The higher cotinine levels associated with a shorter time to cigarette smoking after waking might be due to more intense smoking in response to overnight abstinence. More highly dependent smokers, as defined by the Fagerstrom Tolerance Questionnaire (an earlier version of the FTND) have increased compensatory smoking behaviors such as puff number and puff duration than less dependent smokers when switching to a low nicotine cigarette (25
). There is little data on the time to first cigarette and smoking intensity. One recent study in adult smokers in the United Kingdom measured puffing behaviors in relation to the time of the day that cigarettes were smoked (26
). Cigarettes that were smoked within five minutes after waking were associated with a significantly lower mean total smoke volume than cigarettes smoked afterwards. These data would seem to indirectly contradict our hypothesis, although cotinine measurements were not reported in the UK study. Our studies would need additional data on puffing profiles to make more definitive conclusions on whether the association between time to first cigarette and cotinine could be attributed to more intense puffing behaviors, and whether symptoms of nicotine cravings are greater after waking.
In our study, highly dependent subjects who smoked 10 cigarettes per day had higher cotinine levels than less dependent subjects who smoked 20 cigarettes per day. The lack of a significant linear relationship between cigarette frequency and cotinine in the highly dependent group indicates important physiological or metabolic factors such as the saturation of nicotine uptake and nicotine metabolism may be occurring, and/or reduction in stress that impacts inhalation. In smoking reduction trials, there was no linear relationship between cpd and baseline urinary cotinine in highly dependent subjects (e.g. high FTND, time to first cigarette ≤ 30 minutes, those who had difficulty giving up the first cigarette, and subjects who smoked when ill).
The two suggested phenotypes in this data are not entirely distinct, nor would they be expected to be since TTFC and daily cigarette frequency are just two behavioral measures of nicotine dependence, which is characterized by a physiological desire and craving for nicotine, the perceived good feelings it generates, and unpleasant withdrawal symptoms. There are several behavioral/symptom scales of nicotine dependence in use (26
), and their lack of a high concordance highlights the complexity in defining nicotine dependence. We did not measure the many physiological and psychological attributes of nicotine dependence, which may also independently or collectively help explain much of the remaining variation in cotinine levels. Unmeasured genetic variability in addiction and environmental factors that affect dependence may also account for cotinine variability. In addition, smoking intensity may be due to factors besides nicotine dependence including the tar yield and taste (27
The regression slope for plasma cotinine did not differ by sex but significant differences by sex were found for urinary cotinine. In pooled baseline data from four smoking reduction trials, urinary cotinine levels peaked in men (about 34 cigarettes daily) but continued to rise with cigarettes per day in women (9
). Sex differences in nicotine pharmacology have been reported but it is uncertain whether these differences are due to physiological, hormonal or environmental factors (28
). One possible explanation for the effect of sex on urinary cotinine only is that there are sex differences in the excretion of creatinine, which was used as an adjustment factor for urinary volume. Age was a significant independent predictor of plasma cotinine levels in our data, but the regression of cotinine against cigarette frequency did not differ significantly by age as was reported in the smoking reduction trials (9
These observations underscore the methodological issue of the relative merits of blood, urine and saliva as biological sources for cotinine measurements. All three measures are consistent in determining smoking status. When measuring exposure dose, the levels of biomarkers will depend upon assay sensitivity and specificity, exposure conditions, and variation in metabolism and excretion. Plasma and urinary cotinine are considered comparable measurements of nicotine exposure based on similar rates of elimination (31
). However creatinine production and excretion is lower in the elderly, in women than in men, in whites than in blacks, and in old age (32
). The extent to which creatinine adjustment affects the correlation between urine and blood cotinine levels may vary (35
). In our data, the differences in correlation coefficients between plasma cotinine and unadjusted or adjusted urinary cotinine were minor.
Another methodological issue is that nicotine is converted into over one dozen metabolites. Cotinine accounts for about 10% of nicotine metabolites, but a complete metabolic profile would provide greater accuracy for comparing nicotine uptake between individuals. We are planning such an analysis in the near future.
Nicotine dependence may be a factor in the success of harm reduction programs, where success is measured by decreased cigarette frequency and levels of smoking and disease risk biomarkers (36
). In a meta-analyses of 13 nicotine replacement trials, there was an overall reduction in the average daily numbers of cigarettes smoked, but wide variability in the reduction of cotinine, carbon monoxide, and thiocyanate levels (38
). The reduced consumption of cigarettes might have been offset by nicotine dependent-driven compensatory smoking behaviors (39
). Even if interventions successfully reduce cigarette consumption, concurrent reductions in biological exposure to tobacco smoke might depend upon nicotine dependence. In intervention programs, high dependence may require a proportionally greater reduction in smoking frequency in order to receive similar health benefits as subjects who are less dependent. A limited number of tobacco pharmacotherapy treatment trials in black smokers measured time to first cigarette. Participants who smoked > 30 minutes after waking were more successful quitters (40
). In case-control data, the excess OR for lung cancer increases with smoking frequency up to 20 cigarettes per day, but plateaus with higher cigarette frequency (14
). This parallels the cigarette per day -cotinine relationship, supporting the notion that biomarker levels reflect risk and not just exposure.
A strength of the current study is that subjects were not recruited as part of a smoking reduction trial where a high motivation to quit could possibly bias the findings. Limitations include possible misclassification of nonsmokers as smokers, although none of the participants had blood cotinine levels <5 ng/mL, an optimal cutpoint for distinguishing passive from active smoking (41
). We also assessed misclassification of reported smoking amount in 133 subjects by comparison with the number of cigarette butts saved and stored in a plastic container the week before the study day (0.95 in Blacks and 0.83 in Whites, 1).
Differences in the time to first cigarette may represent a constellation of factors that including genetic variation in nicotine dependence, variation in non-genetic behavioral and social factors, and possibly variation in characteristics of the cigarettes including taste. The fact that a simple question is such a strong predictor of a biomarker of nicotine dependence seems quite noteworthy, particularly since considerable efforts have been made to discover relatively small effects of variants on nicotine dependence in genome-wide scans. Since nicotine dependence (cigarettes per day, time after waking) is the major explanatory variable for the variation in cotinine levels, and since cotinine levels appear to reflect the risk of lung cancer, time to first cigarette may be an important risk factor for lung cancer and should be considered in the design of smoking cessation programs.