7.4 Diet, Exercise, and Disease

A free GCSE Biology lesson for AQA (8461) from OBA School: obesity, type 2 diabetes and cardiovascular disease; BMI and waist-to-hip ratio; the benefits of exercise. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

• Explain how an unbalanced diet can lead to obesity and why obesity is a risk factor for disease
• Calculate body mass index (BMI) and waist-to-hip ratio and interpret the results
• Describe type 2 diabetes and its link with obesity
• Explain how regular exercise reduces the risk of cardiovascular disease and type 2 diabetes

Key words

• Obese: Having so much body fat that health is at risk. Usually a BMI of 30 or more.
• BMI: Body mass index: mass in kg ÷ (height in m)². A rough guide to whether a person's mass is healthy for their height.
• Waist-to-hip ratio: Waist measurement ÷ hip measurement. A high value shows fat stored around the abdomen.
• Type 2 diabetes: A disease in which body cells no longer respond properly to insulin, so blood glucose stays too high.
• Metabolic rate: The rate at which the body uses energy for all its chemical reactions.
• Cardio- vascular disease: Disease of the heart or blood vessels, e.g. coronary heart disease, high blood pressure, stroke.

Diet and obesity

If you take in more energy from food than your body uses, the extra is stored as fat. Over time this can lead to being overweight and then obese. Obesity is a risk factor for type 2 diabetes, cardiovascular disease (high blood pressure, coronary heart disease, stroke), some cancers and joint problems.

A poor diet can also cause problems in other ways: too much saturated fat raises blood cholesterol (linked to fatty deposits in arteries); too much salt raises blood pressure; too little of some nutrients causes deficiency diseases.

Worked example: energy balance

A woman needs 9000 kJ a day but eats food containing 9900 kJ a day. About 37 000 kJ of extra energy is stored as 1 kg of body fat. Roughly how many days will it take her to gain 1 kg of fat?
extra energy per day = 9900 − 9000 = 900 kJ
days = 37 000 ÷ 900 = 41.1…

Answer: about 41 days (roughly 9 kg in a year)

Measuring obesity

BMI = mass (kg) ÷ height² (m²)

waist-to-hip ratio = waist measurement ÷ hip measurement (same units)

• below 18.5; Category underweight
• 18.5 – 24.9; Category healthy weight
• 25 – 29.9; Category overweight
• 30 – 39.9; Category obese
• 40 and over; Category severely obese

Worked example: BMI and waist-to-hip ratio

Raj has a mass of 88 kg and is 1.80 m tall. His waist is 98 cm and his hips are 100 cm.
BMI = 88 ÷ 1.80² = 88 ÷ 3.24 = 27.2 (to 1 d.p.), so he is overweight.
waist-to-hip ratio = 98 ÷ 100 = 0.98

Answer: BMI 27.2 (overweight); ratio 0.98, above the 0.9 guide for men, so fat stored round the abdomen raises his risk

Exam tip: BMI does not tell muscle from fat: a very muscular athlete can have a high BMI but little fat. The waist-to-hip ratio shows where fat is stored. Ratios above about 0.85 for women and 0.9 for men are linked with a higher risk of disease.

Type 2 diabetes

In type 2 diabetes, the body's cells no longer respond properly to the hormone insulin (and the pancreas may not make enough), so blood glucose levels stay too high. Over time this damages blood vessels, nerves, eyes and kidneys.

Obesity is the biggest risk factor; age, family history and lack of exercise also matter. Type 2 diabetes is often controlled with a carbohydrate-controlled diet, losing weight and exercise, and sometimes medicines.

Exam tip: A relative risk of 7 means people in that group are 7 times as likely to develop type 2 diabetes as people with a healthy BMI. It does not mean 7% of them get it.

Exercise and health

Regular exercise reduces the risk of cardiovascular disease and type 2 diabetes because it:

• uses up energy, so less is stored as fat, helping to control body mass
• builds muscle, which raises the metabolic rate
• lowers blood pressure and improves the balance of cholesterol in the blood, so fatty deposits are less likely to build up in arteries
• helps body cells respond better to insulin

Key fact: In questions about exercise, link each benefit to a disease: e.g. "exercise lowers blood pressure, which reduces the risk of coronary heart disease".

Common mistakes

• Squaring the BMI wrongly: square the height first, then divide the mass by it. 88 ÷ 1.80², not (88 ÷ 1.80)².
• Mixing up type 1 and type 2 diabetes. In type 2 the cells stop responding to insulin; it is strongly linked to obesity.
• Saying obesity "causes" heart disease in everyone. It is a risk factor: it increases the chance.
• Thinking BMI measures fat. It only compares mass with height.

Revision questions

Try these, then check your answers below each one.

Question 1. Ben has a mass of 81 kg and a height of 1.80 m. Calculate his BMI to 1 decimal place.

Answer: 25.0

Working: 1.80² = 3.24. BMI = 81 ÷ 3.24 = 25.0 kg/m².

Question 2. A person has a BMI of 32.4. Which category are they in? (Choose from: Healthy weight, Overweight, Obese, Underweight.)

Answer: Obese

Working: A BMI of 30 – 39.9 is classed as obese.

Question 3. True or false? In type 2 diabetes, body cells no longer respond properly to insulin.

Answer: True

Working: True. The cells stop responding to insulin, so blood glucose stays too high.

Related lessons

• Previous lesson: 7.3 Smoking and the Risk of Disease
• Next lesson: 7.5 Alcohol and Other Carcinogens
• In this chapter: 7.1 Non-Communicable Diseases
• In this chapter: 7.2 Cancer

Continue in the full course

This lesson is section 7.4 (AGCBH0704) of our free GCSE Biology course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Biology course.