A new study says excess body fat may not just put you at an increased risk for a heart attack, it lays the groundwork for developing many forms of cancer.

According to the American Institute of Cancer Research, obesity may be to blame for up to one third of all cancer cases, with overweight people at higher risk of developing colon, prostate, or breast cancer.

Obesity expert Dr. George Bray of the Pennington Biomedical Research Center in Louisiana says body fat fundamentally alters the body's systems in ways that can "pave the way" for tumours.

While most of us think of body fat as an inert substance that merely collects on us, recent research suggests that fat is a remarkably active substance. Studies suggest fat cells behave like endocrine cells, constantly producing and secreting a wide variety of hormones and other "growth factors" into the bloodstream.

Bray says that these substances, such as insulin, estrogen and other sex hormones, carry out basic life functions, encouraging cells to grow and divide. But in many obese individuals, excess body fat seems to cause greater amounts of these hormones to be continually pumped into the bloodstream, making it easier for certain cancers to initiate.

The excess hormones urge cells to grow and divide at an accelerated rate, and that can increase the chances that something could go wrong in the process, such as the random mutations that can lead to cancer.

Most unaware of obesity-cancer link

The AICR also found that most of the public does not realize that being overweight accounts for up to a third of all cases of cancer. According to a recent survey, only 25 per cent of American adults surveyed knew that obesity is associated with a higher risk of many forms of cancers.

The poll, conducted in June, showed that most American adults -- nearly 90 per cent -- know that being obese can lead to diabetes and heart disease. But only 25 per cent were aware that obesity increases cancer risk.

And when asked to name major risk factors for developing cancer, 18 per cent mentioned high-fat diets, but only six per cent mentioned obesity and being overweight.

Heart disease is the No. 1 killer in developed countries, but cancer comes in second. After lung cancer -- caused almost exclusively by smoking -- breast, prostate and colon cancer are the biggest cancer killers.

Dr. Bray points out that all of these cancers, along with cancer of the uterus and gallbladder, are associated with obesity.

"If we lost weight, the nation's health would improve and we would have less cancer," Bray says. "Thirty to 40 per cent of all cancers could be prevented by eating a better diet and getting more exercise."

A person who is more than 25 percent above his or her maximum healthy body weight is considered obese. Since 1985, Canada's obesity rate has doubled, with more than 29 per cent of middle aged Canadians considered overweight or obese. Bray says it's crucial to get these numbers down.

"The important thing is to eat less fat, be physically active, get your weight in control. and be more physically active," Bray says.