Experts believe that about 30% of cancers in the US are related in some way to obesity and diet. However, there is actually little certainty when it comes to diet and cancer, according to a report in The Lancet (Vol. 360: 861-868).

The review, written by Timothy J. Key, PhD, and colleagues from the Cancer Research UK Epidemiology Unit at the University of Oxford in England, looked at many studies that have been made by others linking the role of diet to certain cancers.

"Adequate intakes of fruit and vegetables probably lower the risk for several types of cancer," they said, "especially cancers of the gastrointestinal tract. The importance of other factors, including meat, fiber, and vitamins, is not yet clear."

Although the authors noted that there are major differences around the world regarding the types of foods people eat, they pointed out that getting accurate information about what people really eat is the big problem in getting conclusive evidence.

For example, people who were asked about foods they have consumed may not correctly remember what and how much they ate for years past.

And there are the other factors that may make the diet information less reliable when it comes to figuring out the links between diet and cancer.

For example, people who don't eat large amounts of fresh fruits and vegetables may be more likely to be smokers, or may not exercise much, they said. That makes it harder to say whether it's the diet or the other factors that explain some of the links between diet and cancer.

The best-known relationship, according to the article, is between obesity and increased risk of cancers of the esophagus, colon and rectum, breast, endometrium, and kidney.

Another link between diet and cancer is found with alcohol intake. Alcohol increases the risks of cancers of the mouth and oral cavity, the larynx, the esophagus, the liver, and the breast.

What we are more uncertain about, said the researchers, is the relation between the typical diet in developed countries, such as the US, and cancers of the colon and rectum, prostate, and breast.

Other observations included:

• There appears to be a link between meat intake and increased risks of colon cancer. But, vegetarians do not have lower rates of colon cancer. So, it may be how the meat is prepared (high temperature, smoked, or processed) that creates cancer-causing compounds to occur in meat.
• Although high fiber intake was thought to protect against colon cancer, the current thought is that it has no protective effect.
• There is no firm proof that a diet high in fruits and vegetables decreases the risk of lung cancer.
• Obesity increases the risk of breast cancer in postmenopausal women. So does alcohol

In addition to consuming fruits and veggies, the authors wanted to know if cancer can be prevented by taking vitamins.

They noted that there have been some disappointing results from studies where the use of vitamins to prevent cancer was evaluated.

For example, neither beta-carotene nor vitamin E has protected smokers from lung cancer, according to the authors.

On the other hand, calcium and folic acid have looked promising in preventing, slowing down, or eliminating polyps in the colon that can develop into colon cancer.

Many people find themselves confused about their diets after the ping-pong effect of reading and hearing what is good and then what isn't.

The authors explained, "Despite extensive research during the last 30 years, few specific dietary determinants of cancer risk have been established, even for cancers such as colorectal cancer for which most researchers agree that diet probably has important effects."

There is still much to learn, said the authors, and there are large clinical trials underway to answer some of the more important questions.

But, they concluded, "At present, prudent advice is to maintain a healthy weight, restrict alcohol consumption, and select a conventionally balanced diet ensuring an adequate intake of fruit, vegetables, and cereals."