MEDICAL MATTERS: Obesity causes insulin resistance and a chronic rise in the amount of insulin in the blood.
It seems that there is no end to the health risks associated with obesity. One of the latest to be identified is that men are at a higher risk of being killed in car accidents if they are obese.
But strangely, the study found that obese women were not at high risk of death in car crashes.
Clearly, this observation needs further research before it can be accepted as gospel.
But one link with a growing body of evidence is that between obesity and the risk of developing cancer. It now looks like our rising weight levels may contribute to a future increase in the incidence of certain cancers.
One major European study found that around 40 per cent of cancers of the womb, almost 25 per cent of kidney cancers and around 10 per cent of breast and colon cancers could be avoided if we maintained a healthy body weight (a body mass index [ BMI] below 25).
There is now sufficient evidence to link being overweight with an increased risk of cancer. Despite this evidence, the precise biological mechanism linking obesity and cancers is not fully understood.
According to Prof John Reynolds and Dr Aoife Ryan of the Department of Clinical Surgery at St James's Hospital and Trinity College Dublin, writing in a recent issue of Cancerwise, scientific advances have dramatically altered our understanding of the function of fat tissue.
Fat had long been considered to be metabolically inert, merely having a storage function, but we now know that it secretes a variety of biologically active substances.
The mechanism linking fat to cancer may be through alterations in hormone metabolism, including changes to sex steroids and insulin. Sex steroids (androgens and oestrogens) are know to regulate the balance between the proliferation and destruction of cells, an activity that is central to the development of cancers.
In turn this is linked to an increase in a substance called insulin-like growth factor -1 (IGF-1). Both insulin and IGF-1 strongly stimulate cell growth and may favour the selective growth of cancer cells in certain parts of the body. And leptin, a protein produced by fat tissue, has been shown to promote the growth of breast, colon and oesophageal cancer cells.
A high level of oestrogen is thought to be the main mechanism stimulating growth of cancer cells in the womb and also for increasing the risk of breast cancer in post-menopausal women. Several studies have shown an increase in colon cancer risk in people with increased concentrations of IGF-1. Leptin has also been implicated, with a three-fold increased risk of bowel cancer associated with increasing levels of the protein.
There has been a marked increase in cancer of the oesophagus in Western societies; obesity is emerging as the strongest risk factor for this change.
One possible mechanism is the frequency of acid reflux into the gullet of people who are overweight. This is known to cause pre-malignant changes in the cells lining the lower part of the oesophagus.
Given the clear cut links between obesity and the development of certain cancers, avoiding weight gain should eliminate the risk. And, although it makes sense that weight reduction in individuals who are overweight should protect against the development of cancers, few studies have addressed the issue with sufficient accuracy to prove that this is indeed the case.
Dr Muiris Houston is pleased to hear from readers, but regrets he cannot answer individual medical queries.
