# Bioavailability of calcium from tofu as compared to milk in premenopausal women

#### Abstract

Calcium intakes by girls and adult women should be at or above the Recommended Dietary Allowances for the achievement of peak bone mass in early adulthood, and for the maintenance of bone mass thereafter. In the U.S., calcium intake is only 88% of the current RDA. Dairy products provide approximately three-quarters of calcium consumed in the diet. However dairy products may be avoided for reasons of lactose intolerance, vegetarian beliefs, or from perception of dairy products as being high in fat or cholesterol. Alternative food sources rich in calcium are few, and of those, bioavailability of calcium must also be addressed before calcium delivery can be determined. Tofu set with calcium salts and other soy products such as calcium fortified soy milks are rich in calcium, but their bioavailability to humans has not yet been determined. This study investigated the chemical form of calcium in soy from plants hydroponically grown and endogenously labeled with $\sp{45}$Ca, revealing that, although soy had a high concentration of oxalate (8.2 mg/g) it was not bound to calcium but was soluble in nature. Analysis of radiolabeled protein fractions revealed that calcium was not bound to storage proteins but was at least partially associated with phytate in soy. This study also determined the bioavailability of calcium from tofu (49.30% $\pm$ 16.73) as compared to milk (53.26% $\pm$ 11.85) in 10 premenopausal women. Calcium absorption from tofu set with calcium chloride and bovine milk was evaluated with stable isotope methodology. Tofu was prepared with 36 mg of $\sp{44}$Ca as enriched CaCl$\sb2$. Milk was prepared using 36 mg of labeled $\sp{44}$Ca as $\sp{44}$CaCl$\sb2$. Each subject was tested during the follicular phase of the menstrual cycle. Isotope ratios in fecal samples were analyzed with fast atom bombardment mass spectroscopy. Pre-study calcium intakes as determined by dietary records were low, averaging only 71% of the RDA, whereas protein intake was high, averaging 172% of the RDA. Correlation of percent calcium absorption with daily calcium intake (r = 0.46) revealed that 21% of the variability in calcium absorption was accounted for by pre-study calcium intake.

#### Degree

Ph.D.

Weaver, Purdue University.

#### Subject Area

Nutrition|Food science

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