Epidemiological evidence suggests that tobacco smokers weigh less than their non-smoking counterparts. Evidence also suggests that quitting smoking is associated with weight gain. Some researchers have theorised that the habit of smoking may in some way decrease calorie intake, whereby a cigarette may displace other activities such as eating or drinking. However, there is little evidence to suggest that this is the case. Instead, it is more likely that a component of cigarette smoke is able to cause weight loss through physiological means. Nicotine is known to increase resting metabolic rate (RMR) in humans, and it is thought that smoking may as a result increase calorie expenditure due to its content of nicotine. Because RMR can account for up to 70 % of total energy expenditure in a day, this increase might be enough to cause weight loss even if the percentage is small.
One group of researchers1 investigated the effects of the nicotine in cigarette smoke on RMR. A moderate (15 µg per kg body weight) and low (7.5 µg per kg body weight) dosing of nicotine was administered through a nasal spray on 3 separate occasions, and the trial was controlled by use of a placebo containing no nicotine. Following indirect caloriemetry, the RMR of the subjects were assessed and the plasma nicotine levels were used to confirm the reliability of the dosing. The nicotine caused increases of 6 % in the RMR at both the low and moderate doses, which was significantly higher than the placebo which increased RMR by 3 %. The increase in metabolic rate caused by nicotine was equivalent to around 1 kcal per minute during the time the RMR was increased. The plasma nicotine concentration following administration was reflected by an increase in heart rate.
Therefore nicotine in tobacco smoke can explain the mild weight loss effects of smoking. However, mild smoking may be just as effective as heavy smoking as there was no significant difference between the doses in this study. That the placebo can also increase RMR by 3 % is interesting because it suggests that this might be an effective strategy for weight loss. A 3 % increase in RMR would certainly cause weight loss over the long-term and it is amazing that this effect is not utilised more in medicine. The researchers suggested that the act of inhalation may have had acute metabolic effects that increased RMR. This was tested by comparison of a group of subjects inhaling nicotine free spray against a group that rested. Only the act of inhaling caused a 3 % increase in RMR. Therefore it appears that weight loss from cigarette smoking may occur partially though the act of inhalation.
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