How does obesity cause cancer?

Overweight and obesity is the second biggest cause of cancer in the UK. If you are overweight you are more likely to get cancer than if you are a healthy weight.

Being overweight doesn’t mean that you’ll definitely develop cancer. But the risk is higher the more overweight you are and the longer you are overweight for. If you are overweight, avoiding putting on more weight and losing weight will help reduce your risk of cancer.

There are lots of benefits to keeping a healthy weight. These include having increased energy, and less risk of conditions such as heart disease, stroke and joint pain.

How can overweight and obesity cause cancer?

Extra fat in the body doesn’t just sit there, it's active and send out signals to the rest of your body. These signals can tell cells in our body to divide more often, which can lead to cancer.

How can obesity cause cancer?

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The signals released by fat cells can affect:

Growth hormones - too much body fat can cause levels of growth hormones to rise, which tells cells to divide more often. This raises the chance that cancer cells will develop.
Inflammation - immune cells go to areas of the body where there are lots of fat cells. This can lead to inflammation, which causes cells to divide quicker. Over time, this can increase the risk of cancer.
Sex hormones - after the menopause, fat cells produce the hormone oestrogen. This hormone can make cells in the breast and womb divide more often, which increases the risk of cancer developing.

These are the main ways extra body fat can cause cancer that scientists have identified so far. But research continues to find out more.

How do we know overweight and obesity causes cancer?

There is lots of high-quality research showing the link between excess weight and cancer. This research has been going on for decades and has involved millions of people.

We can be sure that obesity causes cancer because the risk increases the more weight is gained and the longer it is held for. There are also good explanations for how fat cells can cause cancer to develop.

Research has shown that overweight and obesity causes 13 different types of cancer:

Breast and bowel (two of the most common cancer types)
Pancreatic, oesophageal and gallbladder (three of the hardest to treat cancers)
Womb and ovarian
Kidney, liver and upper stomach
Myeloma (a type of blood cancer)
Meningioma (a type of brain tumour)
Thyroid

This doesn’t mean that everybody who is overweight will develop cancer. But the risk is higher for people who are overweight or obese.

Am I a healthy weight?

Being overweight increases the risk of health conditions such as type 2 diabetes, raised blood pressure, cardiovascular disease and cancer.

There isn’t one perfect weight for everyone, but there are some tools that can tell you if you are a healthy weight.

Body Mass Index (BMI)

BMI is one way of measuring whether you’re a healthy weight, using your height and weight. BMI isn’t a perfect tool, but it can be used as a good starting point to think about weight. It can be used to tell whether you are in a healthy weight range, underweight, overweight or obese for your height.

Ethnic background also influences what is considered a healthy weight.

For adults with a South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean ethnic backgrounds a BMI of 23 or more is overweight and a BMI of 27.5 or more is obese.

For adults not with these ethnic backgrounds, a BMI of 25 or more is overweight and a BMI of 30 or more is obese.

You can find out more about BMI and healthy weight at NHS healthy weight.

A BMI of 25 or higher increases the risk of 13 types of cancer. Having a BMI higher than 25 doesn’t mean you’ll definitely develop cancer. There are lots of steps you can take to reduce your risk.

Where fat is stored in the body can also affect cancer risk. Some research shows that people who carry a lot of fat around their middle, near the stomach, are at a higher risk of developing some types of cancers. We need more research to understand why this is. But it could be because fat around the middle is very active in sending out signals to other parts of the body.

Waist-to-height ratio

Waist-to-height ratio can be used to tell whether you are carrying too much of your body fat around your tummy. Too much fat around the tummy can increase the risk of some health conditions. If you have a BMI of less than 35, you can use the waist-to-height ratio, as well as BMI, to understand if you are a healthy weight.

To find out your waist-to-height ratio, divide your waist size in centimetres by your height in centimetres. Try to keep your waist size less than half of your height. This is a waist-to-height ratio of 0.5 or below.

The NHS healthy weight website gives instructions on how to measure your waist size.

You can talk to your doctor or nurse if you have any questions about your weight, BMI or waist-to-height ratio.

Our tips on how to keep a healthy weight.

Does obesity cause cancer in children?

Overweight and obesity only causes cancer in adults. There is no link between obesity and childhood cancers.

However, a healthy body weight is important for children too. Children who are obese are around five times more likely to remain obese in adulthood.

It’s important to help children be a healthy weight, so they are more likely to be a healthy weight as adults.

Find out about our action on childhood obesity.

Key references

Brown, K. F. et al. The fraction of cancer attributable to modifiable risk factors in England, Wales, Scotland, Northern Ireland, and the United Kingdom in 2015. Br. J. Cancer 118, 1130–1141 (2018).

Secretan, B. L. et al. Special Report Body Fatness and Cancer — Viewpoint of the IARC Working Group. (2016). https://www.nejm.org/doi/10.1056/NEJMsr1606602?url_ver=Z39.88-2003&rfr_id=ori:rid:crossref.org&rfr_dat=cr_pub%3dwww.ncbi.nlm.nih.gov

World Cancer Research Fund / American Institute for Cancer Research Diet, Diet, Nutrition, Physical Activity and Cancer: a Global Perspective. A summary of the Third Expert Report. (2018).