International A Level Biology / Cardiovascular health and risk
Energy balance, BMI and waist-to-hip ratio
Edexcel International A Level Biology (YBI11), chapter 3: Cardiovascular health and risk, specification point 1.19. Free lesson.
Read the revision notes · International A Level Biology past papers
Energy balance explains how body mass changes, while BMI and waist-to-hip ratio help assess different aspects of CHD risk. Knowing what each measure shows, and what changes can lower risk, helps you interpret health advice.
How energy balance affects body mass
Your body mass depends on the balance between energy in from food and energy out. When energy in is greater, the surplus is stored as fat in adipose tissue, which is fat storage tissue under the skin and around organs. Over time, body mass, BMI and waist measurement rise. Energy out includes the basal metabolic rate, the energy used to stay alive at rest. Most energy out is not exercise. If energy in is lower than energy out, stored fat is respired and body mass falls. A BMI of 30 or more is classed as obese. Obesity raises CHD risk and is linked to high blood pressure, more LDL…
How to calculate and classify BMI
Body mass index (BMI) measures mass in relation to height. Use mass in kilograms and height in metres. Convert any height given in centimetres to metres before calculating. BMI below 18.5: underweight. BMI from 18.5 to 24.9: healthy. BMI from 25 to 29.9: overweight. BMI of 30 or more: obese.
What BMI and waist-to-hip ratio show
Waist-to-hip ratio is waist circumference divided by hip circumference. If both measurements use the same units, those units cancel, so the ratio has no units. The two measures show different things: BMI indicates whether someone is heavy for their height, while the ratio indicates where fat is stored. BMI risk rises at 25 (overweight) and 30 (obese). A high BMI can reflect muscle as well as fat, so a muscular person may have a high BMI but little body fat. The World Health Organization guideline for waist-to-hip ratio is 0.9 for men and 0.85 for women. The ratio does not show overall…
Changes that lower CHD risk
If BMI or waist-to-hip ratio is high, changes can lower risk. State the actual change, then explain how it helps. High blood pressure can damage the endothelium, the thin inner lining of arteries. Take in less energy than you use. Fat stores are respired, so BMI and waist measurement fall. Eat less saturated fat. This lowers LDL cholesterol in the blood, so less plaque builds up. Eat less salt to lower blood pressure and reduce damage to the endothelium. Exercise more to use more energy, lower blood pressure and raise HDL. HDL carries cholesterol back to the liver. Stop smoking. This…
How to give a specific answer
A general statement such as “eat a healthy diet” does not say what should change. Name the change and its direction so the advice is clear. If a question asks specifically about diet, only diet changes count. Instead of “exercise regularly and eat a healthy diet”, say “increase the amount of exercise and reduce intake of saturated fat”.