Obesity is the result of taking in more calories through your diet than you are burning through physical activity1.
The reasons for this calorie imbalance vary from person to person.
It can sometimes be linked to the genes we were born with, or our environments, as well as our individual behaviour and choices. And some drugs and diseases can also contribute to weight gain.
Most people can reach and stay within a healthy weight range by eating healthily, eating smaller amounts and becoming more active.
Extra fat in the body can have harmful effects, like producing hormones and growth factors that affect the way our cells work. An estimated 1 out of every 3 cancer deaths is linked to excess body weight, poor nutrition, and/or physical inactivity. These factors are all related and may all contribute to cancer risk, but body weight seems to have the strongest evidence linking it to cancer. Excess body weight contributes to as many as 1 out of 5 of all cancer-related deaths.
Excess fat changes the levels of sex hormones, like oestrogen and testosterone, in the body. This may increase the risk of cancer. Fat cells also produce many other chemical messengers which affect how the body works.
The hormone insulin is a very important part of how the body uses energy from food. When people are overweight or obese, there is much more insulin present in the body. It’s not clear how this could lead to cancer, although high insulin levels are a common feature of many cancers.
Being overweight or obese is clearly linked with an increased risk of many cancers, including cancers of the 2:
- Breast (in women past menopause)
- Colon and rectum
- Endometrium (lining of the uterus)
Being overweight or obese might also raise the risk of other cancers, such as:
- Non-Hodgkin lymphoma
- Multiple myeloma
- Aggressive forms of prostate cancer
In addition, having too much belly fat (that is, a larger waistline), regardless of body weight, is linked with an increased risk of colon and rectal cancer, and is probably linked to a higher risk of cancers of the pancreas, endometrium, and breast cancer (in women past menopause).
But the links between body weight and cancer are complex and are not yet fully understood. For example, while studies have found that excess weight is linked with an increased risk of breast cancer in women after menopause, it does not seem to increase the risk of breast cancer before menopause.
The timing of weight gain might also affect cancer risk. Being overweight during childhood and young adulthood might be more of a risk factor than gaining weight later in life for some cancers. For example, some research suggests that women who are overweight as teenagers (but not those who gain weight as adults) may be at higher risk for developing ovarian cancer before menopause.
A healthy body weight is important for children as well as adults. By encouraging your children to live a healthy lifestyle, you can help them keep a healthy body weight later on in life. A person’s body weight can be influenced by what they ate when they were children, or even what their mothers ate before they were born.
Children who are very heavy at 2 years of age tend to have a higher chance of being overweight later in life. Some studies have found that people who are overweight or obese as children have higher risks of some cancers later in life.
Excess body weight may affect cancer risk through a number of mechanisms, some of which might be specific to certain cancer types. Excess body fat might affect:
- Immune system function and inflammation
- Levels of certain hormones, such as insulin and estrogen
- Factors that regulate cell growth, such as insulin-like growth factor-1 (IGF-1)
- Proteins that influence how the body uses certain hormones, such as sex hormone-binding globulin
Research on how losing weight might lower the risk of developing cancer is limited. Still, there’s growing evidence that weight loss might reduce the risk of breast cancer (after menopause), more aggressive forms of prostate cancer, and possibly other cancers, too.
Some body changes that occur as a result of weight loss suggest it may, indeed, reduce cancer risk. For example, overweight or obese people who intentionally lose weight have reduced levels of certain hormones that are related to cancer risk, such as insulin, estrogens, and androgens.
While we still have much to learn about the link between weight loss and cancer risk, people who are overweight or obese should be encouraged and supported if they try to lose weight. Aside from possibly reducing cancer risk, losing weight can have many other health benefits, such as lowering the risk of heart disease and diabetes. Losing even a small amount of weight has health benefits and is a good place to start.
- Cancer Research UK. How being overweight causes Cancer. Accessed at http://www.cancerresearchuk.org/about-cancer/causes-of-cancer/bodyweight-and-cancer/how-being-overweight-causes-cancer on July 10, 2015.
- American Cancer Society. Does body weight affect Cancer risk?. Accessed at http://www.cancer.org/cancer/cancercauses/dietandphysicalactivity/bodyweightandcancerrisk/body-weight-and-cancer-risk-effects on July 10, 2015.