Calcium intake increases risk of prostate cancer among Singapore Chinese

Abstract

Consumption of dairy products, the primary source of calcium in Western diets, has been found to be positively associated with prostate cancer. In an Asian diet, non-dairy foods are the major contributors of calcium. Thus, a study of dietary calcium and prostate cancer in Asians can better inform on whether calcium, as opposed to other dairy components is responsible for the dairy foods-prostate cancer association. We examined calcium intake and prostate cancer risk among 27,293 men of the Singapore Chinese Health Study that was established between 1993 and 1998. As of December 31, 2007, 298 incident prostate cancer cases had been diagnosed among the cohort members. Diet was assessed at baseline with a validated 165-item food frequency questionnaire. It is hypothesized that there is greater net absorption of calcium in smaller individuals. Therefore, the calcium-prostate cancer association was also assessed in stratified analyses by median body mass index (BMI). Vegetables were the largest contributor of daily calcium intake in the study population. Overall, we observed a modest, statistically nonsignificant 25% increase in prostate cancer risk for the 4^th (median = 659 mg/day) versus 1^st (median=211 mg/day) quartiles of calcium intake after adjustment for potential confounders. The association became considerably stronger and achieved statistical significance (hazard ratio=2.03; 95% confidence interval: 1.23, 3.34; P for trend=0.01) for men with below median (22.9 kg/m²) BMI. Dietary calcium may be a risk factor for prostate cancer even at relatively low intake.

Introduction

Dairy product consumption has consistently been found to increase risk of prostate cancer in prospective epidemiologic studies (1–3). Dairy products are the major food source of calcium in Western populations, where nearly all of the previous studies were conducted. While the underlying mechanism is unknown, there are several nutrients that may contribute to the observed association between dairy products and prostate cancer, such as calcium, saturated fatty acids, protein, and phosphorus. The focus of this paper is on the role of calcium on prostate cancer risk.

There is biologic plausibility for a role for calcium in prostate carcinogenesis. Intracellular calcium pools have been shown to control prostate cancer cell growth and susceptibility to apoptosis (4). Although tightly maintained, small alterations in calcium homeostasis can result in increased proliferation, differentiation and apoptosis in prostate cancer cells (5, 6). Calcium homeostasis is maintained in part by the calcium-sensing receptor (7), a G protein-coupled cell-surface glycoprotein that is expressed in PC-3 cells (8) and may play a role in malignant progression of prostate tumors (9).

In addition to the experimental evidence, prospective epidemiologic studies of calcium and prostate cancer also support a positive association (1, 2), particularly for high levels of intake (>2000 mg/day) (10). High levels of calcium intake are typical in Western populations, where dairy product intake is high (≈14% consume ≥3 servings/day) (11) and calcium supplementation is common (≈48%) (12). The strong correlation between dairy and calcium intake in previously studied populations made it difficult to evaluate the effects of calcium, independent of residual confounding from dairy intake or supplement use, or sociodemographic factors related to dairy intake and/or supplement use (13).

In Asian diets dairy intake is rarely consumed and thus a minor source of dietary calcium. Instead, the major sources of dietary calcium in Asian diets are from soy foods and green vegetables. Compared to dairy products, tofu and low-oxalate vegetables, such as kale, broccoli, and bok choy provide similar volumetric amounts of bioavailable calcium (14). Utilizing the database of a prospective cohort of Singapore Chinese in whom dairy intake is low and calcium supplementation is rare, we will be able to assess the association between dietary calcium and prostate cancer with less likelihood of residual confounding by dairy intake and supplement use than previous studies. The present study population also allowed us to examine the effect of calcium from non-dairy sources, and thus avoid the potential confounding effect of other components in the dairy products on the calcium-prostate cancer risk association.

Materials and Methods

Study Population

The design of the Singapore Chinese Health Study has been previously described in detail (15). Briefly, the cohort consisted of 27,959 men and 35,298 women recruited between April 1993 and December 1998, from permanent residents or citizens of Singapore aged 45 to 74 years, and who resided in government-built housing estates (86% of the Singapore population resided in such facilities). We restricted the study to individuals belonging to the two major dialect groups of Chinese in Singapore: the Hokkiens and the Cantonese. For these analyses, we used data from the 27,293 men who did not have a history of cancer diagnosis at baseline, based on self-report and computer-assisted record linkage analysis with the Singapore Cancer Registry. The Institutional Review Boards at the National University of Singapore and the University of Minnesota have approved this study.

Identification of incident prostate cancer cases and deaths among cohort members were accomplished by record linkage of the cohort database with respective databases from the population-based Singapore Cancer Registry and the Singapore Registry of Births and Deaths. The nationwide cancer registry has been in place since 1968 and has been shown to be comprehensive in its recording of cancer cases (16). As of April 2008, only 27 cases were known to be lost to follow-up due to migration out of Singapore. As of December 31, 2007 (an average of 11.0 years of follow-up), 298 cohort participants developed invasive prostate cancer. Staging was determined through comprehensive chart reviews by a single urology oncologist (ASW). Cases that had radical prostatectomy were staged according to pathologic criteria, as defined by The American Joint Committee on Cancer (17). The remaining cases were staged according to clinical criteria by D’Amico risk group categories (18). Early disease included cases with pathologically staged organ-confined disease (T1–T2), or clinically staged organ-confined disease and with either D’Amico good or intermediate risk. Locally advanced disease included cases with pathologically staged extra-capsular (T3–T4) or nodal involvement (N1), or clinically staged extra-prostatic extension or regional nodal disease, or clinically organ-confined disease and with D’Amico high risk. Metastatic disease was defined as those with evidence of distant metastases, based on clinical information. In summary, 124 (41.6%) cases had early disease, 99 (33.2%) had locally advanced disease, and 63 (21.1%) had metastatic disease at diagnosis. The remaining 12 (4.0%) cases did not have sufficient information for the determination of disease stage.

Exposure Assessment

Enrollment in the cohort entailed completing a baseline in-person interview in the participant’s home. The questionnaire elicited information on smoking, diet, demographics, current physical activity, occupational exposure and medical history. We used a 165-item quantitative food frequency questionnaire (FFQ), developed for and validated in this population, to assess usual diet over the past year (19). Average daily intake of roughly 100 nutrient and non-nutrient compounds, including calcium, was computed for each study subject based on the Singapore Food Composition Database (19).

Our calculation of the food sources as percentages of dietary calcium in the study population was based on a series of 24-hour dietary recall interviews. Specifically, as part of the FFQ validation study, 24-hour diet recall data were collected from 1,022 (425 men and 597 women) randomly chosen cohort subjects (19). The percent contribution of calcium from a specified food item or food group was calculated as the ratio of A_i / B, where A_i = dietary calcium from a single food (i) summed across all subjects who consumed that food, and B = total dietary calcium summed across the 1,022 subjects.

Statistical Methods

Person-years of follow-up were counted from the date of recruitment to the date of diagnosis of prostate cancer, death, migration, or December 31, 2007, whichever occurred first. Proportional hazards regression methods were used to examine the associations between calcium intake in quartile level (per 1000kcal) and prostate cancer risk, measured by hazard ratios (HRs) and their corresponding 95% confidence intervals (CIs) (20). We considered whether calcium intake was associated with local and advanced disease (locally advanced and metastatic disease combined) separately, to infer whether calcium was related to earlier versus later stages of prostate carcinogenesis.

The linear trend tests for calcium–prostate cancer associations were based on ordinal values of the quartile levels of calcium intake (0, 1, 2 and 3). In all analyses, we adjusted for the following potential confounders: age at baseline interview (years), year of interview (1993–1995, 1996–1998), dialect group (Cantonese, Hokkien), level of education (no formal education/primary school, secondary school/or higher), and any weekly supplement use (yes, no), based on a 10% or greater change in the beta estimate for fourth versus first quartile of total calcium intake and prostate cancer risk. Additional inclusion of the following variables did not materially change any of the study results: body mass index (BMI), weekly physical activity, alcohol consumption, green tea intake, family history of prostate cancer, and dietary intake of total fat, lycopene, phosphorus, vitamin D, and total soy isoflavones.

Based on previous analyses of our data and reports from the literature, we examined whether the association between calcium and prostate cancer risk varied by the following factors: age at baseline (below or above median), weekly physical activity (yes/no), dietary vitamin D (below or above median), and phosphorus (below or above median), by assessing the fitness of interaction terms in adjusted models. We also evaluated whether the calcium-prostate cancer risk association varied by BMI level (below or above median, 22.9 kg/m²) because it is hypothesized that there is greater net absorption of calcium in thinner individuals (21, 22). Statistical computing was conducted using SAS version 9.1 (SAS Institute Inc., Cary, NC, USA). All P-values were two-sided and considered statistically significant if less than 0.05.

Results

Increasing quartile of calcium intake was associated with less education, more frequent physical activity, higher frequency of green tea intake and any supplement use (Table 1). Less than 5% of the cohort (N=1,311) reported any weekly supplement use and 2.1% of the cohort reported use of calcium supplements on a weekly basis. Although there was no difference by median age, supplement users reported somewhat more frequent physical activity and intake of tea, regardless of type, as well as calcium (from food only) (data not shown). In our cohort, consumption of dairy products was low, with a median daily intake of 19.3 g [interquartile range (IQR) = 51.6 g]. Although milk was the most commonly consumed dairy product, only 24% of the cohort reported drinking a glass of milk (250 ml) at least once a week. The major food sources of calcium (percent contribution to total calcium) in our study population were vegetables (19.3%), dairy (17.3%), grain products (14.7%), soyfoods (11.8%), fruit (7.3%) and fish (6.2%) (Table 2). Intake from any of these food groups were not individually associated with prostate cancer risk, regardless of whether calcium intake was in the model (data not shown).

Table 1.

Distribution of study population characteristics by quartiles of total calcium intake

	Calcium intake by quartiles*
	<266 mg/day	266–371 mg/day	372–521 mg/day	≥523 mg/day
N (%)	6800	6857	6829	6807
Median age (IQR)†, year	58 (13)	56 (13)	55 (13)	56 (13)
% with none or primary education only	72.2	64.6	58.6	52.4
% any weekly physical activity	33.5	41.9	46.3	51.5
Median BMI (IQR), kg/m2	22.9 (3.3)	22.9 (3.5)	22.9 (3.7)	23.0 (3.7)
% at least weekly green tea	25.9	33.7	38.2	40.8
% weekly any supplement use‡	1.3	3.0	5.0	10.0
% weekly calcium supplement use	0.2	0.6	1.9	5.8
Median (IQR)† daily energy intake (kcal)	1213 (451)	1552 (505)	1834 (616)	2196 (866)
Median (IQR)† daily intake§
Calcium from food only (mg)	211 (66.6)	317 (52.1)	433 (73.7)	651 (205)
Calcium from food+supplements (mg)	211 (66.5)	317 (51.8)	435 (72.7)	659 (211)
Saturated fat (g)	9.7 (5.2)	14.2 (6.5)	18.0 (8.5)	23.0 (13.0)
Vitamin D (IU)	46.6 (39.4)	75.5 (48.4)	109 (59.9)	175 (90.1)
Lycopene (µg)	416 (702)	783 (1042)	995 (1221)	1320 (1994)
Red meat (g)	21.1 (19.5)	29.2 (24.6)	35.0 (31.0)	40.2 (40.8)
Total vegetables (g)	64.0 (40.3)	92.1 (50.4)	116 (67.4)	144 (99.6)
Dairy products (g)	3.4 (13.8)	15.4 (27.1)	32.1 (44.5)	134 (226)
Soyfoods (g)	47.3 (44.4)	83.4 (65.5)	117 (93.6)	165 (160)
Total soy isoflavones (mg)	7.4 (7.5)	13.1 (11.2)	19.0 (15.5)	26.7 (27.3)

^*Calcium intake from combined food and supplement sources.

^†Interquartile range, IQR

^‡Supplement use was defined by Yes/No responses to the question, “Did you take any vitamins or minerals at least once each week during the last year?”

^§Intake was based on food sources only, unless otherwise noted.

Table 2.

Daily intake, calcium concentration, and percent contribution to total calcium for selected food groups

Food item/group	Median daily intake (g)*	Average mg calcium per g food group*	Percent contribution of each food group to total calcium intake†
Total vegetables	98.2	3.7	19.3
Green vegetables	58.8	6.2	16.3
Cruciferous vegetables	37.6	9.7	9.3
Dairy products	19.3	16.8	17.3
All grain products	555.7	0.6	14.7
Noodles	47.9	7.8	4.7
Rice	441.4	0.8	3.0
All soyfoods	91.0	4.0	11.8
Total fruit	174.1	2.2	7.3
Citrus fruit	39.1	9.9	4.4
All fish and shellfish	54.2	7.0	6.2

^*These values were derived from intake among all male cohort subjects.

^†The percent contribution to total calcium intake was calculated using 24-hour dietary recalls among a randomly selected subset of cohort subjects (see Methods).

Table 3 shows the association between calcium intake and prostate cancer risk in total subjects and among non-supplement users. There was a modest, statistically nonsignificant positive association between calcium intake, whether based on food sources only or food and supplement sources combined, and prostate cancer risk. Risk was elevated roughly 20% for subjects in the highest versus the lowest quartiles of intake, regardless of supplement use (Table 3). The association was not markedly different for local disease (HR=1.43; 95% CI: 0.81, 2.52; P for trend=0.13) or for advanced disease (HR=1.18; 95% CI: 0.75, 1.87; P for trend=0.6), comparing highest versus lowest quartiles of total calcium intake among all subjects.

Table 3.

Hazard ratios (HR) and 95% confidence intervals (CI) for quartiles (Q) of calcium intake and prostate cancer risk

	Median intake (mg/day)	Total cases (N)	Adjusted HR*	95% CI
Total calcium: food + supplement
Q1	211	60	1.0	---
Q3	317	75	1.20	0.85, 1.69
Q3	435	71	1.10	0.78, 1.55
Q4	659	92	1.25	0.89, 1.74
P for trend			0.3
Dietary calcium: food only
Q1	210	59	1.0	---
Q3	315	78	1.27	0.90, 1.78
Q3	432	75	1.18	0.84, 1.67
Q4	651	86	1.23	0.88, 1.72
P for trend			0.4
Among non-supplement users
Dietary calcium: food only
Q1	210	56	1.0	---
Q3	315	71	1.23	0.88, 1.78
Q3	431	66	1.15	0.80, 1.64
Q4	648	76	1.28	0.90, 1.81
P for trend			0.3

^*Adjusted models included variables for: age at interview (year), dialect group (Cantonese, Hokkien), interview year (1993–1995, 1996–1998), education (no formal education/primary school, secondary school/or higher), and any weekly supplement use (yes/no).

We further evaluated the calcium-prostate cancer association stratified by the median value of BMI (22.9 kg/m²) among all men in our cohort. There was a statistically significant, dose-dependent positive association between total calcium intake and prostate cancer risk in men with below median BMI (Table 4). Although the association for highest versus lowest calcium intake seemed to be stronger for local (HR=2.42; 95% CI: 1.11, 5.28; P for trend=0.02) than advanced disease (HR=1.70; 95% CI: 0.86, 3.35; P for trend=0.2), the two sets of hazard ratios were not statistically different from each other (P=0.4). No association with calcium intake was noted among men with greater than median BMI (P for interaction=0.02) (Table 4). Table 5 shows the association between total calcium intake and prostate cancer risk among men with below median BMI, stratified by duration of follow-up. Although the association appeared stronger among subjects with a longer duration of follow-up, the two sets of hazard ratios were not statistically different from each other (P=0.7).

Table 4.

Hazard ratios (HR) and 95% confidence intervals (CI) for quartiles (Q) of total daily calcium intake and prostate cancer risk by median body mass index (BMI)

Total calcium intake	BMI <median, 22.9kg/m2		BMI ≥ median, 22.9kg/m2
	Total cases (N)	HR (95% CI)*	Total cases (N)	HR (95% CI)*
All cases
Q1 (lowest)	23	1.0 (ref)	37	1.0 (ref)
Q2	37	1.58 (0.94, 2.67)	38	0.95 (0.60, 1.50)
Q3	34	1.44 (0.85, 2.44)	37	0.87 (0.55, 1.38)
Q4 (highest)	56	2.03 (1.23, 3.34)	36	0.77 (0.48, 1.23)
P for trend		0.01		0.2

^*Models included variables for: age at interview (year), dialect group (Cantonese, Hokkien), interview year (1993–1995, 1996–1998), education (no formal education/primary school, secondary school/higher), and any weekly supplement use (yes/no).

Table 5.

Hazard ratios (HR) and 95% confidence intervals (CI) for quartiles (Q) of daily total calcium intake and prostate cancer risk by duration of follow-up, among men with <median body mass index, 22.9kg/m²

Total calcium intake	<10 yrs		≥10 yrs
	Cases, N N=101	HR (95% CI)*	Cases, N N=49	HR (95% CI)*
Q1 (lowest)	17	1.0 (ref)	6	1.0 (ref)
Q2	24	1.41 (0.76, 2.64)	13	2.07 (0.79, 5.46)
Q3	22	1.27 (0.67, 2.39)	12	1.95 (0.73, 5.20)
Q4 (highest)	38	1.70 (0.94, 3.08)†	18	3.03 (1.19, 7.68)†
P for trend		0.1		0.03

^*Models included variables for: age at interview (year), dialect group (Cantonese, Hokkien), interview year (1993–1995, 1996–1998), education (no formal education/primary school, secondary school/or higher), and any weekly supplement use (yes/no).

^†P for difference in relative risk for less than 10 years versus 10 or more years of follow-up among subjects with <median BMI = 0.7.

There was no evidence of effect modification by age or physical activity (data not shown). Vitamin D and phosphorus are determinants of calcium absorptive efficiency (23, 24). Among men with greater than median (92.9 IU/day) vitamin D, the point estimate for highest versus lowest calcium was stronger (HR=1.85; 95% CI: 0.92, 3.73; P for trend = 0.5) than their counterparts with less than median vitamin D intake (HR=1.21; 95% CI: 0.71, 2.04; P for trend = 0.5). There was no difference in the association between calcium and prostate cancer by median phosphorus intake (863 mg/day) (data not shown).

Discussion

We report a 25% increase in prostate cancer risk comparing 4^th (median = 659 mg/day) versus 1^st (median=211 mg/day) quartiles of total calcium intake, using data from a prospective cohort of Singapore Chinese. Among men with less than median BMI, and thus thought to have greater net calcium absorption, we observed a statistically significant, dose-dependent positive association that yielded a 2-fold increased risk between subjects in the highest versus the lowest quartiles of intake. Thus, our results support the notion that calcium plays a risk enhancing role in prostate cancer development.

Our overall finding supports the modest positive association reported from a pooled analysis of prospective epidemiologic studies for calcium intake and prostate cancer risk (1). With one exception (25), all previous prospective studies were conducted among Western populations with diets relatively high in calcium and from primarily dairy food sources. The calcium intake levels in the referent group of all previous prospective studies were higher than among Singapore Chinese (e.g. <266 mg/day). Thus, our study is the first to report a positive association between calcium and prostate cancer risk at such a low calcium level.

Ecologic analyses show a strong correlation between milk intake, the major contributor of calcium intake in Western populations, and prostate cancer incidence (Figure 1) (26), yet several US prospective cohorts recently reported no association between calcium intake and prostate cancer risk (27–31). In addition to the methodologic reasons for discrepant results between ecologic and analytic analyses, it is also possible that a non-linear relationship exists between calcium intake and prostate cancer risk, where a very wide range of intake would be needed at the higher end of intake to be able to observe even a small difference in prostate cancer risk. A similar hypothesis has been used to explain the discrepancy between ecologic and analytic analyses for dietary fat and breast cancer risk (32). Our results suggest that evaluating calcium and prostate cancer risk may be more informative among a population with relatively low calcium intake, given the notion of diminishing returns at higher intake.

Figure 1.

Prostate cancer incidence rates versus per capita milk intake among 38 countries (Adapted from: Zhang and Kesteloot, Nutrition and Cancer 2005;53:65–72)

Calcium intake accounts for only about 26% of interindividual variation in calcium absorption (33), so even at relatively high intake, achieving an internal minimum dose necessary to observe an adverse effect may not be possible without efficient calcium absorption. There is evidence for greater calcium absorption efficiency among Chinese (34, 35), compared to whites (36), and among thinner, compared to heavier individuals (21, 22). Our Singapore Chinese men are relatively thin, especially compared to Western populations. For example, the prevalence of overweight or obese (BMI>25.0 kg/m²) among US men 40 to 59 years of age was 78% (37), compared to 22% among men in the Singapore Chinese cohort.

A strong association and dose-dependent trend was revealed for calcium intake and prostate cancer risk only after we restricted our analyses to men with below median BMI (22.9 kg/m²). While individuals with larger body size, and thus larger gut mass (or intestinal surface area) (38), have greater intestinal absorption of exogenous (e.g. dietary) calcium (39), they also have greater endogenous loss of calcium from the proximal and distal intestine, compared to individuals with smaller body size (21, 22). This hypothesized inverse correlation between body size and calcium absorption efficiency infers that studies conducted in Singapore Chinese may be more likely to reveal the underlying association between calcium and prostate cancer risk, if it exists, than studies conducted among their heavier Western counterparts.

Calcium may play a role in prostate carcinogenesis through its effects on the insulin-like growth factor (IGF) system (40). IGF-1 has mitogenic and anti-apoptotic effects on normal and transformed prostate epithelial cells (41). A recent meta-analysis reported a 20% increase in prostate cancer risk per standard deviation increase in IGF-1 peptide levels (42).

In a subset of the Singapore Chinese cohort (N=312 men, 326 women), we previously evaluated determinants of serum IGF-1 and reported statistically significant positive associations with dietary calcium (β = 0.044, P = 0.007) (43). While prior studies reported similar associations (40, 44), it was not clear whether the associations were due to the strong correlation with dairy products (45), or whether calcium alone was associated with increased IGF-1 levels. Our finding for calcium and IGF-I was not due to correlations with dairy, because dairy intake was low and vegetables were the largest contributor of calcium intake among Singapore Chinese.

There is experimental evidence that an important player in the malignant progression of prostate tumors are factors contributing to the maintenance of calcium homeostasis, or the balanced release of parathyroid hormone (PTH) and 1,25-dihydroxyvitamin D (1,25OH₂D) to maintain extracellular calcium within a tight physiologic range (5, 6, 9). The calcium-sensing receptor (CaR) is also responsible for maintaining calcium homeostasis by sensing extracellular calcium concentrations and by mediating alterations in PTH secretion and renal calcium reabsorption (46, 47). CaR expression in prostate cancer cells was correlated with the proliferative effect of elevated extracellular calcium and the cellular metastatic behavior, suggesting that CaR is a mediator of extracellular calcium-initiated effects on prostate cancer cells (48).

Results from prospective epidemiologic studies are inconclusive on the question of whether stage of disease has an influence on the strength of the calcium-prostate cancer association (28, 49). There is no evidence from the present study that the observed calcium-prostate cancer association varied by stage of disease. However, it should also be noted that our modest study sample size does not allow for a meaningful analysis of this modification effect. In addition, there were likely to be few indolent cases among our study population given that population-wide prostate cancer screening is not recommended in Singapore (50).

The prospective nature of our study design reduced the opportunity for differential misclassification of self-reported diet to bias our findings. The major food sources, or those that contributed over 10% of total daily calcium in our study population were diverse, and included vegetables, dairy, grain products, and soyfoods. Thus, our observed positive association between calcium and prostate cancer risk is unlikely to be driven by any one major food group.

In summary, we reported a positive association with prostate cancer risk for 659 mg/day median calcium intake among a prospective cohort of Singapore Chinese, compared to 211 mg/day median intake. When we restricted analyses to men with below median BMI, we observed a dose-dependent trend with increasing calcium intake and prostate cancer risk that reached a 2-fold increase, comparing highest to lowest intake. These data suggest a positive association with calcium at relatively low levels and from primarily non-dairy food sources. In conclusion, our findings warrant further experimental exploration into the possible roles of calcium, as opposed to other dairy product components, in prostate carcinogenesis.