Explain how body weight is managed through energy balance, evaluate healthy versus unhealthy approaches, and interpret BMI

What this dot point is asking

The syllabus wants you to explain how body weight is managed through energy balance, judge healthy against unhealthy approaches such as crash dieting, and calculate and interpret body mass index. The central idea is that safe weight change is gradual and based on a balanced diet plus activity, and that extreme diets do more harm than good.

The answer

Weight management is energy balance

Body weight depends on the balance between energy taken in and energy used:

• To lose weight: create a small energy deficit (eat a little less, move a little more) so the body uses its fat stores.
• To gain weight: create a small energy surplus, ideally from nutritious foods, so the body builds tissue.
• To maintain weight: keep intake equal to output.

The healthy approach in every case is gradual, because the body and habits adjust slowly.

Why crash diets fail

A crash diet supplies very little energy and too few nutrients. Problems include:

• Nutrient shortage: too little protein, vitamins and minerals, causing tiredness, poor immunity and ill health.
• Muscle and water loss: much of the rapid early loss is water and muscle, not fat.
• A slower metabolism: the body conserves energy, making further loss harder.
• Rebound: the diet is too hard to sustain, so weight is regained, often with more, when normal eating returns.

A sensible weight-loss plan

• A small, steady deficit so weight is lost gradually, not crashed off.
• A balanced diet that still meets all nutrient needs: more vegetables, fruit, fibre and lean protein, less fat, sugar and refined carbohydrate.
• Smaller portions and fewer sugary drinks and snacks.
• Regular physical activity, which increases energy output and protects muscle.
• Long-term habit change, not a temporary fix.

Body mass index (BMI)

Body mass index estimates whether a person's weight is healthy for their height:

A common healthy range is about $18.5$ to $24.9$; below this is underweight, $25$ and above is overweight, and higher still is obese. BMI is a useful guide but not perfect, because a very muscular person can have a high BMI without excess fat.

Examples in context

Example 1. Swapping snacks rather than starving. Someone wanting to lose weight who replaces deep-fried snacks and bubble tea with fruit and water creates a daily energy deficit without going hungry. This gradual swap is sustainable, unlike skipping meals, which often leads to overeating later.

Example 2. Healthy weight gain for an underweight athlete. A young athlete needing to gain weight adds extra nutritious, energy-dense foods such as nuts, wholegrains, milk and lean meat, building a small surplus. This adds muscle and weight healthily, rather than relying on sugary or fatty junk food.

Try this

• Cue. A person is $75\ \text{kg}$ and $1.70\ \text{m}$ tall. Calculate their BMI. Work out $75 \div 1.70^2 = 75 \div 2.89 = 25.9$, just into the overweight range.
• Cue. Give three reasons a crash diet is an unhealthy way to lose weight. Recall nutrient shortage, muscle and water loss, slowed metabolism, and weight rebound.
• Cue. Describe three features of a sensible weight-loss plan. Recall a small steady deficit, a balanced nutrient-rich diet, smaller portions, and regular activity.