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Cigarette smoking and dimensions of depressive symptoms: longitudinal analysis among Finnish male and female twins.

INTRODUCTION
The association of smoking and depression is established, but it remains unclear which depression dimensions are linked to smoking patterns.


METHODS
The associations between smoking and depression dimensions were investigated among 4,980 male and 5,997 female Finnish twins. Longitudinal cigarette smoking patterns in 1975-1981 included multiple categories describing consistency and change. Depression was measured with the Beck Depression Inventory (BDI) in 1990. Preexisting depressed mood was screened with the life satisfaction scale strongly correlated with BDI. The BDI dimensions were negative attitudes toward self (NATS), performance impairment (PI), and weight loss (WL). Multiple logistic regressions and conditional logistic regressions for discordant twin pairs were conducted.


RESULTS
Controlling for confounders, two smoking patterns predicted all later depression dimensions. The NATS dimension showed a significant Sex × Smoking interaction: the associations of persistent smoking (odds ratio = 1.6, 95% CI = 1.3-2.0; p < .001) and inconsistent former (current in 1975 and former in 1981) smoking (1.6, 1.2-2.2; p = .001) with NATS remained significant among men only. Independently of gender, inconsistent former smoking predicted PI (1.2, 1.0-1.5; p = .04) and persistent smoking predicted WL (1.5, 1.3-1.8; p < .001). Consistent former smokers (former smokers in 1975 and 1981) had no elevated risk for any dimensions. Controlling for familial confounding, the association of persistent smoking with later NATS was replicated among discordant twin pairs (1.6, 1.1-2.2; p = .006).


CONCLUSIONS
Persistent smoking and inconsistent former smoking predict all depression dimensions, although such associations with the NATS dimension are independent among men only. Long-term abstinence (consistent former smoking) does not predict risk for any depression dimensions.