STRESS AND CANCER
One other factor that may be associated with cancer is stress. It is possible to put a number on stress, so to speak, by measuring the amount of cortisol in blood or saliva. Cortisol is a steroid hormone released by the adrenal gland in response to signals from the brain. Normally secretion is maximal early in the morning and declines, thereafter, during the day (diurnal variation) but it is increased by food, fasting, exercise or stress. Its role is to provide energy when required by stimulating sugar production and the breakdown of lipids/fat and proteins, resulting in raised blood sugar levels and blood pressure. However, cortisol can also indirectly increase appetite and promote fat deposition. Studies of breast cancer patients have shown that about two-thirds may have abnormal cortisol profiles (higher, relatively constant or maximal at abnormal times of day) and this group survive for significantly shorter times (3.2 versus 4.5 years) than those with normal cortisol profiles. The reason may be suppression of the immune system by raised cortisol levels because these patients have reduced numbers of natural killer cells in their blood. The second strand of cortisol association is the evidence that night shift work, which disrupts normal diurnal rhythms, is associated with increased incidence of breast cancer. This has been attributed to melatonin suppression but may well be due to perturbed cortisol variation. Despite all this, in the confusing way that cancer has, cortisol-type steroids can suppress the growth of some tumours and have been used as therapeutic drugs.
Ideally, the demonstration that a specific agent causes cancer requires both strong epidemiological evidence of an association between the two and knowledge of a mechanism by which the effect can be achieved. These conditions have been met for a number of well-known causes – chronic infection, radiation and tobacco use. All of these can be mutagenic, although for infection the effect is usually indirect as a consequence of long-term inflammation. For alcohol, the association is overwhelmingly established although, beyond its action as a local anaesthetic that perturbs cell signalling, the mechanism remains obscure. Despite considerable media attention, the evidence for an association between electromagnetic fields and cancer, either from power lines or mobile phones, remains insufficient for any of the regulatory authorities to have recommended increased stringency in the current exposure guidelines. As with alcohol, the epidemiological evidence linking obesity to enhanced risk of a range of cancers is clear. The mechanism is less well established but is likely to involve hormonal imbalance, especially of oestrogen, a powerful promoter of cell growth proliferation. Poor diet predisposes to a variety of types of cancer. However, many foodstuffs contain a mixture of essential nutrients and compounds that have carcinogenic potential. Thus the most informed advice is to eat a balanced diet and avoid supplements unless they are medically prescribed.