy requirements
A balanced diet is not a diet where every food component is eaten in equal amounts. It is a diet that supplies the right nutrients, in suitable proportions, for the body's needs. Those needs change with the person's activity level, age and pregnancy, so "balanced" always means matched to the individual.
What balanced diet means
Food is useful to the body because it supplies materials for respiration, growth, repair and healthy functioning. The Edexcel list of required diet components is:
- carbohydrate
- protein
- lipid, including fats and oils
- vitamins
- minerals
- water
- dietary fibre
Balanced diet
A balanced diet contains the correct types and appropriate proportions of food components needed to maintain health.
The word appropriate matters. Carbohydrate, protein and lipid are needed in relatively large amounts because they provide energy or building materials. Vitamins and minerals are needed in much smaller amounts, but they are still essential. Water and dietary fibre are not used as respiratory fuels, but the body still depends on them for transport, temperature control and healthy movement of food through the gut.
Examiners often test the difference between a food source and a function. A source is where the nutrient comes from, such as rice as a source of carbohydrate. A function is what the nutrient does in the body, such as carbohydrate providing glucose for respiration.
Carbohydrate, protein and lipid
Carbohydrates, proteins and lipids are the large nutrient groups most often linked to energy and body structure.
| Component | Good sources | Main functions |
|---|---|---|
| Carbohydrate | Bread, rice, pasta, potatoes, cereals, fruit and foods containing sugars | Main source of glucose for respiration, so it supplies energy for cells |
| Protein | Meat, fish, eggs, dairy foods, beans, lentils, nuts and soya products | Growth and repair of tissues; making body proteins such as enzymes and some hormones |
| Lipid (fats and oils) | Vegetable oils, butter, margarine, nuts, seeds, dairy foods, oily fish and fatty meat | Concentrated energy store; insulation; protection around organs; part of cell membranes |
Carbohydrate is usually taught first because it is the body's quick, common fuel. During digestion, many carbohydrates are broken down into glucose. Cells use glucose in respiration to transfer energy for movement, active transport, temperature control and making larger molecules.
Protein has a different main role. It is made of amino acids, which cells use to build new proteins. That is why protein is especially important for growth, replacing damaged cells and repairing tissues after injury.
Lipids are energy-rich and can be stored in the body. They also help form cell membranes and provide insulation under the skin. The exam wording includes fats and oils, so remember that lipid can be solid or liquid at room temperature depending on its type.
Carbohydrate is mainly for energy, protein is mainly for growth and repair, and lipid is an energy-rich store with structural and insulating roles.
Common trap: do not say that only carbohydrate provides energy. Lipid and protein can also be used in respiration, but carbohydrate is the main energy source in a typical balanced diet.
Vitamins and minerals
Vitamins and minerals are needed in small amounts, but a shortage can still cause serious problems because they support specific body functions. Edexcel names vitamins A, C and D, and the mineral ions calcium and iron.
| Component | Good sources | Main functions |
|---|---|---|
| Vitamin A | Dairy foods, eggs, oily fish, liver, carrots, sweet potatoes, spinach and red peppers | Helps vision in dim light; helps keep skin and body linings healthy |
| Vitamin C | Citrus fruits, peppers, strawberries, blackcurrants, broccoli, Brussels sprouts and potatoes | Helps maintain healthy skin, blood vessels, bones and cartilage; helps wound healing |
| Vitamin D | Sunlight on skin, oily fish, egg yolk and fortified foods such as some spreads and cereals | Helps the body absorb calcium; supports healthy bones and teeth |
| Calcium ions | Milk, cheese, yoghurt, calcium-fortified soya drinks, leafy green vegetables and fish eaten with bones | Builds and maintains strong bones and teeth; helps muscle contraction and blood clotting |
| Iron ions | Red meat, liver, beans, nuts, dried fruit and fortified breakfast cereals | Needed to make haemoglobin in red blood cells for oxygen transport |
The functions link together. For example, calcium ions help make bones hard, while vitamin D helps the body absorb calcium. Iron ions are not "for energy" directly; they help haemoglobin carry oxygen, and oxygen is then used in aerobic respiration.
When answering source questions, give named foods rather than whole food groups if you can. "Fruit" is sometimes too vague for vitamin C; "oranges" or "blackcurrants" is clearer. "Dairy foods" is acceptable for calcium, but "milk" or "cheese" is even more precise.
Water and dietary fibre
Water and dietary fibre can be easy to underestimate because they are not usually listed as "energy-giving" nutrients. In a balanced diet, both are essential.
Water is needed because many substances dissolve in it. Blood plasma is mostly water, so water helps transport glucose, amino acids, mineral ions, hormones, urea and carbon dioxide. Cells also need water as a reactant or solvent for many chemical reactions, and sweating helps transfer thermal energy from the body to the surroundings.
Good sources of water include drinks and water-rich foods such as fruit and vegetables. The important exam idea is not a fixed daily volume; it is that water prevents dehydration and allows transport, temperature control, excretion and normal cell reactions.
Dietary fibre is plant material that humans cannot fully digest. It is found in wholegrain cereals and bread, brown rice, fruit, vegetables, beans, lentils, nuts and seeds. Fibre adds bulk to the contents of the intestine, helping food and waste move through the gut. This reduces the risk of constipation.
Dietary fibre
Dietary fibre is indigestible plant material in the diet that adds bulk to gut contents and helps food move through the intestines.
Do not confuse dietary fibre with protein fibre, muscle fibre or nerve fibre. In this lesson, fibre means roughage from plant foods.
Why energy requirements vary
An energy requirement is the amount of energy a person needs from food to match the energy they use and to support any extra growth or development. Food energy is commonly measured in kilojoules, kJ, or kilocalories, kcal.
Energy requirements vary for three named reasons in this cluster: activity level, age and pregnancy.
| Factor | How energy requirement changes | Biological reason |
|---|---|---|
| Activity level | More active people need more energy than less active people | Muscle contraction needs energy transferred by respiration, so more movement increases energy use |
| Age | Children and teenagers need energy for growth; requirements also change as body size, muscle mass and activity change with age | Building new tissue needs energy and materials; older people may need less energy if they are less active or have less muscle mass |
| Pregnancy | Pregnancy increases energy requirement | Energy is needed for growth of the fetus, placenta and maternal tissues, and for changes in the mother's body |
This is not a memory test for one "correct" daily calorie value. The stronger answer is comparative: a more active person needs more energy than a sedentary person; a growing teenager needs energy for growth; a pregnant person needs extra energy for developing tissues.
Energy balance means energy intake from food matches energy use by the body over time. If intake is consistently greater than use, the extra energy can be stored, mainly as fat. If intake is consistently less than use, stored energy is used.