How being overweight causes cancer

Overweight man's waist is measured by doctor

Extra fat in the body can have harmful effects, like producing hormones and growth factors that affect the way our cells work. This can raise the risk of several diseases, including cancer. It's thought that more than 1 in 20 cancers in the UK are linked to being overweight or obese.

Which cancers are affected?

Body diagram showing cancer types associated with obesity

Research has shown that many types of cancer are more common in people who are overweight or obese, including:

  • breast cancer, in women after the menopause
  • bowel cancer
  • womb cancer
  • oesophageal (food pipe) cancer
  • pancreatic cancer
  • kidney cancer
  • liver
  • upper stomach
  • gallbladder
  • ovarian
  • thyroid
  • myeloma (a type of blood cancer)
  • meningioma (a type of brain tumour)

This list includes 2 of the most common types of cancer – breast and bowel cancers - and 3 of the hardest to treat – pancreaticoesophageal and gallbladder cancers.

What leads to people being overweight or obese?

Simply put, obesity is the result of taking in more calories through your diet than you are burning through physical activity.

The reasons for this calorie imbalance vary from person to person. It can sometimes be linked to the genes we were born with, or our environments, as well as our individual behaviour and choices. And some drugs and diseases can also contribute to weight gain.

Most people can reach and stay within a healthy weight range by eating healthily, eating smaller amounts and becoming more active.  Visit our tips and advice on keeping a healthy weight.

Why is being overweight linked to so many different types of cancer?

Fat cells in the body are active and produce hormones and proteins that are released into the bloodstream and carried around the body. Because they are spread through the circulation, these ‘chemical messengers’ can affect many parts of the body, and increase the risk of several different types of cancer.

Fat cells can also attract immune cells to body tissues. These immune cells release chemicals that cause long-lasting inflammation which can raise the risk of cancer.   

How does being overweight change hormone levels in the body?

Excess fat changes the levels of sex hormones, like oestrogen and testosterone, in the body. This may increase the risk of cancer.

The hormone insulin is a very important part of how the body uses energy from food. When people are overweight or obese, there is much more insulin present in the body. It’s not clear how this could lead to cancer, though it could be because insulin affects the levels of growth factors available to cells which tell them to divide.

Belly fat has harmful effects

When too much fat is carried around the belly, it can do even more damage. So-called ‘apple’ shapes are linked to bowel, kidney, oesophageal, pancreatic, and breast cancers.

It isn’t clear exactly why this is, but it could be to do with how quickly certain chemicals from fat can get into the blood.

A healthy body weight is important for children as well as adults

One in five children are overweight or obese before they begin primary school, and one in three children are overweight or obese by the time they leave.

Obese children are more likely to grow into obese adults, and obese adults are more at risk of cancer. It is possible overweight children may be at increased risk of cancer as adults, regardless of what they grow up to weigh, but the evidence isn’t clear. 

By encouraging your children to lead a healthy lifestyle, you can help them keep a healthy body weight as a child as well as later on in life. 

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