3.29 Smoking and body weight

Tobacco use and overweight and obesity are the leading causes of morbidity and mortality in Australia.¹ It is generally thought that people who smoke have a lower body weight compared to those who have never smoked. However, body weight appears to vary among people who smoke, with body weight increasing as smoking intensity increases. In addition, people who smoke tend to have greater abdominal adiposity (fat in the stomach area) which is associated with a higher risk of morbidity and mortality.^2,3 It is also important to note that the health benefits of smoking cessation far outweigh the health risks associated with extra body weight – see Section 7.1.11.3.

3.29.1 Smoking, body weight, and BMI

Body weight is commonly assessed using the body mass index (BMI, kg/m²), a measure that is calculated by dividing an individual’s weight by the square of their height. While using this measure comes with several limitations, it is often used in practice to determine whether an individual is underweight (BMI < 18.5 kg/m²), a normal weight (BMI between 18.5 and 24.9 kg/m²), overweight (BMI between 25 and 29.9 kg/m²), or obese (BMI > 30 kg/m²).⁴

Many observational studies have shown that people who smoke tend to have a lower BMI compared to people who do not smoke and people who have quit smoking.^5-12 A repeated cross-sectional study involving 21 twin cohorts across 14 countries, found that on average, twins who smoked had a lower BMI than their twin who did not smoke.⁵ Moreover, twins who had quit smoking were found to have a higher BMI on average, than their twin who continued to smoke.⁵ Similarly, a large cross-sectional study conducted in the United Kingdom found that people who currently smoke were less likely to have a BMI of more than 30 kg/m² compared to people who have never smoked (OR: 0.83, 95% confidence interval: 0.81 - 0.86).⁶ In addition, a longitudinal study that monitored the weight of males found that at baseline, participants who smoked had a lower BMI by an average of 1.6 kg/m² compared to those who did not smoke and those who formerly smoked. This study further demonstrated that BMI increased by an average of 1.6 kg/m² among participants who quit smoking during the follow-up period.⁷

One of the reasons that may explain why people who smoke tend to have a lower BMI than those who do not smoke relates to nicotine and its effect on metabolism – see Chapter 6 for further information on nicotine and its influences on the human body. Nicotine suppresses appetite and increases energy expenditure, thereby theoretically supporting a lower risk of gaining weight.^13-16

The physiological mechanisms underpinning this phenomenon are complex and not entirely understood. However, there have been suggestions that nicotine acts on neurons in the hypothalamus, the part of the brain that regulates hunger, satiety, and energy expenditure.^14,16 Another potential mechanism that may explain this phenomenon involves leptin, a hormone secreted by adipose tissue that reduces appetite and energy intake.^16-18 It is worth noting that research investigating the effect of nicotine on leptin is inconsistent,¹⁷ and the physiological mechanisms through which nicotine regulates body weight warrant further exploration.

In terms of eating behaviours and patterns, a 2024 cross-sectional study found that people who smoke have lower odds of snacking between meals, eating as a reward or to cheer themselves up, and eating out of boredom, stress or anger compared to those who do not smoke. This study also found that people who smoke have higher odds of skipping meals and going more than three hours without food.¹⁹ However, people who smoke were found to have higher odds of adding salt and sugar to their food, finding it hard to leave food on their plate, overeating, and eating fried food more frequently compared to those who do not smoke.¹⁹

While people who smoke tend to have a lower BMI than people who do not smoke, it appears that as smoking intensity increases BMI also increases.^8,20,21 This may be partly explained by the clustering of risk factors. For instance, a higher smoking intensity is likely to be associated with physical inactivity and a poor quality diet,^22-25 both of which are factors that promote overweight and obesity.²⁶

3.29.2 Smoking and adipose tissue distribution

The distribution of adipose tissue around the body is influenced by a range of factors, one of which may be smoking. While people who smoke tend to have a lower BMI, they also tend to have greater abdominal adiposity than people who have never smoked.^25,27 Abdominal adiposity is commonly assessed by measuring the circumference of an individual’s waist or via the waist-to-hip ratio, which is calculated by dividing an individual’s waist circumference by their hip circumference. Some cross-sectional studies have demonstrated that people who smoke are likely to have a higher waist-to-hip ratio compared to people who do not smoke.^28-30 There may be some evidence to suggest that the relationship between smoking and abdominal adiposity is dose dependent, with one study finding that levels of abdominal adiposity are higher among people who heavily smoked compared to those who smoked at lighter levels.⁸

It is important to note that the relationship between smoking and abdominal adiposity may be confounded by other risk factors. While a recent study found that the relationship between smoking and abdominal adiposity is independent of socioeconomic status, alcohol consumption, and other risk-taking behaviours,³¹ the role of physical activity and diet were not explored.³² Similar to the relationship between smoking intensity and BMI, the relationship between smoking and abdominal adiposity may be partly explained by the clustering of other risk factors.^32-35 Further research is necessary to establish the mechanism by which the relationship between smoking and adipose tissue distribution is influenced by physical activity and diet.

3.29.3 The relationship between smoking cessation and body weight

For many people, smoking cessation results in some weight gain.^36,37 A meta-analysis investigating this relationship, found that people who quit smoking gained an average of 4–5 kilograms in their first year of abstinence.³⁶ However, it should be noted that estimates of weight gain following smoking cessation are highly variable, and are dependent on the study design, the characteristics of the sample, and the follow-up period. See Section 7.1.11.3 for more information on the relationship between smoking cessation and weight gain, and Section 7.8.5 for a discussion on the interventions used to minimise weight gain associated with smoking cessation.

3.29.4 Contribution of smoking, and overweight and obesity to morbidity and mortality

Several studies have found that the co-occurrence of smoking, and overweight and obesity is associated with a greater risk of morbidity and mortality.^24,38-41 A 2023 study investigating the combined effects of smoking, overweight and obesity, and physical activity levels on mortality, found that mortality rates (number of deaths per 100,000 people per year) for those who smoked, were obese, and physically inactive, were higher compared to people who had two, one, and none of these risk factors. For example, the mortality rate among people who currently smoked, were obese, and physically inactive was 3.31 times greater than those who never smoked, were of normal weight, and physically active. Moreover, the mortality rate among people who currently smoked and were either physically inactive, or overweight and obese was 2.27 times greater, and for those who never smoked, were overweight and obese, and physically inactive, the mortality rate was 1.21 times greater.²⁴

A 2023 meta-analysis assessing the synergistic associations of risk factors on cancer incidence and death, found that smoking and extra body weight independently and jointly increased the risk of cancer incidence and death. However, the authors note that due to the limited number of studies included in the meta-analysis, a significant synergistic effect of smoking and extra body weight on cancer incidence and death could not be established.⁴¹

Test your knowledge

References

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