Body weight and cancer: the evidence

Major studies confirm that being overweight or obese increases your risk of various cancers. 1,2,3,4 The World Health Organisation (WHO) says that overweight and obesity are the most important known avoidable causes of cancer after tobacco. 5

Researchers estimate that overweight and obesity are behind around 17,000 cases of cancer each year in the UK. 6 This number may well increase in the future since more cancers are being linked to obesity, and the number of people who are overweight is increasing.

Scientists have estimated that anywhere between 7% and 15% of breast cancer cases in developed countries are caused by obesity. 6 - 9 Over a hundred studies show that women who are overweight or obese and have been through the menopause have higher breast cancer risks. 3,10

Two large studies funded by Cancer Research UK - the EPIC study and the Million Women Study - have found that obese women have a 30% higher risk of postmenopausal breast cancer than women with a healthy weight. 4,10 This means that if the average lifetime risk of breast cancer is one in nine, an obese woman’s lifetime risk is one in seven.

Obesity does not increase the risk of breast cancer in women before their menopause. 11,12 However, women who are overweight or obese at this point in their lives often find it hard to lose weight after their menopause.

Putting on weight over time can also increase the risk of breast cancer. Studies have found that:

• putting on 2-10 kg (4.4 - 22 lb) after the menopause increases the risk of breast cancer by 30% 13
• putting on 25 kg (55 lb) after the age of 18 increases the risk of breast cancer by 45% 8

Obesity is one of the most important causes of bowel cancer. 2,5 Some groups have estimated that obesity causes about 11-14% of bowel cancer cases. 6, 7,9

Many large studies have found that bowel cancer is more common in overweight or obese people. 3,14,15,16 Two of the largest studies so far, including the EPIC study, have found that obese men have about 50% higher risks of bowel cancer than those with a healthy weight. 15,16

When BMI is used to measure body fat, studies tend to find that only obese men have a higher risk of bowel cancer. But when researchers use waist circumferences or waist-to-hip ratios, both obese men and women have higher risks of bowel cancer. 16 This suggests that for women at least, fat around the stomach is more of a problem than fat elsewhere on the body. 17

A large body weight is one of the most important causes of womb cancer. A 2011 study estimated that more than a third of womb cancers in the UK are caused by being overweight or obese. 6

Studies have consistently found that obese people are three to four times more likely to develop womb cancer than people with a healthy bodyweight. 4,18,19

Being overweight or obese increases the risk of a type of oesophageal cancer (cancer of the foodpipe) called “oesophageal adenocarcinoma”.

Being overweight doubles the risk of developing this cancer, 20 and being obese can triple the risk. 1,21 Experts have estimated that in the UK it causes about 1 in 5 cases of this type of cancer. 6 In fact, the rates of oesophageal adenocarcinoma in white UK men are among the highest in the world and rising. Some studies have suggested that this type of cancer may be becoming more common because of rising levels of obesity. 20,21

Studies have consistently found that people who are overweight or obese are also more likely to develop pancreatic, 22,23 kidney, 24 and gallbladder cancers. 25 Studies have estimated that having a high body weight accounts for nearly a quarter of kidney and gallbladder cancers. 6, 26

And there is more and more evidence that being overweight or obese could increase the risk of many other types of cancer, including:

• brain cancer 27
• leukaemia 4,28
• liver cancer 29
• multiple myeloma 4,30
• non-Hodgkin lymphoma 4,31
• ovarian cancer, before the menopause 32
• aggressive prostate cancer 33
• thyroid cancer 34

The way that fat is distributed around the body can also affect the risk of cancer. Apple-shaped people who put on weight around their stomach may have higher risks than pear-shaped people who put on weight around their hips.

Scientists measure belly fat using either waist circumference (the length of tape that goes around your waist) or waist-to-hip ratio (how wide your waist is compared to your hips). Studies have found that people with larger waists or waist-to-hip ratios have higher risks of breast cancer, 35 bowel cancer, 17 kidney cancer, 36 and pancreatic cancer. 22

Obesity most likely increases the risk of cancer by raising levels of hormones such as oestrogen and insulin.

In early life, oestrogen is mainly produced by a woman’s ovaries, but this stops after menopause. Instead, fat in the body becomes the main source of oestrogen and obese women have up to twice as much oestrogen as women with a healthy weight. 37 They also have lower levels of SHBG, or ‘sex hormone binding globulin’, which mops up oestrogen in the body 38. This is almost certainly why obesity increases the risk of breast and womb cancers.38,39, 40

Obesity also increases levels of insulin in the body. It’s not clear how this could lead to cancer, although high insulin levels are a common feature of many cancers. High insulin levels could explain why being obese increases the risk of bowel, kidney and pancreatic cancer. 41,42, 43

Obesity could also cause cancer through other means, including:

• increasing the risk of oesophageal cancer by causing ‘gastric acid reflux’, a condition where the stomach’s acids are briefly pushed back into the throat. 44 This damages the lining of the oesophagus.
• increasing the risk of gallstones, which in turn increase the risk of gallbladder cancer. 25
• being associated with physical inactivity or unhealthy diets.

Studies have shown that overweight and obese people are more likely to develop cancer than people with a healthy body weight. It therefore makes sense that losing weight can help to reduce the risk of cancer, and scientists are now trying to confirm this with large studies. 45 - 48

One study found that women who lost 20 pounds or more had 11% lower risks of cancer overall compared to women who had never lost that much weight. 49 Another study found that women who lost 10kg since menopause and kept the weight off more than halved their risk of breast cancer. 46 Other studies have found similar results for breast and other types of cancer

When people try lose weight through short-term fixes, in most cases, the end up putting the weight back on. It’s unclear how this ‘weight cycling’ affects the risk of cancer. But at least one study found that women whose weight had gone up and down by over 10 pounds, more than ten times, had higher risks of kidney cancer than those whose weight was stable. 36 While this was just a single study, it does suggest that the best way to reduce the risk of cancer is to maintain a healthy bodyweight over time.

Eating habits established in childhood often endure after many years. In 1993, a group of scientists showed that at least half of obese children were still obese as adults 50. And this proportion is likely to be even higher now. Another study of Scottish and English children found that people who are obese as children have higher risks of some cancers later on in life 51.

Many studies have shown that children with obese parents are much more likely to be obese themselves 52. But this may not be due to parenting styles. Some research from Cancer Research UK indicates children’s attitudes towards food may be due to genes. By inheriting certain genes from their parents, some children may have a higher chance of being overweight or obese 53.