Relation of Serum Ascorbic Acid to Serum Vitamin B12, Serum Ferritin, and Kidney Stones in US Adults

ORIGINAL INVESTIGATION

Joel A. Simon, MD, MPH; Esther S. Hudes, PhD, MPH

Background: Concern has been raised that high levels was marginally associated with higher serum vitamin B12 of ascorbic acid consumption may lead to potential ad- levels: each 57-µmol/L (1.0-mg/dL) increase in serum verse effects, such as vitamin B12 deficiency, iron over- ascorbic acid level was associated with a serum vitamin load, and kidney stones. B12 level increase of 27 pmol/L (36 pg/mL) (P = .10). In addition, serum ascorbic acid level was not associated with Objective: To examine the relation of serum ascorbic correlates of vitamin B12 deficiency, such as higher MCV acid level, which reflects intake, to serum vitamin B12 level, levels, macrocytosis (MCV Ͼ100), or lower hemoglo- serum ferritin level, and kidney stones. bin concentrations. Serum ascorbic acid level was not independently associated with serum ferritin levels. How- Methods: We analyzed data collected on a random ever, among women only, serum ascorbic acid levels were sample of the US population enrolled in the Second Na- associated in a nonlinear fashion with prevalence of el- tional Health and Nutrition Examination Survey, 1976- evated serum ferritin levels (P = .02). We found no as- 1980. We analyzed data using linear and logistic regres- sociation between serum ascorbic acid level and preva- sion models. Serum ascorbic acid, serum vitamin B12, lence of kidney stones in women or men (both PϾ.05). hemoglobin, red blood cell mean corpuscular volume (MCV), and serum ferritin levels were measured using Conclusions: Serum ascorbic acid levels were not asso- standardized protocols. History of kidney stones was de- ciated with decreased serum vitamin B12 levels (or indica- termined by self-report. tors of vitamin B12 deficiency), prevalence of kidney stones, serum ferritin levels, or—among men—prevalence of el- Results: After multivariate adjustment, serum ascorbic evated serum ferritin levels. Serum ascorbic acid levels were acid level was associated with higher serum vitamin B12 associated with prevalence of elevated serum ferritin lev- levels among women in regression models that assumed els among women. Although the clinical relevance of these a linear relationship; each 57-µmol/L (1.0-mg/dL) in- findings is uncertain, it seems prudent to suggest that crease in serum ascorbic acid level (range, 6-153 µmol/L women with a genetic susceptibility to iron overload should [0.1 to 2.7 mg/dL]) was independently associated with consider moderating their intake of ascorbic acid. a serum vitamin B12 level increase of 60 pmol/L (81 pg/ mL) (PϽ.001). Among men, serum ascorbic acid level Arch Intern Med. 1999;159:619-624

ANY AMERICANS con- ond National Health and Nutrition Ex- sume ascorbic acid amination Survey (NHANES II), a study supplements daily.1 Al- that surveyed a large random sample of the though high levels of American population. ascorbic acid ingestion Mhave been judged safe in healthy persons,2-4 concern has been raised regarding the re- RESULTS lation of such ingestion to vitamin B12 deficiency, iron overload, and kidney stones.5-8 Most studies that have examined these re- Baseline characteristics of women and men lations have analyzed data from small num- are presented in Table 1. Men had higher bers of subjects7,9-15 or from laboratory dietary iron intakes and serum ferritin lev- experiments.5,16-18 The use of ascorbic acid els compared with women but roughly From the General Internal supplements may have important public comparable serum vitamin B12 levels. Ap- Medicine Section, Medical health implications, particularly if adverse proximately 3% of women and 5% of men Service, Veterans Affairs health effects result as a consequence. reported a history of kidney stones. Preva- Medical Center, San Francisco, Calif (Dr Simon), and To examine the relation of blood lence of high serum ferritin levels was ap- Department of Epidemiology ascorbic acid level, which reflects usual di- proximately 5% among women and 8% and Biostatistics, University of etary intake, to serum vitamin B12 level, se- among men. A varying number of sub- California, San Francisco rum ferritin level, and kidney stones, we jects in our sample gave a history of us- (Drs Simon and Hudes). analyzed data from participants of the Sec- ing vitamin C supplements, and, as ex-

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Downloaded From: https://jamanetwork.com/ on 09/24/2021 METHODS published by the NHANES II Expert Scientific Working Group on iron status of the US population.22 These cut points SUBJECTS were more than 200 µg/L in men and more than 150 µg/L in women aged 20 to 44 years; more than 300 µg/L in men NHANES II was a national probability survey of more than and 200 µg/L in women aged 45 to 64 years; and more than 19 400 µg/L in men and more than 300 µg/L in women aged 20 000 Americans conducted between 1976 and 1980. Par- 22 ticipants, whose age ranged from 3 to 74 years, were in- 65 to 74 years. terviewed and examined by study personnel at 2 visits.19 A history of kidney stones was established by a posi- We analyzed data from adult participants between the ages tive response to either of the following 2 questions: “Have of 20 and 74 years. Data were available from 10 735 par- you ever had kidney stones?” and “Did a doctor ever tell ticipants who answered questions regarding a history of you that you had kidney stones or stones in the ureter?” kidney stones. Serum vitamin B12 levels and serum ferritin levels were available for 1798 participants and 3407 par- STATISTICAL METHODS ticipants, respectively. We examined the distribution of ascorbic acid concentra- MEASUREMENTS tions and other variables of interest using sample weights. We used simple and multiple linear regression models to NHANES II questionnaire data included self-reported age, examine the relation of serum ascorbic acid level to serum race, sex, menopausal status, years of education com- vitamin B12 level, hemoglobin concentration, MCV, and se- pleted, level of leisure time physical activity, history of smok- rum ferritin level. For the dichotomous outcomes, ie, pres- ing, level of alcohol intake, gout, and use of diuretic medi- ence of elevated serum ferritin levels and kidney stones, cations, hormones, and nutritional supplements. Nutrition we used simple and multiple logistic regression models. data were collected using a food frequency questionnaire Level of education, level of physical activity, and alcohol and 24-hour diet recall. We calculated body mass index (a consumption were analyzed as ordinal variables in the mul- measure of weight in kilograms divided by the square of tivariate models. height in meters) from weight and height data recorded dur- Analyses were performed using Stata software that in- ing the physical examination. The questionnaires, dietary cluded survey commands for the analysis of complex sur- methods, and examination procedures used in NHANES vey data.24 We calculated linear regression coefficients and II have been described elsewhere in detail.19 95% confidence intervals (CIs) to estimate differences in Serum ascorbic acid levels were measured at the Cen- serum vitamin B12 level, hemoglobin concentration, MCV, ters for Disease Control and Prevention by the dinitrophenyl- and serum ferritin level associated with each 57-µmol/L (1.0- hydrazine method using a standardized protocol.20 Be- mg/dL) increase in serum ascorbic acid level. Odds ratios cause there were a small number of extreme serum ascorbic and their 95% CIs were calculated using logistic regres- acid values of questionable validity (ranging as high as 1028 sion to estimate the relative prevalence of elevated serum µmol/L [18.1 mg/dL]), we excluded participants with ascor- ferritin levels and kidney stones. Hosmer-Lemeshow good- bic acid levels in the top 0.5% of the sample (n = 54). Ascor- ness-of-fit tests25 modified for weighted data were per- bic acid levels for the remaining 99.5% of the participants formed using a SAS macro program.26 Two-tailed P values ranged from 6 to 153 µmol/L (0.1-2.7 mg/dL). of .05 were considered to be statistically significant, unad- 27 Serum vitamin B12 levels were measured in a sub- justed for multiple comparisons. group of participants using the radioassay method de- Figures examining the relation of serum ascorbic acid 20 21 scribed by Gunter et al and Lau et al. Serum ferritin lev- level to serum vitamin B12 level and hemoglobin concen- els were measured at the University of Kansas Medical tration were calculated using linear and quadratic coeffi- Center, Kansas City. For both serum vitamin B12 and se- cients derived from linear regression models. Predicted lev- rum ferritin assays, a subsample of persons was selected els of the outcome variables were calculated within each as follows: all persons whose NHANES II numbers ended 6-µmol/L (0.1-mg/dL) serum ascorbic acid level and the low- in 8 and persons with 1 or more abnormal hematologic val- ess smoothing procedure was applied to produce the plots.28 ues (red blood cell count, hemoglobin, hematocrit, red blood Figures that examined the association between serum ascor- cell mean corpuscular volume [MCV], white blood cell bic acid level and prevalence of elevated serum ferritin level count). An additional ferritin subsample of participants was were used to plot the adjusted prevalence of elevated se- also randomly chosen at the end of the survey for ferritin rum ferritin level as a function of serum ascorbic acid level. analysis.22 A 2-site immunoradiometric assay was per- Logistic coefficients were used to compute individual pre- formed and samples with high serum ferritin levels (Ͼ200 dicted probability of elevated serum ferritin level. Next, we µg/L) and low serum ferritin levels (Ͻ20 µg/L) were reas- computed the mean of these probabilities within each 6- sayed.22,23 To identify participants with elevated serum fer- µmol/L (0.1-mg/dL) serum ascorbic acid level and then ap- ritin levels, we used the age- and sex-specific cut points plied the lowess smoothing procedure to produce the plots.28

pected, participants with the highest serum ascorbic acid We examined whether serum ascorbic acid level was levels were more likely to report such use. Approxi- associated with serum vitamin B12 level in several models mately 1% of participants in the lowest serum ascorbic (Table 2). In univariate, age-adjusted, and multivariate acid quintile used supplements compared with 6% to 12% models, serum ascorbic acid level was associated with of participants in the highest quintile. Mean dietary ascor- higher serum levels of vitamin B12 in women; in the mul- bic acid intakes for women and men were 94 mg/d and tivariate model, each 57-µmol/L (1.0-mg/dL) increase in 105 mg/d, respectively. serum ascorbic acid level was independently associated with

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Downloaded From: https://jamanetwork.com/ on 09/24/2021 Table 1. Characteristics of the Study Subjects Table 2. Relation of Serum Ascorbic Acid Level (5785 Women and 5214 Men) (per 57 µmol/L [1 mg/dL]) to Serum Vitamin B12 (pmol/L)*

Women, Men, Women Men Characteristic Mean (SD) Mean (SD) P* No. Slope (95% CI) No. Slope (95% CI) Serum ascorbic acid level, 62 (28) 51 (28) Ͻ.001 µmol/L [mg/dL] [1.1 (0.5)] [0.9 (0.5)] Univariate model 908 46 (15 to 77)† 890 27 (−3 to 58) Age-adjusted model 908 44 (13 to 75)† 890 26 (−4 to 57) Serum B12 level, pmol/L, 489 (181) 483 (146) .46 [pg/mL]† [663 (246)] [654 (198)] Multivariate model 881 60 (32 to 88)‡ 880 27 (−6 to 60) Serum ferritin level, µg/L‡ 60 (65) 119 (99) Ͻ.001 Age, y 42.5 (15.6) 42.1 (15.3) .19 *Multivariate models adjusted for age, race, body mass index, smoking, Dietary iron intake, mg/d 10.7 (5.7) 16.2 (8.9) Ͻ.001 education level, alcohol intake, level of physical activity, and vitamin B Dietary calcium intake, mg/d 600 (442) 899 (659) Ͻ.001 supplement use. CI indicates confidence interval. Body mass index, kg/m2 25.1 (5.6) 25.5 (4.0) .01 †PϽ.01. ‡PϽ.001. White, % 87 88 .37 Current smokers, % 34 42 Ͻ.001 Level of alcohol 738 consumption, % (1000) ,

Ն6 drinks/wk 6 20 12 1 to 5 drinks/wk 51 56 Ͻ.001 664 Ͻ1 drink/wk 42 25 (900) Level of education, % High school or greater 68 69 590 Some high school 18 16 .03 (800) No high school 14 15 pmol/L(pg/mL) Level of leisure time 516 physical activity, % (700) Frequent 31 39 Predicted Level of Serum Vitamin B Seldom 56 47 Ͻ.001 443 Never 13 15 (600) 6 (0.1)42 (0.8) 80 (1.4) 116 (2.1) 153 (2.7) History of kidney stones, %§ 3 5 Ͻ.001 Serum Ascorbic Acid Level, µmol/L (mg/dL) Diuretic therapy, %࿣ 13 6 Ͻ.001 History of gout, %¶ 2 4 Ͻ.001 Figure 1. Relation between serum ascorbic acid concentration and serum vitamin B12 level among 888 women (O) and 880 men (+) enrolled in the Second National Health and Nutrition Examination Survey (NHANES II), *P values for continuous variables and categorical variables were obtained 1976-1980, based on the multivariate model noted in Table 2. by linear regression and logistic regression, respectively, taking into account sample weights and survey design effects. All numbers reflect sample weights. acid levels were associated with indicators of vitamin B12 †N = 908 and 890 for women and men, respectively. ‡N = 1795 and 1612 for women and men, respectively. deficiency, such as hemoglobin concentration, red blood §N = 5618 and 5117 for women and men, respectively. cell MCV, and macrocytosis (MCV Ͼ100). Among par- ࿣N = 5750 and 5187 for women and men, respectively. ticipants in whom measurements of serum vitamin B12 ¶N = 5782 and 5213 for women and men, respectively. were performed, we found no association between serum ascorbic acid level and hemoglobin concentrations a serum vitamin B12 level increase of 60 pmol/L (81 pg/ in men (P = .34) but found a significant curvilinear re- mL) in women (PϽ.001). Among men, each 57-µmol/L lation among women (P = .001) (Figure 2). We also ex- (1.0-mg/dL) increase in serum ascorbic acid level was mar- amined whether serum ascorbic acid level was a corre- ginally associated with a serum vitamin B12 level increase late of red blood cell MCV and prevalence of macrocytosis. of 27 pmol/L (36 pg/mL) (P = .10). Multivariate models Serum ascorbic acid level was not associated with either controlled for the effects of a number of variables, includ- MCV or macrocytosis in women or men (all PϾ.08). Fur- ing age, race, smoking, alcohol consumption, and use of thermore, self-reported ascorbic acid supplement use was vitamin B supplements. Because we also identified signifi- not independently associated with serum vitamin B12 lev- cant curvilinear relations between serum ascorbic acid and els in either women (P = .92) or men (P = .11). serum vitamin B12 levels (P = .09 in men and .04 in wom- We also assessed the relation between serum ascor- en) and because the proportion of variance explained by bic acid and ferritin levels (Table 3). In univariate mod- both the linear and nonlinear models was almost identi- els, serum ascorbic acid level was significantly associ- cal, we also present these relations using models contain- ated with serum ferritin level among women (PϽ.01) but ing linear and quadratic terms (Figure 1). not among men (P = .99) The association between ascor- Less than 1% of participants had low serum vita- bic acid and iron status among women was attenuated min B12 levels (Ͻ221 pmol/L [300 pg/mL]) consistent with after multivariate adjustment (P = .05) and further at- vitamin B12 deficiency. Serum ascorbic acid level was as- tenuated after controlling for the additional effects of sociated with a decreased prevalence of low serum vita- menopausal status and hormone use (P = .14). Serum min B12 levels in women (P = .001) but was not associ- ascorbic acid level was not associated with elevated se- ated with prevalence of low serum vitamin B12 levels in rum ferritin levels in men (P = .59) (Table 4) but was men (P = .58). Because the assay used in NHANES II can- independently associated in a nonlinear fashion with el- not distinguish between active and inactive analogs of evated serum ferritin levels in women (P = .02) vitamin B12, we also examined whether serum ascorbic (Figure 3). Women with the highest serum ascorbic acid

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Downloaded From: https://jamanetwork.com/ on 09/24/2021 145 COMMENT Concern has been expressed that high levels of ascorbic 140 acid consumption may be placing many Americans at risk for adverse health outcomes, such as vitamin B12 deficiency, iron overload, and kidney stones.5,6 Several stud-

135 ies that have examined these issues were performed in a laboratory setting 5,16-18 or on small numbers of subjects.7,9-15 Among a representative sample of the adult US population surveyed between 1976 and 1980, we found 130 no evidence that higher serum ascorbic acid levels re-

Predicted Hemoglobin Concentration, g/L sulting from ascorbic acid consumption adversely af- fected serum vitamin B12 levels, prevalence of kidney 125 6 (0.1)42 (0.8) 80 (1.4) 116 (2.1) 153 (2.7) stones, or, among men, prevalence of elevated serum fer- Serum Ascorbic Acid Level, µmol/L (mg/dL) ritin levels. We did, however, find a nonlinear relation between serum ascorbic acid level and prevalence of el- Figure 2. Relation between serum ascorbic acid concentration and hemoglobin concentration among 888 women enrolled in the Second evated serum ferritin levels among women, which ap- National Health and Nutrition Examination Survey (NHANES II), 1976-1980, peared similar to the relation between serum ascorbic acid based on a multivariate model that included menopausal status, hormone level and hemoglobin concentration among women. use, and all variables noted in Table 2.

SERUM VITAMIN B12 LEVELS

Table 3. Relation of Serum Ascorbic Acid Level Using the same vitamin B12 radioassay used in NHANES (per 57 µmol/L [1 mg/dL]) to Serum Ferritin Level (µg/L)* II, Herbert and Jacob5 reported that the addition of ascorbic acid to homogenized test meals resulted in the destruc- Women Men tion of vitamin B12. These findings, however, could not be No. Slope (95% CI) No. Slope (95% CI) replicated.16 Herbert and colleagues11 later reported that 4 Univariate model 1795 11 (3 to 18)† 1612 0 (−13 to 13) of 18 patients with spinal cord injury who received 2 g/d Age-adjusted model 1795 6 (−2 to 13) 1612 −1 (−13 to 12) of ascorbic acid for periods up to several years were vita- Multivariate model 1752 6 (−2 to 13) 1596 −1 (−13 to 11) min B12 deficient. Others, however, have judged these findings to be, at least in part, the result of an artifact of the *Multivariate models adjusted for age, race, body mass index, smoking, assay used to measure vitamin B levels.2 Examination of education level, alcohol intake, level of physical activity, dietary iron intake, 12 and use of iron supplements (including pills and injections). For women, the relation of high intakes of ascorbic acid in 10 patients multivariate models also adjust for menopausal status and hormone use. with spinal cord injury29 and among 40 children with my- CI indicates confidence interval. elomeningoceles30 failed to reveal an association between †PϽ.01. ascorbic acid and vitamin B12 deficiency. To our knowledge, there have been no large population-based studies levels (Ͼ116 µmol/L [Ͼ2.1 mg/dL]) had an approxi- examining the relation of ascorbic acid to vitamin B12 sta- mately 2-fold increase in prevalence of elevated serum tus. We found no evidence that serum ascorbic acid levels ferritin levels compared with women whose serum ascor- are associated inversely with serum vitamin B12 levels. On bic acid levels ranged between 43 and 80 µmol/L (0.8 and the contrary, it appeared that higher ascorbic acid serum 1.4 mg/dL). Ascorbic acid supplement use was not in- levels were, on average, associated with higher serum vi- dependently associated with serum ferritin levels or el- tamin B12 levels among women and men. These findings evated serum ferritin levels among men (P = .34 and .83, could be the result of a correlated intake of these nutri- respectively). Among women, there was no indepen- ents, either in the diet or in nutritional supplements. dent association between ascorbic acid supplement use and serum ferritin levels (P = .81), but we did observe a SERUM FERRITIN LEVELS trend toward a lower prevalence of elevated serum ferritin levels with supplement use (P = .08). Although serum ferritin levels may be elevated as a con- Approximately 3% of women and 5% of men re- sequence of inflammatory conditions, they generally reported a history of kidney stones. Serum ascorbic acid flect total body iron stores and are considered the best mea- level was not associated with an increased prevalence of sure to assess iron status in epidemiologic studies.31 Ascorbic kidney stones in women (multivariate odds ratio = 0.78; acid enhances the absorption of nonheme iron32 and, there- 95% CI, 0.56-1.10; P = .15). A marginally significant in- fore, may aid in the treatment of iron-deficiency anemia. verse association was observed between serum ascorbic However, ascorbic acid may be detrimental to individuals acid level and kidney stones in men (multivariate odds who carry the gene for hemochromatosis.8 In assessing the ratio, 0.72; 95% CI, 0.51-1.02; P = .06) (Table 5). No prevalence of elevated serum ferritin levels based on age- data were available regarding the chemical composition and sex-specific cut points,22 we found no association be- of the self-reported kidney stones. Ascorbic acid supple- tween ascorbic acid and prevalence of elevated serum fer- ment use was not independently associated with preva- ritin levels among men but observed a nonlinear relation lence of kidney stones in either women (P = .87) or men among women. To our knowledge, the relation between (P = .46). ascorbic acid and elevated serum ferritin levels has not been

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Downloaded From: https://jamanetwork.com/ on 09/24/2021 Table 4. Relation of Serum Ascorbic Acid Level (per 57 µmol/L [1 mg/dL]) to Prevalence of Elevated Serum Ferritin Level*

Women Men

No. OR (95% CI) No. OR (95% CI) Univariate model 1795 0.99 (0.48 to 2.05) 1612 0.76 (0.44 to 1.33) Age-adjusted model 1795 0.90 (0.44 to 1.84) 1612 0.76 (0.44 to 1.32) Multivariate model 1752 1596 Serum ascorbic acid 0.10 (0.02 to 0.49)† 0.85 (0.45 to 1.60) Serum ascorbic acid2 2.49 (1.20 to 5.16)‡

*Elevated serum ferritin levels were present in 83 women (3.7%) and in 133 men (8.3%). Multivariate models adjusted for age, race, body mass index, smoking, education level, alcohol intake, level of physical activity, dietary iron intake, and use of iron supplements (including pills and injections). For women, multivariate models also adjusted for menopausal status and hormone use. P values for goodness-of-fit tests for the multivariate models were .83 for women and .48 for men. OR indicates odds ratio; CI, confidence interval. †PϽ.001. ‡PϽ.05.

examined in other large population-based studies. One small nonrandomized study of 17 healthy college students re- 0.20 vealed no adverse effect from high-dose ascorbic acid in- 14 gestion (2 g/d) on serum ferritin levels and iron stores. 0.15 The clinical significance of our findings in women is uncertain. Several reviews have concluded that, for most individuals, high levels of ascorbic acid consumption are 0.10 safe.2-4 Furthermore, other analyses of NHANES data have

reported improved health outcomes and lower mortality Serum Ferritin Level 0.05 rates among individuals with high serum or intake levels of ascorbic acid.33-35 Predicted Prevalence of Elevated 0 6 (0.1)42 (0.8) 80 (1.4) 116 (2.1) 153 (2.7) KIDNEY STONES Serum Ascorbic Acid Level, µmol/L (mg/dL)

Figure 3. Relation of serum ascorbic acid concentration to prevalence of Most kidney stones are composed of calcium oxalate or elevated serum ferritin levels among 1752 women enrolled in the Second uric acid. Although not observed uniformly,12,36,37 some National Health and Nutrition Examination Survey (NHANES II), 1976-1980, studies have reported that urinary oxalate and uric acid based on the multivariate model noted in Table 4. excretion are increased by the ingestion or administra- tion of large amounts of ascorbic acid.2 Studies that examined the relation of ascorbic acid to oxalate and uric Table 5. Relation of Serum Ascorbic Acid Level acid excretion have generally examined small numbers (per 57 µmol/L [1 mg/dL]) to Prevalence of Kidney Stones* of subjects.2 We were able to examine the relation of serum ascorbic acid level to history of kidney stones among Women Men a random sample of more than 10 000 Americans and No. OR (95% CI) No. OR (95% CI) found no evidence to indicate that high serum ascorbic Univariate model 5618 0.88 (0.66 to 1.16) 5117 0.86 (0.65 to 1.14) acid levels increased the prevalence of kidney stones. On Age-adjusted 5618 0.76 (0.57 to 1.02) 5117 0.82 (0.62 to 1.07) the contrary, among men, each 57-µmol/L (1.0-mg/dL) model increase in serum ascorbic acid level was independently Multivariate model 5525 0.78 (0.56 to 1.10) 5044 0.72 (0.51 to 1.02) associated with an approximately 28% decrease in the prevalence of kidney stones (P = .06). These findings agree *A history of kidney stones was reported by 193 women (3.4%) and 309 2 men (6.0%). Multivariate models adjusted for age, race, body mass index, with the conclusions of others that in general, high lev- smoking, education level, level of physical activity, alcohol intake, history of els of ascorbic acid ingestion do not result in increased gout, dietary calcium intake, and use of diuretic medications. P values for kidney stone formation and may, in fact, be associated goodness-of-fit tests for the multivariate models were .37 for women and .36 for men. OR indicates odds ratio; CI, confidence interval. with a decreased prevalence of kidney stones.38 The main limitation of our study derives from the cross-sectional nature of the analyses. Thus, we cannot be decreasing urinary calcium excretion.13 Because most certain that the outcomes did not affect ascorbic acid con- people are unlikely to know their serum ferritin level or sumption and, in turn, serum ascorbic acid levels. For ex- vitamin B12 level, it seems less probable that these asso- ample, it is possible that persons with kidney stone dis- ciations resulted from individuals changing their ascor- ease decreased their ascorbic acid intake and consequently bic acid intake. The serum vitamin B12 radiodilution as- lowered their blood ascorbic acid level, thereby produc- say used by NHANES II measures both biologically active 39 ing the ostensible protective association between ascor- and inactive vitamin B12 analogs. Hence, we cannot be bic acid and kidney stones. Although we cannot exclude certain that the higher levels of serum vitamin B12 associ- this possibility, it is also biologically plausible that ascor- ated with serum ascorbic acid levels reflect higher levels bic acid may, in fact, lower the risk of kidney stones by of the biologically active form of the vitamin. However,

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Downloaded From: https://jamanetwork.com/ on 09/24/2021 5. Herbert V, Jacob E. Destruction of vitamin B by ascorbic acid. JAMA. 1974; examination of the reliability of the serum vitamin B12 ra- 12 230:241-242. diodilution assay, when compared with the microbio- 6. Herbert V. The antioxidant supplement myth. Am J Clin Nutr. 1994;60:157-158. logic vitamin B12 assay, suggests that the comparative rank 7. Stein HB, Hasan A, Fox IH. Ascorbic acid-induced uricosuria: a consequence of order of serum values is likely to be similar across as- megavitamin therapy. Ann Intern Med. 1976;84:385-388. 40 8. Herbert V, Shaw S, Jayatilleke E. Vitamin C supplements are harmful to lethal says. Higher serum ascorbic acid levels were not associ- for the over 10% of Americans with high iron stores. FASEB J. 1994;8:A678. ated with lower hemoglobin concentrations, higher red Abstract 3934. blood cell MCVs, or increased prevalence of macrocyto- 9. Berger L, Gerson CD, Yu¨ T-F. The effect of ascorbic acid on uric acid excretion with a commentary on the renal handling of ascorbic acid. Am J Med. 1977;62:71-76. sis, which further suggests that serum ascorbic acid lev- 10. Tiselius HG, Almgard LE. The diurnal urinary excretion of oxalate and the effect els, which reflect ascorbic acid intake, are unlikely to be of pyridoxine and ascorbate on oxalate excretion. Eur Urol. 1977;3:41-46. 11. Herbert V, Jacob E, Wong KT, Scott J, Pfeffer RD. Low serum vitamin B12 levels associated with decreased serum levels of biologically ac- in patients receiving ascorbic acid in megadoses: studies concerning the effect tive vitamin B12. Although serum ascorbic acid levels re- of ascorbate on radioisotope vitamin B12 assay. Am J Clin Nutr. 1978;31:253-258. flect intake patterns over days to weeks,41 we had only a 12. Mitch WE, Johnson MW, Kirshenbaum JM, Lopez RE. Effect of large oral doses of ascorbic acid on uric acid excretion by normal subjects. Clin Pharmacol Ther. single measurement of serum ascorbic acid, which may 1981;29:318-321. not reflect longer-term intake. Finally, because the geno- 13. Schmidt KH, Hagmaier V, Hornig DH, Vuilleumier JP, Rutishauser G. Urinary oxa- type of participants is not known, we cannot comment on late excretion after large intakes of ascorbic acid in man. Am J Clin Nutr. 1981; 34:305-311. the relation of serum ascorbic acid levels to elevated se- 14. Cook JD, Watson SS, Simpson KM, Lipschitz DA, Skikne BS. The effect of high rum ferritin levels among individuals categorized by their ascorbic acid supplementation on body iron stores. Blood. 1984;64:721-726. genetic susceptibility to iron overload. 15. Urivetzky M, Kessaris D, Smith AD. Ascorbic acid overdosing: a risk factor for calcium oxalate nephrolithiasis. J Urol. 1992;147:1215-1218.

There are 2 notable strengths to these analyses. First, 16. Newmark HL, Scheiner J, Marcus M, Prabhudesai M. Stability of vitamin B12 in NHANES II surveyed a large random sample of the US the presence of ascorbic acid. Am J Clin Nutr. 1976;29:645-649. population using standardized procedures and proto- 17. Newmark HL, Scheiner JM, Marcus M, Prabhudesai M. Ascorbic acid and vitamin B12. JAMA. 1979;242:2319-2320. cols. Hence, our findings should be generalizable. Sec- 18. Marcus M, Prabhudesai M, Wassef S. Stability of vitamin B12 in the presence of ond, the measurement of serum ascorbic acid levels on ascorbic acid in food and serum. Am J Clin Nutr. 1980;33:137-143. 19. National Center for Health Statistics. Plan and Operation of the Second National such a large number of persons provides a more accu- Health and Nutrition Examination Survey, 1976-1980. Hyattsville, Md: National rate and precise assessment of ascorbic acid status as a Center for Health Statistics; 1981. US Dept of Health and Human Services pub- correlate of serum vitamin B level, serum ferritin level, lication (PHS) 81-1317. 12 20. Gunter EW, Turner WE, Neese JW, Bayse DD. Laboratory Procedures Used by and history of kidney stones than studies using dietary the Clinical Chemistry Division, Centers for Disease Control, for the Second Health and supplement estimates only. and Nutrition Examination Survey (HANES II), 1976-1980. Atlanta, Ga: US Dept of Health and Human Services, Public Health Service, Centers for Disease Con- Establishing the safety of high levels of ascorbic acid trol, Center for Environmental Health, Nutritional Biochemistry Branch; 1985.

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