Bioavailability, calcium

The in vitro estimate of potential availability was defined, somewhat arbitrarily, as calcium solubility (18,000 x g supernatant) after complete digestion. Potentially available calcium was expressed as a percentage of the total food calcium (Figure 1). With the exception of a low in vitro calcium solubility value for whole milk, our earlier data compared reasonably well with calcium bioavailability information in the literature (7.) ... 

Experiments were conducted to determine if varying the conditions in the in vitro digestion procedure would affect post-digestion calcium solubility and in some cases, exchangeability. This was done with two purposes to test the use of the in vitro digestion procedure for studying factors which might influence calcium bioavailability and to use the results to modify the standard procedure. 

Bioavailability. In principle, the in vitro procedure provides a relatively fast and inexpensive means to study calcium bioavailability as a characteristic of foods. A knowledge of the chemistry of... 

Choice of Potential Bioavailability Criterion. It is usually assumed that calcium must be soluble and probably ionized in order to be available for absorption ( ). For the in vitro procedure, as a first approximation we chose calcium solubility after centrifugation at 18,000 x g as the measure of potential bioavailability (Figure 1). We assumed that this would probably overestimate the available calcium and later work based on fractionation might define the bioavailable calcium more precisely. The data in Table IV illustrate how the choice of criterion for "solubility" could affect the in vitro estimate of potential availability, even if in vitro conditions closely resembled in vivo conditions. Since our in vitro criterion unexpectedly underestimated calcium bioavailability for two of the three foods in the direct in vivo - in vitro comparison (8), it was necessary to determine the in vitro digestion conditions which might be limiting solubility before addressing the choice of appropriate criterion. 

Many other dietary factors have been reported to affect calcium bioavailability. Phytate, fiber, cellulose, uronic acids, sodium alginate, oxalate, fat (only in the presence of steatorrhea), and alcohol have been reported to decrease calcium bioavailability (15). Lactose and medium chain triglyceride increase it (15). FTuoride also affects calcium retention primarily by stimulating bone formation thereby decreasing calcium excretion (33-38). The effects of fluoride on calcium utilization have been variable (34,38,39). 

Strategies for determining calcium bioavailability The term bioavailability implies that fraction of a nutrient, drug or toxicant that is utilized relative to the amount consumed. Calcium is fed to the test subject in amounts below what the subject will utilize. This ensures that all of the calcium provided can be absorbed and metabolized. Then, that fraction... 

Our studies do not resolve the question of phytate vs fiber for the effect of wheat bran on dietary calcium bioavailability. Phytate level clearly affected apparent absorption of calcium in HS-II in the presence of an amount of the water insoluble fraction of dephytinized bran equivalent to 12 g of untreated bran and the phytate supplied as sodium phytate. An additional trial using untreated bran and the same amount of fiber as the water insoluble fraction with sodium phytate could resolve the question of fiber vs phytate. In HS-I, the balances were positive when a relatively large amount of bran, 36 g/day, was consumed. Calcium intakes were possibly higher than most men consume, but under the dietary conditions imposed for 15 days, the phytate and fiber of 36 g of bran did not express an adverse effect on calcium balance. 

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. 

Several comprehensive reviews on oxalic acid have been published in which effects on calcium metabolism were discussed (7-11). A review of studies on the effect of oxalic acid on calcium bioavailability in rats and humans is presented in this paper. 

In the studies on humans there appeared to be decreased calcium balances when 200 g or more of spinach per day was included in the diet. In two of the studies in which women were fed spinach, calcium intakes were below the Recommended Dietary Allowance of 800 mg/day (37). Some studies were conducted for short period of a week or less, which may not be sufficient time to adjust to a change in diet. From measurement of calcium excretion in urine after a test meal, it was shown that the calcium in oxalate-containing vegetables was less well-absorbed than that of milk or of vegetables not containing oxalic acid. However, this would not necessarily affect calcium balance, since the total amount of calcium in the diet would have to be considered. The effect of a combination of oxalic acid and fiber on calcium bioavailability should be further investigated. 

High levels of dietary zinc were associated with marked decreases in bone calcium deposition and in the apparent retention of calcium in male weanling albino rats. Marked increases in fecal calcium levels were also observed in the zinc-fed rats. Excessive dietary zinc was associated with a shifting of phosphorus excretion from the urine to the feces. This resulted in an increase in fecal phosphorus and provided an environmental condition which would increase the possibility of the formation of insoluble calcium phosphate salts and a subsequent decrease in calcium bioavailability. The adverse effect of high dietary zinc on calcium status in young rats could be alleviated and/or reversed with calcium supplements. 

Most of the research on the influence of zinc on calcium bioavailability has been in connection with zinc toxicity or the effects of high levels of dietary zinc on various animal systems. Such studies and/or investigations have been conducted on a variety of animal species and humans, but those studies which have revealed a possible effect of zinc on calcium bioavailability have generally involved the lamb, pig, and rat. This paper will be primarily a discussion of the effects of high levels of dietary zinc on calcium status in the rat. The effect of zinc on phosphorus status, however, has been included because there is the possibility that the effect of zinc on calcium bioavailability may be dependent upon the phosphorus status of the system. 

Basu and Nath (59), studied calcium absorption in young men eating diets varied in levels of calcium. Diets were either nearly fat-free or contained fat supplied primarily by butterfat, sesame oil, peanut oil, coconut oil or mustard seed oil. Feeding of all test fats except coconut oil resulted in a slight decrease in fecal calcium, thus, an assumed increase in calcium bioavailability. However, feeding of coconut oil (a highly saturated fat) resulted in increased fecal and urinary losses of calcium. 

The mechanisms by which various forms of dietary fiber influence calcium bioavailability apparently also differ. In some cases, apparent dietary fiber effects on calcium bioavailability may be secondary to effects on bile acid and salt secretion and reabsorption or to other dietary components. 

Luccia, B. H. D., and Kunkel, M. E. (2002b). Psyllium reduces relative calcium bioavailability and induces negative changes in bone composition in weanling Wistar rats. Nutr. Res. 22, 1027-1040. 

Mason, A. C., Weaver, C. M., Kimmel, S., and Brown, R. K. (1993). Effect of soybean phytate content on calcium bioavailability in mature and immature rats. /. Agric. Food Chem. 41, 246-249. 

Mehansho, H., Kanerva, R. L., Hudepohl, G. R., and Smith, K. T. (1989a). Calcium bioavailability and iron-calcium interaction in orange juice. /. Am. Coll. Nutr. 8, 61-68. 

Perales, S., Barbera, R., Lagarda, M. J., and Parre, R. (2006). Fortification of milk with calcium Effect on calcium bioavailability and interactions with iron and zinc. /. Agric. Food Chem. 54, 4901 906. 

Pointillart, A., and Gueguen, L. (1993). Calcium bioavailability in pig fed a dry milk-based diet versus a standard diet containing Ca citrate malate. In "BioavailabiUty 93 Nutritional, Chemical and Food Processing Implications of Nutrient Availability". (U. Schlemmer, ed.), pp. 1-5. Ettlingen, May 9-12 [Proceedings]. 

Alaska pollack back bone Calcium bioavailability Jung et al. (2006)... 

In addition, gelatin peptides have shown to accelerate absorption of dietary calcium in animal models increasing calcium bioavailability (Kim et al., 1998). Jung et al. (2006) reported that fish bone peptides (FBP) could inhibit the formation of insoluble Ca salts in neutral pFI. During the experimental period, Ca retention was increased and loss of bone mineral was decreased by FBP II supplementation in ovariectomized rats. The levels of femoral total Ca, bone mineral density, and strength were also significantly increased by the FBP diet to levels similar to those of the casein phosphopeptide diet group. 

UHT processing and storage have no effect on the total calcium content or calcium bioavailability. Calcium, phosphorus, and magnesium are shown to be equally bioavailable to rats from UHT milk, raw milk, and traditionally processed milk (Katz et al. 1981). Also, human infants retain similar amounts of calcium, potassium, and phosphorus whether fed UHT milk or conventionally pasteurized milk (Renner 1980 Mehta 1980). Data to date indicate no significant changes in the nutritional value of UHT milk under controlled heat treatment and subsequent storage. 

Cashman KD. Calcium intake, calcium bioavailability and bone health. BrJ Nutr. 2002 87(suppl 2) S169-S177. 

Kitts, D.K. and Yuan, Y.V. 1992. Caseinophosphopeptides and calcium bioavailability. Trends Food Sci. Technol. 3, 31-35. 

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