3144 JOURNAL OF FOOD SCIENCE—Vol. 67, Nr. 8, 2002 © 2002 Institute of Food Technologists Further reproduction prohibited without permission Sensory and Nutritive Qualities of Food JFS: Sensory and Nutritive Qualities of Food Bioavailability of Calcium from Tofu as Compared with Milk in Premenopausal Women C.M. WEAVER, R.P. HEANEY, L. CONNOR, B.R. MARTIN, D.L. SMITH, AND S. NIELSEN ABSTRACT: Using a cross-over design, two studies were conducted to measure calcium absorption from calcium-set tofu compared to milk in healthy, premenopausal women. In “study 1,” calcium absorption from tofu set with CaCl 2 was determined in Caucasian women by fecal recovery of the stable isotope, 44 Ca. In “study 2,” calcium absorption was determined in Asian women from tofu set with CaSO 4 by appearance of 45 Ca in serum after 5 h. Analysis of the studies, both separately and pooled, showed that calcium absorption was similar between calcium- set tofu and milk. Calcium-set tofu is a concentrated source of bioavailable calcium. Keywords: calcium bioavailability, tofu, humans Introduction M EDIAN INTAKES OF CALCIUM IN WOMEN ARE LOWER THAN THE recommended intakes in the U.S.A. and many other coun- tries (Institute of Medicine 1997). While over 70% of dietary calcium in the United States comes from dairy products, many individuals do not consume dairy products in sufficient quantities to meet the recommended intakes. Calcium-rich soy products are popular al- ternative calcium choices for some populations, particularly in Asia. Although soybeans are high in both oxalate and phytate, calcium from whole soybeans is well tolerated by lactase-deficient individ- uals. They also have a fractional absorption efficiency comparable to that of milk, although inversely related to phytate content (Heaney and others 1991). On the other hand, calcium fractional absorption from fortified soymilk is only 75% of cow’s milk (Heaney and others 2000). Among the few calcium-fortified foods, tofu provides calcium in concentrations comparable to milk in an acceptable serving-size portion. United States Department of Agriculture (USDA) Hand- book 8 lists the calcium content of milk as 117 mg/100g and tofu as 128 mg/100g. Poneros and Erdman (1988) found that calcium in tofu was highly bioavailable to rats, 107% compared to 100% for CaCO 3 . But bioavailability of calcium from tofu has not been stud- ied in humans. The purpose of this study was to compare in humans the bio- availability of calcium from tofu to that for milk, using 2 different methodologies and populations. In the 1 st study, calcium absorp- tion was determined in Caucasian women by fecal recovery of a stable calcium isotope. In the 2 nd study, calcium absorption was determined in Asian women using a 5-h post-administration blood sample of 45 Ca. Materials and Methods T HIS STUDY’S PROTOCOLS WERE APPROVED BY THE PURDUE AND Creighton University’s “Human Subjects in Research” Commit- tees. Healthy Caucasian women, aged 19 to 24 at Purdue Univer- sity and healthy Asian women, aged 20 to 38.5, at Creighton Uni- versity were recruited from those campuses. Exclusion criteria included individuals with predisposing conditions known to influ- ence calcium absorption (thyroid dysfunction, diabetes mellitus, kidney disease, hypertension, gastrointestinal disorders, steroid therapy, pregnancy and lactation, past or current eating disorders, and amenorrhea) as well as women who were taking medications known to impact calcium absorption directly. Preparation of labeled test foods Milk. In study 1, each subject received 36 mg of 44 Ca as 44 CaCl 2 (prepared from 44 CaCO 3 by dissolving in a molar excess of HCl, fil- tering with a Buchner funnel, and freeze-drying), added to 67.3 grams of liquid 2% lowfat milk. Extrinsic labeling with 44 CaCl 2 was done the night before ingestion to assure equilibration of the cal- cium. Extrinsic labeling of milk by this method previously has been shown to give values for calcium absorption not different from in- trinsically labeled milk (Nickel and others 1996). Six representative samples (0.5 mL each) of labeled milk were chosen randomly and evaluated for 44 Ca enrichment and calcium content. Tofu. In study 2, whole milk was labeled by the addition of 5Ci 45 CaCl 2 per serving and stored at 4 °C for 14 to 16 h until administra- tion to the subjects. Preparation of the tofu was the same for both studies except for the calcium salts and coagulants (Shurtleff and Aoyagi 1984). Silk- en tofu was manufactured in individual portions to prevent ex- pression of whey (Shen and others 1991). In study 1, silken tofu was set with CaCl 2 containing 36 mg 44 Ca as 44 CaCl 2 and 94.5 mg glu- cono-delta-lactone (GDL) (Griffith Labs, Scarborough, Ontario, Canada) to maximize stable isotope recovery. In study 2, 45 CaSO 4 was used as the coagulant. Tofu was prepared the evening before ingestion. Washed soybeans (150 g) were soaked overnight, then made into a paste by placing the beans into an Osterizer ® for 1.5 min on the highest setting. The paste was filtered under pressure through 2 layers of cheesecloth to produce soymilk. Percent solu- ble solids, relating to percent protein, were evaluated with a hand- held refractometer. A target of 10% soluble solids in the soymilk was set. If the soymilk soluble solids’ values were low, the soy paste was filtered until the desired solids level was reached. Soymilk was heated to 85 °C and held for at least 5 min to inactivate trypsin in- hibitors. One hundred mL of the heat-treated soymilk, plus coag- ulants, were added to a preweighed container. Covered tofu sam- ples were tempered in an 85 °C water bath for 1 h to set the curd. Samples were refrigerated until consumption. Containers were weighed following consumption to determine amount of tofu con- sumed. Subjects were asked to consume the whole portion quanti- tatively.