UK GCSE level age ~14-16, ~US grades 9-10 Biology revision notes re-edit 14/05/2023 [SEARCH]

Diet and exercise : 5. Obesity and measures of obesity - body mass index and waste to hip ratio

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(5) Obesity and measures of obesity

(a) Introduction

(Remember obesity is only one measure of healthiness and well-being)

(see also on health 4 page on noncommunicable diseases page

If you consume more energy rich foods from your diet than your body uses, your body will store the excess energy rich foods as fat, hence you put on weight.

If you body weight is over 20% than the recommended weight for your height, you are classed as clinically obese.

Obesity can result in health problems including diabetes, high blood pressure and heart disease.

There is no such thing as the 'perfect weight', we come in all shapes and sizes, but there are limits within which we should be to be healthy!

In the medical profession, a doctor can't just simply say that somebody is overweight, without reference to some kind of statistical index, usually by one/both of the ratios described below.

Equally healthy people can have quite different weights, but there are some reasonably good indicators as to when your weight is not what it should be 'ideally'.

In rich developed countries we are often dealing with 'overweight' people eating too much rich fatty food, but in poorer underdeveloped countries we are dealing 'underweight' people, particularly young children suffering from malnutrition.

Malnutrition means lack of proper nutrition, caused by not having enough to eat, not eating enough of the right things, or being unable to use the food that one does eat.

(b) The Body Mass Index

The body mass index is a 'rough' guide to help the medical profession decide whether you are underweight, normal, overweight or obese - based on your height and height.

The higher your BMI the more fat you are carrying - but not necessarily unhealthily.

Body Mass Index (BMI) = (body mass in kg) / (height in m)2

When measured, you then consult a table of BMI values to se where you fit in!

Table of BMI values (from

Body mass index Weight description
less than 18.5 underweight
18.5 to 24.9 normal
25.0 to 29.9 overweight
30.0 to 40.0 moderately obese
over 40.0 very obese

If you eat too much fatty sugary foods and don't take enough exercise, most people will put on weight and too much of it. You are taking in too much energy rich food for your daily needs.

The excess energy releasing food is stored as fat and gives you a raised BMI value.

Having a higher than normal BMI value increases your risk of developing type 2 diabetes.

The BMI calculation is easy e.g. for a person with a body mass of 70.0 kg and a height of 1.80 m,

The BMI = m / h2 = 70 / 1.802 = 21.6 (3 sf, kg/m2), suggesting the person is 'normal'.

Note of caution: Having a high BMI is not always unhealthy e.g. athletes train hard to build up extra muscle which is heavier than fat, so they will tend to have higher than 'normal' BMI values and would not be classed as overweight.

Note from 2020 on the covid-19 flue pandemic:

Research has shown that obese people are more likely to be seriously ill with the virus than non-obese people - more severe fever, more likely to be hospitalised and more likely to die!

The waist-to-hip ratio should also be taken into consideration (see below).

(c) The waist-to-hip ratio

Where the body stores fat is quite important.

If a lot of fat is stored around the abdomen you get a 'fat tummy' and this is associated with an increased risk of developing type 2 diabetes - hence the importance of diet and exercise.

Comparing the circumferences of waist and hips is another measure of whether you are 'overweight' and concentrates around one area where we can accumulate to much fat - the 'tummy'.

waist-to-hip ratio = circumference of waist (cm) / circumference of hips (cm)

The higher your waist-to-hip ratio the more fatty tissue you are carrying around the middle of your body.

When measured, you then consult a table of BMI values to se where you fit in!

Table of waste-to-hip ratios (from

Health risk waist-to-hip ratio (women) waist-to-hip ratio (men
low less than 0.81 less than 0.96
moderate 0.81 to 0.85 0.96 to 1.00
high (overweight) over 0.86 over 1.00

The Waist to Hip calculation is easy e.g. for a woman of waist measurement of 32 cm and a hip measurement of 40 cm,

The waist to hip ratio  = 32 / 40 = 32 / 40 = 0.80 (3 sf, no units), suggesting the person is 'fine' (borderline low-moderate).

If you are female and your waist-to-hip ratio is over 0.86 you are classed as overweight.

If you are male and your waist-to-hip ratio is over 1.00 you are classed as overweight.

If you are above the moderate waist-to-hip ratio values you are carrying too much fat around your 'middle' - referred to as abdominal obesity.

Having a higher than normal waist-to-hip ratio increases your risk of developing type 2 diabetes.



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