How does obesity cause cancer – the science behind the warnings

Feet on a weighing scale

Dr Giotra MitrouDr Panagiota Mitrou is Director of Research Funding & Science External Relations at World Cancer Research Fund International.

We live in a society where excess weight is becoming more and more the social norm – even though our own evidence has shown that being overweight or obese can have drastic health consequences. Too few people still do not know that being overweight or obese increases the risk of 11 cancers, including some of the most common such as breast, prostate and bowel.

So, maybe the question we need to be asking is why does the body respond negatively to excess body fat?

A key thing to understand is that fat cells change the environment inside the body. They release a range of chemicals that can make cells near them act differently leading to cells potentially becoming cancerous. This process happens in various ways in different parts of the body, leading to different types of cancer developing. We don’t yet know all the biological mechanisms that are responsible for obesity increasing cancer risk, but there are 3 main explanations that underpin the process.

Inflammatory response

When there is a large collection of fat cells, immune system cells gather around the fat cells to ‘tidy up’ the area around them. This is known as an inflammatory response. However, when immune cells do this, they also release chemicals known as cytokines. Cytokines can encourage cells to divide more rapidly than they die off, this build up of cells can lead to cancer growth. This is one of the main theories of how body fat can lead to cancer, but more research is needed to fully understand how this process works.


Fat cells release significant amounts of oestrogen and the more fat cells you have, the more oestrogen will be released. Oestrogen can cause cells in the womb and breasts, which are particularly sensitive to oestrogen, to divide uncontrollably – this process can cause cancer to form. This explains why women who have post-menopausal breast cancer and womb cancer often have higher levels of oestrogen.


The hormone insulin is an important part of how the body uses energy from food. Overweight and obese people often have increased levels of insulin in their blood. This is because excess body fat leaves the cells increasingly resistant to the effects of insulin, so-called ‘insulin resistance’, and then the pancreas is forced to make more insulin to try and compensate, this is known as ‘hyperinsulinemia’. Insulin resistance and hyperinsulinemia over a period of time can affect the levels of growth factors, which are a green light signal for cells to divide. Both insulin and these growth factors encourage uncontrolled cell division and promote the development of cancer.

Research around the ‘why’ of cancer and obesity is still in its infancy and we have only recently started to better understand the reasons behind how excess fat can lead to cancer. It is only with more research into this area that we will be able to fully understand the dangerous role that being overweight or obese plays in cancer.

World Cancer Research Fund has identified which cancers are linked to obesity through its Continuous Update Project and is now working on a method to help scientists improve their understanding of mechanisms, which will give us vital knowledge of how lifestyle factors affect our cancer risk.