Benign prostatic hyperplasia (BPH) is a highly prevalent disease of older men caused by nonmalignant, unregulated growth of the prostate gland,1 and it is a major cause of lower urinary tract symptoms (LUTS).

Despite an extensive research effort, the underlying aetiology of BPH/LUTS has still not been established. Recently, metabolic syndrome (MetS) has been thought to play an important role in the aetiologies of LUTS/BPH. In contrast with results from the United States and from European countries, no results from Asian populations have shown a positive association between MetS and LUTS/BPH.2 However, studies from Asia have had the following limitations: First, a limited number of men aged 50 years old and older, usually the clinically important age for developing LUTS/BPH, have been enrolled. Gao et al3 collected data from 3,103 men, but the median age for their study population was in the 30s. Additionally, other studies from South Korea and Japan have enrolled fewer than 600 men (>50 years old).4-6 Second, most of these studies have used only self-reporting of the International Prostate Symptom Score (IPSS) questionnaire in evaluating LUTS/BPH.3-5 This practise introduces a potential for response bias, as the respondents might inaccurately report their urinary symptoms. Additionally, some patients might not fully understand the meanings of the questions posed. Third, serum testosterone levels were not evaluated in any of the data from Asian populations.3-6 This omission might be a limitation, considering that many studies have established the potential relationship between testosterone levels and MetS, and recent studies have indicated inverse associations between testosterone and LUTS.7

To control for these factors in the present study, we administered the IPSS questionnaire, transrectal ultrasonography, and uroflowmetry, and we tested for residual urine volume and serum testosterone, coupled with a full metabolic work-up, in a large middle-aged population (1,224 men in their 50s). Using the data collected, we then investigated the relationship between MetS and LUTS/BPH.

To our knowledge, these are the first data assessing LUTS/BPH using uroflowmetry among studies evaluating the associations between BPH/LUTS and MetS. Most studies have assessed LUTS/BPH using the IPSS only or in combination with transrectal ultrasound. There are no data concerning uroflowmetry or postvoid residual urine volume. Our results indicate that MetS is not clearly associated with BPH/LUTS in Korean men in their 50s.

We defined BPH as an IPSS >7, maximal flow rate <15 ml/s, and prostate gland volume ≥20 ml. This definition is open to dispute. However, this definition has been widely used in investigating the prevalence of BPH.10,11 According to this definition, the rate of BPH was 14.2% in subjects without MetS and 17.2% in subject with MetS. There was no significant difference between the two groups. In addition, LUTS severity, depending on the IPSS questionnaire, was no different between the two groups, nor was the IPSS total score or QoL score. Additionally, the IPSS subscore (obstructive symptoms and irritative symptoms) was not significantly different (data not shown). After adjusting for age and testosterone, MetS was not associated with BPH, IPSS total score, prostate gland volume, maximal flow rate or postvoid residual urine volume. The prostate volume and postvoid residual volume were significantly greater in subjects with MetS. Nevertheless, the differences (1 ml in prostate size and 6 ml in postvoid residual urine volume) were too small to accept them as clinically significant differences.

In a recent review article,2 results from an Asian population did not consistently show a positive association between MetS and LUTS/BPH. The results of the present study correspond with the results of earlier studies. In data from China3 collected from 3,103 men with a median age of 35 years, MetS was not associated with the severity of LUTS according to IPSS (multivariate OR=0.97, 95% CI=0.67~1.39) after adjusting for age, smoking status, and alcohol consumption. Data from 900 men in Japan, with a mean age of 56.7 years in the non-MetS group and 56.4 years in the MetS group who underwent digital rectal examinations of the prostate and who completed the IPSS, did not show a clear association between LUTS and MetS. Data from 538 men in South Korea with a mean age of 48.8 years did not show differences in scores on the IPSS or on an overactive bladder questionnaire short form.4 In another Korean study of 348 men assessing IPSS and transrectal ultrasonography, no significant differences were found in the mean IPSS score (11.1 in MetS vs. 12.3 in non-MetS) or prostate volume (20.7 ml in MetS vs. 19.4 in non-MetS).6 However, these studies had clear limitations, as discussed in introduction. We tried to compensate for the weaknesses of the former studies. Our data are from the largest study sample of men in their 50s to date. Furthermore, we evaluated the prevalence of BPH synthetically, depending on uroflowmetry, transrectal ultrasonography, and IPSS, and we also evaluated associations between BPH/LUTS and MetS after adjusting for confounding factors, including testosterone. In addition, we evaluated single occupational clusters, with the result that both subjects in the MetS and non-MetS groups had similar socioeconomic backgrounds. However, there was no clear association between LUTS/BPH and MetS.

We speculate that there were several reasons that a positive association between MetS and LUTS/BPH was not found in the present study or formerly among Asian data, unlike non-Asian data.

The reason for this difference might be population characteristics. We collected data from policemen through prostate health examinations. In general, policemen exercise more regularly than the general population. A recent meta-analysis showed that moderate to heavy physical activity was associated with a decreased risk of BPH/LUTS. The OR for moderate activity was 0.74 (moderate activity: 95% CI=0.60~0.92, p=0.005) and for heavy activity was 0.74 (95% CI=0.59~0.92, p=0.006).12 We speculate that the protective effect of physical activity could counterbalance the negative effects of MetS for BPH/LUTS.

A second explanation for our findings is eating habits. In general, Asians eat vegetables frequently compared to people in Western countries. Such eating habits would reduce the effects of MetS on LUTS/BPH. A cross-sectional study including 2,397 men older than 60 years of age showed that men in the top four quintiles for vitamin E, lycopene, and selenium had a non-statistically significant 25% to 50% reduced odds of LUTS, compared with men in the bottom quintile. Similarly, a case-control study including 1,369 cases and 1,451 controls reported that the risk of BPH significantly decreased with increasing intake of carotene (OR=0.80), alpha-carotene (OR=0.83), beta-carotene (OR=0.82), and cis-beta-carotene (OR=0.82).13

Several limitations of the present study warrant mentioning. First, the cross-sectional nature of the data set rendered causal inferences problematic. Additionally, because this study was performed at a single institution and was conducted in a single occupational cluster, there might be a potential for selection bias. However, despite these limitations, we believe that the results of this study are highly relevant because it is the first study to use uroflowmetry, transrectal ultrasound, and IPSS simultaneously, and because it is the largest study to date recruiting men in their 50s among studies evaluating BPH/LUTS and MetS.

In conclusion, the present results have shown that LUTS/BPH is not clearly associated with MetS in Korean men in their 50s. Future investigations should explore the temporal relationship between MetS and LUTS in Asian men. The confirmation of these associations in a prospective, longitudinal data set would be informative.