Under- and Overreporting of Energy Intake using Urinary Cations as Biomarkers: Relation to Body Mass Index
Jianjun Zhang1,
Elisabeth H. M. Temme1,
Satoshi Sasaki2 and
Hugo Kesteloot1
1 Department of Epidemiology, School of Public Health, Catholic University of Leuven, Leuven, Belgium.
2 Epidemiology and Biostatistics Division, National Cancer Center Research Institute, Kashiwa, Japan.
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ABSTRACT
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Under- and overreporting of energy intake in relation to body mass index (BMI, kg/m2) were examined by using 24-hour urinary sodium and potassium as biomarkers. The data were obtained from 2,124 men and 1,998 women aged 2574 years who participated in the 19811984 Belgian Interuniversity Research on Nutrition and Health study conducted in Belgium. The ratios of dietary intake to urinary excretion of sodium and potassium, as a measure for relative underreporting, were inversely associated with BMI (for men, ß = 0.019 for sodium ratio and ß = -0.026 for potassium ratio; for women, ß = -0.017 for sodium ratio and ß = -0.019 for potassium ratio; allp < 0.0001) independent of age, smoking, alcohol intake, and educational level. Since 77% of dietary potassium was reported to be excreted in the urine, subjects for whom the (dietary potassium x 0.77)/urinary potassium ratio was <1 were considered underreporters and >1 as overreporters. The percentage of underreporters increased with increasing pooled sex-specific deciles of BMI (ß = 1.88,p < 0.0001) and was higher than the percentage of overreporters in 13 of 20 deciles. At a BMI of 25.4, the percentage of under- and overreporters equalized. In conclusion, the relative underreporting of energy intake and the percentage of underreporters increased with increasing BMI. Am J Epidemiol 2000;152:45362.
biological markers; body mass index; cations; energy intake; nutrition assessment; obesity
Abbreviations:
BIRNH, Belgian Interuniversity Research on Nutrition and Health; BMI, body mass index; D-Na/D-K, ratio of dietary sodium to dietary potassium; U-Na/U-K, ratio of urinary sodium to urinary potassium
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INTRODUCTION
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Accurate assessment of habitual dietary intake is the cornerstone of epidemiologic studies on the relation between diet and disease. Black et al. (1
) evaluated energy intake data obtained from 37 published dietary surveys of adults reporting on 68 subgroups by computing a ratio of reported energy intake to basal metabolic rate. Energy intake was found to be underestimated in 46 of the 68 groups. In a subsequent study (2
), the sex ratio of energy intake was investigated by using data derived from 81 published dietary surveys conducted in 28 countries. The male-to-female sex ratio of energy intake was higher than the sex ratios of urinary excretion of cations; therefore, the former is likely to be overestimated in most dietary surveys analyzed. These two studies examining dietary surveys worldwide indicate that food intake data gathered by using prevailing dietary assessment methods must be validated.
Several methods have been used to validate dietary survey data, for instance, the 7-day weighed food record, the ratio of energy intake to basal metabolic rate, and the doubly labeled water technique (3





10
). The self-limitations or the complexity and high cost of these methods prevent most of these validation measures from being used as a "gold standard" in large epidemiologic studies. Determination of urinary biomarkers of dietary intake, such as urinary nitrogen for protein intake and urinary potassium for potassium intake, is relatively easy and less expensive. More importantly, urinary biomarkers can objectively reflect dietary intake of the nutrients they represent (11
, 12
). Thus, it seems practical and appropriate to validate reported dietary intake against these biomarkers. To date, however, few studies have used this approach to assess the accuracy of reported dietary intake (13
, 14
).
Some (15

18
) but not all (19
20
) studies show that overweight persons tend to underreport their dietary intake more frequently and/or to a greater extent than normal-weight persons. Sex, smoking, alcohol intake, educational level, and dieting behavior also are reported to be determinants of misreporting of dietary intake (4
, 5
, 17
, 21
). Overreporting of dietary intake is less documented in the literature (4
, 22
). The relative importance of under- and overreporting of energy intake is still a matter of debate.
In this study, we evaluated whether and to what extent body mass index (BMI) is related to under- and overreporting of dietary energy intake by using 24-hour urinary sodium and potassium as its biomarkers. Furthermore, we examined whether this relation is independent of other proposed determinants of dietary assessment error. The data analyzed in the present study were obtained from a population sample of 2,124 men and 1,998 women who participated in the Belgian Interuniversity Research on Nutrition and Health (BIRNH) study.
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MATERIALS AND METHODS
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Study population
The aims, design, methodology, and results of the BIRNH study have been described in detail elsewhere (21
, 23
). Briefly, a randomized sample of the population was drawn from each of the 42 counties in Belgium by using the voting lists. In view of the low response rate (38.6 percent for men and 34.4 percent for women), an additional random sample of 10 percent of the nonrespondents was screened. No significant difference in dietary habits was found between respondents and nonrespondents (24
). The survey of dietary intake and of cardiovascular and other risk factors was carried out from 1981 to 1984 among 5,949 men and 5,353 women aged 2574 years. In a subsample of 2,199 men and 2,064 women randomly selected from all study subjects, a single 24-hour urine sample was obtained. This subsample of subjects was used in our analysis. Thirty-two men and 28 women were considered to have incomplete 24-hour urine samples and were thus excluded, since their urinary excretion of creatinine fell outside the 99 percent tolerance interval. Because of missing values, an additional 43 men and 38 women were excluded from the data set. Therefore, the final population available for analysis consisted of 2,124 men and 1,998 women.
Survey of dietary intake and of anthropometric and lifestyle variables
The dietary habits of all study participants were assessed by using a 24-hour food record. All participants surveyed were given a self-administered questionnaire covering dietary intake and lifestyle and socioeconomic characteristics, and they were asked to record their intake of all food items for a whole day preceding the day of the interview. All completed questionnaires were checked and verified by trained dietitians in the presence of the participants. Dietary intake of energy, sodium, and other nutrients was calculated from the food consumption table of Paul and Southgate (25
) and dietary intake of potassium from the Dutch food consumption table (26
). Discretionary salt added to the diet was not measured in the survey. On the day of the interview, body weight was measured to the nearest 1 kg and body height to the nearest 1 cm after subjects had removed their heavy garments and their shoes.
Assessment of 24-hour urinary excretion of cations
A single 24-hour urine sample was collected 25 days after the day on which dietary intake was recorded. Prior to urine collection, special instructions were given to participants on how to minimize urine loss. All urine samples were measured in the Central Laboratory of St. Rafaël Hospital, University of Leuven, Belgium. Urinary sodium and potassium levels were determined by emission flame photometry (27
). Urinary creatinine was measured by Jaffé's method (28
).
Statistical analysis
All statistical analyses were performed for men and women separately by using version 6.12 of the SAS software package (SAS Institute, Inc., Cary, North Carolina). In this study,p values of <0.05 were considered statistically significant.
BMI was calculated as body weight (kg) divided by the square of body height (m2). Dietary intake and urinary excretion of sodium and potassium and their ratios (dietary/urinary), as well as other variables, were analyzed according to quintiles of BMI. The univariate regression coefficients between all these variables and BMI were calculated for 2,124 men and 1,998 women. To examine whether dietary intake and urinary excretion of sodium and potassium and their ratios were significantly and independently associated with BMI, multiple linear regression analysis with the backward elimination procedure was performed. Dietary intake and urinary excretion of sodium and potassium and their ratios were treated as respective dependent variables; BMI, age, smoking, alcohol intake, and educational level were considered independent variables. To meet study goals, BMI was forced into the multivariate models.
In multiple regression analysis, smoking, alcohol intake, and educational level were used as categorical variables. The smoking variable was divided into never smokers, former smokers, and current smokers. Smoking any type of tobacco defined a current smoker. Two classes of alcohol intake were created: nondrinkers and drinkers. Educational level was grouped into the three categories of low, medium, and high corresponding to incomplete or complete primary school, high school, and professional higher education or university, respectively. Never smokers, nondrinkers, and persons with a low level of education were considered the reference groups in multivariate analysis.
As 77 percent of dietary potassium has been reported to be excreted in the urine (29
), the (dietary potassium x 0.77)/urinary potassium ratio was developed to assess the prevalence of under- and overreporting of potassium intake. Subjects whose ratio was <1 were defined as underreporters and those whose ratio was >1 as overreporters. The percentage of underreporters was calculated for each BMI decile for both sexes. By definition, the percentage of overreporters in each BMI decile was thus equal to 100 percent minus the percentage of underreporters. Because sex was not significantly associated with the BMI-decile-specific percentage of underreporters, pooled data (deciles for each sex) were used for this analysis.
To explore to what extent the value of the correction factor chosen (0.77) influenced the results obtained, all related analyses were repeated by using two supplementary values, 0.75 and 0.79. To clarify whether reported and actual food intakes were qualitatively different in obesity, the ratios of dietary sodium to dietary potassium (D-Na/D-K) and of urinary sodium to urinary potassium (U-Na/U-K), all measured as mmol/mmol, were calculated. The correlation of these two ratios with BMI was examined.
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RESULTS
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The general characteristics of the population studied are described in table 1. The men were slightly older than the women. Although the variation in BMI was greater for women than for men, mean BMI did not differ significantly between the sexes.
Table 2 presents information on dietary intake and urinary excretion of sodium and potassium and their ratios and other variables and on univariate regression of these variables on BMI for 2,124 men and 1,998 women. Dietary intake of sodium and potassium and the ratios of dietary intake to urinary excretion of these cations were inversely and significantly associated with BMI for both sexes (p < 0.001 to p < 0.0001), with the exception of dietary potassium for men (p > 0.05). A positive, significant relation was found between urinary excretion of sodium and potassium and BMI for men and women (all p < 0.0001). Data on dietary intake and urinary excretion of sodium and potassium and their ratios, and other variables stratified by BMI quintiles, are given in table 3 for men and table 4 for women. For both sexes, dietary intake of sodium and potassium and the ratios of dietary intake to urinary excretion of these cations decreased progressively with increasing quintiles of BMI, whereas urinary excretion of sodium and potassium displayed an opposite pattern.
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TABLE 2. Dietary Intake and urinary excretion of sodium and potassium, and their ratios and other variables,* and univariate regression of these variables on body mass index (kg/m2), for 2,124 men and 1,998 women who participated in the BIRNH study, 19811984
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TABLE 3. Dietary Intake and urinary excretion of sodium and potassium, and their ratios and other variables,* by quintiles (Q) of body mass index, for men who participated in the BIRNH study, 19811984
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TABLE 4. Dietary Intake and urinary excretion of sodium and potassium, and their ratios and other variables,* by quintiles (Q) of body mass index, for women who participated in the BIRNH study, 19811984
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Tables 5, 6 and 7 show the results of multiple regression analysis of dietary intake of sodium and potassium, urinary excretion of sodium and potassium, and their ratios, respectively, versus BMI, adjusted for other potential determinants of the misreporting of dietary intake. A weak, inverse, and significant association was observed between BMI and dietary intake of sodium (mmol/24 hours) for men (ß = -0.68,p < 0.05) and of potassium (mmol/24 hours) for women (ß = -0.38,p < 0.01) (table 5). Urinary excretion of sodium (mmol/24 hours) (ß = 2.74 for men; ß = 2.80 for women) and potassium (mmol/24 hours) (ß = 1.32 for men; ß = 0.59 for women) was strongly, positively, and significantly associated with BMI (allp < 0.0001) (table 6). The ratios of dietary intake to urinary excretion of sodium (ß = -0.019 for men; ß = -0.017 for women) and potassium (ß = -0.026 for men; ß = -0.019 for women) were significantly and inversely associated with BMI (allp < 0.0001). For men, except for BMI, age, smoking, alcohol intake, and high educational level also were significantly associated with the ratios of dietary intake to urinary excretion of sodium, potassium, or both. For women, only BMI was significantly related to these ratios of sodium and potassium, except that age was significantly associated with the potassium ratio (table 7).
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TABLE 5. Multiple regression analysis* of dietary intake of sodium and potassium (mmol/24 hours) on body mass index and other variables for 2,124 men and 1,998 women who participated in the BIRNH study, 19811984
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TABLE 6. Multiple regression analysis* of urinary excretion of sodium and potassium (mmol/24 hours) on body mass index and other variables for 2,124 men and 1,998 women who participated in the BIRNH study, 19811984
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TABLE 7. Multiple regression analysis* of sodium and potassium ratios (24-hour dietary intake/24-hour urinary excretion) on body mass index and other variables for 2,124 men and 1,998 women who participated in the BIRNH study, 19811984
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As shown in figure 1, the percentage of subjects whose (dietary potassium x 0.77)/urinary potassium ratio was <1 increased progressively and significantly with increasing pooled sex-specific deciles of BMI (ß = 1.88,p < 0.0001). Of the 20 decile points, 13 were above the reference line (at 50 percent), and the remaining 7 were below the reference line. This distribution indicates that for 13 of the 20 decile points, underreporters predominated over overreporters. At a BMI of 25.4, the regression line crossed the reference line; thus, at this BMI value, the percentage of under- and overreporters equalized. For 6 of the 10 decile points for subjects whose BMI was <25.4, the percentage of underreporters was <50 percent. For 9 of the remaining 10 decile points for subjects whose BMI was >25.4, the percentage of under-reporters was >50 percent. The BMI-decile-specific percentage of underreporters of potassium intake ranged from 35.8 percent (BMI 5 20.4) to 62.6 percent (BMI 5 32.5) for men and from 36.9 percent (BMI 5 19.7) to 67.5 percent (BMI 5 30.9) for women. According to our definition, the BMI-decile-specific percentage of overreporters of potassium intake varied from 37.4 to 64.2 percent for men and from 32.5 to 63.1 percent for women.

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FIGURE 1. Percentage of subjects whose (dietary potassium (DK) x 0.77)/urinary potassium (UK) ratio was <1 (y), by definition considered underreporters of dietary energy intake. Relation to pooled sex-specific deciles of body mass index (BMI) (x) for 2,124 men and 1,998 women who participated in the Belgian Interuniversity Research on Nutrition and Health study, Belgium, 19811984. The regression (solid) line and 95% confidence limits for mean predicted values are given. The regression line crosses the reference line (at 50%) at a BMI of 25.4, a value at which the percentage of under- and overreporters equalizes.
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The Pearson's correlation and regression coefficients between pooled sex-specific deciles of BMI and the percentage of subjects whose (dietary potassium x correction factor)/urinary potassium ratio was <1 were 0.82 and 1.85, 0.83 and 1.88, and 0.84 and 1.79 when the correction factors 0.75, 0.77, and 0.79, respectively, were used (allp < 0.0001). The BMI values at the points at which the regression line crossed over the reference line were 23.5 for correction factor 0.75, 25.4 for correction factor 0.77, and 26.9 for correction factor 0.79.
For the men, both D-Na/D-K (r = -0.056, p < 0.01) and U-Na/U-K (r = -0.004, p = 0.85) correlated inversely with BMI. D-Na/D-K (r = -0.018, p = 0.42) correlated inversely and U-Na/U-K (r = 0.080, p < 0.001) correlated positively with BMI for the women.
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DISCUSSION
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A main advantage of this study is that the data on dietary intake and urinary excretion of sodium and potassium were obtained from the same large population sample, with a wide age range for both sexes. A previous study showed, for both sexes, an independent, significant, and positive relation between dietary intake and 24-hour urinary excretion of sodium and potassium (all p < 0.001) (11
). This finding demonstrates that 24-hour urinary sodium and potassium may serve as valid biomarkers of dietary intake of these nutrients. The 24-hour urinary excretion of sodium and potassium also reflects the difference in energy intake resulting from various levels of physical activity, as physical activity is an important determinant of energy expenditure and consequently energy intake (11
). Our method therefore provides an alternative way to evaluate under- and overreporting of energy intake objectively and simultaneously. The validity of the approach proposed could still be increased by measuring 24-hour urinary total nitrogen and/or urea as biomarkers of
protein intake.
The present study showed that for sodium and potassium, dietary intake decreased and urinary excretion increased with increasing quintiles of BMI. An inverse, significant relation between the ratios of dietary intake to urinary excretion of sodium and potassium and BMI persisted for both sexes even after adjustment for age, smoking, alcohol intake, and educational level. These findings indicate the relative underreporting of dietary cation intake by subjects with a higher BMI. From the data in tables 1, 2, and 7, it can be calculated that an increase of two standard deviations of BMI would result in a decrease in the ratios of dietary intake to urinary excretion of sodium and of potassium by 16.2 and 12.0 percent for men and 22.4 and 12.4 percent for women, respectively.
In four consecutive balance studies of sodium and potassium, Holbrook et al. (29
) revealed that 86 percent of the ingested sodium and 77 percent of the ingested potassium were excreted in the urine. Development of the (dietary potassium x 0.77)/urinary potassium ratio, based on the findings of this cation balance study and other studies (30
, 31
), enabled us to examine the prevalence of under- and overreporting of dietary intake of potassium in the same population sample. Because discretionary salt added to the diet was not measured, the prevalence of under- and overreporting of sodium intake could not be evaluated by using the same approach. Our results showed that BMI determined both the nature and the prevalence of misreporting of potassium intake. In this Belgian population, underreporters predominated among subjects with a higher BMI (>25.4) and overreporters among subjects with a lower BMI (<25.4). The prevalence of underreporters increased and the prevalence of overreporters decreased with increasing BMI. As a whole, the prevalence of underreporters was higher than the prevalence of overreporters.
In the literature, the magnitude of underreporting of dietary intake is expressed in two major ways: as the percentage of underreported amount and as the percentage of underreporters (4
, 7
, 22
). In addition, the definition of "underreporting" vvaries considerably among different studies (17
, 18
, 32
, 33
). These factors make a direct comparison of the magnitude of underreporting in different dietary surveys unreliable. In the relevant papers (4
, 6
, 7
, 15
, 17
, 20
, 22
, 32

35
), the percentage of underreported amount of energy intake varied from 12 to 32.8 percent for men and from 18 to 34.1 percent for women (6
, 7
, 15
, 20
, 22
, 34
). The percentage of subjects who underreported their energy intake ranged from 16.3 to 81 percent for men and from 15.8 to 81 percent for women (4
, 17
, 22
, 32
, 33
, 35
). Although the definition and magnitude of underreporting of energy intake were different, most of these studies (4
, 6
, 15
, 17
, 32

35
) reached a consensus that the percentage of underreporters or of underreported amount of energy intake increases with increasing BMI or amount of adiposity.
Until now, few investigations have addressed the issue of overreporting of dietary intake. The percentage of subjects overreporting their energy intake was found to be below 9 percent (4
, 22
). The BMI-decile-specific percentage of overreporters in this study was high among subjects with a low BMI. Therefore, the phenomenon of overreporting in dietary studies deserves attention.
The ratio of reported energy intake to basal metabolic rate used as a reference parameter to validate dietary intake data was proposed by Goldberg et al. in 1991 (36
). This frequently used method is simple and can address the issue of underreporting of energy intake in large epidemiologic studies. However, it has two major limitations. First, the Goldberg et al. method does not consider physical activity at the individual level. The cutoff value for defining underreporting for a given population is arbitrary and is calculated by assuming that the level of physical activity is the same for all persons; for instance, basal metabolic rate x 1.55 defines a sedentary lifestyle (36
). Therefore, this method is able to identify only the underreporters whose dietary intake data are obviously implausible. Second, when the Goldberg et al. method is applied, the basal metabolic rate must be estimated by using the various regression equations, since it is impractical to measure this rate in a large population sample. It has been reported that the basal metabolic rate estimated by using eight equations available in the literature accounted for only 3052 percent of the variance in measured basal metabolic rate (37
). This rate is overestimated when six of these eight equations are used, including the most frequently used Schofield equation (37
39
). This error in predicting basal metabolic rate results in systematic underestimation of the ratio of reported energy intake to basal metabolic rate. Consequently, the degree of underreporting of energy intake is overestimated.
In the present study, age, smoking, alcohol intake, and educational level also influenced the accuracy of reported dietary intake. Older persons and men with a high level of education tended to underreport their cation intake. Overreporters of sodium and potassium intake were overrepresented among male former and current smokers, which conforms with the findings of some (4
) but not all (5
, 32
) studies. We found that male alcohol drinkers seemed to overreport their sodium intake. However, smoking, alcohol intake, and educational level had no significant effect on the accuracy of reported intake of sodium and potassium among women.
The main uncertainty of the present study concerns the precision of the correction factor, 0.77, for evaluating the prevalence of under- and overreporting of potassium intake. Many factors may influence the percentage of dietary potassium excreted in the urine, for instance, the absolute level of potassium intake, the seasons during which potassium balance studies are conducted, race, and cooking methods (29
31
, 40
). The percentage of potassium intake excreted in the urine was found to be 76 percent for White Americans (30
) and 79 percent for 154 Japanese men and 69 Japanese women (31
). In the present Belgian population sample of men and women, 76 and 79 percent, respectively, of the calculated dietary intake of potassium was excreted in the urine. All these values are close to the correction factor (77 percent) used in this study. A Finnish study (40
) reported that 92 percent of ingested potassium was excreted in the urine, a finding perhaps due to the fact that the Finnish cook potatoes with the skin intact, thereby minimizing loss of potassium during food preparation.
Our analysis suggests that use of different correction factors (0.75 and 0.79) had no marked effect on the strength of the relation observed between BMI and the percentage of subjects whose (dietary potassium x correction factor)/urinary potassium ratio was <1. However, it changed the BMI cutoff value above which underreporters predominated. Therefore, both the absolute and relative magnitudes of the prevalence of under- and overreporters of potassium intake derived from the data in figure 1 should be considered with caution, since they substantially depend on the value of the correction factor chosen. It is certain, however, that the percentage of underreporters increases significantly with increasing BMI.
Another uncertainty is the correctness of the potassium content of food groups as obtained from the food composition tables (25
, 26
). Furthermore, the 24-hour urine samples may have been incomplete for some proportion of participants. The urinary output of creatinine and the urinary volume increased with increasing BMI quintiles for both sexes. This finding suggests that the error in the 24-hour collection of urine, if any, would have been systematic for all participants and not restricted to specific subgroups. The time difference of 25 days between urine collection and dietary survey may have weakened the power of urinary cations as biomarkers of energy intake for persons with changing dietary habits. However, in a study conduced in Korea and Belgium (41
), mean 24-hour urinary excretion of potassium and sodium was found to be stable for 3 consecutive days. This observation was confirmed by a Japanese study in which 24-hour urine samples were obtained daily for 381 days from 12 healthy men (42
). The monthly group mean of urinary potassium excreted and its coefficient of variation ranged from 49.6 to 57.4 mmol/24 hours (mean, 52.9 mmol/24 hours) and from 7.2 to 10.6 percent (mean, 10.2 percent), respectively (42
). These data indicate that the urinary collection undertaken several days after the dietary survey probably did not influence our conclusions significantly.
A 1-day food record is not optimal for characterizing individual habitual dietary intake, but it provides a valid estimate of dietary intake on a group basis (3
). Assessment of nutrient values by using a 1-day food record is likely to introduce classification errors when the relation between diet and disease is investigated; however, this kind of error would result in a bias toward attenuating rather than enhancing the relation (43
).
Discretionary salt intake was not measured for all BIRNH study participants, but data were available from a subsample of 1,239 men and women. Analysis of the data from this subgroup showed that the higher the calculated sodium intake, the more frequently salt was added to food, a finding in agreement with results from the First National Health and Nutrition Examination Survey (44
). The fact that salt added to food was not considered could have resulted in a systematic error that would not have invalidated the overall results. This statement is supported by the finding that when urinary sodium or potassium is used as a reference parameter for validation of dietary survey data, similar conclusions are reached. Underreporting of alcohol intake is well recognized (35
), which could have introduced a bias. As men consume more alcohol than women do, this underreporting could be of greater importance for men. Therefore, caution also should be exercised when the findings of the BIRNH study are extrapolated to other studies conducted among different age groups or when different dietary assessment methods are used.
Whether excretion of sodium and potassium in the urine versus feces could be related to obesity is unclear. It has been reported that 98 and 85 percent of dietary sodium and potassium, respectively, are absorbed in the gut, and these proportions are quite constant over a wide range of intake levels (29
).
An important question in nutritional epidemiology is whether obese subjects eat or report differently from nonobese persons. The dietary origin of sodium and potassium is qualitatively different; for example, fruit, vegetables, potatoes, wine, and tea contain much potassium but little sodium (25
, 26
). Therefore, the D-Na/D-K and U-Na/U-K ratios can be used as a proxy of different types of reported and actual food intake, respectively. Results of this study show that for men and women, both D-Na/D-K and U-Na/U-K correlated very weakly with BMI (all r < 0.08), suggesting that, qualitatively, both reported and actual food intakes are largely independent of BMI.
In summary, our study demonstrated that in this large Belgian population sample, relative underreporting of dietary intake of sodium and potassium, as biomarkers of energy intake, increased with increasing BMI. This association was independent of age, smoking, alcohol intake, and educational level. Moreover, BMI determined both the nature and magnitude of misreporting of potassium intake. The prevalence of underreporting was higher among subjects with a higher BMI (>25.4) than among those with a lower BMI (<25.4). Overall, the prevalence of underreporting was higher than the prevalence of overreporting. However, overreporting of potassium intake was present for a substantial percentage of subjects with a lower BMI. This finding points out that the problem of overreporting should not be overlooked in dietary surveys. Our findings suggest that BMI-related under- and overreporting of dietary intake of cations should be considered when the direction and strength of the relation between dietary intake and disease are interpreted. More dietary surveys, especially those conducted in large population samples, should be validated against the urinary biomarkers of dietary intake.
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ACKNOWLEDGMENTS
|
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The BIRNH study was supported by National Fund for Scientific Research grant 3.9002.79 and the Algemene Spaar- en Lijfrentekas (parastatal insurance company), Brussels, Belgium. This research project was funded by the Unilever Chair in Nutritional Epidemiology, Catholic University of Leuven, Belgium.
Participating universities and principal staff were as follows: Gent State UniversityDr. Gaston Verdonk, Dr. Karel Vuylsteek, Dr. Guy De Backer, Greet Haelterman, and Chris Seynaeve; Catholic University of LeuvenDr. Jozef V. Joossens, Dr. Hugo Kesteloot, and Jef Geboers; Free University of Brussels (U.L.B.)Dr. Marcel Graffar, Dr. Marcel Kornitzer, Dr. Claude Thilly, Werner Vanneste, Dr. Michèle Dramaix, Francoise Kittel, Liliane Ravet, Anne Van Hemeldonck, and Henri Darquennes; Free University of Brussels (V.U.B.)Dr. Anne-Marie Depoorter; Liège State UniversityDr. Gilberte Reginster-Haneuse; International Agency for Research on Cancer, Lyon, FranceDr. Albert Tuyns.
The authors thank Roos Struyven for her editorial assistance.
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NOTES
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Reprint requests to Dr. Hugo Kesteloot, Department of Epidemiology, School of Public Health, Catholic University of Leuven, Capucijnenvoer 33, B-3000, Leuven, Belgium (e-mail: Hugo.Kesteloot{at}med.kuleuven.ac.be).
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Received for publication April 20, 1999.
Accepted for publication October 11, 1999.