“Experts have linked eight more cancers to being overweight or obese, nearly tripling the list from five to 13,” the Daily Mail reports.
This is the latest finding of the International Agency for Research on Cancer (IARC), a group of cancer experts from around the world that look at risk factors for cancer.
What is the basis for these reports?
The headlines are based on a report published in the peer-reviewedNew England Journal of Medicine.
The report is not exactly new research, but a review of previously published studies that looked at the link between weight and cancers.
It is the result of a working group of international cancer researchers who met to review the evidence in April this year.
What’s the link between fat and cancer?
The IARC looked at research into the reasons why being overweight may cause cancer.
They found strong evidence that sex hormones and inflammation – both of which are affected by weight – are involved in cancer formation.
They also reviewed evidence from experiments on rats, which found animals fed a calorie-restricted diet were less likely to develop a range of cancers, and obese animals were more likely to get cancer.
They reviewed studies in humans, animals and basic science to see whether the group’s previous conclusions, published in 2002, needed to be updated.
The group’s new report concludes that, “the absence of excess body fatness lowers the risk of most cancers”, also saying that losing weight intentionally may help prevent cancer.
They list 13 cancers where they say there is “sufficient” evidence to conclude that being a healthy weight reduces the risk of cancer, three where there is “limited” evidence, and eight where the evidence is “inadequate”.
The cancers they identify as having sufficient evidence to link them to weight are:
- oesophageal cancer
- gastric cardia – a type ofstomach cancer
- bowel cancer
- liver cancer
- gallbladder cancer
- pancreatic cancer
- breast cancer in postmenopausal women
- womb cancer
- ovarian cancer
- kidney cancer
- meningioma – a type of brain tumour
- thyroid cancer
- multiple myeloma – cancer of the white blood cells
The degree of increased risk ranged from an almost fivefold increase for oesophageal cancer in the highest BMI category compared with people with a normal weight (relative risk [RR] 4.8; 95% confidence interval [CI] 3.0 to 7.7), to a 10% increased risk of postmenopausal breast cancer (RR 1.1, 95% CI 1.1 to 1.2).
What is the link between cancer and weight?
Scientists have known for some time that people who are overweight have an increased risk of certain cancers compared with people of a healthy weight.
A healthy weight is usually defined as having a body mass index (BMI) of 18.5 to 24.9. People are classed as overweight if their BMI is 25 to 29.9 and obese if their BMI is 30 or over. BMI is calculated from weight and height.
Almost all of the evidence linking being overweight and cancer is from epidemiological studies, which look at large groups of people and then calculate how likely people of different weights are to have been diagnosed with cancer, compared with people of a healthy weight.
Many of these studies also try to take account of other factors that can affect cancer risk, such as whether people smoke, whether they exercise, and how healthy their diet is.
But it’s hard to account for all other factors, so individual studies can’t really show whether being overweight causes cancer.
When reviewed together, however, and when studies show that the more overweight someone is, the more likely they are to get cancer, the chances are higher that the research is showing that weight has a causal effect.
A report by the IARC in 2002 said there was enough evidence to say being overweight increased the risk of eight cancers, all of which are included in the new list of 13.
Since then other studies have strengthened the evidence, so the IARC now feels it has enough evidence to list these 13 cancers.
How does weight and cancer affect you?
Carrying excess body weight has a number of health risks, including a greater chance of having a heart attack or stroke, as well as being linked to a raised risk of the cancers listed above.
The easiest way to keep to a healthy weight is to avoid putting weight on, but if you already weigh more than you like, diet and exercise can help you achieve a healthier weight.
Talk to your GP or see our 12-week plan to lose weight through healthy eating and physical activity.
Weight is not the only factor that affects the risk of cancer. Although there’s no proven way to avoid cancer altogether, you can lower your risk of getting cancer if you:
- eat a healthy, balanced diet
- maintain a healthy weight
- stay physically active
- drink less alcohol
- stop smoking
- protect your skin from sun damage
Analysis by Bazian. Edited by NHS Choices. Follow NHS Choices on Twitter. Join the Healthy Evidence forum.
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