What is the biggest preventable cause of cancer after smoking?

What is the biggest preventable cause of cancer after smoking?

By Emma Shields, NZ Registered Dietitian

Read on and you’ll find out:

  • How we know obesity causes cancer

  • What types of cancer obesity causes

  • How obesity causes cancer

  • What we need to keep a healthy weight

When most people learn that  overweight and obesity is the biggest preventable cause of cancer after smoking, they’re shocked.

Obesity causes 13 different types of cancer, including common types like breast and bowel and hard to treat ones like pancreatic and oesophageal cancer. Other types include kidney, womb, ovarian and liver cancer.

With many people living with obesity (here obesity is defined as excess body fat that can impair health) in New Zealand, it’s an important issue to talk about.

How can we be sure obesity causes cancer?

We know obesity is a cause of cancer thanks to consistent results from decades of research involving millions of people showing a link between body fatness and cancer. Because of this, we can confidently rule out other explanations such as chance or other related factors.

We also know the risk of cancer increases the more weight is gained (called a dose-response relationship). Plus, we have good explanations for how obesity can cause cancer.

International organisations like the International Agency for Research on Cancer, the World Cancer Research Fund, and Cancer Research UK all agree that obesity is a cause of cancer.

How does obesity cause cancer?

The reason obesity is linked to so many cancer types lies in the role fat plays in the body. Fat is active, telling other cells what to do, which can affect things like cell growth. And when there’s too much in the body, the signals it sends around the body can cause damage.

These signals can affect:

  • Growth hormones – higher levels of body fat can increase levels of growth hormones like insulin, which tell cells to divide more often
  • Sex hormones – after menopause, oestrogen made by fat cells tell the cells in the womb and breast to divide faster
  • Inflammation – higher levels of inflammation due to extra body fat can cause cells to divide faster.

All of the above can increase the chance of cancer cells developing which can continue to divide and cause a tumour.

These are the three main reasons scientists have identified so far, but research continues to help us fully understand all the ways obesity can cause cancer.

How can we all keep a healthy weight?

There’s no doubt there are steps we can all take individually to keep or achieve a healthy weight and reduce our risk of cancer. Whether it’s cutting down on sugary drinks or upping the steps you do each day, small changes can make a big difference.

But we live in a world that makes taking those steps really hard for most people.

That’s why the real solutions lie in government action. Around the world, actions like sugary drink taxes and restrictions of junk food marketing have been introduced to help create healthier environments. When will New Zealand follow suit?


Emma Shields is a NZ Registered Dietitian. She was previously Health Information Manager at Cancer Research UK. During her time there, she was the spokesperson for a national campaign to raise awareness about the link between obesity and cancer and push the UK Government to introduce policies that help make it easier to keep a healthy weight.