Metabolic Balance Studies

3 ABSORPTION AND METABOLISM OF BORON 4.3.1 Metabolic Balance Studies 

The metabolic balance study by Sutherland et al. (1999) highlighted a potential interaction between boron and zinc. The anthors reported boron as being in negative balance during a period of zinc repletion, as compared with the positive boron balance observed during consumption of the baseline diet and the low zinc diet. An interaction has been observed also with calcium whereby postmenopausal women who were supplemented with boron showed a decrease in the excretion of calcium (Hunt et al., 1997), and an increase in serum ionized calcium (Nielsen, 2004). Based on these data and others derived from experimental animals, boron is postulated to interact with a range of other micronutrients, particularly under situations of metabolic stress (Nielsen, 1991,2008) however, the mechanisms remain to be elucidated. 

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Two metabolic balance studies were conducted using healthy adult men to study the effect of phytate on bioavailability of dietary calcium. Dietary treatments were each 15 days in duration. In the first study, a mean daily calcium balance of 208+58 (SD) mg was observed when 2.0 g of phytate from 36 g of whole wheat bran was consumed daily with 1100 mg of calcium, phytate/calcium molar ratio 0.11. Calcium balance was 184+87 mg when 36 g of dephytinized bran was consumed with the same intake of calcium, phytate/calcium molar ratio 0.01. In the second study, calcium intake was 740 mg/day. 

Two metabolic balance studies conducted in our laboratory have yielded information relative to the effect of phytate and dietary fiber on calcium bioavailabilty. In the first study, a relatively high intake of dietary fiber was consumed with a 10-fold difference in phytate intake from wheat bran. In the second study three levels of phytate were consumed with a low amount of dephytinized bran as the principal dietary fiber source. The two higher phytate levels in the latter study were attained using sodium phytate. 

Law of Constant Heat Summation (1840-1843) were to provide the theoretical foundations for the metabolic balance studies of Voit and his pupils (Chapter 3) which established the quantitative link between food consumption and energy output, and thus the beginnings of the modem study of nutrition. 

In addition to the classical symptoms of zinc deficiency mentioned above, the following unusual conditions have been reported liver and spleen enlargement, abnormal dark adaptation and abnormalities of taste. Several laboratory procedures for diagnosing zinc deficiency are available. Measurement of zinc levels in plasma is useful in certain cases. Levels of zinc in the red cells and hair may be used for assessment of body zinc status. More accurate and useful parameters are neutrophil zinc determination and quantitative assay of alkaline phosphatase activity in neutrophils. Determination of zinc in 24 h urine may help diagnose deficiency if sickle cell disease, chronic renal disease and liver cirrhosis are ruled out. A metabolic balance study may clearly distinguish zinc-deficient subjects. 

The effect on zinc balance of a 10-fold difference in dietary molar ratio of phytate/zinc was tested in a metabolic balance study with 10 adult men. The mean zinc balance was 2.7 mg per day vrtien the dietary molar ratio of phytate/zinc was about 12 and 2.0 mg per day when the ratio was about 1. Menus consisted of foods commonly consumed in the United States. The mean daily intake of zinc was 17 mg and of neutral detergent fiber 16 g. The molar ratios of phytate/zinc were attained by using 36 g per day of whole or dephytinized wheat bran. Analysis of hospital and self-chosen diets Indicate that the majority of the United States population consume diets with molar ratios of phytate/zinc less than 10, but which provide less than the recommended dietary allowance of zinc. The balance results are discussed in relation to magnitude of the zinc intake, the type of food consumed and the role of adaptive responses in maintaining adequate zinc nutriture. 

We conducted a human metabolic balance study to test the concept of dietary phytate/zinc molar ratio as a predictor of zinc bioavailability to humans. Using unaltered or enzymatically dephytinlzed wheat bran with ordinary foods we attained phytate zinc molar ratios of about 1 and 12 with relatively high intakes of dietary fiber in the menus and found no difference in zinc balance. Retrospectively, the result may be qualified on the basis of the magnitude of the zinc Intake and possible adaptive or homestatic responses over the period of the study. A second study was then conducted and a wider range of phytate/zinc molar ratio was provided than in the first study. We will briefly outline the first study, give a progress report on the second study and, with some information on phytate intakes obtained by our laboratory, discuss the nutritional implication. 

Although the balance results indicate that in human diets a phytate/zinc molar ratio of 12 is not a deterrent to adequate zinc nutriture in adult men, qualifications as discussed above raise questions in interpreting the practical application. We have pursued two questions (1) What is the molar ratio of phytate/ zinc in our typical diets (2) What would be the effect of lower zinc intakes in a metabolic balance study with phytate/zinc molar ratio as the variable Part of question 1 has been completed, but only preliminary Information is available on question 2. 

The authors thank Priscilla Steel, Sheryl Cottrell and staff of the Human Study Facility, BHNRC Dr. Phylis Moser and Taryn Moy, College of Human Ecology, University of Maryland, College Park, and David Hill, VMNL, BHNRC for their cooperative efforts toward the metabolic balance studies. 

Methods for studying zinc bioavailability in humans include metabolic balance studies, radioisotopic techniques, stable isotope techniques, circulating zinc response, and perfusion techniques. These methods of bioavailability, along with other zinc-related studies, also are covered in detail. 

Several approaches have /been used to determine absorption of trace elements in humans. The most frequently used method has been balance studies, in which the amount of a mineral ingested is compared with the amount eliminated in the feces. However, absorption calculated from total mineral eliminated in fecal collections generally differs greatly from true absorption, since some of the mineral eliminated in the feces is of endogenous origin (1). A number of other difficulties with metabolic balance studies, such as variation in intestinal transit time and inadequate analytical precision, limit their usefulness and often result in conflicting results (2). 

For metabolic balance studies, coEections of stool are usuaEy made over a 72-hour period. Many balance studies are carried out in conjunction with research on the metabolism of such elements as calcium. It is important for such studies that a patient be on a controEed diet for a sufficiently long time before the commencement of the study, so that a steady state has been attained. 

From Beisel WR, Goldman RF, Joy RJT. Metabolic balance studies during induced hyperthermia in man. J Appl Physiol 1968 24 1-10. 

Direct measurement of total dietary intake over several days together with measurement of ah outputs in urine, feces, or by other routes can be used to estimate positive or negative balances. Because of the intrinsic difficulty of ensuring complete collection of aU materials with minimal contamination, metabolic balance studies can have systematic errors. ... 

We conducted a metabolic balance study, in conjunction with the Human Study Facility at Beltsville, to compare mineral balance of humans consuming whole bran or dephytinized bran continuously over several weeks. Results of this study have been published in part (17. 18. 19), and the results for iron will be summarized here. Ten adult men consumed menus that... 

Beonnee F, Salle BL, Putet G, Rigo J and Sen-TEEEE J (1992) Net calcium absorption in premature infants results of 103 metabolic balance studies. Am J Clin Nutr 56 1037-1044. 

Trivalent chromium is the most stable form in the food supply. Absorption estimates for chromium(III), based on metabolic balance studies or on urinary excretion from physiological intakes, range from 0.4 to 2.5% (Doisy et al. 1971 Bunker et al. 1984 Anderson and Kozlovsky 1985 Offenbacher etal. 1986 Anderson 1987). Because of analytical problems associated with the measurement of chromium absorption, several investigators have used the urinary excretion of chromium as an indicator of absorption. When dietary chromium intake was 10 pg per day, 2% of that amount was absorbed (estimated as urinary excretion), whereas at a chromium intake of 40 pg only 0.4-0.5% of the chromium was recovered in the urine (Anderson and Kozlovsky 1985). 

Bunker, V.W., Hinks, L.J., Lawson, M.S. and Clayton, B.E. (1984). Assessment of zinc and copper status of healthy elderly people using metabolic balance studies and measurement of leucocyte concentrations, Amer. J. Clin. Nutr., 4Q, 1096-1102 Burguera, M., Burguera, J.L, Cergio Rivas, P., and Alarcon, O.M. (1986). Determination of copper, zinc, and iron in parotid saliva by flow injection with flame absorption spectrophotometry. Atomic Spectrosc., Z. 79-81... 

Bunker VW, Hinks LJ, Stansfield MF, et al. 1987. Metabolic balance studies for zinc and copper in housebound elderly people and the relationship between zinc balance and leukocyte zinc concentrations. Am J Clin Nutr 46 353-359. 

Other approaches have been used to estimate nutrient requirements, such as metabolic balance studies and nutrient analysis of human milk. However, both of these approaches adopt an arbitrary target value to assess response to various nutrient intakes. For the premature infant, whose actual existence outside the womb is unnatural , it is impossible, other than by arbitrary definition, to define and justify goals. Therefore, although the intrauterine accretion data are based on relatively few observations and may be an underestimate of true intrauterine accretion, they presently represent the most appropriate guideline for establishing minimum growth and nutrient accretion goals in the newborn premature infant. 

If the catheter is not radio-opaque, 0.5 ml of water soluble contrast material should be injected into the catheter. After correct catheter placement has been confirmed, the hypertonic solution is continuously and finitely delivered to the infant by means of a constant infusion pump. If metabolic balance studies are to be done, the infant should be maintained on a metabolic bed to facilitate accurate collection of all external secretions. 

Blood lead values in childhood vary with age and differ from those of adults living in the same environment. Conventional metabolic balance studies in children have limitations for the study of factors modifying lead absorption. Feeding studies, and the use of both isolated and in-vivo gut loops in laboratory animals have shown that major minerals, lipids and proteins, modify lead uptake and absorption. Intrinsic ma-turational changes modify the response to ingested lead and are relat-... 

Metabolic balance study, 8 children 3 months—8 years old, Pb intake 5—17 g/kg/day 53 Variable diet versus age, older children in group Alexander etal. (1974)... 

Unabsorbed ingested Pb is simply excreted with feces. Absorbed Pb is retained in the short term and long term. Short-term retention studies in humans have principally involved adult volunteers ingesting isotopic Pb labels. The limited information for young children is derived from metabolic balance studies. Estimates of retention in the near term depend on the methodology employed in the estimates. Long-term retention of absorbed Pb in humans and experimental animal models entails deposition in bone and teeth, topics presented earlier in this chapter. 

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