Human Nutrition
Components of a Balanced Diet
A balanced diet contains all the nutrients the body needs in the correct proportions:
| Component | Main sources | Functions |
|---|---|---|
| Carbohydrate | Bread, pasta, rice, potatoes, sugar | Energy supply |
| Protein | Meat, fish, eggs, beans, lentils | Growth, repair, making enzymes and hormones |
| Lipid (fat/oil) | Butter, oils, nuts, fatty fish | Energy storage, insulation, cell membranes, fat-soluble vitamins |
| Vitamin A | Liver, dairy, carrots (β-carotene) | Maintenance of healthy skin and eyes; night vision |
| Vitamin C | Citrus fruit, peppers, broccoli | Healthy skin and immune system; prevents scurvy |
| Vitamin D | Oily fish, eggs, dairy; synthesised in skin by sunlight | Absorption of calcium; healthy bones and teeth; prevents rickets |
| Calcium | Dairy, leafy greens, fortified foods | Bone and teeth formation; muscle contraction; nerve function |
| Iron | Red meat, spinach, legumes, fortified cereals | Making haemoglobin in red blood cells; oxygen transport; prevents anaemia |
| Water | Drinks, fruit, vegetables | Solvent for reactions; transport medium in blood; temperature regulation |
| Dietary fibre | Wholegrain, vegetables, pulses | Adds bulk to food; keeps gut moving; prevents constipation |
Deficiency Diseases
| Deficiency | Disease / Consequence |
|---|---|
| Vitamin C | Scurvy (bleeding gums, poor wound healing) |
| Vitamin D | Rickets (soft, deformed bones in children) |
| Iron | Anaemia (fatigue, pale skin, breathlessness) |
| Calcium | Weak bones and teeth; muscle cramps |
Energy Requirements
Energy requirements vary depending on several factors:
| Factor | Effect on energy requirement |
|---|---|
| Activity level | More active → more energy needed (muscles work harder) |
| Age | Children and teenagers need more relative to body mass (growth); elderly need less (less active) |
| Pregnancy | Increased energy needed (supporting foetal growth and metabolism) |
| Sex | Males typically have higher requirements (more muscle mass; higher metabolic rate) |
Metabolic Rate
A person’s basal metabolic rate (BMR) is the energy used at rest to maintain basic functions (breathing, heartbeat, etc.). Total energy requirement = BMR + energy for activity.
Consequences of an Unbalanced Diet
| Condition | Cause |
|---|---|
| Malnutrition | Too little food overall, or deficiency of specific nutrients |
| Obesity | Chronic excess energy intake over expenditure; excess stored as fat |
| Type 2 diabetes | Associated with obesity; cells become resistant to insulin |
| Cardiovascular disease | Excess saturated fat and salt raise cholesterol and blood pressure |
Exam Questions
Q: State the function of iron in the diet.
Iron is needed to make haemoglobin in red blood cells. Haemoglobin carries oxygen around the body. A deficiency of iron causes anaemia.
Q: Explain why a pregnant woman has a higher energy requirement than a non-pregnant woman of the same age.
During pregnancy, extra energy is needed to support the growth and development of the foetus, form new tissues (placenta, increased blood volume), and power the increased metabolic demands of both mother and developing baby.
Q: State two differences between the functions of carbohydrates and lipids in the body.
Carbohydrates are the body’s primary source of quick-release energy. Lipids are used mainly for long-term energy storage, insulation and making cell membranes. Lipids provide more than twice the energy per gram compared to carbohydrates.
Q: Explain why dietary fibre is important even though it is not digested.
Dietary fibre adds bulk to food in the large intestine. This stimulates peristalsis (muscular contractions that move food through the gut), preventing constipation and reducing the risk of bowel cancer.
Exam Tip
When asked about vitamins or minerals, state both the source AND the function, and ideally link to the deficiency disease. A complete answer scores more marks than just naming the source.