Dr Thomas Stuttaford's next online forum will be live after 1pm on June 11. The topic is: diagnosis and treatment of prostate cancer. To ask the doctor your question on this topic and to read other recent topics he has answered click here

Q1: If the contents of your article are true, how is it that, after years of eating my way through acres of brocolli and Brussel sprouts, tomatoes, apples etc, I then developed bowel cancer? I have never smoked and am a size 12. To what extent does diet counteract the contribution of faulty genes or damaged DNA? Judith Pepler

A1: The facts of my piece on the effect of different foods on the incidence of various diseases, including cancers, is so far as I know true. I sincerely hope it is true as the information was obtained from an international conference of food scientists and oncologists, all of whom seemed to be well informed, scarily intellectual and well known academics. The fallacy is in your expectations, or perhaps in my failure to make the point abundantly clear that my youngers and betters were making at the conference.

Cancers are usually the result of many different factors. Only occasionally does a patient have a specific gene, such as BRCA1 or 2 that causes a grossly increased liability of breast cancer or the gene that gives rise to polyposis coli and a strong likelihood of developing cancer of the colon. Conversely many people may have a less well defined but still an obviously increased familial tendency to develop malignancies or possibly an increased sensitivity to environmental factors that may in some cases prove to be cancer inducing.

The effect of one specific environmental factor may well be small, but even so the patient's liability to the cancer may still be larger than usual. Therefore if this environmental factor can be deleted from the patient's life their chances of developing the cancer would be reduced, but not eliminated altogether.

Similarly if there is an environmental factor that is known to increase a patient's resistance to cancer adding it to the patient's lifestyle or diet will make it less likely that they will develop cancer, but will certainly not render them immune to it. The reduction or increase in the incidence of cancer by altering the factors in someone's life may make a significant difference. However any improvement or worsening in the cancer rate may only be detectable by studying the statistics. For example prostate cancer is less likely in those who eat whole tomatoes, especially if they are cooked, rendered into paste or taken as tomato juice (or of course taken in supplement form if the supplement is made from the whole tomato rather than one constituent of the fruit.) The vegetables and fruits you took will have reduced your chances of developing cancer of the colon but there may have been other factors that would have prevailed despite the care you took over your diet and lifestyle.

Weight does have a significant influence on a liability to cancer. There is now a mountain of evidence that shows that skinny people, and those who take adequate exercise, are less liable to develop cancer. Your relative slimness will now be a factor that will help, but no more than that, your chances of being spared any recurrence. Keep up the good work and enjoy your diet. Faulty genes are one of the risk factors and they, together with any other relevant risk factor, usually achieve their damaging influence by uncovering weaknesses in the DNA structure so that faulty lines of cells develop.

Q2: One of my closest friends had breast cancer and after a partial mastectomy and drug treatment has been clear for four years. She's now 55 and although she eats a lot of fruit and vegetable, she also eats a lot of cakes and biscuits and bread, so that her weight is ballooning. She's at least three stone more than she should be. Is it possible that overweight could bring back her cancer? Mary Wood, South Benfleet, Essex

A2: Yes. There is every hope that your friend will have been cleared of her cancerous cells. This will depend on how early the cancer was detected and on the nature of the cells in the cancerous tissue. However, it is also true that in a few cancers there is a distinct possibility of a recurrence however soon it was diagnosed. Any recurrence is made much more likely if the patient is overweight. In one large study it was found that a new, very expensive and interesting drug was able to reduce the recurrence rate in some forms of breast cancer. Some of the gloss was taken off the trial as another study showed that almost similar advantages were provided for a control group of patients if instead of having the new drug they dieted heavily and didn't allow their weight, and hence their remaining breast tissue, to increase excessively. Breasts are especially sensitive to excess weight as plump women produce more oestrogen, and oestrogen levels are an important factor in the causation of breast cancer.

Patients who have had a cancer should keep their waist measurements down. In women the waist should certainly under 35 inches and preferably under 32 - in men the waist should be under 40 inches and preferably under 37. A heavy carbohydrate diet especially if the carbohydrate has a high glycaemic index (that is, is rich in very easily absorbed and utilised carbohydrate such as refined sugars) is especially fattening and has other undesirable biochemical effects on metabolism.

Q3: When I look at the range of woman I have known who have had cancer, some recovered and some, alas, in decline or even dying, there is only one really obvious, discernible common denominator and that is their psyche. Nearly all of those who have fared the worse have driven personalities, on the go all the time, can't stop, never ease up on themselves. The only exception was a very dear aunt who didn't like to both the doctor until it was too late. To what extent do you take account of personality in the treatment of cancer? Name and address withheld