The exact mechanisms underlying post-cessation weight gain still are not well understood. According to the principles of energy balance, smoking cessation must lead to either an increase in energy intake, and/or a decrease in energy expenditure (viz., metabolic rate, physical activity) to promote weight gain (33).
The available data indicate that physical activity does not play a role in the relationship between smoking and body weight (20,33,34). Cross-sectional studies comparing activity levels in smokers and non-smokers have failed to find discrepancies that would account for the difference in body weight between the two groups (20,35). In fact, studies finding a relationship between smoking status and physical activity have typically found smokers to be less active than non-smokers (36-38). Additionally, physical activity does not appear to decrease following smoking cessation (33,39-41). Those studies finding changes following smoking cessation have reported increases in physical activity (42-44). Thus, physical activity does not appear to figure independently in either the difference in body weight between smokers and non-smokers, or in post-cessation weight gain.
Energy intake appears to play an important, although complicated, role in the relationship between smoking and body weight (34). Despite the fact that they tend to have lower body weights, smokers consume as much, or more energy than non-smokers (37,45,46).
Additionally, smoking cessation is associated with increased energy intake, at least acutely. Several studies of short-term cessation (1 day to 7 weeks) have documented increases in total energy (41,47-49) although negative findings also have been reported (39,43,50). Despite considerable variability in methodology, studies typically show an immediate increase in energy intake of 250 to 300 kilocalories per day following smoking cessation (51,52).
Long-term changes in intake following smoking cessation, however, have been less consistent (52). Unfortunately, few studies have examined changes in energy intake beyond a few months post-cessation. One study, however, assessed changes in dietary intake among women who quit smoking for a period of 1 year. Caan et al. (53) found increases of 163 and 125kcal/day at 1 and 6 months post-cessation, respectively. Levels of energy intake had returned to baseline, however, by the 1-year follow-up. These results suggest that increases in energy intake following smoking cessation probably do not extend much beyond 6 months, which may help to account for the fact that most of the weight that is gained after quitting smoking occurs within this time period (32,53-55).
In addition to short-term increases in total energy intake, smoking cessation has been associated with changes in specific components of dietary intake. Selective increases in dietary fat (56), carbohydrates (57), sucrose (56,58), and alcohol (41) have been observed following smoking cessation. Overall, increases in dietary intake after smoking cessation appear to be due to between-meal snacking, rather than from a general increase in food consumption during meals. Gilbert and Pope (59) found that energy intake from meals was similar during 24-hour periods of ad libitum smoking and abstinence, but that intake from between-meal snacks increased 50% in men and 94% in women during abstinence.
Given that women generally have greater concerns about post-cessation weight gain, as well as greater actual weight gain, gender differences in the mechanisms of post-cessation weight gain are of major interest. There is evidence that changes in energy intake associated with smoking cessation may differ by gender, but the exact relationship is unclear. While several studies have reported differences in energy intake as a function of gender, they have disagreed on the nature of the relationship. Klesges et al. (39), for example, found increased intake of polyunsaturated and monoun-saturated fat in women during a week of abstinence, but no changes in dietary intake for men. Conversely, Hatsukami et al. (60) observed a greater increase in total energy intake in men than women following 4 days of cessation. Hall et al. (56) found that both women and men increased their intake of total energy, fat, and sucrose immediately after quitting. Men decreased their average total energy intake by nearly 1000 kcal from the first week after cessation to 4 months (3014 to 2119 kcal) and maintained this lower level at 6 months (2035 kcal). In contrast, total energy intake by women remained stable (1841,2077, and 1867 kcal at 1 week, 4 months, and 6 months, respectively). Increased energy intake predicted weight gain at 6 months for women, but not for men. Thus, information on the influence of gender on changes in energy intake following smoking cessation is incomplete, but suggests significant and sustained post-cessation energy intake increases in women, which are associated with weight gain.
Studies examining the relationship between smoking and metabolic rate have been inconclusive. There is considerable indirect evidence that metabolic factors influence the weight-controlling properties of smoking. The fact that smokers are no more active than non-smokers and consume as much or more energy, yet weigh less, suggests that metabolism may play a role in the relationship between smoking and body weight (34).
Several studies have documented acute metabolic increases due to smoking or nicotine administration (61-64). At least one study did not find any acute effect of smoking on metabolic rate (65) and in general, there appears to be tremendous individual variation in the metabolic response to smoking and smoking cessation (1,62). There is evidence that the acute effects of smoking may be more pronounced during light physical activity than during rest (63,66), at least among men, and for normal weight smokers than the obese (61). Thus, it is possible that the acute metabolic effects of smoking may factor into the difference in body weight between smokers and non-smokers, although it remains unclear whether these effects are strong and persistent enough to have a substantial impact on body weight.
Studies that have directly examined the chronic metabolic effects of smoking have produced inconsistent results. Cross-sectional studies comparing resting energy expenditure (REE) in smokers and non-smokers have typically found little or no differences between the groups (38,67). The few studies that did find differences failed to control for the thermic effects of nicotine by allowing smokers to smoke before the assessments, which could have resulted in an overestimation of the chronic effects of smoking on metabolic rate (68).
Only a few prospective studies have examined metabolic changes during long-term smoking cessation, and conflicting results have been found. Mof-fatt and Owens (40) compared changes in metabolic rate among 36 women who quit for 60 days (n = 12), quit but relapsed 30 to 60 days post-cessation (n = 6), continued smoking (n = 8), or were non-smokers (n = 10). Resting metabolic rate (RMR) was assessed as oxygen uptake at baseline, 30 and 60 days post-cessation. At baseline, RMR was higher in smokers than non-smokers. No changes in RMR were observed for non-smokers or continuing smokers. Smoking cessation resulted in a 16% decrease in RMR at day 30. Both relapsers and abstinent subjects showed trends for RMR to rebound toward baseline at day 60. Despite the trend for RMR to return toward baseline, weight continued to increase throughout the 60-day follow-up. The authors estimated that 39% of the weight gain among quitters was attributable to change in RMR. Dallosso and James (50) reported a 4% decrease in resting metabolic rate following smoking cessation, although the change was only significant when expressed per kilogram of body weight.
In contrast, Stamford et al. (49) did not find changes in oxygen consumption in 13 subjects who quit smoking for 48 days. Additionally, a recent study (69) assessed 24-hour energy expenditure in a respiratory chamber and basal metabolic rate among eight smokers (four men and four women) during regular smoking and after 4 to 8 weeks of abstinence. No significant differences were observed between smoking and non-smoking assessments for either measure of energy expenditure, suggesting that smoking cessation does not produce any chronic alteration in metabolic rate. Other studies also have failed to find chronic changes in resting energy expenditure (REE) after quitting smoking (70-72). Thus, the relationship between smoking and REE remains unclear. One possible explanation is that changes in REE following smoking cessation are influenced by moderators, such as ethnicity or gender. Most studies investigating this relationship have consisted of small, homogeneous samples, making it impossible to investigate these variables. Thus, there is a need to examine changes in REE following smoking cessation in large, diverse samples.
Simultaneously examining the influence of all three energy balance variables would be helpful in understanding the relative contribution of each component. However, to date, only five prospective studies have examined the influence of smoking cessation on all three components of energy balance. Four of these studies utilized relatively short follow-up periods (14 to 60 days). Vander Weg et al. (73) examined changes in energy balance in 95 male and female smokers during 2 weeks of abstinence from smoking. Energy intake increased significantly following cessation (344 kcal/day). There were no changes, however, in REE or physical activity. Stamford et al. (49) examined changes in body weight and energy balance in 13 women following 48 days of abstinence from smoking. There were no changes in either physical activity or REE. Energy intake, however, did increase by an average of 227 kilocalories/day. Perkins et al. (41) investigated changes in energy balance in seven female smokers over a 3-week period consisting of a week of smoking, a week of abstinence, and a return to smoking. Energy intake increased significantly during the week of abstinence, primarily due to an increase in alcohol consumption. REE also changed over the 3-week period. A non-significant decrease in REE was observed during abstinence, followed by a significant increase upon return to smoking. There were no changes in physical activity. Finally, Mof-fatt and Owens (40) examined changes in energy balance in 18 women who quit smoking for 30 to 60 days. Consistent with the other studies, physical activity did not change as a function of smoking status, while energy intake increased significantly following cessation. However, unlike the three previous studies, smoking cessation was associated with a significant decrease in REE.
Klesges et al. (55) assessed the relationships of all three major components of energy balance and weight gain during 12 months of abstinence—the longest follow-up period to be examined to date. The sample included 42 subjects (22 women, 20 men) with biochemically verified sustained abstinence over the 12-month following period. Weight gain among women was predicted by lower baseline REE, higher baseline total energy intake, and increased carbohydrate intake over the year. However, changes in energy balance components (dietary intake, physical activity, and REE) did not predict weight gain among women. Furthermore, no energy balance variables predicted weight gain for men. Future research should attempt to examine more fully potential gender differences in energy balance changes that predict weight gain during extended smoking cessation.
In summary, increases in energy intake appear to be the most consistent energy balance change following smoking cessation. There is no evidence that changes in physical activity generally contribute to post-cessation weight gain. While removal of the acute increases in metabolic rate caused by smoking may also contribute somewhat to post-cessation weight gain, long-term changes in metabolic rate after smoking cessation do not occur reliably.
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