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.

Dr Vishnubala, a GP in York, talks about how obesity can cause cancer, who is at risk, and how to lose weight.  

Which cancers are affected?

Research has shown that many types of cancer are more common in people who are overweight or obese, including cancers of the breast (in women after the menopause), bowel, womb, oesophageal (food pipe), pancreatic, kidney, liver, upper stomach (gastric cardia), gallbladder, ovarian, thyroid, myeloma (a type of blood cancer), and 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 – pancreatic, oesophageal and gallbladder cancers.

Body diagram showing cancer types associated with obesity

What is my risk of developing cancer if I'm overweight or obese?

The risk is greater the more weight you gain and the longer you are overweight for. But that doesn’t mean the damage is done. You can help stack the odds against cancer by avoiding gaining more weight. And evidence is growing to show risk can start to go back down with weight loss. Plus the best ways to lose weight for most people are by eating healthily and moving more which on their own reduce cancer risk.

If you are thinking of losing weight and think you need support, your doctor or nurse will be able to help.

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.

How could obesity lead to cancer?

It's very clear that there is a link between cancer and obesity. But it's still not completely understood how exactly obesity causes cancer. Research so far has identified three leading explanations. It's helpful to start by knowing what fat is, and what it does in the body.

Fat in the body has two main functions. It acts as a store of energy. And it's constantly spreading information and instructions to the rest of your body. These messages affect things like cell growth, chemical reactions in cells, and the body’s reproductive cycles.

So fat doesn’t just sit there doing nothing. It's active, telling your cells what to do. And if there is too much fat in the body, then the signals it sends around the body can cause damage. It's likely that this is how obesity raises the risk of cancer.

  • Growth hormones- too much body fat can cause levels of insulin and other growth factors to rise, which can tell cells to divide more often. This raises the chance that cells will change and lead to cancer.
  • Inflammation- when there are more fat cells in the body, specialised immune cells go to the area, possibly to remove dead and dying fat cells. They release a cocktail of chemicals called cytokines, which can lead to inflammation. This makes cells divide faster, and when this happens over a long time it can raise the risk of cancer.
  • Sex hormones- after the menopause, oestrogen made by fat cells can make cells divide faster in the breasts and womb (two of the cancer types most closely linked to obesity), increasing the risk of cell faults and cancer.

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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