When we think of risk factors for mortality we properly think behaviours (e.g. smoking / obesity) or genetics (e.g. family history). What about psychological factors – can unhappiness increase your risk of risk of cancer? Well, Batty and colleagues  have tackled this problem as follows:
- They assembled 16 prospective cohort studies where behaviours and psychological state had been measured and in which participants were followed up to see if cancer developed.
- They obtained the raw data and obtained an individual patient meta-analysis.
- They adjusted for the usual things known to increase risk of cancer (obesity, smoking, etc).
- They calculated relative risk of cancer according to antecedent psychological state.
They found a positive correlation between psychological distress and risk of cancer. But causality might have run the other way – (occult) cancers may have been the cause of psychological distress, not the other way round. So:
- They ‘left censored’ the data, thereby widening the gap between the point in time where the psychological state was measured and the point where cancer supervened.
The association between psychological state and cancer death persisted, even when they were separated by many years. What is the explanation?
- Failure to fully control for all behaviours (although behaviour could be the mechanism through which the cancer risk is increased in people with depression, in which case they ‘over-controlled’).
- Reduced natural killer cell function.
- Increased steroid levels, which can apparently affect DNA repair in some way.
- Some mechanism yet to be discovered.
In any event, the findings are intriguing, for all that practical implications may be limited.
— Richard Lilford, CLAHRC WM Director
- Batty GD, Russ TC, Stamatakis E, Kivimäki M. Psychological distress in relation to site specific cancer mortality: pooling of unpublished data from 16 prospective cohort studies. BMJ. 2017; 356: j108.