People who are obese can reduce their risk of cancer 'almost immediately' if they lose weight, Irish researchers have said.

According to the team from Cork, there is now 'substantial evidence' that many cancers are related to obesity. In fact, the World Cancer Research Fund estimates that up to 45% of cancers are directly related to being obese.

While this link has long been acknowledged, until now, it was unclear if weight loss lowered this risk of cancer.

"This is partly due to the fact that intentional weight loss and maintenance of long-term normal body weight is relatively uncommon," they explained.

However, they referred to a number of recent studies and reviews, which have shown that weight loss significantly reduces the risk of cancer. This is irrespective of whether the weight loss occurred as a result of lifestyle changes or as a result of bariatric (weight loss) surgery, such as the insertion of a gastric band.

The researchers, who were based at University College Cork, Mercy Hospital and Cork University Hospital, emphasised the importance of physical activity, as it is 'inextricably linked' to weight loss, but also appears to reduce the risk of cancer in other ways.

"Physical activity most likely influences the development of cancer in many ways including its role in energy balance, metabolism of sex hormnes, insulin regulation and alteration of a number of inflammatory factors, which may influence cancer risk," they explained.

They noted that more than 50 studies have looked at the relationship between physical activity and colorectal (bowel) cancer and these have 'consistently found' that adults who exercise more can reduce their risk of bowel cancer by up to 40% compared to those who lead a sedentary lifestyle.

The team concluded that weight loss 'should be the principal factor to be targeted for cancer risk reduction among obese individuals'.

"This is due to the striking findings of an almost-immediate and substantial impact of weight loss on cancer risk'.

The team made its comments in the Irish journal, Cancer Professional.