Increased calcium bioavailability in mice fed genetically engineered plants lacking calcium oxalate

Abstract

Bioavailable calcium affects bone formation and calcification. Here we investigate how a single gene mutation altering calcium partitioning in the model forage crop Medicago truncatula affects calcium bioavailability. Previously, the cod5 M. truncatula mutant was identified which contains identical calcium concentrations to wild-type, but contains no oxalate crystals. In this study, equal number of male and female mice were randomly grouped and then fed one of four 45Ca-containing diets: M. truncatula extrinsically or intrinsically labeled, and cod5 extrinsically or intrinsically labeled. Absorption of the tracer was determined in the legs one day after consumption. The absorption was similar in the M. truncatula and cod5 extrinsically labeled diets; however, in the intrinsically labeled diets, calcium absorption was 22.87% (P < 0.001) higher in mice fed cod5. Our study presents the first genetic evidence demonstrating the nutritional impact of removing oxalate crystals from foods.

Nutritional impact of elevated calcium transport activity in carrots

“…In previous work using this protocol, we demonstrated that mice fed spinach had absorption values of Ϸ0.24% for intrinsically and Ϸ0.44% for extrinsically labeled diets. These values are consistent with similar studies using rats, validating our procedures (14) and further demonstrating that spinach, which contains high oxalate levels, is a poor source of bioavailable calcium. The sCAX1-expressing carrot lines did not contain more oxalate, and this finding may explain why both intrinsically and extrinsically labeled carrot diets showed reasonable calcium absorption.…”

“…Thus, calcium labeling by using either radioactive or stable isotopes in the carrots can be used to study calcium bioavailability in feeding studies. (14). We have validated this mouse model by using various diets, and our results compared favorably to previous feeding studies in rats (15).…”

“…The C57BL/6 (Charles River Laboratories) mice were housed in cages with ad libitum access to water and food (AIN93G diet; ref. 32), and the feeding protocols have been described previously (14). The mice ate Ϸ3.5-4 g of diet per day at 6 -7 weeks of age (n ϭ 120).…”

“…These compounds can bind the calcium freely, and some plants also can bind calcium during the growth process (18)(19)(20). In the case of spinach, a poor source of bioavailable calcium, the incorporation of a 45 Ca tracer in bone is reduced by 50% in the intrinsically labeled diets compared with the extrinsic diets when fed to rats or mice (14,15). Although sCAX1-expressing carrots contained very low concentrations of oxalic acid, it is still imperative to measure calcium bioavailability from intrinsically labeled carrots.…”

“…Carrots were germinated and grown hydroponically (14). After 90 days, the solution was supplemented with 5 mg 42 Ca per liter for the control carrots and 2.5 mg 42 Ca per liter for the sCAX1-expressing carrots.…”