Saliva has many functions in the oral cavity: it is responsible for the digestion of food, serves to protect and lubricate mucous membrane, and facilitates ingestion of food and speech production. Besides, saliva has a major role in maintaining oral health and oral hygiene. In addition to helping washing away pathogenic bacteria and debris nutrients, proteolytic enzymes and antibodies from saliva can destroy microorganisms on the mucosa and teeth including bacteria that cause tooth decay. The presence of saliva is necessary for the permanent process of remineralization of the teeth for which calcium and phosphate ions from saliva are required (1).
Lack of saliva represents a major health problem because it completely compromises oral function and oral health. Along with aggravated function, dry mouth is subject to the development of inflammation, fungal infection, the rapid development of caries, inflammation of major salivary glands, and bad breath.
The amount of saliva can be measured objectively with sialometry. Daily secretion of saliva is 0.8-1.2 l. Salivation from 0.4 to 0.5 ml/min is considered normal, while the quantity of saliva 0.2 -0.4 ml/min indicates oligosialia and a quantity of less than 0.2 ml /min hyposalivation (2).
Patients usually experience hyposalivation as a subjective feeling of dry mouth or xerostomia.
Causes of hyposalivation are numerous. Internal and external factors are salivary gland diseases, various organic diseases as well as various mental conditions and diseases, treatment with radiation, chemotherapy as well as a number of different drugs (> 500) which have a documented side effect of dry mouth or xerostomia. It is known that smoking tobacco affects general and oral health. Also, evidence suggests that smoking is one of the external factors which reduces secretion of saliva, however, research findings are contradictory (3). Therefore, the aim of this study was to assess the effect of tobacco smoking on quality and quantity of salivation and the oral hygiene status of subjects.
Materials and methods
The study included 60 subjects - randomly selected patients of Clinical Department of Oral Medicine and the Clinical Department of Fixed Prosthodontics. The study was approved by the Ethics Committee of the School of Dental Medicine, University of Zagreb, and the subjects were included in the study after signing the informed consent and voluntarily agreeing to participate in research. The inclusion criteria were age 18 and above of patients, the absence of salivary gland diseases and the fact that they were not irradiated in the head and neck area. All subjects were divided into two groups of 30 subjects: a test group consisted of smokers and a control group of nonsmokers. The distribution of subjects shows that the average age of smokers (37.8 years) was lower than non-smokers (47.7 years). Regarding the distribution by gender, in both groups, the majority of subjects were females: in the group of smokers 63.33% and in the group of non-smokers 66.67%, which means that there were more female smokers.
All subjects were asked the questions from the questionnaire that was developed for this research. The questions included: general information, information about smoking (number of cigarettes smoked per day, length of smoking), information on systemic diseases and medications taken by the patient.
Saliva of every subject was collected and measured in a graduated tube by spitting method. Saliva was measured from 9 to 12 am and at least 2 hours after the last meal. First, the amount of unstimulated saliva was measured (QNS) during 5 minutes, after which each subject rinsed their oral cavity with 2% aqueous solution of citric acid, and the amount of stimulated saliva (QSS) was measured for 5 minutes as well. From these values the amount of saliva produced in ml per time unit was obtained (ml / min), which was used in data processing.
The quality of saliva was assessed visually when taking each sample and was divided into five categories: sticky and thick, foamy and watery.
In each of the subjects, clinical examination was performed to detect and record condition of oral hygiene and presence of teeth staining and coated tongue. Oral hygiene status was recorded according to the plaque index (Silness and Löe, 1964) (4).
Data were collected and analyzed using Microsoft Excel (Microsoft, USA). Statistical analysis was made by the program MedCalc v11 (MedCalc Software, Belgium). Values of quantitative differences between groups were analyzed by an independent t-test, while for categorical and qualitative variables, Fischer's exact test was used. For the analysis of more than two groups, the Kruskal-Wallis test was used, while the correlations between variables were analyzed by Pearson correlation coefficient - r. For the level of significance, p <0.05 was determined.
The effect of tobacco smoking on salivation
The results of the comparison of the amounts of saliva between smokers and non-smokers show that statistically significant difference was not found in the amount of unstimulated (Qns) and stimulated saliva (Qss) between smokers and non-smokers (Figure 1).
Figure 2 shows a comparison of the quantity of saliva Qns and Qss in relation to the age of smokers and non-smokers. The results show that in the group of smokers with increase of age the amount of unstimulated saliva was significantly reduced (p = 0.0008), compared to non-smokers with no significant difference (p = 0.2195). A similar result was obtained for stimulated saliva as well, the amount of which significantly reduced by the increase in age of smokers (p = 0.0002) compared to non-smokers (p = 0.2786).
The results of the correlation of the duration of smoking and the amount of saliva of smokers (Qns and Qss) also show a statistically significant difference (p = Qns for 0.0186 and p = 0.0083 Qss). The amount of unstimulated and stimulated saliva decreases significantly with increasing of the smoking duration (Figure 3).
However, the amount of saliva is not significantly associated with the number of cigarettes smoked per day (Qns p=0.6811, Qss p=0.5552).
Results show the difference in the quality of saliva. In the group of smokers, the largest number of subjects had thick saliva, while among non-smokers the thin and watery saliva prevailed.
The effect of drugs on salivation
Smokers used fewer drugs (40%) than non-smokers (50%). The most commonly used drugs were antihypertensives (51.5%). The average number of drugs per patient in the group of smokers was 0.83, and in the group of non-smokers 1.3.
Comparison of saliva (unstimulated, Qns and stimulated, Qss) of smokers who use drugs and smokers who do not use drugs is shown in Figure 4. Smokers who use medications have significantly less saliva (Qns) compared to smokers who do not use them, while the Qss difference was not significant.
Figure 5 shows a comparison of the quantity of saliva (Qns and Qss) of non-smokers who take drugs and those who do not take them. The difference in the quantity of unstimulated saliva (Qns) was not significant (p = 0.0694), in contrast to the stimulated saliva (Qss) where there was a significant difference (p = 0.0186). Drugs do not significantly affect the amount of unstimulated saliva in non-smokers (Qns), but significantly reduce the amount of stimulated saliva (Qss).
Comparison of the amount of saliva (Qns and Qss) between smokers who do not use drugs and non-smokers who use them, showed no statistically significant difference in the amount of saliva, neither in the Qns (p = 0.1074) nor in Qss (p = 0.2411).
The effect of tobacco smoking on oral hygiene
The average value of the plaque index per smoker is greater than the average value per non-smoker (1: 0.8) which indicates a poorer oral hygiene of smokers.
It is evident that the plaque index increases with the length of smoking, meaning that the oral hygiene of smokers is poorer the longer they smoke. This correlation is statistically significant (p = 0.0386). Also, teeth staining was found in 53.3% of smokers as opposed to non-smokers among whom 30% had staining. In 20% of smokers the coated tongue was observed.
The aim of this study was to determine whether smoking tobacco affects salivation.
Assuming that tobacco smoking is harmful to the entire organism we expected that it would have a negative impact on the function of the salivary glands in terms of reduced quantity and quality of saliva which was proved in this study. In addition, we observed the effect of smoking on the condition of oral hygiene, appearance of pigmentation on the teeth and coated tongue. The results showed no statistically significant difference in the amount of unstimulated and stimulated saliva in smokers compared to non-smokers. We explain these results by the fact that the majority of smokers were younger and the period of smoking was shorter. Therefore the function of salivary glands was preserved.
In literature, we came across contradictory results of similar research. Khan et al. observed that long-term smoking does not adversely affect salivary reflex and salivation (5). They also noticed that there was a short-term increase in the amount of saliva in smokers by stimulating taste receptors by nicotine. The mechanical, chemical and thermal stimulation of salivary glands by cigarettes during smoking can stimulate a short-term increase of the amount of saliva (6). Iida et al. explained chemical stimulation on an animal model and demonstrated that nicotine and cytisine acted on nicotinic receptors as agonists and stimulated the secretion of saliva (7). Nicotine and cytisine also act as sialogogue wherein the cytisine may be a better therapeutic candidate to serve as a sialogogue for xerostomia patients. Authors Field and Duke have shown that there is a psychological stimulation of salivation in smokers displaying smoking requisites: participants also showed salivary reactivity to smoking cues, with males showing a decrease in salivation, and females showing an increase, but only when cigarettes were perceived as unavailable (8). On the other hand, Rad et al. (9) found that there were significant differences in the amount of saliva between smokers and non-smokers, and that long-term smoking causes saliva to significantly decrease. This was confirmed by our results, which showed that in smokers there is an inverse correlation between age and the amount of saliva, while in non-smokers, this correlation does not exists. A decrease in the amount of saliva in older smokers is associated with the duration of smoking. The results have shown that smokers have a modified quality of saliva compared to non-smokers. Most of the smokers had thick saliva while in non-smokers watery saliva prevailed.
When harmful effect of cigarettes impacts the salivary glands, the first to be affected is parotid gland whose role is secretion of watery saliva. The loss of its function is compensated by submandibular and sublingual glands which secrete mucous saliva. This explains thicker saliva in smokers. Recent research confirms that smoking negatively affects the quality of saliva. Substances from cigarette smoke destroy protective macromolecules of saliva, enzymes and proteins, and thus saliva loses its protective role and becomes an agent in carcinogenesis and development of oral and oropharyngeal cancer (10-12).
This study, in contrast to other similar studies (9, 13-15) included the effect of drugs on salivation in both groups, which could potentially reduce the amount of saliva. Non-smokers took more drugs compared to smokers and the most often used drugs were antihypertensives that are known to affect the salivation and have a xerostomic effect. We observed no significant difference in the amount of saliva of smokers who do not use drugs and non-smokers who use them. Therefore, the usage of drugs and smoking equally affects salivation. Similarly, our results show that the amount of unstimulated saliva is lower in smokers who use drugs. Reduced amount of saliva can be explained as a result of the cumulative effect of drugs and smoking over time. The results of the impact of drugs on salivation in smokers could not be compared with similar studies by other authors since such data were not found in the available literature.
Noticed deterioration of oral hygiene among smokers is consistent with previous research (15). Some results showed that long-term smoking causes oral hygiene to deteriorate and it increases the severity of gingival disease (16). Also, the study by Chang et al. showed positive association between poor oral hygiene and increased risk of head and neck cancer which appeared to differ between alcohol or cigarette consumption (17).