**4. Discussion**

The aim of the current systematic review was to provide benchmark data on the association between hookah smoking and obesity. Five studies, comprising a total of 16,779 adolescent and adult participants and age range between 13–75 years and conducted in Iran, Syria, Jordan, Pakistan, and Tunisia, met the inclusion criteria and were reviewed, revealing one major finding: All five studies included in our systematic review showed a higher prevalence of obesity and/or a higher association between obesity (abdominal obesity, BMI percentile ≥ 95th, or BMI ≥ 30 kg/m2) and hookah smoking than the corresponding values for nonsmokers and cigarette smokers (when comparisons were available) regardless of gender and among all ages. This finding is considered to be strong and robust because (i) data were derived from well-conducted, large-sample, population-based studies with a low risk of bias; (ii) the finding was not contradicted in any of the included studies; (iii) the same finding has also been reported in clinical samples (i.e., not the general population) [39]; and (iv) this finding was confirmed by a meta-analysis.

## *4.1. Clinical Implications*

Our findings have some implications, especially for the general population. Firstly, it is important to discuss the association between hookah smoking and obesity among young adults, perhaps through educational interventions in schools and universities and in work settings [40,41]. In addition, the common public belief that hookah smoking may be healthy, since hookah smoke contains fruit flavours and the water in the bottom of the hookah can eliminate the toxicity of tobacco compared with cigarettes, should be contradicted. On the contrary, we found that the adverse e ffects of hookah smoking could be even greater than those of cigarette smoking. In fact, several types of cancer (e.g., lung cancer) have been linked to hookah smoking [42]. Moreover, it causes coronary artery disease [39], an increased heart rate and high blood pressure [43], respiratory diseases [10], dental problems [44], and osteoporosis [45], as well as infections when sharing a hookah [45].

It is unclear why smoking hookah is associated with obesity; we speculate that the potential mechanisms behind this association may be multiple. However, two factors may have a major impact. Firstly, smoking a hookah requires sitting, and a hookah-smoking session may last for two hours. Some individuals may repeat the session two or three times a day [46], and this unavoidably facilitates a sedentary lifestyle (unlike cigarettes), which reduces energy expenditure [47]. Also, the hookah is smoked during social events where smokers spend time together and talk as they pass the mouthpiece around in environments (e.g., restaurants and co ffee shops) rich in eating stimuli, which could increase the exposure to and consumption of high-calorie foods [47]. All in all, it has been shown that hookah smoking is associated with less healthy lifestyle habits in both men and women [48].

#### *4.2. Strengths and Limitations*

This systematic review has certain strengths. To the best of our knowledge, this is the first systematic review to investigate the association between hookah smoking and obesity. Despite the fact that few studies met the inclusion criteria and were included in our systematic review, the finding is considered to be strong, with definite evidence for the association between hookah smoking and obesity. This needs to be underlined due to the increasing trend of this smoking habit, especially among young people. However, this systematic review also has certain limitations. In particular, our results should be interpreted with caution with regard to the association between hookah smoking and obesity, since the cross-sectional design of the studies included in our systematic review indicates only simple associations at best and does not provide solid information regarding any causal relationships between conditions [49]. In other words, these studies lack evidence to determine whether hookah smoking may lead to obesity, since very few studies have longitudinally investigated the "real" e ffects of hookah smoking [50]. Moreover, the included studies in our systematic review were conducted only in low-middle income countries (i.e., Middle East); therefore, our findings may not be generalized on a global scale. Finally, none of the included studies clearly examined if the average number of sessions (i.e., per day or week) or years (i.e., months and years) of hookah smoking are related to a higher risk of obesity. All these shortcomings in the current research indicate the need to design longitudinal studies to clarify the real e ffect of hookah smoking on the onset and progression of obesity and weight-related comorbidities, especially in Western countries (i.e., US and Europe).
