Balanced and Unbalanced Diets: Eating Right for a Healthy Body
Extension topic – this whole chapter is Extension Part content in the 2025 junior secondary science curriculum.
Eating enough is not the same as eating right! In this chapter you will learn what a balanced diet is, why different people need different amounts of energy, how to calculate the energy value of food from a label, and what happens when a diet is unbalanced. (The food substances and their jobs are in the chapter "Nutrition and health".)
1. What Is a Balanced Diet?
A balanced diet contains different food substances in the right proportion and the right amount to keep the body healthy.
The three "rights"
1. Right kinds – all the food substances: carbohydrates, lipids, proteins, vitamins, minerals, dietary fibre and water.
2. Right proportion – more grains, vegetables and fruits; less fatty, sugary and salty food.
3. Right amount – about as much energy as your body uses: not too little, not too much.
The Healthy Eating Food Pyramid
In Hong Kong, the Healthy Eating Food Pyramid is a simple guide to the right proportion of foods:
Bottom – eat the most: grains (rice, noodles, bread, oats).
Next – eat more: vegetables and fruits.
Middle – eat moderately: meat, fish, eggs and alternatives (e.g. tofu); milk and alternatives.
Top – eat the least: fat/oil, salt and sugar.
Drink enough water every day too. Memory aid: "Most from the bottom, least from the top."
A balanced day in Hong Kong
Breakfast: wholemeal bread with egg, low-fat milk, a banana
Lunch: rice with steamed fish, choy sum and an orange
Dinner: rice, beef with broccoli, tofu soup and fruit
All day: plain water instead of sugary drinks
For a different person, the amounts change: a construction worker needs bigger portions; a young child needs smaller ones.
Key takeaway: A balanced diet has all the food substances in the right proportion and the right amount.
2. Different People, Different Energy Needs
The body needs energy all the time – to keep the heart beating, breathe, keep warm, move, grow and repair. Food energy is measured in kilojoules (kJ) or kilocalories (kcal).
\(1\ \text{kcal} \approx 4.2\ \text{kJ}\)
Factors affecting energy requirement
1. Age – Teenagers grow fast and are often active, so they need a lot of energy. Young children have smaller bodies and need less in total; older adults usually need less as they are less active.
2. Sex – On average, males need more energy than females of the same age and activity, because they usually have bigger bodies with more muscle.
3. Occupation – A construction worker doing heavy physical work needs more energy than an office worker who sits most of the day.
4. Growth and pregnancy – Growing children need extra energy and protein to build new cells. A pregnant woman needs extra for the growing baby.
Did you know? Even when you are fast asleep, your body still uses energy to keep your heart beating, your lungs breathing and your body warm.
Key takeaway: Energy requirement depends on age, sex, occupation (activity) and growth or pregnancy.
3. Calculating the Energy Value of Food
Reading a nutrition label
In Hong Kong, most prepackaged foods carry a nutrition label showing energy plus seven core nutrients ("1+7"). Values are usually given per 100 g (or per 100 mL for drinks), in kJ, kcal or both.
The energy formula
\(\text{energy taken in} = \dfrac{\text{energy per 100 g}}{100} \times \text{mass eaten (g)}\)
From kcal to kJ, multiply by 4.2; from kJ to kcal, divide by 4.2.
Worked example 1: breakfast cereal
A cereal has 1600 kJ per 100 g. Mei eats 40 g. Energy taken in:
\(\dfrac{1600}{100} \times 40 = 16 \times 40 = 640\ \text{kJ}\)
Worked example 2: instant noodles
Noodles give 450 kcal per 100 g. A cup contains 80 g. Find the energy in kcal and in kJ.
Step 1: \(\dfrac{450}{100} \times 80 = 360\ \text{kcal}\)
Step 2: \(360 \times 4.2 = 1512\ \text{kJ}\)
Worked example 3: chips or an apple?
Chips: 2200 kJ per 100 g, so a 30 g bag gives \(22 \times 30 = 660\ \text{kJ}\).
Apple: 220 kJ per 100 g, so a 150 g apple gives \(2.2 \times 150 = 330\ \text{kJ}\).
The small bag of chips gives twice the energy – but the apple also gives vitamin C and dietary fibre!
Common mistake: Treating the "per 100 g" value as the energy of the whole packet. Always use the mass you actually ate, and do not mix up kJ and kcal.
Key takeaway: Divide the energy per 100 g by 100, then multiply by the mass eaten. \(1\ \text{kcal} \approx 4.2\ \text{kJ}\).
4. Too Little of a Food Substance
If a diet lacks a food substance for a long time, deficiency diseases may develop.
Proteins and dietary fibre
Protein (fish, eggs, beans) – needed for growth and repair. Too little causes poor growth, weak muscles and slow healing of wounds.
Dietary fibre (vegetables, fruits, wholegrains) – helps food move along the gut. Too little causes constipation.
Vitamins A, C and D
Vitamin A (carrots, liver) – too little causes night blindness: poor vision in dim light.
Vitamin C (citrus fruits, guava) – too little causes scurvy: bleeding gums and slow healing of wounds.
Vitamin D (egg yolk, oily fish; skin makes it in sunlight) – helps us absorb calcium. Too little causes rickets in children: soft bones that may bend.
Minerals: calcium, iron and iodine
Calcium (milk, tofu, small fish with bones) – too little leads to weak bones and teeth.
Iron (red meat, liver, green vegetables) – needed to make haemoglobin, which carries oxygen in red blood cells. Too little causes anaemia: pale skin, tiredness, shortness of breath.
Iodine (seafood, seaweed, iodised salt) – needed by the thyroid gland in the neck. Too little causes goitre, a swelling in the neck.
Memory aid: Iron – "I'm tired" (anaemia). Iodine – "I've got a lump in my neck" (goitre).
Did you know? Long ago, sailors on long voyages often got scurvy because they had no fresh fruit for months. Carrying lemons and limes on board helped prevent it.
Key takeaway: Learn the pairs: protein – poor growth; fibre – constipation; A – night blindness; C – scurvy; D – rickets; calcium – weak bones and teeth; iron – anaemia; iodine – goitre.
5. Too Much of Some Foods: Health Risks
Eating too much fat, sugar or salt for years increases the risk of some health problems ("risk" means a higher chance, not a certainty).
Too much fat: cardiovascular diseases
Too much fat, especially animal fat, can raise blood cholesterol. Fatty deposits build up inside arteries and narrow them, like mud inside a garden hose. This raises the risk of cardiovascular diseases such as heart attacks and strokes.
Too much sugar: diabetes
Extra sugar gives extra energy, stored as fat. This can lead to obesity, which raises the risk of diabetes (type 2), where the blood sugar level stays too high. Sugar also causes tooth decay (see "Dental care").
Too much salt: hypertension
Too much salt can raise blood pressure. Blood pressure that stays high is called hypertension, which strains the heart and blood vessels.
Smart choices: drink water instead of sugary drinks, order milk tea "less sweet", choose steamed rather than deep-fried dishes, and eat instant noodles only once in a while.
Key takeaway: Too much fat, sugar and salt raises the risk of cardiovascular diseases, diabetes and hypertension.
6. Under-eating and Over-eating
Energy balance
Energy in equals energy out: weight stays about the same.
Energy in is more: extra energy is stored as fat, so weight goes up.
Energy in is less: the body uses stored fat, so weight goes down.
It is like an Octopus card: eating adds value, activity spends it. Regular exercise helps too (see "Keeping our bodies healthy").
Effects of under-eating
Eating too little for a long time can make a person underweight, weak and tired, with low immunity (catching infections easily) and poor growth. Anorexia nervosa is a serious eating disorder in which a person eats far too little from fear of gaining weight; they need help from doctors and family.
Effects of over-eating
Taking in more energy than you use for a long time can make a person overweight and then obese. Obesity raises the risk of cardiovascular diseases, diabetes and hypertension, and strains joints such as the knees.
BMI: a rough guide
The Body Mass Index (BMI) is a quick check of whether weight is healthy for a person's height.
\(\text{BMI} = \dfrac{\text{mass (kg)}}{\left(\text{height (m)}\right)^2}\)
Worked example 4: calculating BMI
An adult has a mass of 55 kg and a height of 1.60 m: \(\text{BMI} = \dfrac{55}{1.60^2} = \dfrac{55}{2.56} \approx 21.5\). For adults in Hong Kong, 18.5 to 22.9 is taken as the healthy range.
Common mistake: Using height in cm, or forgetting to square it. Change 160 cm to 1.60 m first.
Remember: BMI cannot tell muscle from fat, and teenagers are still growing, so doctors compare a teenager's BMI with charts for their age and sex. Everyone grows at their own pace – if you are worried about your weight, talk to a parent, a teacher or a doctor rather than dieting on your own.
Key takeaway: Under-eating causes underweight, weakness and low immunity; over-eating causes overweight and obesity. BMI is only a rough guide.
Quick Review
1. Balanced diet = different food substances in the right proportion and amount.
2. Food pyramid: most grains; least fat/oil, salt and sugar.
3. Energy needs depend on age, sex, occupation and growth or pregnancy.
4. \(\text{energy} = \dfrac{\text{energy per 100 g}}{100} \times \text{mass eaten}\); \(1\ \text{kcal} \approx 4.2\ \text{kJ}\).
5. Deficiencies: protein, fibre, vitamins A, C, D, calcium, iron and iodine each cause a specific problem.
6. Too much fat, sugar, salt: cardiovascular diseases, diabetes, hypertension.
7. Under-eating: underweight, low immunity. Over-eating: obesity. BMI is a rough guide.
Great job! Small daily choices – fruit instead of chips, water instead of a sugary drink – add up to a healthier you!