Eating lots of fruit and vegetables and limited amounts of red meat and sugary foods is the way to protect against cancer, say researchers.
The government recommends five portions of fruit and veg per day
Three separate studies in the Journal of the American Medical Association show the benefits of a Mediterranean-style diet.
As well as protecting against bowel cancer, eating a plant-based diet is good for the heart, say experts.
Such diets offered no extra protection against breast cancer, however.
But US research released days ago showed adding olive oil to your diet can cut the risk of developing breast cancer.
The three new studies join thousands of research papers looking at the effect of diet on cancer risk.
Dr Steve Heggie, a scientist at World Cancer Research Fund, said: "The best advice is still as it stands to eat lots of fruit and vegetables."
He said the research showing no effect on breast cancer was important, but that it was vital to look at all available evidence rather than the conclusions of one study.
He said the World Cancer Research Fund was currently compiling all the available data on diet and cancer, involving some 10-20,000 studies in total, and would publish results in 2006.
What you eat
The first of the JAMA studies, conducted by researchers at Johns Hopkins in the US and Yonsei University in Korea, followed more than 1.2 million people for ten years.
The people with higher blood sugar levels, regardless of whether they were diabetic or not, were at increased risk of developing and dying from cancer.
The authors believe glucose intolerance might be one way that obesity increases cancer risk, and that rising obesity rates might increase future cancer rates.
The second study, by Dr Ann Choa and colleagues at the American Cancer Society in Atlanta, looked at the relationship between meat consumption and colorectal cancer risk among nearly 150,000 people.
People who ate the highest amounts of red meat (up to about a kg per week) in the study were 50% more likely to get colon cancer than those who ate the least amount of red meat.
In the third study, researchers at the University Medical Centre in Utrecht, the Netherlands, found eating fruit and vegetables or drinking juices had no effect on breast cancer risk among more than 250,000 women.
But the authors said a modest benefit could still exist for some women.
In an accompanying editorial, Dr Walter Willett of the Harvard School of Public Health in Boston, said the benefits of a plant-rich diet were far reaching.
"Reductions in blood pressure and epidemiological evidence for lower risks of cardiovascular disease provide sufficient reason to consume these foods in abundance.
"The relation between red meat consumption and cancer may not be conclusive, but prudence would suggest that red meat, and processed meats in particular, should be eaten sparingly to minimise risk.
"When combined with other healthful diet and lifestyle factors, it appears that approximately 70% of colon cancer can potentially be avoided."
Amanda Vezey, care advisor at Diabetes UK said the blood sugar research was interesting.
"The study indicates that obesity may increase the risk of cancer and for people with Type 2 diabetes, being the right weight is an important part of managing their condition."
Cancer Research UK's Professor Kay-Tee Khaw, said: "These papers add to the growing evidence about the role of lifestyle factors in cancer.
"For particular cancers such as breast cancer, other factors such as reproductive history and hormonal status are a major risk, but this study provides no good reason to change current general dietary recommendations.
"Dietary patterns with high fruit and vegetable intake and limited red and processed meat intake are those most consistent with good health including lower overall cancer, cardiovascular disease and mortality rates.
"Obesity is a well documented risk factor for many cancers. The Korean study confirms previous reports that diabetes or a raised glucose level may increase cancer risk and this may well be one of the mechanisms through which obesity may influence cancer risk."