9 Effect of Oxalic Acid on Calcium Bioavailability
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JUNE L. KELSAY Carbohydrate Nutrition Laboratory, Beltsville Human Nutrition Research Center, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705 Review of studies on the effect of oxalic acid on calcium bioavailability in rats and in humans indicates that most of the research was done between 1930 and 1950. Decreased availability of calcium in young rats was reported when spinach containing oxalic acid was fed with low calcium diets. The extent of the effect of oxalic acid on calcium availability was shown to be related to levels of calcium and oxalic acid, as well as the presence of vitamin D in the diet. In human studies there was generally no effect of oxalic acid on calcium balance; however, in a few studies decreased calcium balances were reported. There is recent evidence that oxalic acid consumed along with a moderately high level of fiber intake may have adverse effects on calcium balance of human subjects. Decreased calcium balances in children fed vegetables were reported by Sherman and Hawley (1.) in 1922. The vegetables fed were carrots and spinach, and the decreased balances were at that time attributed to the crude f i b e r in the vegetables. However, r e s u l t s of e x p e r i ments conducted on rats in the 1930's led investigators to believe that the consumption of spinach as a source of calcium was undesirable due to the presence of o x a l i c acid in large amounts. Oxalic acid can combine with calcium to form a s a l t complex of low s o l u b i l i t y . Spinach has been reported to contain o x a l i c acid in amounts from 458-1050 mg/100 g ( 2 - 6 ) , depending on the variety of spinach. Similar amounts of o x a l i c acid are found in other green vegetables such as beet leaves, Swiss chard, lamb's quarter, purslane, dandelions, and poke. Rhubarb, cashew nuts, almonds, walnuts, lady f i n g e r , cocoa, and dried tea leaves also contain considerable amounts of o x a l i c a c i d . Several comprehensive reviews on o x a l i c acid have been published in which effects on calcium metabolism were discussed (7-11). A review of studies on the e f f e c t of o x a l i c acid on calcium b i o a v a i l a b i l i t y in rats and humans i s presented in t h i s paper. This chapter not subject to U.S. copyright. Published 1985 American Chemical Society
Kies; Nutritional Bioavailability of Calcium ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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NUTRITIONAL BIOAVAILABILITY OF CALCIUM
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Studies on Rats Bloom (12) reported that in both older rats (63 days of age) and younger rats (33 days of age) the amount of calcium retained was less when 5% dried spinach, either raw or cooked, was included in the diet in 1-week balance studies (Figure 1). The experimental diets contained about 0.4% calcium, and one drop of cod l i v e r o i l was given each rat every day. Most of the calcium excretion on the spinach diet was in the feces. Retentions of calcium on the basal low f i b e r diet and on diets containing f i l t e r paper in amounts to equal the crude f i b e r in spinach or 12 times the crude f i b e r in spinach were high. There was no s i g n i f i c a n t difference in calcium retentions between raw and cooked spinach. The low retention of calcium from spinach could not be attributed to the presence of crude f i b e r in the d i e t . In a study by Fincke and Sherman (13), the calcium of spinach was not u t i l i z e d as well as that from m i l k ; however, the calcium of k a l e , which i s low in o x a l i c acid ( 3 , 4 ) , was about as available as that from m i l k . The calcium u t i l i z a t i o n factor was determined by d i v i d i n g the weight of calcium stored by the weight of calcium ingestion. Rats 4 weeks old were fed for 60 days a diet in which most of the calcium was supplied by skim m i l k , or in which half of the skim milk was replaced by dried spinach or dried kale in amounts to provide the same amount of calcium. The diets contained about 0.3% calcium and 10% butter f a t . It was concluded that the poor u t i l i z a t i o n of the calcium of spinach was due to the o x a l i c acid in spinach. T i s d a l l et a l . (14) obtained poor weight gains and low calcium retention in rats fed 10% d r i e d , cooked spinach with a low calcium (0.041 g percent) d i e t . Rats 28 days of age were placed on a low calcium d i e t , or the low calcium diet containing 10% dried spinach, 5.83% dried tomatoes, or 1.88% dried spinach for 4 weeks each. Calcium contents of whole rats were determined at the end of the experiment. Weight gains and calcium retentions were better when the animals were fed tomatoes than when they were fed spinach, even though the calcium intake on the higher level of spinach was about 20 times greater than when tomatoes were f e d . The r e s u l t s of experiments conducted by MacKenzie and McCollum (15) indicate that the effect of dietary o x a l i c acid on the rat depends on the composition of the d i e t . There was no e f f e c t on rate of growth or calcium excretion of 50 g rats fed for 10 weeks a diet containing 0.6% calcium, 0.7% phosphorus, and optimum vitamin D, when levels of potassium oxalate up to 2.5% were f e d . The percent bone ash on the 2.5% oxalate diet was somewhat lower than on the control d i e t . On a 0.35% calcium, 0.35% phosphorus, and vitamin D-free d i e t , 1.7% potassium oxalate resulted in r e s t r i c t e d growth and bone formation of weanling r a t s . T i s d a l l and Drake (16) found that the addition of 10% dried spinach to a low calcium"~cTiet containing 2% cod l i v e r o i l resulted in a reduction in body calcium (Figure 2 ) . The spinach diet was compared with one containing an equal amount of calcium as calcium carbonate, and with a t h i r d containing an equal amount of calcium carbonate plus an amount of o x a l i c acid equal to that present in the spinach. Three-week-old rats were fed the diets for 4 weeks.
Kies; Nutritional Bioavailability of Calcium ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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9.
KELSAY
Effect of Oxalic Acid on Ca
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Figure 1. Calcium intake and excretion of older rats (63 days) and younger rats (33 days) on a low f i b e r diet (1), raw spinach (2), cooked spinach (3), f i l t e r paper (4), and high f i l t e r paper (5). Dietary intake i s indicated by the top of the bar. Balance i s indicated by the bottom of the bar. Adapted from Ref. 12. a Q W
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