A newly released report by the British cancer charity Cancer Research UK (CRUK) has finally put a number on what epidemiologists and cancer scientists have known for a long time: obesity is one of the biggest causes of cancer, and is set to become the most common cause of cancer in women by 2043. Obesity mainly drives breast, colon and uterine cancer, as well as playing a big role in the development of esophageal and kidney cancer. It is estimated that up to 10% of all cases of cancer are directly caused by patients being overweight, which is especially concerning when one takes into consideration that “only” about 30% of the population is morbidly obese. With an obesity epidemic on the rise and other environmental causes of cancer slowly becoming eradicated (such as smoking or the prevalence of asbestos in modern construction), obesity is set to play an even bigger role in the future.

The first question audiences asked when presented with this data is – how does it work? Why does obesity drive cancer? This is more easily apparent when looking at cancers of the digestive tract, which bear the brunt of unhealthy eating habits (which carry with them unhealthy amounts of swallowing, digesting and bowel movements). The answer when it comes with an apparently unrelated diseases like breast cancer can be more complicated, and far more interesting.

One aspect of the complicated picture of obesity and breast cancer is hormone production. Fat cells have been shown to produce high levels of the hormone estrogen, which is one of the two key female sexual hormones and is usually only produced by very specific cells in the female body, namely in the ovaries and from the placenta in pregnant women. In the healthy female body, estrogen is a signal for breast cells to start growing and dividing – ultimately in preparation for breast-feeding after pregnancy. This mechanism is the reason why pregnant women see their breasts grown in size and why during the healthy monthly cycle breasts become more tender and swollen when estrogen levels peak. However, in obese patients estrogen levels are elevated constantly, which leads to breast cells constantly growing and dividing. Unfortunately, forcing cells to constantly grow and divide is most often how cancer forms.

Every time a cell divides, it needs to duplicate all of its genetic information. Every time a copy is made, there is a chance mistakes may be introduced. Much like copying the same book over and over by hand, every new copy brings the possibility of making a new mistake. While copy mistakes in a book can be rectified, however, mistakes in the genetic code of a cell can, under the right circumstances, turn it into a dangerous, cancerous cell. Cancer cells are different from their healthy counterparts because the divide uncontrollably: breast cancer cells that grow in the high-estrogen environment created by obesity become addicted to the hormone and use it as a tool to keep dividing more and more, faster and faster, until they form a tumor.

The picture of breast cancer and obesity is far more complicated than this small snippet of cellular narrative. For instance, obese patients have more difficulty performing self-examination, which is often how breast cancer is caught early enough for treatment to be effective. There is much we still have to learn. What is clear, however, is that obesity is the next big challenge facing public health. We need to be ready to face it, or face the consequences.