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While most people are aware of the biggest cause of cancer in the UK – smoking – many are unaware of the second biggest: being overweight or obese.

In fact, only 15% of UK people are aware of the link between obesity and cancer.

The evidence linking bodyweight and cancer has been building for decades, with new evidence still emerging.

The International Agency for Research on Cancer (IARC) released a report saying there is now strong evidence linking bodyweight to even more cancer types than was previously thought. Bringing the total tally to 13 types of cancer.

It’s important to remember that although cancer risk is increased in people who are overweight or obese, this doesn’t mean they’ll develop cancer. Cancer risk is affected by many different factors – including whether you smoke, how old you are, and your genes.

So where does this evidence come from, and just how does what someone weighs affect their risk of cancer?

What is a healthy weight?

Studies have shown that people who are classified as overweight or obese are at a higher risk of cancer.

And while there are lots of ways to measure if someone is overweight, the most commonly used way is the Body Mass Index (BMI).

BMI is a measurement used to see whether a person’s weight differs from what’s expected for their height.

Having a BMI of between 25 and 30 means a person is overweight, while having a BMI of 30 or more means a person is obese. For most adults, although it isn’t perfect, BMI is fairly accurate – unless you’re someone who is carrying a lot of muscle, like professional athletes.

The evidence shows that the higher a person’s BMI, the higher their risk of cancer. There’s also some evidence that where we carry fat also matters – people who carry fat particularly around their stomachs are at an increased risk compared to people who carry fat around their hips.

What’s the evidence?

As mentioned above, the evidence linking bodyweight and cancer has been building for decades. We’ve blogged about it before, and it’s something that both the World Cancer Research Fund and IARC label as a major preventable cause of cancer.

Similarly to smoking, carrying too much weight doesn’t just affect one type of cancer. There’s now a large body of evidence that shows carrying too much weight increases the risk of many different types of cancer.

This includes some of the most common, such as breast and bowel cancers, but also some of the hardest to treat, such as oesophageal cancer.

And as our graphic below shows, the most recent IARC review brings the total number of cancers linked to obesity to 13.

The effect weight has on each type of cancer is complex. And the effect is particularly strong in womb cancer. Studies have shown that obese women have more than twice the risk of developing womb cancer compared to women who are a healthy weight.

To put this into perspective, out of every 1000 women who are a healthy weight in the UK, around 16 would develop womb cancer at some point in their lives. But among 1000 women who are obese, 41 would develop womb cancer. That’s 25 more women who would develop womb cancer because of their bodyweight. And overall around 2,900 extra womb cancer cases every year in the UK are caused by extra bodyweight.

Womb cancer rates have sharply increased over the last 20 years, and we think that rising bodyweight is largely behind this rise.

But the effect of weight is also large in cancers that affect both men and women, including oesophageal and kidney cancers. Around a quarter of kidney cancers and more than a fifth of oesophageal cancers are caused by obesity.

How does weight affect cancer risk?

Researchers are still trying to figure out exactly how carrying extra weight causes cells to become cancerous, but it’s likely down to the chemical signals that are released from the extra body fat.

Some body fat is essential. It’s our back-up energy store and it makes chemical signals that help keep our bodies in check. But when we have too much body fat, it can have harmful effects.

We’ve blogged about the three main theories in detail before. In a nutshell, the extra fat we carry releases hormones and other growth-promoting signals around our bodies. It also causes inflammation. And each of these has an effect on how often our cells divide. And it’s these changes in cell division that are most likely behind the increased risk of cancer.

Does losing weight reduce the risk of cancer?

Whatever the exact mechanism is, there is strong evidence to show that carrying extra weight increases the risk of many different types of cancer.

But whether cancer risk decreases after someone loses weight isn’t so clear. Designing this type of research is particularly hard to do, because it’s difficult to separate people who lose weight through lifestyle changes from people who may have lost weight because they already have cancer.

And it’s hard to find lots of people for studies who have lost weight and keep it off over a long period of time.

But overall, research so far suggests that weight loss does help reduce the risk of cancer.

What can you do?

What’s important to remember is that although cancer risk is increased in people who are overweight, this doesn’t mean they’ll develop cancer.

Cancer risk is affected by many different factors – including whether you smoke, how old you are, and your genes.

And dropping down to a healthy weight has many health benefits, including reducing your risk of heart disease and diabetes.

If you’d like to lose weight, there are lots of free resources online to help. We’ve developed the Ten Top Tips programme with Weight Concern, which uses scientific evidence to help build long term healthy habits around taking in fewer calories and burning more energy through activity. There’s also the One You website from Public Health England that’s packed with good info. And you can talk to your doctor if you think you need more support.

But perhaps what’s most important to state is that nobody should feel bad about their bodyweight or feel blamed for their cancer.

We’d like to prevent more cases of cancer, and one way of doing this is by raising awareness of what causes cancer so people know how to reduce their risk.

Only 15% of people in the UK know there’s a link between carrying extra weight and cancer. So to try and tackle this we’re running a campaign to see if we can help increase awareness with posters, radio adverts and using social media.

Being aware that carrying extra bodyweight can cause cancer is important – in the UK, it’s estimated to cause over 18,000 cancers every year. But awareness alone isn’t enough. That’s why we’re also asking the Government to do more to tackle obesity.

Keeping a healthy weight isn’t a cast iron guarantee against cancer, but it can greatly stack the odds of avoiding cancer in your favour.

Casey Dunlop is a health information officer at Cancer Research UK

Update 26/02/18: This post has been updated with new stats as part of our campaign to raise awareness of the link between bodyweight and cancer.