by RP Holmes · 1995 · Cited by 112 — Dietary oxalate is currently believed to make only a minor contribution ( < 20 % ) to urinary oxalate excre-.
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Scanning Microscopy Scanning Microscopy Volume 9Number 4 Article 16 10-22-1995 Dietary Oxalate and Its Intestinal Absorption Dietary Oxalate and Its Intestinal Absorption Ross P. Holmes Wake Forest University, rholmes@isnet.is.wfu.edu Harold O. Goodman Wake Forest University Dean G. Assimos Wake Forest University Follow this and additional works at: https://digitalcommons.usu.edu/microscopy Part of the Biology Commons Recommended Citation Recommended Citation Holmes, Ross P.; Goodman, Harold O.; and Assimos, Dean G. (1995) “Dietar y Oxalate and Its Intestinal Absorption,” Scanning Microscopy: Vol. 9 : No. 4 , Article 16. Available at: https://digitalcommons.usu.edu/microscopy/vol9/iss4/16 This Article is brought to you for free and open access b y the Western Dairy Center at DigitalCommons@USU. It has been accepted for inclusion in Scanning Micr oscopy by an authorized administrator of DigitalCommons@USU. For more information, please contact digitalcommons@usu.edu.
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Scanning Microscopy, Vol. 9, No. 4, 1995 (pages 1109-1120) 0892-953X/95$5. 00 +. 25 Scanning Microscopy International, Chicago (AMF O’Hare), IL 60666 USA DIETARY OXALATE AND ITS INTESTINAL ABSORPTION Ross P. HolmesŁ, Harold 0. Goodman, and Dean G. Assimos Departmeqt of Urology, Bowman Gray School of Medicine, Wake Forest University, Medical Center Blvd., Winston-Salem, NC 27157, U.S.A. (Received for publication July 21, 1995 and in revised form October 22, 1995) Abstract Dietary oxalate is currently believed to make only a minor contribution ( < 20 % ) to urinary oxalate tion. A recent prospective study of stone disease gested that dietary oxalate may be a significant risk factor. This observation led us to re-evaluate the tion of dietary oxalate to urinary oxalate excretion. Previous studies have been hampered by inaccurate food composition tables for oxalate and inadequate methods for studying intestinal oxalate absorption. This evidence as well as factors that modify oxalate absorption are reviewed. New approaches to measure food oxalate and intestinal oxalate absorption have been examined. lary electrophoresis appears to be well suited for the analysis of the oxalate content of food. Two individuals consumed an oxalate-free formula diet for 7 days. This diet decreased urinary oxalate excretion by an average of 67 % (18.6 mg per 24 hours) compared to oxalate cretion on self-selected diets. The absence of detectable oxalate in feces by day 6 of the diet suggested that the intestinal absorption was minimal. However, an effect of the formula diet on endogenous oxalate synthesis not be excluded. Restoring oxalate to the formula diet increased urinary oxalate excretion and illustrates that this experimental protocol may be well-suited for ing oxalate absorption and factors that modify it. Our results suggest that the intestinal absorption of dietary oxalate makes a substantial contribution to urinary late excretion and that this absorption can be modified by decreasing oxalate intake or increasing the intakes of calcium, magnesium, and fiber. Key Words: Dietary oxalate, intestinal oxalate tion, urinary oxalate excretion, oxalate-free diet, oxalate analysis. Ł Contact for correspondence: Ross P. Holmes, address as above. Telephone number: (910) 716-4231 FAX number: (910) 716-0174 E-mail: rholmes@isnet.is. wfu.edu 1109 Introduction Oxalate is ubiquitous in the plant kingdom and is consumed in normal human diets as a component of nuts, fruits, vegetables and grains [25]. Oxalate forms an insoluble salt with Ca2+ which may lead to lithiasis or deposition of crystals in tissues. Human sues have limited mechanisms to degrade oxalate. tunately, only a fraction of the oxalate ingested is sorbed into the body in normal individuals. The der is either broken down by intestinal bacteria or creted in feces. This is clearly not the case in uals with enteric hyperoxaluria where either resection or bypass of a segment of the bowel or small bowel dysfunction causes hyperabsorption of dietary oxalate in the colon [15, 21]. This hyperabsorption is believed to sult from a changed colonic milieu due to malabsorption of fat and bile salts in the upper intestinal tract [21]. An alteration in intestinal flora has also been proposed as a contributing factor [2]. Dietary oxalate is usually cited as contributing 10-20 % of the oxalate excreted in urine in normal uals [25, 51], while the remainder is derived from genous synthesis. However, as we shall document low, the amount of oxalate that is ingested and the tion that is absorbed is uncertain, making these factors of unknown significance in calcium oxalate stone ease. The thesis will be developed that the intake of alate is underestimated because of inaccurate techniques used to assay food oxalate, and that absorption is estimated because of the difficulty in assessing uptake in the large intestine. This thesis is supported by a recent prospective study of health professionals which showed that a low dietary intake of Ca2+ was a prominent risk factor for stone formation [12]. The most likely nation for this observation is that increasing dietary Ca2+ intake decreases oxalate absorption. This finding argues that increases in urinary oxalate may have larger effects on stone risk than increases in urinary Ca2 + spite the higher urinary Ca2+ excretion produced by a higher Ca2+ intake [37]. This effect of dietary ca2+ would also suggest that intestinal oxalate absorption may be a more significant risk factor than previously recog-
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R.P. Holmes, H.O. Goodman and D.G. Assimos nized. Individuals with an apparent diet-induced hyper-oxalate oxaluria have been identified in a calcium stone-forming population (20]. Dietary Oxalate Intake One of the first estimates of the dietary intake of oxalate was by Archer et al. [3] who examined the diets of 6 individuals in 1957 and reported intakes to be an average of 920 mg/day. Zarembski and Hodgkinson [52] estimated the intake of oxalate in English diets to be 70-150 mg/day using a revised analytical procedure for oxalate determination in food. Hodgkinson [25] mated that a derived typical diet contained 130 mg of oxalate, a level consistent with their previously reported results and a widely cited value. Although there are many reported values for the alate content of certain foodstuffs (reviewed in [25, 34, 52]), the values reported by Zarembski and Hodgkinson (52] and Kasidas and Rose [34] are the most sive and the most widely used. These values figure prominently (for instance, in Massey et al. [43] and Ney et al. [45]), and are used by health professionals in fering dietary advice to Ca oxalate stone-formers. The validity of the compositional data in advising stone tients is, of course, no greater than the accuracy of the methods used to obtain them. Only multi-stepped and indirect methods have been previously used in these compositional assays and they are subject to ences and inaccuracies. For example, the method used by Hodgkinson and Zarembski (26] involved an ether extract of an acidified sample followed by a precipitation of oxalate with Ca. Both the ether extraction and cipitation are unlikely to be quantitative and the amount extracted may vary widely in different foods. The duction of oxalate to glycolate is also a critical step in this procedure and an incomplete reduction will timate oxalate [49]. The method developed by Kasidas and Rose [34] was an enzyme-linked assay that utilized oxalate oxidase. Inhibitors of the enzyme have to be moved for assays of urinary oxalate with oxalate oxidase (49]. Such inhibitors are also likely to occur in foods and without any pre-purification steps to remove tors from food extracts, an oxalate oxidase-based dure is likely to underestimate oxalate. Other sources for the oxalate content of foods include a 1984 USDA publication (24] reporting values for many vegetables which has been used by some tigators (e.g., by Massey et al. [43]). The methodology employed to assay oxalate content was not stated, and hence, the usefulness of these data is uncertain. cant amounts of oxalate, 100 mg/100 g, were reported in most vegetables including asparagus, beans, broccoli, brussels sprouts, cabbage, carrots, cauliflower, celery, 1110 " .D 0 "' .D : sulfate 3.7 3.8 V 3.9 4.0 4.1 4.2 Migration Time (min) Figure 1. The analysis of oxalate in Bran Flakes (from Post, Kraft Foods, White Plaines, NY) by capillary trophoresis. The sample was homogenized in 9 volumes of 1 M H3PO4 using a Polytron homogenizer and diluted 1/100 for analysis as described for urinary oxalate. The large peak before sulfate is chloride, and the phosphate peak has been cut-off at the end of the profile. chives, eggplant, garlic, lettuce, parsley, sweet potato, and turnips, as well as in the more familiar values for spinach and beet. These values were at least one order of magnitude higher than previous estimates and more recent ones we have obtained using capillary phoresis. Based on these limitations, it is clear that the actual amount of oxalate in normal diets is still uncertain. We have adapted a method developed for the assay of oxalate in urine (28] to measure the oxalate content of foods. The method relies on high performance capillary electrophoresis (CE) with the measurement of anions by indirect ultra-violet (UV) absorbance detection. A Waters (Milford, MA) Quanta 4000 CE was used and the data obtained were analyzed on a microcomputer ing Millennium® (Waters) software. Anions are rated on a 60 cm x 75 µm fused silica capillary column using 10 rnM sodium chromate and 0.5 rnM trimethylamrnonium bromide (a modifier of motic flow) as the electrolyte. Oxalate had a migration time of 4 minutes using a constant current of 25 amps. Because much of the oxalate in many plant species is present as crystalline calcium oxalate (38], procedures that dissolve this oxalate have to be employed. Samples were prepared by homogenization (10% w/v) in 1 M H3PO4 using a Polytron® homogenizer (Brinkmann struments, Westbury, NY), heating at 55°C for 1 hour and clarified by centrifugation and filtration. Increasing the acid concentration, the temperature, the time of
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Dietary oxalate and its intestinal absorption Table l. The oxalate content of foods in mg per 100 g. Food Colorimetric Enzymatic Assay [52) Assay [34) Spinach 780 750 Celery 17.5 20 Tomato 5.3 2 Carrots 22.7 4 Chocolate 124 117 Com Flakes®1 5 2 Weetabix 3.2 Tea 7a 5.5b V-8® juice2 Orange juice 0.25 a2 minute infusion of 2 g tea in 100 ml of boiled water 1Kellogg Co., Battle Creek, MI; incubation, or re-extraction of the pellet did not increase the yield of oxalate. As an example, the analysis of Bran Flakes breakfast cereal is shown in Figure 1. The Bran Flakes contained 141 mg oxalate/100 g. Repetitive analyses of this product showed that intra-assay ty (CV) was 2.4 % and the inter-assay variability, 3. 3 % . The recovery of 20 mg added oxalate was 92 ± 3 % . These results indicate that for the analysis of Bran Flakes the assay is both accurate and reproducible. Oxalate values reported for some commonly sumed food items using several different methods are listed in Table 1 and compared with values obtained by CE. As the oxalate level in plants may vary depending on the particular genetic strain of the plant or the growth conditions for the plant [38), some of the variability tween results could be due to strain or growth ences. Similar arguments may apply to manufactured products such as bread, Weetabix, and chocolate which may vary in processing as well as in ingredients. The overall trend observed in Table 1 was for food oxalate estimates to be lower using colorimetric and enzymatic procedures than using CE. This underestimation ularly applied to the cereal grains and their products. As the current food pyramid for daily nutrition mended by the USDA calls for the consumption of 8 servings of cereal products per day, such products could be important sources of oxalate in an American diet. Seeds of all description appear to be rich sources of late. Costello et al. [11] reported that sesame seeds tained 2800 mg/100 g and, as such, must be the richest source of oxalate yet identified. The discovery that the 1111 Gas Chromatography USDA Handbook CE [8, 9) #8-11 [24) 618 970 645 190 61.2 50 5.7 500 9.6 140 1.9 76.1 280a 7.5a 5.2 5.8 0.4 0.4 b2 min infusion of 1 g tea in 100 ml of boiled water. 2Campbell Soup Co., Camden, NJ. protein, germin, which is important in seed germination, is actually an oxalate oxidase suggests an important logical role for oxalate in plants [36]. The bran portion of wheat seeds is enriched with oxalate and CE analysis indicates that it contains 524 mg/100 g. In view of the number of bran-enriched foods currently available in the supermarket, bran is becoming an important source of oxalate in the diet. The results clearly indicate that a reliable method for oxalate analysis of foods is required, particularly if intestinal oxalate absorption is a cant factor in calcium oxalate stone disease. more, it is possible that absorption can be substantially modified with prudent dietary choices. Measurement of the Intestinal Absorption of Oxalate Three methods have been used to measure the amount of ingested oxalate absorbed in the intestine. There are problems associated with each of these ods creating much doubt about the validity of current timates of oxalate absorption. The methods will be ferred to as the isotopic method, the load method, and the daily excretion method, and are reviewed below. Isotopic method On the surface, this method would appear to be the most straightforward. 14C-oxalate is ingested orally and the 14C-oxalate appearing in urine is used as an index of the amount of oxalate absorbed. The two major lems with this procedure involve the way the dose is ministered and the mixing of the isotopic dose with un-
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R.P. Holmes, H.O. Goodman and D.G. Assimos labelled oxalate in the large intestine. The way the tope has been administered has varied from providing it in the fasting state with 8 mg of sodium oxalate [10, 50] to including it with complex meals [16, 39]. In some studies, the isotope was included in a formula diet [16, 48]. The critical factor in the method of administering the isotope is whether the isotope is free, complexed or crystalline when it reaches absorption sites. Evidence from absorption studies indicates that a much larger take of 14C-oxalate occurs when the isotope is given in the fasting state as sodium oxalate, compared with administration with any nutrients. In the study by wick et al. [10], 29 ± 4 % of the fasting, sodium oxalate dose was absorbed in 36 hours in normal subjects pared with 6.6 ± 0.9 % when the isotopic dose was given with food. This larger uptake of sodium oxalate in the fasting state strongly implies that absorption of oxalate occurs as the free, charged anion and not as a neutral complex when bound to Ca or Mg. This is ther supported by the evidence reviewed below that the intake of Ca, Mg and other ions suppresses intestinal oxalate absorption. Thus, the absorption of 14C-oxalate from the administered dose depends on how much has crystallized, how much has complexed with Ca and Mg, and how much is free. This is further complicated by the changing milieu of the intestinal contents. Questions that arise include how much of the crystalline Ca oxalate that would undoubtedly form in the formula diets solves in the stomach, and how much would lize when the pH increases in the small intestine? How do intestinal excretions and absorptions influence the solubility of oxalate? Erickson et al. [16] have shown, for instance, that a hyperabsorption of Ca in the intestine increases oxalate absorption. The second complication arises when the ed dose reaches the large intestine where substantial mixing with previously ingested food will occur. The amount of previously ingested Ca, Mg, oxalate, phorus and other ions will affect the distribution of label into crystalline, complexed and free forms. Thus, the dietary intake in the 24-48 hours prior to the study will influence the absorption of the dose. This mixing of testinal contents coupled with the Jong transit time in the large intestine makes the study of oxalate absorption in the large intestine using the isotopic method very difficult. Not surprisingly in view of these complications, large ranges in the percentages of administered doses sorbed have been reported. The largest amount of 14oxalate reported to be absorbed in normal subjects was by Chadwick et al. [ 10] who found an average of 29 % absorbed (range 8-55 % ) in 36 hours when given with dium oxalate in the fasted state. This absorption would presumably have increased further with longer collec-1112 tions. Most studies [10, 16, 39, 48, 50] report an sorption of 6-12 % in 24 hours for 14C-oxalate ingested with food or formula under normal conditions. Prenen et al. [46] reported, however, that absorption was low, 1. 7-2.6 % over 96 hours, when the isotope was given with oxalate-rich foods. This low absorption may be lated to the incorporation of the isotope into crystals fore reaching an absorption site. An absorption of 6-12 % of dietary oxalate would contribute 7.8-15.6 mg oxalate to a 24 hour urine sample if dietary oxalate is an average of 130 mg/day, and 12-24 mg if it is 200 mg/ day. Even the lowest figure, 7.8 mg, would represent a contribution of > 25 % to the average excretion of 29 mg/day in 101 individuals consuming self-selected diets [29]. Load method This method has been used primarily to obtain data on the bioavailability of oxalate in food and to study modification of oxalate absorption in the small intestine. A baseline urine collection is obtained, which in one study was 6-12 hours after a meal [4], 9-12 hours in another [9], and 11-14 hours in the most recent study [8]. This baseline oxalate value was used to examine changes following either a sodium oxalate load or tion of a food item of known oxalate content. Food vided post-load was an oxalate-free formula diet. The increase in urinary oxalate excretion over the next 8 hours [8, 9] or 48 hours [4] was determined and used to calculate the oxalate absorbed. For a sodium oxalate load the amount absorbed was 9 % over a 48 hour period [ 4]. For the 8 hour post-load evaluations, absorption ranged from 43 % for pecan oxalate to zero for berry juice oxalate [8, 9]. The main weakness in this experimental approach is the reliance on a pre-load urine collection as the baseline value. This value may change over time due to tions in endogenous synthesis and to differences in the colonic absorption of oxalate which will also contribute to urinary oxalate during the experimental period. The amount of oxalate available for absorption may depend on the food ingested 12-60 hours prior to the studies. Such factors may account for the unrealistically high absorption of oxalate from pecans. Daily excretion method The basis for this method is the equilibration of individuals on two controlled diets with a different late content, the collection of 24 hour urine samples ter equilibration, and calculating the amount of oxalate absorbed from the differences in dietary oxalate and the amount of urinary oxalate excreted. Criteria that have to be fulfilled for this method to be valid are (a) that equilibration to the diets has occurred, (b) that urinary oxalate excretion on the diets is constant (i.e., endoge-
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Dietary oxalate and its intestinal absorption nous synthesis does not fluctuate), and (c) that the diet is well controlled. Four studies [15, 17, 41, 53], two from the same laboratory, have utilized this approach. Only criterion (b) was satisfied and then only in two of the studies [ 41, 53]. The variability obtained in these studies, however, was less than what we have observed in individuals on a diet coritrolled in magnesium, fiber, sodium, potassium and protein [30]. It is difficult to know whether the difference in the results is due to ferences in the diets, differences in the study pants, or differences in the methods for oxalate nation. Equilibration to the high oxalate diets was not achieved in any of these studies and the mean excretions on the first 3 or 4 days of the diet were used. Finch et al. [17] showed that equilibration on a diet was not tained until days 6 or 7, which is similar to what we show below. Despite recognizing this time required for equilibration, Finch et al. [17] used shorter study ods. Experimental details of the diets were scant in all studies making it impossible to evaluate criterion (c). Those details supplied suggested that variable menus were used rather than the sai;ne menu each day. bility in the menus is important as the magnesium and fiber content of these diets were not controlled and could have varied and thereby influenced the results. The ranges in oxalate absorption obtained in these studies were 2.6-4.1 % [53], 0.1-13.6% [41], 5-7% [15], and 1.3-22.2 % [17]. This large range in values is consistent with methodological problems being associated with the studies. Sites of Intestinal Absorption of Oxalate Oxalate absorption can occur by paracellular fluxes along the entire gastrointestinal tract [7]. To date, an active uptake mechanism has been identified only in the colon of experimental animals but may also occur in man [21, 22]. The amount of oxalate absorbed in each intestinal segment will depend on the amount of oxalate solubilized, transit times, and the absorptive properties of the luminal epithelial cells. In most studies, an initial peak of oxalate absorption occurs in 2-5 hours [16, 40, 46, 48, 50] which is very similar to the absorption of calcium [14]. The timing of this peak indicates that it is occurring in the small intestine. Barilla et al. [4] have argued that this absorption site is in the proximal section of the small intestine based on observations that the tial peak of absorption was still observed in individuals with the distal portion of the small intestine resected. It has been ascertained that the bulk of paracellular calcium absorption in the small intestine occurs in the ileum, principally because the transit time is longest in this ment [15]. If oxalate absorption in the small intestine occurs predominantly by paracellular routes, the bulk of 1113 this absorption may also occur in the ileum. tion that the ileum is an important absorption site for alate in humans is warranted as well as a comprehensive study of its absorptive properties. It has been suggested that the stomach is an important absorption site [23], but the nature of the protocol in blocking gastric emptying and the use of patients who had been on long-term tric tube feeding, indicates that the significance of this absorptive site should be investigated by other proaches in normal individuals. Some absorption does occur in the first hour and dissolution of food-borne crystalline oxalate in gastric juices could be time ent leaving the question of the extent of stomach oxalate absorption open. Because of the difficulty in measuring absorption processes in the large intestine in humans, there is a lack of direct evidence showing that the large intestine is a significant site of absorption in normal individuals. There is substantial indirect evidence described below, however, based on studies with individuals with bowel resections, the kinetics of 14C-oxalate absorption, and isolated rabbit intestinal segments. In individuals with enteric hyperoxaluria secondary to intestinal disease or intestinal surgery, the colon is clearly the site of absorption of intestinal oxalate [13, 44]. As patients who had resection of the proximal ascending colon veloped hyperoxaluria, the distal colon was implicated in the hyperabsorption [13]. These studies do not exclude a role for the proximal colon in hyperabsorption of late, however. In several studies where an oral dose of 14C-oxalate was given to normal individuals, the centage of the dose absorbed after 8 hours ranged from 0.4-10.1 % with the bulk of the studies in the 7 % range [27, 39, 40, 48, 50]. Based on estimates of the transit of food through the intestinal tract [47], some of this sorption may occur in the proximal part of the large testine. Experiments with segments from the large tine of rabbits have indicated that net oxalate absorption by active transport can be detected only in the distal lon [21, 21]. The relative contributions of active and paracellular transport to intestinal oxalate absorption along the human intestinal tract are yet to be determined. Modification of Absorption Several factors have been identified that modify testinal oxalate absorption. They include Ca, Mg and fiber. Other compounds that bind to Ca, Mg, or late, such as phosphate and phytate, could also modify oxalate absorption but have not been studied as yet. The extent of bacterial degradation of oxalate is another tor that may be potentially important. Ca and Mg decrease oxalate absorption presumably by binding to oxalate and decreasing its “free” level. Fiber decreases
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Dietary oxalate and its intestinal absorption 24,——————–, -22 U 20 C) oi 18 E -;; 16 0 -~ 14 Q) u 12 X w a> 10 8 re 5 6 4o~~-~2–3~~4~~5–6~~7~~8-~9-~10 Days Figure 2. Changes in urinary oxalate/creatinine tion in two individuals consuming an oxalate-free mula diet. The formula diet documented in Table 2 was ingested for 7 days followed by oxalate supplementation for the 2 days denoted by arrows. It was consumed 6 times daily beginning at 7:00 AM at 2.5 hourly vals. Caloric requirements of the individuals were sessed by their age (A 47, B 70), sex (male), weight (A 77 kg, B 65 kg), body frame, and level of activity, and were 2500 for A and 1800 for B. Oxalate was ured by CE [28] and creatinine as previously described [29] in 24 hour urine collections. —–·———————studying the effects of adding oxalate and modifiers of oxalate absorption back to the diet. The number of lections required to accurately reflect excretion levels will depend on the variability in the endogenous sis of oxalate. The formulation of a diet used for testing in two individuals is shown in Table 2. Sodium nate was used as the source of protein. Com oil, com syrup, sucrose and vanilla extract were purchased at a local supermarket. a-Cellulose (Avicel® PH101) was obtained from FMC (Philadelphia, PA) and added at 12.5 g/l. All other additions were of food grade quality. A multi-vitamin/mineral supplement {Centrum Silver®; Lederle, Lederle, NY) was crushed and added to the formula with the Ca and Mg content of the supplement taken into consideration. Two male individuals consumed this formula diet for 7 days. On self-selected diets, individual A excreted a mean of 29.7 mg/day and individual B 26.7 mg/day based on four 24 hour urine collections. The changes observed in urinary oxalate excretion are shown in ure 2. By days 6 and 7, urinary oxalate had declined to a mean of 10.5 mg in A and 8.6 in B, a mean decrease of 18.6 mg. Stool samples were monitored by a CE analysis to follow changes in fecal oxalate excretion. The analysis of feces obtained before ingestion of the 1115 oxalate sulfate 3.6 3.7 3.8 3.9 4.0 4.1 Migration Time (min) Figure 3. The analysis of oxalate in fecal samples. The upper pherogram shows the analysis of oxalate in feces from individual A while consuming a self-selected diet. For analysis, 0.5 g of feces was mixed with 4.5 ml of 1 M H3P04 and incubated at 55°C for 1 hour. late matter was removed from a 1 ml aliquot by gation in a microfuge for 2 minutes at 15,000 g. The supernatant was diluted 1/100 for CE analysis as scribed for urinary oxalate [28]. The lower pherogram shows the analysis of oxalate in a stool sample from dividual B after consumption of the oxalate-free diet for 6 days. formula diet is shown in Figure 3. Fecal oxalate could be detected on day 4 of the formula diet in both uals (25 gig in A and 42 gig in B) but in neither ual at day 6. Based on a signal to noise ratio of 3:1, these assays indicated that the fecal samples contained < 10 g/g fecal wet weight. An analysis of feces lected on day 6 is shown in Figure 3. These results showed that it takes at least 5 and possibly even 6 days to reduce intestinal oxalate to below detectable levels in the gut. Urinary oxalate declined by 65.1 % in A and 69 .1 % in B when comparing mean excretions on days 6 and 7 of the formula diet with their mean excretions on self-selected diets. Thus, dietary sources of oxalate may have contributed the majority of the oxalate excreted by these two individuals on self-selected diets. However, it remains possible that endogenous synthesis is pressed by the composition of the formula or by the six times a day formula ingestion. It was estimated from recorded dietary intakes that both these individuals sumed 200 mg oxalate/day on their self-selected diets based on the preliminary CE analyses of the majority of the foods ingested. Thus, the mean amount absorbed, 18.6 mg, is close to the average of 10% absorption tected in 14C-oxalate absorption studies reviewed above.
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R.P. Holmes, H.O. Goodman and D.G. Assimos On days 8 and 9, the formula was supplemented with sodium oxalate (AR grade; Fisher, Pittsburgh, PA) equivalent to 200 mg oxalate per 2500 kcal. This tion of oxalate to the diet increased urinary oxalate cretion of individual A to 70 % and B to 76 % of the els obtained on self-selected diets. The restoration of urinary oxalate excretion to near levels on self-selected diets by adding an amount of oxalate to the formula equivalent to the normal intake suggests that the formula diet was not drastically altering endogenous synthesis, although the possibility that the formula diet has fied the permeability properties of the intestinal lium has to also be considered. On day two of the late-containing diet, the increment in urinary oxalate cretion would be equivalent to an absorption of 6.1 % in A and 5.1 % in B. A period longer than two days may have been required to re-equilibrate the gut with oxalate and maximize oxalate absorption so these values most likely underestimate absorption. A factor that may cause enhanced oxalate absorption on self-selected diets is the ingestion of oxalate and cations that modify absorption at different times during the day. We believe that this experimental system can be manipulated not only to study the timing of ingestion of nutrients but also to study, with some precision, the effects of dietary late, Ca, Mg, and fiber on oxalate absorption. The contribution of dietary oxalate to urinary late excretion in these two individuals is large because they have a low rate of endogenous oxalate synthesis. This endogenous synthesis is apparently genetically termined due to the presence of two co-dominant alleles of a gene that regulates oxalate synthesis (16; Holmes RP, Goodman HO, Assimos DG, manuscript in tion]. These alleles create 3 classes of oxalate excretors, low (49 % of the Caucasian population), intermediate (46%), and high (5%) excretors. The high oxalate cretors (hyperoxaluric) have the highest risk of stone disease with intermediate excretors having approximately twice the risk of low excretors (18). The proportional contribution of dietary oxalate to urinary oxalate tion obviously differs between these classes. Whereas, we have shown experimentally above that dietary oxalate may contribute 67 % to urinary oxalate excretion by low oxalate excretors, the contributions are estimated to be 49 % for intermediate excretors and 32 % for high tors, based on the mean oxalate excretion of these tory classes. The assumption was made that an average of 19 mg of oxalate, the mean absorption of the two dividuals studied, was absorbed from the diet by each excretory class. A more precise estimate of the bution of dietary oxalate to urinary oxalate excretion in each excretory class will depend on determining whether the formula diet modifies endogenous oxalate synthesis and in accurately classifying individuals into excretory 1116 groups. Conclusions (1). The amount of oxalate ingested in typical ern diets is not known with certainty due to incomplete and inaccurate food composition tables for oxalate. What is regarded today as a healthy diet rich in whole grain products, fruits and vegetables may contain close to 200 mg of oxalate per day. Ingestion of significant amounts of wheat bran-enriched products would increase the intake of oxalate further, as would the ingestion of spinach, beets, peanuts, and chocolate. A less healthy diet (rich in animal protein, fat and refined sugars) may result in an intake of less than 100 mg of oxalate. (2). The amount of ingested oxalate that is ed in the intestine is also not known with certainty. It appears to range between 5 and 15 % depending on the co-ingestion of Ca, Mg and fiber. This level of tion would contribute 10-30 mg with an intake of 200 mg of oxalate per day. (3). The use of oxalate-free formula diets will make it possible to study, in more detail, intestinal oxalate sorption and factors that modify it. Preliminary studies suggest that ingestion of such diets for 6 days will be quired to eliminate detectable oxalate from the intestinal tract. (4). The use of formula diets to study oxalate sorption and its modification should make it possible to confirm whether lowering the intake of oxalate and creasing the intakes of Ca, Mg and fiber are viable egies for decreasing urinary oxalate excretion in calcium oxalate stone-formers. References (1) Allison MJ, Dawson, KA, Mayberry WR, Foss JG. (1985). Oxalobacter Jormigines gen. nov., sp. nov.: oxalate-degrading anaerobes that inhabit the intestinal tract. Arch. Micro. 141: 1-7. (2) Allison MG, Cook HM, Milne DB, Gallagher S, Clayman RV. (1986). Oxalate degradation by testinal bacteria from humans. J. Nutr. 116: 455-460. (3) Archer HE, Dormer AE, Scowen EF, Watts RWE. (1957). Studies on the urinary excretion of late by normal subjects. Clin. Sci. 16: 405-411. (4) Barilla DE, Notz C, Kennedy D, Pak CYC. (1978). Renal oxalate excretion following oral oxalate loads in patients with ileal disease and with renal and absorptive hypercalciurias. Effect of calcium and sium. Amer. J. Med. 64: 579-585. [5] Bataille P, Charransol G, Gregoire I, Daigre JL, Coevoet B, Makdassi R, Pruna A, Locquet P, Sueur JP, Fournier A. (1983). Effect of calcium restriction on re-
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