What you need to know
- State the functions and sources of the main nutrients.
- Explain why energy and nutrient needs differ between people.
- Describe effects of nutrient deficiency, excess and obesity.
- Interpret food labels and calculate food energy, BMI and waist-to-hip ratio.
- Evaluate what health measurements can and cannot show.
Nutrients and their functions
| Nutrient | Main functions | Example sources |
|---|---|---|
| Carbohydrate | Major source of glucose for respiration. | Bread, rice, potatoes, fruit. |
| Protein | Growth and repair; makes enzymes, antibodies and some hormones. | Meat, fish, eggs, pulses, nuts. |
| Lipids | Energy store, insulation, protection and cell membranes. | Oils, butter, nuts, seeds. |
| Vitamins | Required in small amounts for healthy body processes. | A varied diet of plant and animal foods. |
| Minerals | Elements needed for structures and processes; for example iron in haemoglobin and calcium in bones. | Varied foods; sources depend on the mineral. |
| Fibre | Adds bulk, supports peristalsis and reduces constipation. | Wholegrains, fruit and vegetables. |
| Water | Solvent, transport medium, reactant and temperature control. | Drinks and water-rich foods. |
A diet that provides sufficient energy and all essential nutrients in proportions that meet an individual's needs.
What changes dietary needs?
Energy requirements rise with physical activity. They also vary with age, body size, sex, pregnancy, growth and environmental temperature. A growing teenager needs materials for new tissue as well as energy; an endurance athlete may need much more energy than an inactive adult.
Energy intake and energy transferred by respiration are not perfectly balanced from one meal to the next. Over time, a sustained intake greater than expenditure promotes storage of energy, mainly as fat. A sustained energy deficit causes stored materials to be used and body mass to fall.
Malnutrition, deficiency and excess
Malnutrition means an unbalanced nutrient or energy intake. It includes undernutrition, overnutrition and a shortage or excess of particular nutrients.
| Imbalance | Possible consequence | Biological link |
|---|---|---|
| Too little protein and energy | Poor growth, loss of muscle and reduced repair. | Insufficient amino acids and respiratory substrates. |
| Iron deficiency | Anaemia, tiredness and breathlessness. | Less haemoglobin can reduce oxygen transport. |
| Vitamin D or calcium deficiency | Weak bones. | Mineralisation of bone is impaired. |
| Too little fibre | Constipation. | Less bulk stimulates the gut less effectively. |
| Long-term energy excess | Obesity and increased disease risk. | Excess energy is stored and can affect metabolism and organs. |
Obesity is associated with increased risk of type 2 diabetes, cardiovascular disease and some cancers. It is a risk factor, not a guarantee that a person will develop disease.
Misconception AlertAll dietary fat is unhealthy.Select to reveal the correctionSelect to hide the correction
Lipids are essential nutrients used for energy storage, insulation and cell membranes. Health effects depend on the type and amount consumed.
Using nutritional and health data
Compare foods using the same mass or realistic portion size. Check energy, saturated fat, sugars, fibre, protein and salt rather than judging a product from one value.
BMI does not distinguish muscle from fat and adult categories are not directly appropriate for growing children. Waist measurements indicate fat distribution but are affected by technique. Sound health judgements use several measurements and clinical context.
Iodine, Benedict's, Biuret and emulsion tests identify classes of molecule. They do not by themselves show whether a whole diet is balanced.
Quick retrieval check
1. Give two functions of lipids.
2. Why might an athlete need more carbohydrate than an inactive person?
3. Calculate BMI for a 72 kg adult who is 1.80 m tall.
4. Why can BMI misclassify a muscular athlete?
Exam connection
A snack contains 18 g carbohydrate at 17 kJ g−1 and 6 g lipid at 37 kJ g−1. Calculate the energy supplied by these nutrients.
Show the answer
Carbohydrate: 18 × 17 = 306 kJ. Lipid: 6 × 37 = 222 kJ. Total = 528 kJ.