ARTICLE IN PRESS
JOURNAL OF
FOOD COMPOSITION
AND ANALYSIS
Journal of Food Composition and Analysis 18 (2005) 723–729
www.elsevier.com/locate/jfca
Short Communication
Oxalate content of legumes, nuts, and grain-based flours
Weiwen Chai, Michael Liebman
Department of Family and Consumer Sciences (Human Nutrition), University of Wyoming, University Station,
P.O. Box 3354, Laramie, WY 82071, USA
Received 14 January 2004; accepted 15 July 2004
Abstract
About 75% of all kidney stones are composed primarily of calcium oxalate and hyperoxaluria is a
primary risk factor for this disorder. Since absorbed dietary oxalate can make a significant contribution to
urinary oxalate levels, oxalate from legumes, nuts, and different types of grain-based flours was analyzed
using both enzymatic and capillary electrophoresis (CE) methods. Total oxalate varied greatly among the
legumes tested, ranging from 4 to 80 mg/100 g of cooked weight. The range of total oxalate of the nuts
tested was 42–469 mg/100 g. Total oxalate of analyzed flours ranged from 37 to 269 mg/100 g. The overall
data suggested that most legumes, nuts, and flours are rich sources of oxalate.
r 2004 Elsevier Inc. All rights reserved.
Keywords: Dietary oxalate; Kidney stones; Legumes; Nuts; Flours
1. Introduction
About 75% of all kidney stones are composed primarily of calcium oxalate (Williams and
Wandzilak, 1989) and hyperoxaluria is a primary risk factor for this disorder (Goldfarb, 1988;
Robertson and Hughes, 1993). Urinary oxalate originates from a combination of absorbed
dietary oxalate and endogenous formation from oxalate precursors such as ascorbic acid and
glyoxylate (Williams and Wandzilak, 1989). Restriction of dietary oxalate intake has been
Corresponding author. Tel.: +1-307-766-5597; fax: +1-307-766-5686.
E-mail address: [email protected] (M. Liebman).
0889-1575/$ - see front matter r 2004 Elsevier Inc. All rights reserved.
doi:10.1016/j.jfca.2004.07.001
ARTICLE IN PRESS
724
W. Chai, M. Liebman / Journal of Food Composition and Analysis 18 (2005) 723–729
suggested as a treatment to prevent recurrent nephrolithiasis in some patients (Massey et al.,
1993).
Legumes, nuts, and different types of grain-based flours are commonly consumed throughout
the world. Soybeans and other legumes such as lentils, red kidney beans, and white beans have
been previously analyzed for oxalate (Massey et al., 2001; Hönow and Hesse, 2002). The oxalate
content of nuts has been reported to be relatively high (Massey et al., 1993) and there are
published values in the literature for almonds, cashews, hazelnuts, peanuts, pecans, pistachios,
and walnuts (Hodgkinson, 1977; Brinkley et al., 1981, 1990; Noonan and Savage, 1999; Hönow
and Hesse, 2002). However, comprehensive reports of oxalate concentrations in either legumes or
nuts have not been published. In addition, there are few reported data on the oxalate contents of
different types of flour products. Thus, the purpose of the present study was to determine the
oxalate content of various types of legumes, nuts, and flours.
2. Materials and methods
2.1. Samples
The following types of legumes were assessed for oxalate: azuki beans, anasazi beans, black
beans, garbanzo beans, great northern beans, large lima beans, mung beans, navy beans, october
beans, pink beans, pinto beans, red kidney beans, small red beans, small white beans, soybeans,
lentils, green split peas, yellow split peas, and blackeye peas. The types of nuts assayed were
almonds, cashews, hazelnuts, macadamia nuts, peanuts, pecans, pine nuts, pistachio nuts, and
walnuts. The types of finely ground flours were barley flour, buckwheat flour, brown rice flour,
dark rye flour, semolina flour, soy flour, unbleached white flour, whole wheat flour, and
corn meal. All legumes, nuts, and flours were purchased from local establishments in
Laramie, Wyoming.
2.2. Sample preparation
Legumes were soaked overnight, the water discarded, and then boiled in distilled deionized
water until they reached a soft consistency. The cooking times computed from when the water
began to boil were: 2 h for all kinds of beans tested; 1 h and 30 min for blackeye peas; 1 h and
15 min for green split peas and yellow split peas; and 30 min for lentils. For moisture
determinations, cooked legumes, nuts, and all flour samples were dried to constant weight at 80 1C
in an Imperial II incubator for 48 h. Dried legumes were ground in a Pavoni coffee mill until the
particles could not be further homogenized. Nuts, in their original pre-dried state, were also
ground in a Pavoni coffee mill until the particles could no longer be further homogenized. Predried flour samples were used for oxalate extraction.
2.3. Total and soluble oxalate extraction
Total oxalate from all test foods and soluble oxalate from a sampling of the test foods
were extracted according to a previously described method (Ross et al., 1999). A 5 g sample
ARTICLE IN PRESS
W. Chai, M. Liebman / Journal of Food Composition and Analysis 18 (2005) 723–729
725
of dried, finely ground food was accurately weighed into a 500 mL beaker. Approximately
50 mL of 2 M H3PO4 (for total oxalate) or 50 mL of distilled deionized water (for soluble
oxalate) were added. The contents were then mixed and the beaker was placed in a shaking water
bath at 80 1C for 30 min. Aqueous samples containing the extracted oxalate were then
quantitatively transferred to 100 mL volumetric flasks, cooled, made up to volume with distilled
deionized water, and mixed. The diluted extractions were centrifuged at 3000 rpm for 20 min
and the supernatant was filtered through Whatman number 1 filter paper. The use of H3PO4
to extract total oxalate has been previously reported (Holmes et al., 1995). The use of 2 M H3PO4
in the present study rather than 2 M HCL used by Ross et al. (1999) was necessary to prevent
distortion of the oxalate peak by Cl molecules in the subsequent capillary electrophoresis (CE)
analysis.
One type of legume (navy beans), one type of nuts (almonds), and one type of flour (corn meal)
were randomly selected for assessing the degree of methodological variation associated with
oxalate extraction. Two extractions were carried out for each of these food samples followed by
the determination of oxalate by the enzymatic method subsequently described.
In addition to the method described in the previous section, total oxalate from cooked soybeans
and lentils was also extracted using the method of Ohkawa (1985). A 10 g slurry (5 g sample with
5 g distilled deionized water) was homogenized with 20 mL 2 M HCL and the mixture was
centrifuged at 10 000 rpm for 5 min. The supernatant was transferred to a 100 mL volumetric flask.
The extraction was repeated two additional times. The supernatant was collected in the same
flask, diluted to volume (100 mL) with distilled deionized water, and analyzed for oxalate by the
enzymatic method subsequently described.
2.4. Sample analysis using capillary electrophoresis (CE) method
The filtered samples were diluted 10-fold with distilled deionized water. Standard curves were
constructed with known concentrations of oxalate added to a solution containing 65 mg/L NaCl,
11 mg/L Na2SO4 and 5% (volume/volume) 2 M H3PO4.
A Biofocus (Bio-Rad Company, CA) 3000 CE system with a negative power supply was used to
analyze oxalate content. Samples were detected at a wavelength of 254 nm. Separation was
conducted on a 75 mm internal diameter 60 cm length polyimide-coated fused-silica capillary
(Polymicro Technologies, Phoenix, AZ).
The background electrolyte solution contained 10 mmol/L sodium chromate (Aldrich Chemical
Company, Inc, Milwaukee, WI) and 0.5 mmol/L tetradecyl-trimethyl-ammonium bromide (Sigma
Chemical Co, St. Louis, MO). The pH (8.1) was not adjusted. The electrolyte solution was
degassed using vacuum before use. Samples were introduced at 10 KV for 10 s. Separations were
conducted at a constant voltage of 16 KV. The capillary was washed for 1 min with 0.1 mol/L
KOH, 1 min with 0.1 mol/L HCL, and 2 min with the electrolyte solution between samples
(Holmes, 1995). The oxalate peak was identified and calculated by comparison of the retention
time and peak area to the standard oxalate curve.
An oxalate recovery determination was conducted by adding known levels of oxalic
acid (3.0 or 6.0 mg) to 100 mL of a 10-fold diluted almond sample which had been
extracted with 2 M H3PO4 as previously described. Each of these samples was analyzed by CE
in duplicate.
ARTICLE IN PRESS
726
W. Chai, M. Liebman / Journal of Food Composition and Analysis 18 (2005) 723–729
2.5. Sample analysis using enzymatic method
Oxalate from filtered food samples was also measured in duplicate by using a Sigma oxalate kit
(Sigma Diagnostics, St. Louis, MO). The method is based on the oxidation of oxalate by oxalate
oxidase followed by detection of the H2O2 produced during the reaction (Li and Madappally,
1989). Lyophilized (control) urine samples (Sigma Diagnostics, St. Louis, MO) providing predetermined oxalate levels of 20–30 mg/L were analyzed with each assay to ensure good quality
control.
2.6. Statistical analysis
Pearson correlation coefficients (r) were computed to assess the strength of the association
between oxalate levels measured by CE and enzymatic methods and to determine whether there
was an association between oxalate and moisture contents within each of the three types of foods
tested. All P values o0.05 were considered to designate statistical significance.
3. Results and discussion
The measurement of oxalate by the CE method agreed well with its measurement by the
enzymatic method. The Pearson correlation coefficient (r) between oxalate levels as determined by
the two methods for all food samples was 0.99. However, the CE method did not provide an
accurate measurement when the oxalate concentration of a sample was relatively low (i.e.,
p1.5 mg/L). The CE oxalate recovery determination yielded recoveries of 98% and 94% for the
addition of 3.0 and 6.0 mg oxalic acid to the almond acid extract, respectively. There was also a
good agreement in total oxalate contents between the two extracts obtained from each of the three
randomly selected food samples (i.e., 52 and 58 mg/100 g for navy beans; 491 and 503 mg/100 g for
almonds; and 52 and 55 mg/100 g for corn meal). No significant correlations between oxalate and
moisture contents were observed for any of the three types of foods analyzed (legumes, nuts, and
flours).
Data from this study indicated that nuts contain high levels of total oxalate, ranging from 42 to
469 mg/100 g (approximately equivalent to 12–131 mg/serving of 28 g) (Table 1). Kidney stone
patients who form calcium oxalate-containing stones are advised to limit their intake of foods
which contain 410 mg oxalate per serving, with total oxalate intake not to exceed 50–60 mg/day
(Chicago Dietetic Association, 2000). Using these guidelines, none of the nuts assessed could be
recommended for kidney stone patients. The values for total oxalate contents of almonds and
pecans were about 3.5 and 5 times higher than the 131 mg/100 g and 12 mg/100 g reported in the
previous literature (Brinkley et al., 1981, 1990). The difference may be due to the different
methods used for oxalate extraction and analysis. Variations among almond or pecan cultivars
may also partly contribute to the different oxalate levels reported. The oxalate values obtained for
peanuts (140 mg/100 g) and cashews (262 mg/100 g) were close to the 116 and 231 mg/100 g
previously reported (Brinkley et al., 1990; Noonan and Savage, 1999). The oxalate values reported
by Hönow and Hesse (2002) for almonds (383 mg/100 g), hazelnuts (167 mg/100 g), and pistachios
(57 mg/100 g) were similar to the values presently reported (Table 1).
ARTICLE IN PRESS
W. Chai, M. Liebman / Journal of Food Composition and Analysis 18 (2005) 723–729
727
Table 1
Oxalate content of various types of nutsa
Nuts
Almonds (roasted)
Cashews (roasted)
Hazelnuts (raw)
Pine nuts (raw)
Peanuts (roasted)
Walnuts (raw)
Pecans (raw)
Pistachio nuts
(roasted)
Macadamia nuts (raw)
Oxalate content (mg/100 g wet weight)
Moisture content
(g/100 g wet weight)
Enzymatic
method
CE method
Mean of the
two methods
491
263
221
199
131
77
66
51
447
260
223
196
148
70
62
46
469
262
222
198
140
74
64
49
1.7
1.3
3.9
1.9
1.1
2.6
2.6
1.9
43
40
42
1.5
a
Sample n=1 for all types of nuts; wet weight refers to original store-bought weight prior to Imperial II incubator
drying.
There was a wide oxalate range of 4–80 mg/100 g in various types of cooked legumes (Table 2).
The consumption of legumes such as anasazi beans, small white beans, great northern beans, pink
beans, black beans, navy beans, soybeans, and small red beans should be considered carefully for
kidney stone patients since total oxalate in 1 serving (1 cup, approximately 170 g) of these cooked
legumes exceeds 50 mg. Legumes containing low levels of total oxalate such as green split peas,
yellow split peas, and blackeye peas could be recommended for kidney stone patients. Total
oxalate content of soybeans and lentils were much lower than values reported by Massey et al.
(2001). However, the presently reported values for lentils, red kidney beans, and white beans were
similar to those reported by Hönow and Hesse (2002).
Oxalate from cooked soybeans and lentils was also extracted according to the method reported
by Ohkawa (1985) to assess whether different extraction methods would significantly affect
oxalate levels. The two extraction methods yielded almost identical oxalate results. Holmes et al.
(1995) reported that altering extraction conditions by increasing acid concentration, temperature,
and time of incubation, or re-extraction of the pellet did not increase oxalate yield.
Almonds and black beans were further analyzed for soluble oxalate since they are commonly
consumed and contain high total oxalate levels. The soluble oxalate content of almonds was
153 mg/100 g which was about 31% of the total oxalate content. The soluble oxalate content of
black beans was 4 mg/100 g of cooked weight which was about 5% of the total. Thus, the
proportion of soluble oxalate in almonds was about 6-fold greater than that in black beans. Since
soluble oxalate in foods appears to be more bioavailable than insoluble oxalate (Albihn and
Savage, 2001), almond consumption could be considered to be a much higher risk for individuals
with hyperoxaluria as compared to the consumption of black beans. Future research should
estimate the proportions of soluble oxalate in other high oxalate-containing nuts and legumes and
determine whether the pattern is similar to that found in almonds and black beans.
The analyzed flours contained relatively high levels of total oxalate, ranging from 37 to 269 mg/
100 g (Table 3). These results may be of use to kidney stone patients since there are few reported
ARTICLE IN PRESS
728
W. Chai, M. Liebman / Journal of Food Composition and Analysis 18 (2005) 723–729
Table 2
Oxalate content of various types of cooked legumesa
Legumes
Oxalate content (mg/100 g wet weight)
Anasazi beans
Small white beans
Great northern beans
Pink beans
Black beans
Navy beans
Soybeans
Small red beans
Pinto beans
October beans
Azuki beans
Red kidney beans
Garbanzo beans
Mung beans
Lentils
Large lima beans
Green split peas
Yellow split peas
Blackeye peas
Moisture content
(g/100 g wet weight)
Enzymatic
method
CE method
Mean of the
two methods
85
77
77
75
73
58
57
36
29
28
26
19
9
8
8
8
6
5
4
75
78
72
75
71
56
55
33
25
27
23
13
—b
—b
—b
—b
—b
—b
—b
80
78
75
75
72
57
56
35
27
28
25
16
9
8
8
8
6
5
4
71
66
69
64
66
65
70
64
74
66
63
68
65
71
72
67
66
69
59
a
Sample n=1 for all types of legumes; wet weight refers to cooked (boiled and drained) weight prior to Imperial II
incubator drying.
b
CE was not able to measure oxalate because of the low oxalate concentration of the sample.
Table 3
Oxalate content of various types of floursa
Flours
Buckwheat
Soy
Whole wheat
Barley
Corn meal
Dark rye
Semolina
Unbleached
white
Brown rice
a
Oxalate content (mg/100 g wet weight)
Moisture content
(g/100 g wet weight)
Enzymatic
method
CE method
Mean of the two
methods
271
187
68
59
55
52
48
41
267
179
66
53
52
49
48
38
269
183
67
56
54
51
48
40
6.8
4.5
6.7
7.0
6.6
6.7
7.2
6.4
38
35
37
6.8
Sample n=1 for all types of flours; wet weight refers to original store-bought weight prior to Imperial II incubator
drying.
ARTICLE IN PRESS
W. Chai, M. Liebman / Journal of Food Composition and Analysis 18 (2005) 723–729
729
data on the oxalate levels of various types of flours. Diets which are heavily based on flour
products may increase pre-disposition to calcium oxalate-containing kidney stones in susceptible
individuals.
Oxalate bioavailability can be defined as the percentage of oxalate absorbed from an oxalatecontaining food. The ability of various oxalate-containing foods to increase urinary oxalate
excretion and pre-disposition to stone formation depends on both oxalate content and
bioavailability (Brinkley et al., 1981). Thus, it is also important for future studies to determine
the oxalate bioavailability of high oxalate-containing legumes, nuts, and flours.
Acknowledgements
This study was supported by a grant from the VP Foundation, Graham, North Carolina.
References
Albihn, P.B.E., Savage, G.P., 2001. The bioavailability of oxalate from oca (Oxalis tuberosa). Journal of Urology 166,
420–422.
Brinkley, L., MgGuire, J., Gregory, J., Pak, C.Y.C., 1981. Bioavailability of oxalate in foods. Urology 17 (6), 534–538.
Brinkley, L.J., Gregory, J., Pak, C.Y.C., 1990. A further study of oxalate bioavailability in foods. Journal of Urology
144, 94–96.
Chicago Dietetic Association, 2000. Manual of Clinical Dietetics. American Dietetic Association, Chicago, IL, p. 475.
Goldfarb, S., 1988. Dietary factors in the pathogenesis and prophylaxis of calcium nephrolithiasis. Kidney
International 34, 544–555.
Hodgkinson, A., 1977. Oxalic Acid in Biology and Medicine. Academic Press, New York, pp. 193–212.
Holmes, R.P., 1995. Measurement of urinary oxalate and citrate by capillary electrophoresis and indirect ultraviolet
absorbance. Clinical Chemistry 41 (9), 1297–1301.
Holmes, R.P., Goodman, H.O., Assimos, D.G., 1995. Dietary oxalate and its intestinal absorption. Scanning
Microscopy 9 (4), 1109–1120.
Hönow, R., Hesse, A., 2002. Comparison of extraction methods for the determination of soluble and total oxalate in
foods by HPLC-enzyme-reactor. Food Chemistry 78, 511–521.
Li, M.G., Madappally, M.M., 1989. Rapid enzymatic determination of urinary oxalate. Clinical Chemistry 35,
2330–2333.
Massey, L.K., Roman-Smith, H., Sutton, R.A.L., 1993. Effect of dietary oxalate and calcium on urinary oxalate and
risk of formation of calcium oxalate kidney stones. Journal of the American Dietetic Association 93, 901–906.
Massey, L.K., Palmer, R.G., Horner, H.T., 2001. Oxalate content of soybean seeds (glycine max: leguminosae),
soyfoods, and other edible legumes. Journal of Agricultural and Food Chemistry 49, 4262–4266.
Noonan, S.C., Savage, G.P., 1999. Oxalate content of foods and its effect on humans. Asia Pacific Journal of Clinical
Nutrition 8 (1), 64–74.
Ohkawa, H., 1985. Gas chromatographic determination of oxalic acid in foods. Journal of the Association of Official
Analytical Chemists 68 (1), 108–111.
Robertson, W.G., Hughes, H., 1993. Importance of mild hyperoxaluria in the pathogenesis of urolithiasis—new
evidence from studies in the Arabian peninsula. Scanning Microscopy 7, 391–401.
Ross, A.B., Savage, G.P., Martin, R.J., Vanhanen, L., 1999. Oxalate in oca (New Zealand yam) (Oxalis tuberosa Mol.).
Journal of Agricultural and Food Chemistry 47, 5019–5022.
Williams, H.E., Wandzilak, T.R., 1989. Oxalate synthesis, transport and the hyperoxaluric syndromes. Journal of
Urology 141, 742–747.