Weight and cancer risk: the second biggest preventable cause

This is the least-known major fact about body weight in the UK. Only around 15% of people are aware of the link, which leaves it doing a great deal of damage quietly.

The headline

Excess weight is the second biggest preventable cause of cancer in the UK, after smoking ^[1].

A detailed analysis of UK cancer cases in 2015 found that overweight and obesity accounted for 6.3% of all cases — 7.5% in women and 5.2% in men, with smoking at 15.1% ^[2]. Cancer Research UK’s public figure is over 18,000 cases a year.

Which cancers

A 2016 review by the International Agency for Research on Cancer brought the total to 13 cancers linked to excess body fat ^[3], including bowel, post-menopausal breast, womb, oesophageal adenocarcinoma, pancreatic, kidney, liver, gallbladder, gastric cardia, ovarian and thyroid cancers, along with meningioma and multiple myeloma.

The strength of the link varies a great deal between them.

Womb cancer, specifically

The clearest and largest link, and the one least talked about.

Women with obesity have more than twice the risk. In absolute terms: of 1,000 women at a healthy weight, around 16 develop womb cancer in their lifetime; of 1,000 women with obesity, around 41 — 25 extra cases per 1,000 ^[1].

Around 2,900 extra womb cancer cases a year in the UK are caused by excess weight, and womb cancer rates have risen sharply over the past two decades, largely attributed to rising body weight.

This is also why postmenopausal bleeding should always be investigated promptly, regardless of weight.

Why fat affects cancer risk

Cancer Research UK describes three main mechanisms ^[1]:

Hormones. Fat tissue produces oestrogen, particularly after the menopause, and higher oestrogen levels drive cell division in the womb lining and in breast tissue.

Growth signals. Higher insulin and insulin-like growth factors encourage cells to divide more often.

Inflammation. Excess fat tissue produces a chronic low-grade inflammatory state, and inflammation increases cell turnover.

More cell divisions means more opportunities for the errors that lead to cancer.

The honest uncertainty

Does losing weight reduce the risk?

Cancer Research UK is careful here, and it is worth reproducing their position rather than improving on it: whether cancer risk decreases after someone loses weight is not so clear, while research so far suggests that weight loss does help ^[1].

The reason it is hard to establish is methodological. Undiagnosed cancer itself causes weight loss, which confuses the picture, and relatively few people maintain substantial loss for long enough to study.

So the honest framing is: the risk from carrying excess weight is well established; the benefit of losing it is likely and supported but not proven to the same standard. That is still a reason to act — it is simply not a reason to overstate.

What this is not

It is not a reason for alarm about a particular body. Most people with a high BMI will not develop any of these cancers, and plenty of people at a healthy weight do.

It is also not a reason to skip screening. Bowel, breast and cervical screening apply regardless of weight, and attendance is more protective than any single risk factor is dangerous.

When to speak to a clinician

See a GP promptly if you have:

• bleeding after the menopause, or unusual bleeding between periods — always investigate
• a change in bowel habit lasting three weeks or more, or blood in your stool
• difficulty swallowing, or food sticking
• unexplained weight loss
• a new lump anywhere, or a change in a mole
• persistent bloating, particularly if you are a woman over 50
• blood in your urine
• a cough lasting three weeks or more, or coughing blood

Take up screening invitations when they arrive. Bowel, breast and cervical screening are the highest-value things on this page.

This article is for general information and isn’t a substitute for advice from your own clinician. If you feel unwell, contact your GP or NHS 111. In an emergency, call 999.