In the largest study of its kind, UK researchers found further evidence of the link between carrying extra weight and cancer. They followed five million subjects for seven years and tracked whether each individual developed any of the 22 most common cancers.
The new study, published in the Lancet today, found that extra weight was strongly linked to ten of the 22 cancers. In particular: uterus, gallbladder, kidney, cervix, thyroid, leukemia, liver, colon, ovarian and breast cancers.
While overweight and obesity have long been associated with a number of other health conditions — including Type-2 diabetes, heart disease, stroke, and infertility — this research adds to the body of evidence demonstrating that one's weight is related to one's risk of developing some types of cancer.
"The number of people who are overweight or obese is rapidly increasing both in the UK and worldwide," said study leader Dr Krishnan Bhaskaran, from the London School of Hygiene & Tropical Medicine. His team worked with others from the Farr Institute of Health Informatics. "It is well recognized that this is likely to cause more diabetes and cardiovascular disease. Our results show that if these trends continue, we can also expect to see substantially more cancers as a result."
Even people on the high end of the normal BMI range were at an increased risk of some cancers. Dr. Bhaskaran said this demonstrates that those "at the top end of the 'healthy' range may still have a somewhat higher risk of some cancer types than people in the middle or lower end of the 'healthy' range."
The cancer-weight link isn't entirely straightforward
Interestingly, the researchers also found evidence that people with a high body mass index (BMI) were at a slightly reduced risk of prostate cancer and pre-menopausal breast cancer. So the relationship between cancer and obesity can be specific to the type of disease that develops in an individual.
Dr. Bhaskaran pointed out that while uterine cancer increased substantially at a higher body mass index, for other cancers the increases in risk were more modest. "For some cancers like breast cancer occurring in younger women before the menopause, there even seemed to be a lower risk at higher BMI. This variation tells us that BMI must affect cancer risk through a number of different processes, depending on the cancer type."
When asked about the cause of the link between weight and cancer risk, Dr. Bhaskaran said that "it's very unlikely that there is a single answer to how excess body weight increases cancer risk, and that's clear from the variation in the effects we see for different cancers and in different groups of people."
Some possible explanations for the link include the fact that having more body fat changes the levels of certain hormones, in particular insulin and sex hormones including estrogen, in a way that may favor tumor growth. "Fat tissue also secretes signalling proteins that are thought to help 'turn on' cell division, which could help to start new tumors growing," he added.
Based on this accumulating evidence about the weight-cancer link, Dr. Bhaskaran said he has this message for people: "There are lots of reasons to aim for and maintain a healthy weight, which we already know reduces the risks of cardiovascular disease and diabetes. This research confirms the extent to which maintaining a healthy weight is important for reducing the risk of many common cancers."