Oxalate content and calcium binding capacity of tea and herbal teas

Abstract

Thirty-two commercially available teas consisting of green, oolong and black teas were bought from supermarkets in Christchurch, New Zealand in June 2001. Fifteen herbal teas were also purchased at the same time. The soluble oxalate content of the infusate made from each of the teas was determined using high pressure liquid chromatography. The mean soluble oxalate contents of black tea in tea bags and loose tea leaves were 4.68 and 5.11 mg/g tea, respectively, while green teas and oolong tea had lower oxalate contents, ranging from 0.23 to 1.15 mg/g tea. The soluble oxalate content of the herbal teas ranged from not detected to 3.00 mg/g tea. A regular tea drinker consuming six cups of tea/day would have an intake of between 26.46 and 98.58 mg soluble oxalate/day from loose black tea, 17.88 and 93.66 mg soluble oxalate/day from black tea in tea bags and a maximum of 18.0 mg/day from herbal teas. The oxalate intake from the regular daily consumption of black teas is modest when compared to the amounts of soluble oxalate that can be found in common foods. However, oxalate in black teas has the potential to bind to a significant proportion of calcium in the milk, which is commonly consumed with the black teas.

Bioavailability of soluble oxalate from tea and the effect of consuming milk with the tea

“…McKay et al (1995) showed that steeping tea for different times and stirring the tea had a significant effect on the oxalate content of the final extraction. From their data a regular tea drinker consuming six cups of tea per day would consume between 65 and 124 mg soluble oxalate per day, which is comparable to the results obtained in the study carried out by Charrier et al (2002). If black tea is taken with added milk or is consumed with calcium-containing meals then less oxalate will be absorbed as some soluble oxalate extracted from the tea leaves will bind to calcium and continue down the digestive tract without being absorbed (Massey, 2000).…”

“…Over 24 h there was a highly significant (P < 0.01) reduction in oxalate output when milk was consumed with the two teas. Charrier et al (2002) showed that a regular tea drinker consuming six cups of tea=day would have an intake of between 17.9 and 93.7 mg soluble oxalate=day from black tea in tea bags. While this intake of oxalate would occur on a daily basis these amounts are modest when compared to the soluble oxalate content of some common foods (Noonan & Savage, 1999;Albihn & Savage, 2001b).…”

“…Charrier et al (2002) showed that, if 25 ml of non-fat milk (containing 160 mg Ca=100 ml) was consumed with the same two brands of tea, then the calcium in the milk would have the capacity to bind to between 168 and 177% of the oxalate in the two teas.…”

“…In some publications it is not clear whether the oxalate content has been measured in tealeaves or in brewed tea. More recently Charrier et al (2002) have shown that tea sold loose or in tea bags contains between 1.5 and 6.8 mg soluble oxalate=g tea. They went on to show that the consumption of six cups of tea made from either loose black tea or black tea in tea bags would result in an intake of between 16 and 102 mg soluble oxalate=day.…”

Simultaneous determination of selected anti-nutritional components in Asiatic plants using ion chromatography

“…Oxalates can be determined by various analytical methods, such as: KMnO 4 titration [7], high-performance liquid chromatography (HPLC) with UV detection [8], HPLC with enzymatic detection [9], capillary electrophoresis [10], ion chromatography [11], chemiluminescence [12] and spectrophotometry NIR [13].…”