Basu et al. the effect of all other variables specified in the ordinal variables: categories of age, education, largest difference in urinary InAs% was for model. This represents the independent effect type of housing, urinary arsenic, and BMI urinary creatinine, with those in the high- of the nutrient on the specified urinary methy- (Table 1). est tertile of creatinine having 19.8 InAs% lated arsenical. In addition to nutritional fac- compared with those in the lowest tertile, Results tors, the other variables in the model were age, who had 27.7% (difference 7.8%; p ≤ 0.001). sex, housing, education, presence or absence The distribution of participants according to Those in the highest tertile of plasma lyco- of skin lesions, body mass index (BMI), and age, sex, education, and quality of housing pene also had lower InAs% (20.5% compared total urinary arsenic. Housing and education is shown in Table 1. Men had lower urinary with 26.3%; p = 0.01). The largest differ- were modeled as categorical variables, whereas InAs% (men, 22.2%; women, 24.5%; p = ences in MMA% were for dietary animal fat, age, BMI, and urinary arsenic were modeled as 0.05) and higher MMA% than did women plasma retinol, plasma homocysteine, dietary continuous variables. (men, 8.9%; women, 7.0%; p = 0.01). Those retinol, and dietary animal protein, followed The important advantages of using tertiles with arsenic-caused skin lesions had lower by serum folate and urinary creatinine, all of of nutritional factors in the analyses, rather urinary InAs% (21.2% vs. 24.7% for those which were associated with p-values < 0.01. than continuous variables, include the fact without skin lesions; p = 0.02), and higher The largest difference (2.3%) was for ani- that there is no implicit assumption about urinary DMA% (70.4% vs. 67.2%; p = 0.03). mal fat (p < 0.001). When we repeated the the functional form of any dose–response Table 2 presents the mean ± SD of the dietary analysis with animal fat as a continuous vari- relationship. In addition, outliers often have factors intake per day and the mean concen- able, the result was the same (p < 0.001). The undue influence on regression analyses, and trations of blood micronutrients. strongest association with DMA% was for dividing into tertiles avoids this problem, as The differences in mean urinary InAs%, urine creatinine, with those in the highest ter- outliers are merely one member of the tertile MMA%, and DMA% between those in the tile of creatinine having a DMA% of 71.4% to which they belong. However, we repeated highest and lowest tertiles of each dietary compared with those in the lowest tertile, the analyses that produced the key findings intake factor and blood micronutrient are using continuous variable regression and presented in Table 3, ranked in order of the Table 2. Mean intake per day of dietary factors, obtained similar results. p-Values were based magnitude of the absolute differences, exclud- mean plasma and serum concentrations of micro- on chi-square tests of trend for the following ing those factors having a p-value > 0.2. The nutrients, and mean urine creatinine concentra- tions, with SDs for the 405 participants. Table 1. Distribution of the percentage of urinary total arsenic in inorganic form (InAs%), monomethylated Dietary intake/micronutrient (MMA%), and dimethylated (DMA%), according to other factors for the 405 participants. concentration Mean ± SD Characteristics n (%) InAs% MMA% DMA% Diet survey Calcium (mg/day) 484 ± 332 Age (years) Carbohydrate (g/day) 448 ± 161 < 15 28 (6.9) 20.4 6.8 72.8 Carotene (µg/day) 3,546 ± 7,161 15–29 91 (22.5) 23.3 8.4 68.3 Energy (kJ/day) 9,204 ± 3,091 30–44 135 (33.4) 23.6 8.3 68.1 Fat, animal (g/day) 3.91 ± 4.24 45–59 89 (21.9) 24.8 7.9 67.3 Fat, vegetable (g/day) 21.8 ± 14.9 ≥ 60 62 (15.3) 19.5 9.3 71.2 Fiber (g/day) 5.3 ± 3.47 p a 0.19 0.06 0.14 Folate (µg/day) 164 ± 102 Sex Iron (mg/day) 13.9 ± 7.0 Female 154 (38.2) 24.5 7.0 68.5 Niacin (mg/day) 22 ± 8.1 Male 251 (61.8) 22.2 8.9 68.9 Phosphorus (mg/day) 1,130 ± 435 p b 0.05 0.01 0.2 Protein, animal (g/day) 9.7 ± 7.14 Education Protein, vegetable (g/day) 45 ± 17.1 Nonformal 117 (28.9) 22.5 8.2 69.3 Retinol (µg/day) 52.2 ± 66.1 Primary 200 (49.5) 23.5 8.2 68.3 Riboflavin (mg/day) 0.6 ± 0.3 Secondary 68 (16.8) 22.8 8.0 69.2 Thiamin (mg/day) 1.49 ± 0.6 Tertiary 20 (4.9) 22.4 8.3 69.3 Vitamin B6 (mg/day) 1.3 ± 0.5 p a 0.47 0.48 0.46 Vitamin C (mg/day) 111 ± 115 House type Zinc (mg/day) 9.2 ± 3.3 Kacha 205 (50.6) 22.6 8.3 69.1 Plasma analysis Semipucca 138 (34.1) 24.1 7.9 68.1 Alphatocopherol (µg/dL) 636 ± 214 Pucca 58 (14.3) 22.3 8.5 69.2 Beta-carotene (µg/dL) 83.6 ± 97.3 Missing data 4 (0.9) Beta-cryptoxanthine (µg/dL) 5.74 ± 5.6 p a 0.32 0.25 0.4 Homocysteine (µM/L) 14.8 ± 8.1 Skin lesions Lutein-zeaxanthine (µg/dL) 66.5 ± 30.6 Present 213 (52.5) 21.2 8.3 70.4 Lycopene (µg/dL) 3.3 ± 4.9 Absent 192 (47.4) 24.7 8.1 67.2 Methionine (µM/L) 19.4 ± 5.7 p b 0.02 0.56 0.03 Retinol (µM/dL) 34.1 ± 10.9 BMI Transthyretin (mg/L) 237 ± 53.1 < 16.9 122 (30.1) 23.5 8.0 68.5 Vitamin B6 (nmol/L) 40.8 ± 45.3 16.9–19.3 121 (29.8) 21.3 8.7 70.1 Serum analysis > 19.3 126 (31.1) 22.2 8.4 69.4 Cholesterol (mg/dL) 155 ± 35.4 Missing 36 (8.9) Cysteine (µM/L) 215 ± 37.5 p a 0.52 0.42 0.73 Folate (ng/mL) 3.4 ± 2.8 Total urinary arsenic (µg/L) Selenium (µM/L) 1.2 ± 0.4 < 14.9 134 (33.1) 20.4 8.5 71.1 Total glutathione (µM/L) 6.3 ± 5.6 14.9–56.3 133 (32.8) 22.5 7.8 69.6 Vitamin B12 (pg/mL) 435 ± 315 > 56.3 138 (34.1) 21.5 8.5 69.9 Urine p a 0.55 0.48 0.71 Urine creatinine (mg/L) 622 ± 531.8 ap-Value from test for trend. bp-Value from t-test. 1310 119 | number 9 | September 2011 • Environmental Health Perspectives ovlume