Microscopic image of fat tissue

Fat and obesity

How does fat relate to obesity?

Body mass index (BMI) is one way to define a healthy weight. The World Health Organization defines obesity as having a BMI of at least 30. Globally, 12.9 per cent of people were obese in 2014. Despite widespread recognition of the problem and public health campaigns designed to address it, obesity rates have doubled since 1980.

We used to think that the amount of fat a person ate in their diet was closely linked to their percentage of body fat. Yet statistics for the UK show that we eat less fat now (as well as less sugar and fewer total calories) than we did in the 1970s but have higher levels of obesity.

This apparent inconsistency may be partly explained by people under-reporting what they are eating. But it also suggests that there are other reasons for the obesity epidemic, such as more sedentary lifestyles where we spend much of our time sitting down – at home, at work or in cars. Broadly speaking, we must be consuming more energy than we expend.

Obesity occurs when excess energy accumulates as fat in cells called adipocytes. It involves increases in both the number of fat cells (hyperplasia) and their size (hypertrophy). Some excess fat cell development is healthy, as without adipocytes, fat would be stored around organs and blood vessels, leading to metabolic disorders. However, there may still be ways to manipulate fat cell development in order to treat obesity.

Lead image:

Scanning electron micrograph of fat tissue which has been computer-coloured orange.

David Gregory and Debbie Marshall/Wellcome Images CC BY NC ND


Further reading

Downloadable resources

About this resource

This resource was first published in ‘Fat’ in December 2015.

Cell biology, Statistics and maths, Health, infection and disease
Education levels:
16–19, Continuing professional development