693 Am J Epidemiol 2004;159:693–701

American Journal of Epidemiology

Copyright © 2004 by the Johns Hopkins Bloomberg School of Public Health

All rights reserved

Vol. 159, No. 7

Printed in U.S.A.

DOI: 10.1093/aje/kwh081

Pancreatic Cancer and Drinking Water and Dietary Sources of Nitrate and Nitrite

Angela Coss1, Kenneth P. Cantor2, John S. Reif1, Charles F. Lynch3, and Mary H. Ward2

1 Department of Environmental and Radiological Health Sciences, Colorado State University, Fort Collins, CO. 

2 Occupational and Environmental Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer 

Institute, National Institutes of Health, Department of Health and Human Services, Bethesda, MD. 

3 Department of Epidemiology, University of Iowa College of Public Health, Iowa City, IA.

Received for publication April 24, 2003; accepted for publication October 24, 2003.

N-Nitroso compounds, known animal carcinogens, are formed endogenously from drinking water and dietary

sources of nitrate and nitrite. The authors conducted a population-based case-control study of pancreatic cancer

in Iowa to determine whether increased consumption of nitrate and nitrite from drinking water and dietary sources

was associated with risk. They linked detailed water source histories to nitrate measurements for Iowa community

water supplies. After exclusions for insufficient data, 1,244 controls and 189 pancreatic cancer cases were

available for analysis. Among controls, the median average nitrate level (1960–1987) was 1.27 (interquartile

range, 0.6–2.8) mg of nitrate nitrogen per liter of water. No association was observed between pancreatic cancer

risk and increasing quartiles of the community water supplies’ nitrate level. Increasing intake of dietary nitrite from

animal sources was associated with an elevated risk of pancreatic cancer among men and women (highest

quartile odds ratios = 2.3, 95% confidence interval: 1.1, 5.1, for men and 3.2, 95% confidence interval: 1.6, 6.4,

for women). In contrast, dietary nitrate intake showed an inverse association with risk among women and no

association among men. This study suggests that long-term exposure to drinking water nitrate at levels below the

maximum contaminant level of nitrate nitrogen (10 mg/liter) is not associated with pancreatic cancer; however,

the consumption of dietary nitrite from animal products may increase risk.

diet; nitrates; nitrites; pancreatic neoplasms; water; water pollution 

Abbreviations: CI, confidence interval; OR, odds ratio.

Pancreatic cancer is the fourth leading cause of cancer

death among both men and women in the United States (1).

Smoking is the only established risk factor. Chronic pancreatitis and a family history of pancreatic cancer are predisposing factors but both are rare. High consumption of meat,

animal protein, and fat has been associated with increased

risk, whereas fruits and vegetables have often been linked to

a reduced risk (2). Despite continuing efforts, the etiology of

pancreatic cancer is poorly understood (2–5).

Animal studies provide strong evidence for the carcinogenicity of N-nitroso compounds. N-Nitroso compounds have

been shown to cause tumors in every animal species tested

and to induce cancer in many different organs including the

pancreas (6, 7). Specific N-nitroso compounds cause pancreatic cancer in hamsters, which have been used extensively as

animal models for evaluating potential pancreatic carcinogens because of the morphologic and clinical similarities

with pancreatic tumors in humans (7).

Although the evidence is more equivocal for humans,

several N-nitroso compounds are classified as reasonably

anticipated to be human carcinogens by the National Toxicology Program (8) and are classified as possible or probable

human carcinogens by the International Agency for

Research on Cancer (9). Several of these N-nitroso

compounds can be formed endogenously (in vivo) from

dietary amine precursors and nitrite (9). N-Nitroso

compound formation in healthy individuals occurs primarily

in the stomach through reaction of nitrite with amine and

amide precursors. Nitrite is predominantly derived from

ingested nitrate, although preserved meats and some other

foods can serve as direct sources. Vegetables are the major

source of nitrate exposure when drinking water levels are

Correspondence to Dr. Mary H. Ward, Occupational and Environmental Epidemiology Branch, Division of Cancer Epidemiology and Genetics, 

National Cancer Institute, 6120 Executive Boulevard, EPS 8104, Bethesda, MD 20892-7240 (e-mail: wardm@mail.nih.gov).
694 Coss et al.

Am J Epidemiol 2004;159:693–701

low; however, drinking water contributes substantially to

exposure when nitrate levels are near or exceed the US Environmental Protection Agency’s maximum contaminant level

of 10 mg of nitrate nitrogen (NO3-N) per liter of water. An

increased endogenous formation of N-nitroso compounds

was demonstrated among Nebraskan men drinking water

from private wells with increasing levels of nitrate (10).

Nitrate in drinking water has long been considered a health

threat for its ability to induce methemoglobinemia, and this

health outcome is the basis of the Environmental Protection

Agency’s maximum contaminant level (11). Groundwater

underlying agricultural areas often has elevated nitrate levels

due to agricultural runoff of nitrogen fertilizers. Intake of

nitrate from drinking water and dietary sources may cause

increased exposure to N-nitroso compounds through endogenous nitrosation (10, 12, 13). Vegetables are the primary

dietary source of nitrate; however, they contain vitamin C

and other nitrosation inhibitors (14) and, therefore, high

intakes may not result in high rates of formation of N-nitroso

compounds. We hypothesized that individuals with higher

daily nitrate intake from drinking water and lower intakes of

nitrosation inhibitors may be at elevated risk of pancreatic

cancer.

In spite of the biologic plausibility for a role of drinking

water and dietary sources of nitrate and nitrite in cancer risk,

few epidemiologic studies with historical exposure data have

been conducted. Most of the epidemiologic studies have

focused on gastric cancer (15). Whereas several studies evaluated dietary sources of nitrite and pancreatic cancer risk,

only one study in Iowa (16) evaluated drinking water nitrate

levels.

We conducted a population-based, case-control study of

pancreatic cancer in Iowa. The study was originally designed

to evaluate the possible association between disinfection byproducts and six cancer sites (17, 18). The detailed information on historical water sources, tap water intake, and diet

allowed us to conduct additional analyses to evaluate the

relations between pancreatic cancer and nitrate levels in

drinking water and nitrite intakes from the diet. 

MATERIALS AND METHODS

Study population

The overall study population was described previously

(17, 18). Eligible cases were White residents of Iowa who

were aged 40–85 years, diagnosed with pancreatic cancer

during the years 1985–1987, and without previous diagnosis

of a malignant neoplasm. Microscopic confirmation was an

eligibility criterion for cases in this study. All cases were

confirmed by either positive histology (n = 348) or cytology

(n = 28) as documented by Registry staff through medical

record review. Other races were excluded because of small

numbers. Cases were ascertained through the Iowa Cancer

Registry, a statewide cancer registry that is one of the

National Cancer Institute’s Surveillance Epidemiology and

End Results Program registries.

Controls were frequency matched to cases of all six cancer

types combined by gender, race, and 5-year age groups,

resulting in an approximate 6.5:1 matching ratio for the

pancreatic cancer cases. Controls aged less than 65 years

were selected from computerized state drivers’ license

records, and controls aged 65 years or more were selected

from a randomized listing provided by the US Health Care

Financing Administration, which is now known as the

Centers for Medicare and Medicaid Services. Individuals

with previous cancer diagnoses were excluded.

Interviews

The study was approved by an institutional review board at

the University of Iowa. A letter was sent to study subjects

explaining the study and inviting participation through the

completion of a postal questionnaire. To minimize refusals,

reluctant participants were offered an abbreviated telephone

interview as an alternative to the postal survey. Interviews

were conducted by trained interviewers who were

employees of the University of Iowa. Missing information

for specific questions in the returned postal survey was

retrieved by follow-up telephone contact.

The postal questionnaire contained questions about dietary

habits, smoking history, and a complete residence and

drinking water source history. In the residential history

section, participants were asked to list all towns or cities in

which they lived for 1 year or more from the time of birth, as

well as their primary water source in each location (private

well, community supply, bottled water, or other type of

supply). We also asked about tap water intake and beverages

made with tap water from water sources at home and away

from home. Residential water sources accounted for the

large majority of tap water intake (19). Our data indicated

that drinking water consumed away from the residence was

usually from the same community water supply (i.e., the

same town), so we considered total tap water intake in our

analysis.

Of 429 eligible pancreatic cancer cases, 376 (87.6 percent)

participated by completing the mailed survey (n = 343) or

the abbreviated telephone interview (n = 33). Proxy or selfquestionnaires were mailed on the basis of the information

from the initial telephone contact. Of the 376 respondents,

322 (86 percent) requested proxy questionnaires. Of the 54

direct questionnaires, 31 were completed by the cases themselves and 17 were completed by a proxy; six did not report

the respondent, resulting in 339 (90.2 percent) proxies, 31

(9.1 percent) direct respondents, and six (1.6 percent)

unknown.

Of 999 eligible controls aged less than 65 years, 817 (81.8

percent) participated. Of the 2,034 eligible controls aged 65

years or more, 1,617 (79.5 percent) participated, with 168 (8

percent) by abbreviated telephone interview. There were a

total of 2,064 (84.8 percent) direct respondents and 243 (10.0

percent) proxies, including two sent proxy questionnaires

and 241 sent direct questionnaires who indicated completion

by a proxy, and 127 (5.2 percent) were unknown.

Drinking water nitrate exposure assessment

Details of the exposure assessment for nitrate levels in

community water supplies were published previously (19).

Briefly, an extensive water quality database containing
Nitrate Exposure and Pancreatic Cancer 695

Am J Epidemiol 2004;159:693–701

historical monitoring data for Iowa community water

supplies was available through Iowa’s Center for the Health

Effects of Environmental Contamination. The earliest available nitrate measurements for some communities were made

in 1934. Monitoring data were sparse before the 1970s. In

the 1980s with the full implementation of the Clean Water

Act, nitrate-monitoring data were available for all community water supplies, defined as suppliers serving populations

of 25 or more.

To compute our exposure estimates, we used nitrate

measurement data for finished water samples taken in the

water distribution system after treatment. A finished water

sample should be more representative of the water quality

distributed to households than pretreated water or water

quality data for individual community water supplies’

sources. When more than one finished water nitrate level

was available for a given year, an average was computed.

Towns that purchased water were assigned the water quality

data from the host community water supplies.

We imputed annual nitrate values for years when a town

was missing nitrate data by computing a weighted average of

the annual nitrate means in neighboring years. Weights were

based on the number of years between the missing year and

the year with measurements. The actual measurement could

be before or after the missing year as long as the source did

not change during the period with missing data. The weights

were 1, 0.75, 0.5, and 0.2 for 1–2 years, 3–4 years, 5–6 years,

and 7–9 years, respectively. When there were no data within

10 years, the nitrate level was considered to be unknown.

Yearly nitrate means (actual and imputed data) were

linked to individual residential water source histories by

town and year. Nitrate levels were unknown during periods

when a person lived outside Iowa, resided in a town during a

period with no nitrate data within 10 or more years, or drank

water from a private well. Private wells can have substantially higher nitrate levels than the community water

supplies, but no historical monitoring data were available.

We excluded individuals from our analysis if their drinking

water nitrate levels were unknown for more than 30 percent

of the period after 1960 (187 cases, 1,190 controls). This

decision was based on balancing the completeness of the

exposure data with the available sample size.

The average nitrate exposure level from 1960 onward was

calculated for each case and control. Exposure was calculated to the year of diagnosis for cases and to 1987 (the last

year of case diagnosis) for controls. We also computed the

number of years from 1934 onward that an individual was

exposed to community water supplies with a yearly nitrate

estimate at or above 7.5 and 10 mg of nitrate nitrogen per

liter (the Environmental Protection Agency maximum

contaminant level).

Dietary nitrate and nitrite

Dietary intakes were assessed using a 55-item food

frequency questionnaire. Participants were asked to report

their usual adult intake after excluding any changes to their

diet in the last few years. Dietary intakes of nitrate and nitrite

were computed by multiplying the frequency of intake of

each food item by its nitrate and nitrite concentration and

summing across all foods. As described previously (19),

nitrate and nitrite concentrations were determined from the

literature (20–24). Sex-specific portion sizes were derived

from the Second National Health and Nutrition Examination

Survey (25). Cases and controls with more than five missing

food items were excluded from dietary analyses. Among

those remaining, intakes for missing foods were imputed

using the sex-specific median value among controls.

Statistical analyses

Maximum likelihood estimates of the odds ratios were

calculated using unconditional multiple logistic regression

analysis to estimate the association between the selected

exposure indices and pancreatic cancer. Results were similar

for men and women, so only combined results are presented.

Risk estimates were adjusted for age, gender, and cigarette

use (ever smoked for 6 or more months vs. never). Adjustment for a more detailed smoking variable did not significantly alter results. Therefore, only odds ratios adjusted for

the two-level variables are presented. The dietary odds ratios

were adjusted for total caloric intake by including it as a

continuous variable in the logistic models. For the major

analyses, we evaluated whether excluding proxies who were

not the spouses changed our findings because other respondents may be less knowledgeable about the subject. Trend

tests for categorized exposure variables were performed by

assigning ordinal scores to the categories and testing for a

non-zero slope. Statistical analyses were performed using

SAS version 8.1 software (SAS Institute, Inc., Cary, North

Carolina).

RESULTS

Approximately 50 percent of cases and 49 percent of

controls were excluded because of insufficient drinking

water nitrate data. The primary reason for exclusion was due

to extensive use of a private well as a drinking water source

after 1960. Table 1 shows the characteristics of the cases and

controls included and excluded from the community water

supplies’ nitrate analysis. The median age was similar for

both included and excluded cases and controls. As expected,

cases and controls included in the community water

supplies’ nitrate analysis had used community water

supplies substantially longer than excluded cases and

controls, and private well use was less common. Bottled

water use was uncommon in both groups. Compared with the

overall study population, the community water supplies’

nitrate analysis population included a greater proportion of

smokers and a greater proportion with more years of formal

education. The proportion of cases and controls in the

community water supplies’ analysis with a history of pancreatitis and a family history of cancer was similar to the overall

study population. As was observed in the overall study population, smoking, pancreatitis, and a family history of cancer

were associated with an increased risk of pancreatic cancer.

We divided the study population into quartiles based on

the distribution of the controls’ community water supplies’

average nitrate levels (median, 1.3 (interquartile range, 0.6–

2.8) mg of nitrate nitrogen per liter). We observed no associ-
696 Coss et al.

Am J Epidemiol 2004;159:693–701

ation between pancreatic cancer risk and increasing levels of

nitrate in public drinking water supplies (ptrend = 0.28) (table

2). Lagging the exposure by 10 years did not substantially

change the results (data not shown).

Overall, cases reported a slightly higher total tap water

intake of 2.5 (standard deviation, 1.29) liters/day than did

controls, whose total tap water intake was 2.3 (standard deviation, 1.13) liters/day (p value = 0.07). Total tap water intake

was multiplied by the community water supplies’ average

nitrate level to obtain the average nitrate consumption from

drinking water. No association was found between pancreatic cancer and increasing quartiles of nitrate consumption

TABLE 1. Distribution of selected characteristics among individuals included in the community water supply 

nitrate analysis, Iowa, 1985–1987

* Individuals with 70% or greater of their person-years (after 1960) using a community water supply with a known

nitrate estimate.

† SD, standard deviation.

‡ Numbers do not sum to total because of missing information.

§ Individuals were categorized as former smokers if they quit smoking more than 2 years before.

Included in analysis* Excluded from analysis

Cases (n = 189) Controls (n = 1,244) Cases (n = 187) Controls (n = 1,190)

Mean SD† Mean SD Mean SD Mean SD

Community water supply in Iowa 

(years) 48.7 15.5 48.8 15.2 18.8 21.1 17.4 19.6

Private wells in Iowa (years) 13.5 14.5 13.7 14.2 40.3 26.6 45.0 24.5

Bottled water (years) 0.06 0.84 0.21 2.1 0 0.08 0.72

Cases Controls Cases Controls

No. % No. % No. % No. %

Gender

Male 102 54.0 775 62.3 100 53.5 826 69.4

Female 87 46.0 469 37.7 87 46.5 364 30.6

Age (years)

40–54 11 5.8 120 9.7 19 10.2 122 10.3

55–64 47 24.9 290 23.3 45 24.1 258 21.7

65–74 74 39.2 469 37.7 62 33.2 436 36.6

75–85 57 30.2 365 29.3 61 32.6 374 31.4

Education‡

Less than high school 55 29.7 377 30.5 74 40.2 452 38.2

High school 76 41.1 465 37.7 67 36.4 457 38.6

Greater than high school 54 29.2 393 31.8 43 23.4 275 23.2

Marital status‡

Married (or living as married) 130 68.8 921 74.1 127 67.9 928 78.05

Not married 59 31.2 322 25.9 60 32.1 261 21.95

Cigarette smoking§

Never smoker 62 32.8 531 42.7 74 39.6 576 48.4

Former smoker 60 31.8 437 35.1 56 29.9 425 35.7

Current smoker 67 35.5 276 22.2 57 30.5 189 15.9

Pancreatitis‡

No 137 84.1 1,140 99.1 126 88.1 1,085 98.9

Yes 26 15.9 10 0.9 17 11.9 12 1.1

Cancer in the family‡

No 66 39.5 635 53.6 76 44.4 560 49.4

Yes 101 60.5 550 46.4 95 55.6 573 50.6

Pancreatic cancer in family‡

No 158 94.6 1,164 98.2 158 92.4 1,110 98.0

Yes 9 5.4 21 1.8 13 7.6 23 2.0
Nitrate Exposure and Pancreatic Cancer 697

Am J Epidemiol 2004;159:693–701

from drinking water. Likewise, when we stratified the

average nitrate analysis by the median tap water intake in the

controls (2.1 liters/day), we found no association between

pancreatic cancer and the nitrate level among those with both

high and low tap water intakes (data not shown).

We evaluated the association of drinking water nitrate

levels and pancreatic cancer stratified by dietary vitamin C

intake and smoking status, factors known to affect endogenous nitrosation. Results were similar among those with

vitamin C intake below and at or above the median and

among never smokers and current or past smokers (data not

shown).

Few individuals in this population were ever exposed to

community water supplies’ nitrate levels at or above 7.5 or

10 mg of nitrate nitrogen per liter. When exposure at these

higher levels did occur, it was usually for only a few years

(median years of ≥10 mg/liter, 2; interquartile range, 1–2).

We found no significant association between pancreatic

cancer risk and years of exposure to community water

supplies at or above these levels (table 3). Those with 1–2

years of exposure at 10 mg/liter or more had a 50 percent

elevated risk of borderline significance. However, the risk

among subjects exposed for more than 2 years at this level

was not elevated (odds ratio (OR) = 0.6, 95 percent confidence interval (CI): 0.2, 1.9).

We conducted analyses including only individuals with 90

percent or greater years of nitrate measurements after 1960

(132 cases, 870 controls) to further reduce possible misclassification by unknown nitrate levels. The results were not

substantially different from those presented here, although

the odds ratio for the highest quartile of average nitrate was

somewhat lower (OR = 0.7, 95 percent CI: 0.4, 1.2). We also

evaluated whether the risk estimates changed if we excluded

those who requested and completed proxy questionnaires but

for whom the respondent was not the spouse and was therefore less likely to share the same residential and water source

history (68 cases). Our results were similar to those reported

here.

We evaluated the association between pancreatic cancer

risk and total years of using a private well as a drinking water

source. Shallow wells in Iowa tend to have higher nitrate

levels than do deep wells (26), so we also evaluated the association with years of shallow (<50 feet or 15.24 meters) well

use. We found no association between pancreatic cancer risk

and years of well use overall or years using a shallow well.

The odds ratio for shallow well use greater than 10 years was

0.6 (95 percent CI: 0.4, 1.0) (table 4).

Dietary intake of nitrate showed a different association

with risk among men and women (table 5). Among men,

there was no association with nitrate intake from foods;

however, among women, the risk decreased with increasing

quartiles of intake (ptrend = 0.04). The inverse association

among women was not found when we limited the analysis

to self-respondents and proxies who were the husbands of

the subject. Higher intake of dietary nitrite was associated

with a modest increased risk of pancreatic cancer in both

men and women (table 5). We evaluated animal and plant

TABLE 2. Odds ratios and 95% confidence intervals for 

pancreatic cancer, by average nitrate level in drinking water 

from 1960 onward, Iowa*

* Analysis included only those individuals with 70% or greater

(after 1960) person-years using a community water supply with

known nitrate estimates.

† Odds ratios adjusted for age, gender, and cigarette use.

‡ CI, confidence interval.

Average nitrate 

(nitrate nitrogen, 

mg/liter) 

Cases 

(n = 189)

Controls 

(n = 1,244)

Odds 

ratio†

95% 

CI‡

<0.6 50 311 1.0

0.6–<1.3 62 311 1.2 0.79, 1.8

1.3–2.8 28 311 0.54 0.33, 0.89

>2.8 49 311 0.99 0.64, 1.5

TABLE 3. Odds ratios and 95% confidence intervals for 

pancreatic cancer by number of years with a community water 

supply nitrate level at or above 7.5 and 10 mg of nitrate nitrogen 

per liter after 1934, Iowa*

* Analysis included only those individuals with 70% or greater

(after 1960) person-years using a community water supply with a

known nitrate estimate.

† Odds ratios adjusted for age, gender, and cigarette use.

‡ CI, confidence interval.

Years of nitrate 

nitrogen at or above 

level

Cases 

(n = 189)

Controls 

(n = 1,244)

Odds 

ratio†

95% 

CI‡

7.5 mg/liter

0 140 952 1.0

1–4 19 119 1.1 0.66, 1.9

>4 30 173 1.2 0.79, 1.9

10 mg/liter 

0 144 1,007 1.0

1–2 42 199 1.5 1.0, 2.2

>2 3 38 0.58 0.18, 1.9

TABLE 4. Odds ratios and 95% confidence intervals for 

pancreatic cancer cases and controls by years of shallow (<50 

feet*) private well use, Iowa, 1985–1987†

* Metric equivalent: 15.24 meters.

† Analysis included only those individuals with 70% or greater

known person-years from 1960 onward using a community water

supply with nitrate estimates.

‡ Odds ratios adjusted for age, gender, and cigarette use.

§ CI, confidence interval.

Years Cases 

(n = 189)

Controls 

(n = 1,244)

Odds 

ratio‡

95% 

CI§

0 162 959 1.0

1–10 8 91 0.57 0.27, 1.2

>10 19 194 0.63 0.38, 1.0
698 Coss et al.

Am J Epidemiol 2004;159:693–701

sources of nitrite separately (table 5). Among men and

women, we found a significant positive association between

higher animal nitrite intake and pancreatic cancer (ptrend =

0.02 and <0.01 for men and women, respectively). Odds

ratios were similar after limiting the analysis to only selfrespondents and proxies who were spouses of the subject.

Plant sources of nitrite were not significantly associated with

risk among men and women before or after exclusion of

nonproxy respondents.

DISCUSSION

The average nitrate level in community water supplies over

an approximately 25-year period was not associated with

pancreatic cancer risk in this study population. Average exposure levels were relatively low. For the large majority (75

percent) of individuals, less than 10 percent of daily nitrate

intake came from drinking water; most was dietary (primarily

vegetables). The majority of nitrate intake can come from

drinking water when levels approach the maximum contaminant level of 10 mg/liter. However, we estimated that only 1

percent of the study population received approximately 50

percent of their daily nitrate intake from water. Those with 1–

2 years of exposure at or above the maximum contaminant

level of nitrate nitrogen (10 mg/liter) had a 50 percent elevated

risk that was of borderline significance; however, the risk was

not elevated for exposure for a longer period at this level. Our

analysis of exposure at or above the maximum contaminant

level was limited because of the small number of subjects who

had experienced any exposure at this level. Controlling for

known pancreatic cancer risk factors and potential nitrosation

modulators did not alter our drinking water nitrate findings.

Dietary nitrate, mostly of vegetable origin, was not associated with an increased risk of pancreatic cancer. Among

women but not men, increasing intake was associated with

decreasing risk. In contrast, higher intakes of dietary nitrite

derived from animal sources were associated with significantly increased risks of pancreatic cancer among both men

and women.

Measurements of nitrate levels from private wells were not

available for this study; consequently, individuals with the

highest potential nitrate exposures from drinking water were

not included in the analysis. We did not find an association

between pancreatic cancer risk and years of private well use

overall or years of shallow well use. Well depth is an important predictive factor for nitrate levels in Iowa and other

areas of the United States (26, 27). However, other factors

are also important determinants of nitrate levels in private

wells including well construction, location with respect to

nitrogen sources such as agricultural fields, septic tanks and

animal feedlots, and characteristics of the aquifer in which

the well is located (28, 29). Lack of information about these

factors likely resulted in substantial misclassification of

nitrate exposure in our analysis of duration of private well

exposure.

TABLE 5. Odds ratios and 95% confidence intervals for pancreatic cancer cases and controls by quartiles of 

consumption of nitrate and nitrite from the diet, Iowa, 1985–1987*

* Analysis included only those with an adequate dietary history (less than six missing foods).

† Odds ratios adjusted for age, cigarette use, and caloric intake.

‡ CI, confidence interval.

Men Women

Cases 

(n = 141)

Controls 

(n = 1,247)

Odds 

ratios†

95% 

CI‡

Cases 

(n = 122)

Controls 

(n = 639)

Odds 

ratios†

95% 

CI

Dietary nitrate 

(mg/day)

Dietary nitrate 

(mg/day)

<58 26 298 1.0 <63 39 164 1.0

58–82 33 311 1.1 0.63, 1.9 63–90 33 157 0.99 0.58, 1.7

83–117 39 311 1.2 0.70, 2.0 91–126 24 158 0.64 0.36, 1.1

>117 43 327 1.0 0.60, 1.8 >126 26 160 0.53 0.29, 0.97

Dietary nitrite 

(mg/day)

Dietary nitrite 

(mg/day)

<0.75 15 233 1.0 <0.56 18 144 1.0

0.75–0.98 22 307 1.0 0.52, 2.0 0.56–0.71 32 146 1.8 0.94, 3.4

0.99–1.30 40 333 1.5 0.81, 2.9 0.72–0.93 32 168 1.4 0.72, 2.6

>1.30 64 374 1.5 0.79, 3.0 >0.93 40 181 1.3 0.65, 2.5

Dietary nitrite 

from animal 

sources 

(mg/day)

Dietary nitrite 

from animal 

sources 

(mg/day)

<0.22 9 264 1.0 <0.13 13 148 1.0

0.22–0.31 22 282 2.1 0.95, 4.8 0.13–0.18 32 164 2.4 1.2, 4.7

0.32–0.53 60 359 3.8 1.8, 8.0 0.19–0.26 26 147 1.9 0.94, 4.0

>0.53 50 342 2.3 1.1, 5.1 >0.26 51 180 3.2 1.6, 6.4
Nitrate Exposure and Pancreatic Cancer 699

Am J Epidemiol 2004;159:693–701

Our results for pancreatic cancer risk with average nitrate

level in community water supplies are consistent with results

of a recent cohort study of older women in Iowa (16). Weyer

et al. (16) evaluated pancreatic cancer risk by quartile

cutpoints of the average nitrate level in 1955–1988 for a

women’s Iowa community water supply at the time of

enrollment. Quartile cutpoints of nitrate nitrogen were very

similar to those in our study (<0.36, 0.36–1.00, 1.01–2.46,

>2.46 mg/liter); increasing intake was not associated with

risk (ORs were 1.0, 0.77, 1.20, and 0.65, respectively). There

was potential for some overlap between the women in our

study and in the study by Weyer et al. (16). Female cases

diagnosed with pancreatic cancer after the cohort study’s

baseline survey in 1986 and during 1987 were eligible to be

included in both studies. However, the overlap in cases was

small because the prospective cohort study enrolled only 43

percent of eligible older women, and incident pancreatic

cancer cases in that study were ascertained for an additional

10 years (1988–1998).

A few studies have evaluated dietary intakes of nitrate and

nitrite and pancreatic cancer risk, although none evaluated

animal and vegetable products separately. A case-control

study in 1990 by Howe et al. (30) found no association

between dietary intake of nitrate and nitrite and pancreatic

cancer risk. A case-control study in 1991 by Baghurst et al.

(31) reported a significantly decreased risk associated with

the highest quartile of dietary nitrate intake; there was no

association with dietary nitrite intake. Most studies have

reported elevated risks associated with increased consumption of smoked or processed meats (32). Processed meat

intake, specifically a group consisting of bacon and smoked

ham, was associated with increased risk of pancreatic cancer

in a case-control study conducted in Sweden (33). A casecontrol study using only direct interviews did not find an

association between pancreatic cancer risk and the consumption of processed meat in either men or women (34). Pork

products (including processed and unprocessed) were associated with an increased risk of pancreatic cancer in Louisiana (35). These findings were strongest for individuals

with Cajun ancestry and may be related to preparation

methods.

It is difficult to separate exposure to nitrite from other

aspects of preserved meat consumption, including the

cooking method. Preparation methods such as frying and

grilling have been associated with an increased risk of

pancreatic cancer (33). Future studies that use a more

detailed dietary assessment tool, including questions on food

preparation, will be more informative.

Our findings demonstrate that it is important to consider

the sources of the dietary nitrate and nitrite in the evaluation

of pancreatic cancer risk. Nitrate is derived almost entirely

from vegetables that are known to contain inhibitors of in

vivo nitrosation, which may in part explain their consistent

inverse associations with many epithelial cancers (12, 14).

Therefore, nitrate and nitrite consumed in vegetable products

are not likely to result in significant formation of N-nitroso

compounds. Animal products containing nitrite (primarily

processed meats) are a source of amines and amides, which

are also precursors of N-nitroso compounds. Furthermore, a

number of preformed N-nitroso compounds are found in

processed meats (36, 37). As a result, consumption of nitrite

from animal products should result in more substantial exposure to N-nitroso compounds than plant-based products.

Controlled human studies have demonstrated increased

endogenous nitrosation when subjects are given oral doses of

proline, an amino acid, and then ingest drinking water with

elevated nitrate levels, indicating the potential for the formation of carcinogenic N-nitroso compounds (10, 13). In these

studies, the elevated nitrate levels were higher than the

average nitrate levels for the large majority of our study

population. It is not known if substantial nitrosation occurs at

lower levels.

The proposed pathway linking nitrate to cancer is

complex. Individual variation in the rate of nitrosation

results in various levels of exposure to N-nitroso

compounds. Differences between individuals have not been

totally explained; however, some factors have been identified. Smoking has been identified as a possible nitrosation

enhancer (38, 39). Smokers were found to have significantly

higher rates of N-nitrosoproline production after consuming

nitrate and proline than their nonsmoking counterparts. This

could indicate that smokers are at an elevated risk of exposure to carcinogenic compounds when consuming nitrate in

their drinking water. Vitamin C is a well-documented inhibitor of nitrosation (14). Individuals consuming low levels of

vitamin C may also be at an increased risk. We evaluated

these two possible nitrosation modulators and found no

evidence of interaction.

The strengths of our study include the high response rates

among cases and controls, information on lifetime water

sources and historical nitrate levels for Iowa public water

supplies, and our ability to evaluate effect modification by

vitamin C intake and smoking. Our study did not have sufficient power to evaluate risk at nitrate levels above the

maximum contaminant level because of the lack of exposure

data for private wells and infrequent high exposure among

public water supply users. Studies of exposure to disinfection by-products and cancer risk have often seen the greatest

elevated risks only after 30 or more years of exposure (15,

17, 18). Our exposure period of 27 years would have been

insufficient if a similar induction period occurs for nitrate.

Our drinking water nitrate analyses were limited to those

who used Iowa public supplies with nitrate estimates for 70

percent of their person-years from 1960 onward. By not

including study subjects with a high proportion of years

using a private well, we limited our analysis to nitrate levels

that were largely below an average level of nitrate nitrogen

(5 mg/liter). This exclusion was necessary to limit misclassification due to unknown but likely higher nitrate levels in

private wells. A similar proportion of cases and controls

were excluded from the public supply analyses, and the

mean years of private well use were similar between cases

and controls. Thus, it is unlikely that this exclusion would

bias the results of the public supply nitrate analyses. Misclassification of drinking water nitrate exposure may have

occurred as a result of infrequent monitoring in the early

years of the study. However, cases and controls had similar

numbers of measurements for each decade (19); therefore,

misclassification would likely be nondifferential and would

be expected to bias odds ratios toward the null.
700 Coss et al.

Am J Epidemiol 2004;159:693–701

A limitation of this study was the large number of surrogate respondents among cases. Although the median time

between diagnosis and interview was only 1 year, due to the

severe morbidity associated with pancreatic cancer, only

about 10 percent of cases completed the questionnaire themselves, whereas about 85 percent of controls were selfrespondents. Thus, there was the potential for differential

misclassification of exposure due to the likely poorer quality

of the information among cases. We evaluated this possible

bias by excluding proxy respondents who were not the

spouse. Spouses are likely to have shared a substantial

portion of their residence history with the subject and would

therefore be expected to accurately report residential and

dietary information, whereas other proxy respondents may

be less knowledgeable. Results for the drinking water analysis did not differ substantially from those found using all

proxy respondents. Likewise, excluding proxies who were

not spouses did not alter results for the dietary nitrate and

nitrite analysis in men substantially; however, among

women, the odds ratios for dietary nitrate and nitrite were

attenuated.

In summary, we did not find evidence for an association

between drinking water nitrate levels below the maximum

contaminant level and pancreatic cancer. However, this

hypothesis deserves further evaluation in future studies

among populations with higher drinking water nitrate exposure. Our results suggest a role for nitrite derived from

animal sources as a pancreatic cancer risk factor. Further

research with a detailed analysis of dietary nitrate and nitrite

is needed to more precisely define the relation between Nnitroso compounds and pancreatic cancer.

ACKNOWLEDGMENTS

The authors thank David Riley and Jiji Katamneni of the

Center for Health Effects of Environmental Contaminants in

Iowa for providing us with the nitrate data and Shannon

Merkle at Information Management Systems for programming support. We are grateful to the interviewers and other

study staff whose work and cooperation made this research

possible.

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