All subjects · Biology · WBI11 (Unit 1)

Diet, obesity and reducing heart disease risk

Energy balance, body measurements and lifestyle choices all affect the risk of coronary heart disease (CHD). You need to use the numbers.

Energy balance

If energy taken in from food is greater than energy used (basal metabolic rate plus activity), the extra is stored, mostly as fat, and mass increases. If intake is less than output, stored energy is used and mass falls.

Obesity raises blood pressure and is linked to high LDL and type 2 diabetes, all of which increase CHD risk.

Indicators of obesity

Body mass index: BMI = mass (kg) / height (m)². Healthy range for adults is about 18.5 to 24.9; 25 to 29.9 is overweight; 30 or more is obese.
Waist-to-hip ratio = waist circumference / hip circumference. A high ratio shows fat stored round the abdomen, which is more strongly linked to CHD.

BMI does not distinguish muscle from fat, so a very muscular person can have a high BMI.

BMI	Category
Below 18.5	Underweight
18.5 to 24.9	Healthy weight
25 to 29.9	Overweight
30 and over	Obese

Reducing risk

Diet: less saturated fat and salt, more fruit, vegetables and fibre; energy intake matched to output.
Regular exercise: lowers blood pressure and LDL, raises HDL, helps control mass.
Not smoking.
Keeping blood pressure and cholesterol under control, with medication if needed.

Worked example

A person has mass 81 kg and height 1.80 m. Calculate their BMI and state the category.

BMI = 81 / 1.80²
= 81 / 3.24 = 25.0

Answer: BMI = 25.0: overweight.

Key idea: BMI = mass / height² (25 to 29.9 overweight, 30+ obese). Waist-to-hip ratio shows abdominal fat. Energy in greater than energy out leads to weight gain.

Check yourself

Calculate the BMI of a person with mass 72 kg and height 1.80 m. Give your answer to 1 decimal place.
Show answer
22.2 kg m-2. 72 / 3.24 = 22.2.
A person is 1.60 m tall with mass 82 kg. Calculate their BMI to 1 decimal place.
Show answer
32.0 kg m-2. 82 / 2.56 = 32.03, so 32.0 (obese).
Waist circumference is 90 cm and hip circumference is 100 cm. Calculate the waist-to-hip ratio.
Show answer
0.90. 90 / 100 = 0.9.
A BMI of 27 is classed as
- overweight
- healthy weight
- obese
- underweight
Show answer
overweight. 25 to 29.9 is overweight.
Body mass increases when
- energy output is greater than intake
- intake equals output
- energy intake is greater than energy output
- basal metabolic rate rises
Show answer
energy intake is greater than energy output. Excess energy is stored, mainly as fat.
Why can BMI be misleading for a rugby player?
- It does not distinguish muscle mass from fat
- It ignores body mass
- It uses waist size
- It only works for children
Show answer
It does not distinguish muscle mass from fat. Muscle is dense, so a fit athlete can have a high BMI.
How does regular exercise reduce CHD risk?
- It raises LDL
- It increases blood pressure permanently
- It lowers blood pressure and LDL and raises HDL
- It increases saturated fat intake
Show answer
It lowers blood pressure and LDL and raises HDL. Exercise improves blood pressure and the lipid profile.

Practise with instant feedback

Sources

Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.