Night shiftwork and prostate-specific antigen level in a tire manufacturing factory.

BACKGROUND: Recent studies suggest that night shiftwork may increase the risk of prostate cancer and elevated serum prostate-specific antigen (PSA) level. The purpose of this study was to verify whether rotating night shiftwork affects serum PSA level.
METHODS: This study included 3,195 male production workers who work in a large tire manufacturing factory. Serum PSA levels were measured and the data on related factors were obtained.
RESULTS: The mean serum PSA level was 0.98 ± 0.79 ng/mL. PSA levels were significantly lower in the younger age group, the obese group, and regular exercise group. PSA levels were lower in night shift workers (n = 2,832) compared to day workers (n = 363), but the difference was not statistically significant.
CONCLUSIONS: Unlike previous studies, we did not find any evidence that night shiftwork results in an increase in serum PSA levels. Further research and consistent results are needed to elucidate the association between night shiftwork and the effect on the prostate.

BACKGROUND

The International Agency for Research on Cancer, which is part of the World Health Organization, classified shiftwork involves circadian disruption as probably carcinogenic to humans (group 2A) based on sufficient evidence [1]. While night shiftwork is known to increase the risk of breast cancer, but its association with prostate cancer is still controversial [234]. A previous meta-analysis performed suggests that night shiftwork increase the risk of prostate cancer [5]. Another meta-analysis found no obvious association between night shiftwork and prostate cancer, but subgroup analysis suggests that night shiftwork may increase the risk of prostate cancer in Asian men [6]. In a retrospective cohort of rotating shiftwork, a non-statistically significant elevated risk of prostate cancer was reported (odds ratio [OR]: 1.79, 95% confidence interval [CI]: 0.57–5.68) [7]. A population-based case-control study suggests that a long duration of permanent night work in combination with a long shift length or at least 6 consecutive nights may be associated with prostate cancer [8].

Prostate-specific antigen (PSA) is often elevated in prostate disorders, especially prostate cancer. An earlier study revealed a strong positive association with current shiftwork and elevated PSA level at 4.0 ng/mL or greater (OR: 2.62, 95% CI: 1.16–5.95) [9]. This research supports the notion that sleep disturbance or circadian disruption is associated with elevated PSA. Thus, night shift workers likely have an increased risk of developing prostate cancer. But so far, there is only one study that shiftwork increase PSA level. The aim of this study was to investigate whether PSA level was associated with men performing night shiftwork.

METHODS

Study subjects

Study subjects consisted of 3,295 Korean male production workers in a large tire manufacturing factory who underwent health examinations between October 2015 and November 2015. The production process was a system that runs 24 hours a day. All night shift workers performed the same type of shift. The working schedules of the tire manufacturing factory we studied were as follows: 1) regular day work and 2) rotating shift work. The regular day work schedule started at 08:30 and ended at 17:30 hours daily. It's a schedule that works 5 days a week. Rotating shift workers performed in a four-teams-three-shift system. The rotating shift work schedule was as follows: four teams, three 8-hour shift, 5 day shifts (from 7:00 to 15:00 hours), 1 day off, 5 night shifts (from 23:00 to 7:00 hours), 2 days off, 5 swing shifts (from 15:00 to 23:00 hours), and 2 days off. It is a schedule of the 20-day cycle. And there was no difference in the tasks performed between night shift worker and day worker. We excluded 79 workers under 30 years of age. Night shift workers were defined as current night shift workers performing night shiftwork for more than 3 years. Individuals currently performing daytime work were defined as day workers. Individuals with night shiftwork experience within the last 3 years were excluded from the day workers group. Twenty-one workers who are currently receiving treated for prostate diseases, such as prostate cancer or benign prostate hyperplasia, were excluded. A total of 3,195 workers were selected as the final subjects. All subjects were Koreans.

Data collection

In this study, the PSA test was conducted with health examination by the Occupational Safety and Health Act, as required by the labor union. Therefore, all subjects in the factory participated in the study without exception. Information about prostate diseases was obtained through a structured questionnaire. There was no statistically significant difference in prostate diseases between the night shiftwork group and day work group per χ2 test. All subjects fasted for 12 hours or more before undergoing measurements of serum PSA. Regular exercise was defined as performing regular exercise greater than 30 minutes per day more than twice a week. A drinker was defined as an individual who drinks more than 20 grams of alcohol more than twice a week. Their body mass index (BMI) was calculated using the following formula:

Cases on a current antihypertensive regimen or cases with blood pressure of ≥ 140/90 mmHg were defined as hypertensive. Subjects with diabetes mellitus were defined as those who were being treated for diabetes mellitus or had serum glucose levels ≥ 126 mg/dL. In addition, each subject provided written informed consent before participating.

Statistical analysis

We performed a cross-sectional study of 2,832 rotating night shift workers and 363 regular day workers over 30 years of age. The age range was 30 to 58 years. Since PSA levels did not show a normal distribution pattern, we carried out natural log transformation before analysis. Univariate measures of demographic characteristics were calculated for night shift workers compared to day workers. All categorical variables were compared using chi-square test, and all continuous variables were compared using Student's t-test or analysis of variance. Analyses of covariance were performed to compare the mean serum PSA levels between night shift workers and day workers after adjusting for confounders. The elevated serum PSA was defined as serum PSA ≥ 4.0 ng/mL [10]. The OR with 95% CI of elevated serum PSA for shiftwork was calculated by taking the day worker as the referent using multivariable logistic regression models. SPSS version 23.0 was used to analyze the data (SPSS Inc., Chicago, IL, USA). Statistical significance was defined as p < 0.05.

Ethics statement

This study protocol was approved by the Institutional Review Board (IRB) of Chonnam National University Hwasun Hospital (IRB No. CNUHH-2014-123). In addition, each subject provided written informed consent before participating.

RESULTS

The arithmetic mean of serum PSA level was 0.98 ± 0.79 ng/mL. The mean age of the night shift and day shift workers was 46.9 ± 5.5 years and 45.5 ± 7.5 years, respectively. The night shift workers' age was significantly higher than day workers (p < 0.001). The BMI of the night shift workers and day shift workers was 24.9 ± 2.8 and 25.3 ± 3.0, respectively. The night shift workers' BMI was significantly lower than day workers (p = 0.003). The night shift workers showed a higher percentage of smoking, drinking, and regular exercise, which was not statistically significant. Hypertension and diabetes mellitus were not significantly different between the 2 groups (Table 1).

Table 1

General characteristics of subjects

Variables		Day worker (n = 363)	Night shift worker (n = 2,832)	p-valuea
Age (years)		45.5 ± 7.5	46.9 ± 5.5	< 0.001
	30–39	86 (23.7)	242 (8.5)	< 0.001
	40–44	53 (14.6)	738 (26.1)
	45–49	94 (25.9)	887 (31.3)
	50–54	93 (25.6)	698 (24.6)
	55–59	37 (10.2)	267 (9.4)
Tenure (years)		20.4 ± 9.2	21.5 ± 6.2	0.030
BMI (kg/m2)		25.3 ± 3.0	24.9 ± 2.8	0.003
	< 25	153 (42.1)	1,284 (45.3)	0.263
	≥ 25	210 (57.9)	1,548 (54.7)
Smoking				0.104
	Non-smoker	107 (29.5)	700 (24.7)
	Ex-smoker	118 (32.5)	924 (32.6)
	Current smoker	138 (38.0)	1,208 (42.7)
Drinking				0.667
	No	281 (77.4)	2,159 (76.2)
	Yes	82 (22.6)	673 (23.8)
Regular exercise				0.549
	No	270 (74.4)	2,060 (72.7)
	Yes	93 (25.6)	772 (27.3)
Hypertension				0.216
	No	315 (86.8)	2,521 (89.0)
	Yes	48 (13.2)	311 (11.0)
Diabetes mellitus				0.922
	No	332 (91.5)	2,581 (91.1)
	Yes	31 (8.5)	251 (8.9)
PSA levels (ng/mL)				0.073
	< 1.0	219 (60.3)	1,868 (66.0)
	≥ 1.0, < 2.0	123 (33.9)	791 (27.9)
	≥ 2.0, < 3.0	14 (3.9)	116 (4.1)
	≥ 3.0, < 4.0	1 (0.3)	28 (1.0)
	≥ 4.0	6 (1.7)	29 (1.0)

Values are presented as number (%), arithmetic mean ± standard deviation or p-value.

BMI: body mass index; PSA: prostate-specific antigen.

aComparison by Student's t-test or Pearson χ2 test.

As age increased, PSA levels increased significantly (p < 0.001). The PSA levels were significantly lower in the obese group (BMI ≥ 25) than in the non-obese group (BMI < 25) (p = 0.005). PSA levels were also significantly lower in the regular exercise group than in the group that did not exercise regularly (p = 0.007). There were no significant differences in PSA levels according to smoking, drinking, and night shiftwork. PSA levels were lower in night shift workers compared to day workers, but the difference was not statistically significant. There was still no significant difference after adjusting for the confounders (Table 2). There was no difference in PSA levels according to hypertension and diabetes mellitus in this study. OR and 95% CIs were calculated using a multiple logistic regression analysis, with a PSA of ≥ 4.0 ng/mL as the dependent variable. In all models, the OR was lower in night shift workers but not statistically significant (Table 3).

Table 2

Serum prostate-specific antigen levels (ng/mL) by variables

Variables		No.	Arithmetic (mean ± SD)	Geometric (mean ± GSD)	p-valuea	Adjusted geometric mean	p-valueb
Age (years)					< 0.001		< 0.001
	30–39	328	0.89 ± 0.50	0.78 ± 1.65		0.78
	40–44	791	0.90 ± 0.61	0.77 ± 1.76		0.77
	45–49	981	0.96 ± 0.87	0.80 ± 1.76		0.80
	50–54	791	1.06 ± 0.91	0.85 ± 1.88		0.86
	55–59	304	1.11 ± 0.83	0.91 ± 1.85		0.91
BMI (kg/m2)					0.004		0.005
	< 25	1,437	1.01 ± 0.78	0.84 ± 1.78		0.84
	≥ 25	1,758	0.95 ± 0.81	0.79 ± 1.79		0.79
Smoking					0.642		0.737
	Non-smoker	807	0.96 ± 0.71	0.80 ± 1.84		0.80
	Ex-smoker	1,042	0.99 ± 0.89	0.82 ± 1.77		0.82
	Current-smoker	1,346	0.98 ± 0.76	0.81 ± 1.78		0.82
Drinking					0.768		0.740
	No	2,440	0.97 ± 0.71	0.81 ± 1.79		0.81
	Yes	755	0.99 ± 1.01	0.81 ± 1.79		0.81
Regular exercise					0.006		0.005
	No	2,330	0.99 ± 0.83	0.83 ± 1.78		0.83
	Yes	865	0.93 ± 0.69	0.78 ± 1.82		0.77
Night shiftwork					0.192		0.067
	No	363	1.04 ± 1.14	0.84 ± 1.87		0.86
	Yes	2,832	0.97 ± 0.74	0.81 ± 1.78		0.81

Values are presented as mean ± standard deviation or p-value.

BMI: body mass index; GSD: geometric standard deviation; SD: standard deviation.

aComparison by Student's t-test and analysis of variance; bComparison by analysis of covariance adjusted for age, BMI, and exercise.

Table 3

Night shiftwork and risk for elevated serum prostate-specific antigen (≥ 4.0 ng/mL)

Variables		Adjusted OR	95% CI	β	SE	p-value
Day worka		Reference
Night shiftworkb
	Model 1	0.584	0.24–1.47	−0.539	0.453	0.235
	Model 2	0.596	0.25–1.45	−0.520	0.454	0.255
	Model 3	0.563	0.23–1.38	−0.579	0.457	0.209

Model 1: adjusted for age. Model 2: adjusted for age, body mass index, and exercise. Model 3: adjusted for age, body mass index, exercise, smoking, drinking, hypertension, and diabetes mellitus.

CI: confidence interval; OR: odds ratio; SE: standard error.

aWorkers who have never worked night shiftwork in last 3 years; bWorkers who have been on night shiftwork for the last 3 years.

DISCUSSION

In this study, PSA levels were lower in night shift workers compared to day workers, but the difference was not statistically significant. There was still no significant difference after adjusting for the confounders. Contrary to previous study, we found no evidence that night shiftwork raises the level of serum PSA. Our results suggest that shiftwork may not have an effect on serum PSA levels.

The epidemiological evidence examining the relationship between night shiftwork and breast cancer is now appreciable. Furthermore, possible association between night shiftwork and prostate cancer was also examined. According to the conclusions of a recently published review article, the evidence for prostate cancer is strong, but still insufficient to allow any firm conclusions to be made [11]. There is still debate, but several studies have shown a significant relationship between prostate cancer and shiftwork [67912131415]. The mechanisms are also unclear, but there are the following hypotheses. According to the potential causality pathway, artificial light at night, reaching the human retina, suppresses nocturnal production of melatonin, an anti-oncogenic agent, resulting in the increased risk of cancer development [16]. Disruption of circadian rhythm is responsible for the loss of regulatory function of cells. Vitamin D deficiency, a decline in immunity due to sleep deprivation, and harmful lifestyle factors (such as poor-quality diets, altered mealtimes, less physical activity, and higher BMI) are suggested as causes [1718]. Sleep deprivation suppresses natural killer-cell activity and changes the T-helper 1/T-helper 2 cytokine balance, reducing cellular immune defense and surveillance. Sleep in the modulation of immunity and demonstrate that even a modest disturbance of sleep produces a reduction of natural immune responses and T cell cytokine production [192021].

Unlike previous studies, we show that PSA was lower in the night shiftwork group. Although we could not determine the exact cause, there are possibilities as to why. First, our results suggest that there may be no association between night shiftwork and prostate cancer, or PSA increasing may not be involved in the mechanism in which night shiftwork causes prostate cancer. Second, the effect of including strong healthy workers could reverse the association between shiftwork and serum PSA level. Those who are vulnerable to health hazards are likely to quit night shiftwork and move to day work. And tire manufacturing is a physically burdensome job. So only physically healthy workers are likely to remain in the production workplace, especially night shiftwork. The average work experience of the subjects in this study exceeded 20 years. It is possible that the surviving workers have been adapted to long-term regular night duty schedules. Shift workers' sleep-wake disturbances can be minimized through ergonomic proper scheduling [22]. Third, occupational and recreational physical activity may help reducing prostate diseases. Physical activity may have an inverse association with prostate cancer and benign prostatic hyperplasia risk [2324]. In this study, PSA levels were also significantly lower in the regular exercise group than in the group that did not exercise regularly. There may have been differences in occupational physical activity between day and shift workers. In the future study, objective evaluation of physical activity may be necessary. Fourth, another reason is the possibility of publication bias. Publication bias is defined as the tendency on the parts of investigators, reviewers, and editors to submit or accept manuscripts for publication based on the direction or strength of the study findings. Publishing only results that show a significant finding disturbs the balance of findings and inserts bias in favor of positive results [25]. It is possible that insignificant studies, such as this study, have not been published. More research results should be accumulated to elucidate the impact of night shiftwork on the prostate.

In this study, PSA levels were significantly low in the younger age group, the obese group, and regular exercise group. These results are consistent with previous studies. As age increases, serum PSA level increases. For every unit increase in age, PSA increased by 0.034 ng/mL [26]. Previous studies have shown an inverse relationship between BMI and PSA level. Lower levels of PSA in obese and overweight men could potentially mask prostate cancer. Evidence suggests that prostate cancer screening by serum PSA may be adversely affected by increased BMI. Higher BMI was associated with higher plasma volume, and hemodilution may therefore be responsible for the lower serum PSA concentrations. High BMI has been directly associated with risk of aggressive or fatal prostate cancer [27282930]. Individuals who engage in more sedentary behavior and lower levels of physical activity have higher PSA concentrations [3132].

The limitations of this study are as follows. First, we could not exclude the effect of unknown confounders in this study. PSA is not cancer specific. Serum PSA levels are also increased in various conditions. The tire manufacturing factory uses a variety of chemical hazards including organic solvents and various rubber components, which many workers are exposed to. Previous studies have shown the possibility of chemical exposure and its association with PSA level. Exposure to either dichloro-diphenyl-trichloroethane or dichloro-diphenyl-dichloroethylene may produce a false-negative PSA test when screening for prostate cancer, resulting in an inaccurate clinical diagnosis. And tetrachlorodibenzodioxin exposure might be associated with more aggressive prostate cancer [3334]. Other confounding variables include the effect of ejaculation or constipation on the serum total PSA, which we did not examine [3536]. Future studies will need to consider detailed occupational and personal information. Owing to the limitations of cross-sectional studies, it was not possible to determine if there was a precise causal relationship. In this study, only about 1% (35 out of 3,195) of the total were those with PSA levels of 4.0 ng/mL or higher. It may not be suitable for performing multiple logistic regression models. Therefore, we attempted various analyses, such as Poisson regression analysis, but could not produce other significant results. And, this study focused exclusively on a specific occupation working in a factory and may not generalize to the entire population. Another limitation is the fact that only the total PSA is measured, and the free PSA is not measured. Use of the percentage of free PSA show better improves the specificity in predicting prostate cancer compared with using total PSA alone [37]. Individual differences in internal time (chronotype) is an important factor in shiftwork research. Chronotype plays a key role in a worker's ability to adjust to night shiftwork [3839]. However, there is a limitation in this study that we did not consider it.

CONCLUSIONS

We compared the serum PSA levels of night shift workers and day workers employed in a large manufacturing factory. PSA level was lower in night shift worker compared to the day worker, but it was not statistically significant. Our results suggest that night shiftwork may not influence serum PSA levels. Additional research and consistent results are needed to elucidate the association between night shiftwork and prostate cancer.