Gender difference in unstimulated whole saliva flow rate and salivary gland sizes

Archives of Oral Biology (2006) 51, 1055—1060

www.intl.elsevierhealth.com/journals/arob

Gender difference in unstimulated whole saliva flow rate and salivary gland sizes H. Inoue a,b, K. Ono a, W. Masuda a, Y. Morimoto c, T. Tanaka c, M. Yokota b, K. Inenaga a,* a

Department of Biosciences, Kyushu Dental College, 2-6-1 Manazuru, Kokurakitaku, Kitakyushu, Fukuoka 803-8580, Japan b Department of Cariology and Periodontology, Kyushu Dental College, 2-6-1 Manazuru, Kokurakitaku, Kitakyushu, Fukuoka 803-8580, Japan c Department of Oral Diagnostic Science, Kyushu Dental College, 2-6-1 Manazuru, Kokurakitaku, Kitakyushu, Fukuoka 803-8580, Japan Accepted 20 June 2006

KEYWORDS Unstimulated whole saliva flow rate; Salivary gland size; Gender difference

Summary Objective: A gender difference in the unstimulated whole saliva flow rate (UWSFR) may be due to a difference in the sizes of the salivary glands. In this study, we investigated the relationships among the UWSFR, gland sizes and body sizes of healthy young adult males and females. Design: Unstimulated whole saliva was collected for 5 min by the spitting method in 50 healthy young adults, and the flow rate of the saliva was measured. Heights and weights were measured, and body mass indices (BMI) were calculated. The sizes of the salivary glands were measured by use of a magnetic resonance imaging technique. Results: Parotid and submandibular gland sizes and flow rates in females were significantly smaller than those in males, as were also the weights, heights and BMI. In both males and females, there were significant positive correlations between gland sizes and the flow rates, weights and BMI. The variations of the flow rates were reduced by standardizing them with gland sizes, weights and BMI. Conclusions: These results suggest that the lower UWSFR in females as compared with males is due to the smaller gland sizes due to the smaller body sizes. # 2006 Elsevier Ltd. All rights reserved.

Introduction

* Corresponding author. Tel.: +81 93 582 1131; fax: +81 93 582 8288. E-mail address: [email protected] (K. Inenaga).

A severe reduction of unstimulated whole saliva flow rate (UWSFR) causes dental caries, dry mouth, and inflammation of mucous membrane in the oral cavity.1,2 Unstimulated and stimulated saliva flow rates

0003–9969/$ — see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.archoralbio.2006.06.010

1056 are measured during the diagnosis of salivary gland hypofunction. Particularly, UWSFR is a more important parameter than the stimulated flow rate of saliva, because it may be reduced even when stimulated flow is not affected.3 The accepted ranges for very low, low and normal UWSFR are less than 0.1, 0.1—0.2 and greater than 0.2 mL/min.4 In Europe and America, the range of ‘‘very low’’ UWSFR is used as a criterion of Sjo ¨gren’s syndrome.5 Here, a problem exists in applying the same criterion to both sexes. That is, the UWSFR in the female is generally lower than in the male.6—8 In fact, it has been pointed out that secretion rates in a small, but significant, number of females are inappropriately classified into ‘‘very low’’ and ‘‘low’’ UWSFR.7 What is the cause of the gender difference of UWSFR? Scott determined the volumes of submandibular glands obtained from necropcies and found that female glands were smaller than male glands.9 It has been also reported that the sizes of parotid and submandibular glands determined by a sialographic technique correlated positively and significantly with flow rate of stimulated saliva.10—12 In brief, their studies have suggested that the stimulated salivary flow rate depends on the size of salivary gland, and that the lower flow rates in females were due to the smaller sizes of the glands. However, because it has been reported that parotid gland size does not correlate with the unstimulated flow rate of parotid saliva,13 gender and gland size have long been thought not to be important factors related to UWSFR.14 Recently we have found that parotid and submandibular gland sizes in healthy young humans estimated by magnetic resonance imaging (MRI) correlate positively and significantly with UWSFR.15 The recent study suggests that the gender difference of UWSFR is due to a difference in the size of the gland. If estimation of gland size could be carried out in other ways than by MR imaging or sialography, it would be more convenient. Thus, it is noteworthy that our previous report also showed a positive correlation between the size of the parotid gland and body weight.15 This suggests that the sizes of the salivary glands could be estimated from body profiles, which are obtained from subjects more easily. However, it has been reported that stimulated salivary flow rates are independent of body weight12 and moreover that the correlation between paraffin-stimulated whole saliva flow rates and body profiles is an artifact.16 Thus, the correlations among body profiles, gland sizes and the rates of salivary flow are not fully understood. To address these problems, we investigated correlations among UWSFRs, gland sizes and body profiles in healthy young males and females and evaluated them by standardizing the rates of flow with the gland sizes or the body profiles.

H. Inoue et al.

Materials and methods Subjects The subjects were 50 healthy young adults including 24 males (mean age, 26.0  2.7; range, 20—31) and 26 females (mean age, 24.4  3.2; range, 20—32), chosen in an at-random way from undergraduate and post-graduate students at Kyushu Dental College. There was no significant difference in age between males and females. Body profiles, heights and weights were measured, and then body mass indices (BMI) were calculated. A BMI of >25 is generally used as an indicator of obesity. Most subjects in the study were not obese by these criteria (BMI < 25, n = 21/24 in male, n = 26/26 in female). No subjects were administered drugs or had previous diseases that might affect the salivary secretion. The procedures were explained in full to all subjects and signed consent forms were obtained prior to the start of the experiments. Females were further interviewed about the menstrual cycle. The study design was approved by the Institutional Review Board of Kyushu Dental College.

Collection of unstimulated whole saliva and measurements of salivary parameters UWS was collected for 5 min by the spitting method at 14:00—18:00, and then for additional 5 min periods to examine the parameters of saliva samples. All subjects were instructed to swallow at time zero. Because the results for 5—10 min were the same as those for 5 min, the present study used only the results for 5 min. Na+ and K+ concentrations, pH, total protein concentrations and amylase activities of the saliva samples were measured as described in our previous report.15 Interviews about menstrual cycle were used to divide samples in females into follicular (n = 10) and luteal (n = 15) phases. One female was in the menstrual phase. Salivary estradiol and progesterone levels of 23 of the 25 female samples were measured by means of a commercial enzyme immunoassay kit (SALIMETRICS LLC, USA). Two samples in the luteal phase were too small to measure. The measurements were performed twice and the data were averaged.

Determination of the salivary gland size The sizes of the three major salivary glands were determined by MR images, as described in the previous report.15 All images were taken using a 1.5-T full-body MR system (VISART; Toshiba, Tokyo, Japan) with a circular polarized neck coil to visualize the parotid, submandibular and sublingual glands. The

Gender difference of salivary secreation

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Table 1 Comparisons of a variety of profiles of subjects and salivary parameters between males and females

Profiles of body size Height (cm) Weight (kg) BMIa (kg/m2)

All (n = 50) [mean  S.D.]

Males (n = 24) [mean  S.D. (range)]

Females (n = 26) [mean  S.D. (range)]

p value

163.8  8.7 56.1  9.5 20.8  2.2

171.0  5.3 (163—181) 64.0  6.6 (50—75) 21.8  2.3 (17.9—26.1)

157.0  4.9 (149—166) 49.1  5.3 (42—64) 19.9  1.5 (17.6—23.5)

<0.001 <0.001 <0.001

66.1  18.9 (41.7—135) 24.2  6.6 (12.3—37) 90.3  23.0 (54.6—166)

<0.001 <0.001 <0.001

Bilateral sizes of salivary glands (cm3) PG b 75.5  21.0 SMG c 28.9  8.2 PG + SMG d 104.4  26.7

85.7  18.4 (46—128) 34.1  6.5 (24—45) 119.8  21.7 (77—167)

Flow rates (mL/min)

0.42  0.23

0.50  0.28 (0.20—1.14)

0.35  0.16 (0.10—0.70)

<0.05

Secretion rates of total protein (mg/min)

0.30  0.24

0.38  0.31 (0.1—1.3)

0.22  0.13 (0.04—0.5)

<0.05

a b c d

Body mass index. Parotid glands. Submandibular glands. Both parotid and submandibular glands.

MR images were printed out and the outlines of each salivary gland were then traced manually. All traces were checked by two MR experts (Y.M. and T.T.). The traces were scanned into a personal computer and the averaged areas were calculated by analysis software, Scion Image (Scion Corporation). The sizes of the salivary glands in each subject were then calculated by multiplying the areas by the section thicknesses by Excel 2000 (Microsoft Corporation). The total sizes of salivary glands were then calculated by summation of the bilateral salivary gland sizes.

Statistical analyses The significance in the statistical analyses was assessed using Student’s t-test between males and females and the Pearson correlation coefficient. Relationships yielding p values less than 0.05 were considered to be significant. As index values to

express degree of variation, the coefficients of variation were calculated by dividing the standard variations by the means. All values are expressed as the mean  S.D., and n represents the number of subjects.

Results There were significant gender differences in the sizes of the parotid and submandibular glands, UWSFRs, protein secretion rates, heights, weights and BMIs (Table 1). All values of body profiles were similar to those in a report of The National Nutrition Survey, Japan (2002). In contrast, there were no significant gender differences in the sizes of sublingual glands (males: 10.6  2.6 cm3, females: 9.7  3.5 cm3), concentrations of Na+, K+, total protein concentrations, amylase activities and pHs (data not shown).

Figure 1 Significant positive correlations of salivary gland size with flow rate of unstimulated whole saliva (A), body weight (B) and BMI (C) in males (filled circles, n = 24) and in females (white circles, n = 26). PG + SMG indicates the total bilateral size of both the parotid and submandibular glands. Dashed lines indicate the 95% confidence interval of the regression line.

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H. Inoue et al.

Table 2 Correlation coefficients of flow rates of unstimulated whole saliva with salivary gland sizes and profiles of body profiles Gender

Salivary gland sizes PG

Body profiles

SMG

PG + SMG

Height

Weight

BMI

All Flow rate Height Weight BMI

0.50 *** 0.34 * 0.69 *** 0.76 ***

0.51 *** 0.52 *** 0.67 *** 0.55 ***

0.55 *** 0.42 ** 0.74 *** 0.77 ***

0.29

0.29 * 0.79 ***

0.37 ** 0.29 ** 0.81 ***

Males Flow rate Height Weight BMI

0.44 * 0.30 0.59 ** 0.73 ***

0.51 * 0.04 0.46 * 0.47 *

0.53 ** 0.27 0.64 *** 0.76 ***

0.32

0.08 0.24

0.28 0.34 0.83 ***

Females Flow rate Height Weight BMI

0.42 * 0.11 0.59 ** 0.69 ***

0.34 0.15 0.30 0.29

0.45 * 0.13 0.57 ** 0.65 ***

0.17

0.24 0.68 ***

0.23 0.12 0.80 ***

***

p < 0.001; **p < 0.01; *p < 0.05.

It has been reported that the unstimulated and stimulated flow rates of submandibular saliva change during the menstrual cycle and are highest during the folliclar phase.17 However, in the present study, there was no significant difference in the UWSFRs between the follicular and the luteal phase (follicles: n = 10, 0.35  0.12 mL; luteum: n = 15, 0.33  0.17 mL). It is known that estradiol and progesterone are present in saliva and that their concentrations reflect those in the plasma.18 Generally, estradiol levels predominate in the follicular phase and progesterone levels predominate in the luteal phase. Estradiol levels were significantly higher ( p < 0.05) in the follicular

phase (15.5  6.9 pg/mL, n = 10) than that in the luteal (8.9  5.9 pg/mL, n = 13) phase, and progesterone levels tended to be higher in luteal phase (213.8  100.5 pg/mL) than in the follicular (144.0  61.6 pg/mL) phase, although the difference was not significant ( p = 0.067). The sizes of the parotid and submandibular glands correlated significantly with the UWSFRs (Table 2) or total protein secretion rates in all subjects (data not shown), as found in our previous report.15 There were also significant correlations between UWSFRs and body profile in all subjects (Table 2), and the correlations of gland sizes with weights and BMIs were particularly high. When separated by gender,

Table 3 Standardization of flow rate of unstimulated whole saliva by salivary gland sizes and profiles of body size and coefficients of variation All Mean  S.D. Flow rate Saliva/PG (mL/min/cm3) Saliva/SMG (mL/min/cm3) Saliva/PG + SMG (mL/min/cm3) Saliva/height (mL/min/m) Saliva/weight (mL/min/kg) Saliva/BMI (mL/min/kg/m2)

Males Coefficient of variation (%)

Mean  S.D.

Females Coefficient of variation (%)

Mean  S.D.

Coefficient of variation (%)

5.6  2.5

55.6 44.6

5.8  2.7

56.0 46.6

5.4  2.2

45.7 40.7

14.8  6.6

44.6

14.3  6.6

47.1

15.2  6.8

44.7

4.0  1.7

42.5

4.1  1.9

46.3

3.9  1.6

41.0

2.6  1.4

55.8

2.9  1.7

58.1

2.3  1.1

48.3

7.5  3.8

50.8

7.8  4.2

53.8

7.3  3.4

46.6

20.1  10.3

51.2

22.6  12.0

52.2

17.8  8.0

44.4

Gender difference of salivary secreation the sizes of the parotid and submandibular glands correlated to a significant extent with weights and BMIs in both males and females (Fig. 1), but not with the heights. The UWSFRs showed significant correlations with weights and BMIs among all subjects, but they did not show when separated by gender (Table 2). The total protein secretion rates did not show a significant correlation with body profiles (data not shown). The significant gender differences of UWSFR disappeared after the standardization with gland sizes and body profiles (Table 3). The coefficients of variation were calculated to assess how much the standardization reduced the variations in UWSFR (Table 3). The coefficients of variation in UWSFR were about 10% smaller in females than in males. The standardization with gland sizes reduced the coefficients of variation by about 10% in all the subjects and in the males, and the standardizations with weight and BMI reduced them by several percentage points. In females, the coefficients of variation were reduced about 5% by the standardization with parotid gland sizes and with the total size of the parotid and submandibular glands, and a very few percentage points by the standardization with the BMIs.

Discussion We examined the relationships among UWSFR, gland size and body size in healthy young adult males and females. In comparing the UWSFRs between genders, the effects of steroid hormones on salivary secretion in females cannot be ignored. It is known that the UWSFR is reduced in menopausal and postmenopausal females, and replacement therapy of sex hormones produces a recovery of the salivary flow rate.19—21 Therefore, we examined the effects of the menstrual cycle on the UWSFR in the present study and found no differences in the UWSFRs and other salivary parameters between the luteal and follicular phases, in contrast to the results of a previous study of submandibular saliva.17 Thus, we think that there are no obvious effects of sex steroid hormones during menstrual cycles on UWSFRs in healthy young females. The present study shows significant gender differences in UWSFRs and salivary gland sizes and significant positive correlations of UWSFR with salivary gland sizes in each gender. However, a previous study has reported that the unstimulated parotid flow rate is independent of parotid gland size as estimated by a sialographic technique.13 We believe that the gland sizes are estimated more accurately by the MRI technique than by using the sialographic

1059 technique, because it is very difficult to inject a contrast medium into all of the gland as required by the latter technique. The more exact measurement of gland size in the present study may have given us a significant positive correlation between UWSFR and gland size. There was also a significant gender difference in the rates of total protein secretion, and a significant correlation between rates of protein secretion and gland sizes. However, the latter finding may be secondary to the positive correlation of gland size with UWSFR. We found significant correlations of the UWSFRs with the weights and the BMIs, and significant gender differences in the body profiles of the subjects in the present study. Although correlations of paraffin stimulated whole saliva flow rate with body profiles have been reported, it was concluded that it was an artifact.16 However, we think that the correlations of UWSFRs with body profiles in the present study are not an artifact because the weights and BMIs correlated positively with the salivary gland sizes which correlated with the UWSFRs. The previous studies reported two opposite results as to whether the gender difference in UWSFR in healthy young humans was significant or not. A report showed a significant gender difference in UWSFR of young humans,7 but the other showed no gender difference.6 As this study points out, whether or not a gender difference exists in UWSFR may be dependent on whether or not it exists in body profiles of the subjects. The previous studies did not mention this point. If there were gender differences in weights and BMIs, there would exist significant differences in the UWSFRs. The standardization of UWSFR with gland size was very effective in reducing the variations of UWSFR and removed the gender differences of UWSFR. However, for many researchers and clinicians, it is not easy to determine salivary gland size using MRI because of the expense and inconvenience of the MRI procedure. Hence, we examined the standardization of the UWSFRs with body profiles. The coefficients of variation of UWSFR standardized with weight and BMI tended to decrease, though not as much, as those with gland size. This suggests that the weight and BMI can also be useful to standardize UWSFR. A report casts a problem that a not so small number of females are unnecessarily characterized as being in the low UWSFR group.7 To resolve this problem, the standardizations of UWSFR with gland size or body profiles (weight and BMI) may be useful to diagnose salivary gland hypofunction. In conclusion, the gender difference of UWSFR is due to the difference of gland sizes owing to the differences of body profiles in healthy young humans. Xerostomia is most often observed in older

1060 people.22,23 The present study was limited to young humans in their twenties or thirties to exclude the influence of age. Therefore, it is necessary further to investigate whether or not the gender difference of UWSFR is due to the difference of gland size in older people.

Acknowledgements This work was supported by a grant from academic association in an alumni association of Kyushu Dental College to K.O. and a Grant-in-Aid for Young Scientists (B) from the Ministry of Education, Culture, Sports, Science and Technology, Japan, to Y.M. (14771044).

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