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Smoking increases risk of mental illness reports new study

Robin Wulffson MD's picture
schizophrenia, smoking, genetics, risk factors, mental illness

The harmful effects of smoking such as lung cancer, pulmonary disease, and cardiovascular disease are well known; however, a new study has unearthed a new risk: schizophrenia. German researchers published their findings online on March 26 in the Proceedings of the National Academy of Science.

Schizophrenia is a mental disorder that makes it difficult to tell the difference between real and unreal experiences, to think logically, to have normal emotional responses, and to behave normally in social situations. Sensory input is distorted in schizophrenics. The researchers noted that several variants of a gene known as the transcription factor 4 (TCF4) is known to increases the risk for schizophrenia. One variant, TCF4 rs9960767, is strongly associated with the condition. The researchers found that smoking alters the impact of the TCF4 gene; thus, individuals who carry TCF4 variants who smoke are at increased risk for schizophrenia. In addition, the more these individuals smoke, the greater the risk.

The researchers found that healthy adults who are carriers of the TCF4 and smoke process acoustic stimuli (sound) in a similar manner as schizophrenics. The investigators conducted a multicenter study at six academic institutions throughout Germany. The study group was comprised of 1,821 individuals who were randomly selected from the general population; 1,023 never smoked and 798 were smokers. The subjects underwent genetic testing for 21 TCF4 variants. They used electroencephalography (EEG) to evaluate how the subjects processed simple acoustic stimuli (a sequence of similar clicks). They described their test as a P50 suppression paradigm. Schizophrenics exhibit decreased P50 suppression. Because many schizophrenics are smokers and smoking affect sensory input the investigators evaluated smoking behavior, cotinine plasma concentrations, exhaled carbon monoxide, and the Fagerström Test for Nicotine Dependence (FTND). (Cotine plasma concentration is an indicator of blood nicotine levels.)

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The investigators found that P50 suppression was significantly decreased in carriers of schizophrenia risk alleles (genetic variants) of TCF4: rs9960767, rs10401120rs, rs17597926, and 17512836. The effects of these genes varied by smoking behavior; significant interactions between smoking status and the TCF4 gene were found. Heavy smokers (FTND score of 4 or more) were found to have stronger gene effects on P50 suppression than light smokers and never-smokers.

The researchers concluded: “Our finding suggests that sensory gating is modulated by an interaction of TCF4 genotype with smoking, and both factors may play a role in early information processing deficits also in schizophrenia. Consequently, considering smoking behavior may facilitate the search for genetic risk factors for schizophrenia.”

Take home message:
This study reports yet another negative impact of smoking on one’s health. It also notes that genetic factors affect susceptibility to the harmful effects of smoking. Some individuals are more susceptible to one or more health impacts of smoking than others (i.e., lung cancer, chronic obstructive pulmonary disease (COPD) and cardiovascular disease). Just because a few smokers live a long, reasonably healthy life does not indicate smoking is OK for some people. In addition, smoking is expensive, gives you bad breath, stained teeth, and facial wrinkles.

Reference: Proceedings of the National Academy of Science

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