Differences In Excretion Rates Research Articles

The coevolution of adult body mass and excretion rate between genetically size-divergent brook trout populations

We tested if there was a difference in mass-specific excretion rate between two genetically size-divergent brook trout (Salvelinus fontinalis) populations that can be accounted for by genetic/maternal factors. We conducted laboratory-based common garden experimentation using F1 generation fish, with five to seven families per population at two ages (0.5 year juveniles and 1.5 years adults). We found that genetic/maternal differences in excretion rate on a per gram basis coevolve with genetic divergence in adult body mass between the populations. However, this coevolution has also resulted in no net difference in excretion rate between populations when differences in adult body mass were accounted for. Uncertainty in census estimates and variation in body mass distributions created substantial variation in extrapolated whole-population excretion estimates. No other studies to our knowledge have tested for genetic/maternal divergence in excretion rate between genetically size-divergent fish populations. Genetically based population divergence in body mass, energy allocation in reproduction, and mass-specific excretion rate in the brook trout was likely a result of selection associated with differences in the availability of overwintering habitat between streams.

Enantiomeric determination of α-lipoic acid in urine by LC/MS/MS

An analytical method for the enantiomeric determination of α-lipoic acid in human urine was developed to evaluate the pharmacokinetics of α-lipoic acid, an ingredient in health food. Urine samples were collected over time after administering α-lipoic acid dietary supplement to healthy subjects. The samples were cleaned up by solid-phase extraction using an Oasis® MAX cartridge. In the LC/MS/MS method, CHIRALPAK AD-3R was used as the chiral separation column and acetonitrile-methanol-formic acid (10 mM) (25:25:50, v/v/v) was used as the mobile phase. 13C4 1,2,5,6-d-l-α-Lipoic acid was used as the internal standard. MS/MS was performed by electrospray ionization (ESI) in the negative ion mode using two monitoring ion transitions (m/z 205.0 → 170.9 and m/z 209.0 → 174.9). A calibration curve was prepared in the concentration range of 0.5–100 ng/mL for each enantiomer (r > 0.9999). The limit of detection (LOD, S/N = 3) and the limit of quantification (LOQ, S/N > 10) were 0.1 ng/mL and 0.5 ng/mL, respectively. The intra-day and inter-day accuracy of α-lipoic acid enantiomers at the LOQ level (0.5 ng/mL), the low concentration level (5 ng/mL), the middle concentration level (50 ng/mL), and the high concentration level (100 ng/mL) ranged from 93.7 to 103.1%. The intra-day and inter-day precision were ≦ 6.94% and ≦ 7.05%, respectively. Calculating the mean values of pharmacokinetic parameters revealed that the AUC and Cmax values of d-α-lipoic acid were statistically significantly higher than those of l-α-lipoic acid (p < 0.05). It was suggested that l-α-lipoic acid is more bioavailable than d-α-lipoic acid despite individual differences in excretion rate.

Consumption explains intraspecific variation in nutrient recycling stoichiometry in a desert fish.

Consumer-driven nutrient recycling can have substantial effects on primary production and patterns of nutrient limitation in aquatic ecosystems by altering the rates as well as the relative supplies of the key nutrients nitrogen (N) and phosphorus (P). While variation in nutrient recycling stoichiometry has been well-studied among species, the mechanisms that explain intraspecific variation in recycling N:P are not well-understood. We examined the relative importance of potential drivers of variation in nutrient recycling by the fish Gambusia marshi among aquatic habitats in the Cuatro Ciénegas basin of Coahuila, Mexico. There, G.marshi inhabits warm thermal springs with high predation pressure as well as cooler, surface runoff-fed systems with low predation pressure. We hypothesized that variation in food consumption among these habitats would drive intraspecific differences in excretion rates and N:P ratios. Stoichiometric models predicted that temperature alone should not cause substantial variation in excretion N:P, but that further reducing consumption rates should substantially increase excretion N:P. We performed temperature and diet ration manipulation experiments in the laboratory and found strong support for model predictions. We then tested these predictions in the field by measuring nutrient recycling rates and ratios as well as body stoichiometry of fish from nine sites that vary in temperature and predation pressure. Fish from warm, high-predation sites excreted nutrients at a lower N:P ratio than fish from cool, low-predation sites, consistent with the hypothesis that reduced consumption under reduced predation pressure had stronger consequences for P retention and excretion among populations than did variation in body stoichiometry. These results highlight the utility of stoichiometric models for predicting variation in consumer-driven nutrient recycling within a phenotypically variable species.

Factors affecting intestinal absorption of cholesterol and plant sterols and stanols.

Various factors affect intestinal absorption of cholesterol and plant sterols and stanols. Plant sterols and stanols are generally less absorptive than cholesterol. Differential absorption rates among various plant sterols and stanols have been also reported. Although it was suggested that differential absorption among cholesterol and various plant sterols was determined by difference in excretion rates of sterols and stanols through ATP-binding cassette transporter (ABC) G5/ABCG8 of intestinal cells, our study suggests that affinity for and solubility in bile salt micelles can be important determinants for differential absorption of plant sterols and stanols. It was also suggested that plant sterols were transiently incorporated into intestinal cells and then excreted to intestinal lumen through ABCG5/ABCG8. However, in a rat study, transient incorporation of sitosterol into intestinal cells was not observed, suggesting that sitosterol is differentiated from cholesterol at the incorporation site of intestinal cells. It is well established that plant sterols inhibit intestinal absorption of cholesterol and exert a hypocholesterolemic activity. Plant sterols are solubilized in bile salt micelles as cholesterol. Our study clearly showed that because the sterol-solubilizing capacity of bile salt micelles was limited, plant sterols solubilized in micelles reduced the solubility of cholesterol. This can be the major cause of inhibition of cholesterol absorption by plant sterols. Pancreatic cholesterol esterase accelerates intestinal absorption of unesterified cholesterol. Although it was suggested that cholesterol esterase accelerated esterification of cholesterol incorporated into intestinal cells and acted as a transporter at the surface of intestinal cells, our research revealed that the accelerated cholesterol absorption was caused by hydrolysis of phosphatidylcholine in bile salt micelles. It is thought that hydrolysis of phosphatidylcholine reduces the affinity of cholesterol for the micelles and accelerates the incorporation of cholesterol released from the micelles into intestinal cells.

Assessing the effects of guppy life history evolution on nutrient recycling: from experiments to the field

Summary Trait evolution can occur in response to anthropogenic alterations to ecosystems and can occur on timescales similar to those of ecological processes suggesting that it could alter ecosystem function. In this study, we characterise the effects of life history evolution on nutrient recycling using the Trinidadian guppy (Poecilia reticulata) as a model system. Guppy life history traits evolve in response to predation pressure. When predation pressure is removed, guppy population density and average body size of the population also increase. Therefore, the evolution of guppy life histories involves changes in individual traits and demographic characteristics, both of which can alter nutrient recycling. The relative contributions of these variables to guppy‐driven nutrient recycling are unknown. We synthesise data from published experiments to disentangle how differences in individual traits, population characteristics and environmental conditions contribute to differences in guppy excretion rates. Individual guppies adapted to the absence of predators [low‐predation (LP) guppies] have lower nitrogen and phosphorus excretion rates than individual guppies adapted to predators [high‐predation (HP) guppies]. However, LP guppy populations excrete twice as much nitrogen as HP populations because of their larger average body size and higher population densities. We compare these findings to guppy excretion data collected from HP and LP sites in four rivers in Trinidad. Phenotypic and population differences in excretion rates are consistent with those observed in the experiments. Our study demonstrates that life history evolution can alter nutrient recycling in freshwater ecosystems. Characterizing the combined effects of traits and demographics is essential for understanding the effects of life history evolution on ecosystem processes.

Eco-physiological response of two marine bivalves to acute exposition to commercial Bt-based pesticide

Microbial products based on the entomopathogenic bacterium Bacillus thuringiensis (Bt) are among the most common biopesticides used worldwide to suppress insect pests in forests, horticulture and agricultural crops. Some of the effects of commercial Bt have been recorded for terrestrial and freshwater non-target organisms but little research is available on marine fauna. Nevertheless, due to the contiguity of agro-ecosystems and coastal habitats, marine fauna may be highly influenced by this control method. We studied the effect of a commercial Bt product on the physiological and ecological responses and the energy budget of two of the most frequent marine intertidal bivalves in the Mediterranean, the native Mytilaster minimus and the invasive Brachidontes pharaonis. To test the effects experimentally, we simulated the worst scenarios possible using the average dose applied to fields and a hypothetical accumulation dose. The results showed the feeding rates of both species were affected detrimentally by the different experimental conditions; higher concentrations led to higher respiration rates, however neither species showed any significant difference in excretion rates. The biopesticide had a significant effect on the energy budget, the values decreasing with doses. In addition, it led to high mortality for the worst treatments and, in both species, induced significantly higher cardiac activity than in the controls. These results indicate a measurable effect of Bt commercial products on marine organisms, and great attention should be paid to biopesticides composed by entomopathogenic bacteria and addictive compounds. In addition, the results highlight the urgent need to study not only the effects of anthropogenic pressures on target organisms but also to extend our view to other ecosystems not expected to be influenced. Gaining data at the organismal level should help increase the sustainability of pest control and reduce the consequences of side-effects.

Does Inclusion at Low Levels of Organically Complexed Minerals Versus Inorganic Forms Create a Weakness in Performance or Antioxidant Defense System in Broiler Diets?

Inorganic trace mineral participates, like sulfate or oxide salts, are a critical component in commercial poultry feeds. However, inorganic trace minerals can suffer from high rates of loss due to dietary antagonism which cause a significantly reduce in their bioavailability. As a result, more inorganic trace minerals are supplied than are actually needed. An excess of supplemental inorganically complexed minerals leads to waste and environmental. The use of lower levels of organically complexed minerals in poultry diets has become a common method to solve this problem in recent years. Organically complexed minerals are biotechnologi cal products which inorganic mineral integrated to organic structures such as protein or polysaccharid e. Research regarding this kind of supplementati on is still at a nascent stage, though and not enough data exists to determine optimal levels of organically complexed minerals and to quantify differences in excretion rates between inorganic and organic sources. In addition, it is unclear that using at lower levels of organically complexed minerals in diet whether creates a weakness on systems, in which the mineral plays an active role.

Nutrient recycling by coastal macrofauna: intra- versus interspecific differences

In the context of global change, improving our understanding of how species communities shape ecosystem functioning and stability is a key issue. Therefore, we have to adopt a functional approach by considering the role of organisms in ecosystem processes. Nutrient recycling is important for sustaining primary productivity in aquatic systems but has been largely overlooked for macro-organisms. In a Mediterranean coastal ecosystem we found that per capita ammonium and phosphate excretion rates for the 9 dominant nektonic macrofauna species exceeded those of 3 benthic bivalves by factor 100. Body mass and species identity together significantly explained these interspecific differences in excretion rates. Significant differences in the effect of body mass on nutrient excretion rates were also found among the 7 fish species. More studies are needed to further explore the biological determinants of the intra-and interspecific variability of excretion rates as well as their consequences on marine ecosystem functioning.

Soluble nitrogen and phosphorus excretion of exotic freshwater mussels (Dreissena spp.): potential impacts for nutrient remineralisation in western Lake Erie

Summary1. Recent increases in phytoplankton biomass and the recurrence of cyanobacterial blooms in western Lake Erie, concomitant with a shift from a community dominated by zebra mussels (Dreissena polymorpha) to one dominated by quagga mussels (D. bugensis), led us to test for differences in ammonia‐nitrogen and phosphate‐phosphorus excretion rates of these two species of invasive molluscs.2. We found significant differences in excretion rate both between size classes within a taxon and between taxa, with zebra mussels generally having greater nutrient excretion rates than quagga mussels. Combining measured excretion rates with measurements of mussel soft‐tissue dry weight and shell length, we developed nutrient excretion equations allowing estimation of nutrient excretion by dreissenids.3. Comparing dreissenid ammonia and phosphate excretion with that of the crustacean zooplankton, we demonstrated that the mussels add to nitrogen and phosphorus remineralisation, shortening nitrogen and phosphorus turnover times, and, importantly, modify the nitrogen and phosphorus cycles in Lake Erie. The increased nutrient flux from dreissenids may facilitate phytoplankton growth and cyanobacterial blooms in well‐mixed and/or shallow areas of western Lake Erie.

Urinary excretion of lead during pregnancy and postpartum

We have compared lead isotopic ratios and lead concentrations in 53 spot urine and 59 24-h urine samples from 13 subjects covering the interval from pre-pregnancy through 180 days postpartum to estimate the amount of lead excreted in urine and renal clearance relative to blood. The total amount of lead excreted in 24-h urine samples ranges from 0.8 to 5.9 μg Pb with an arithmetic mean of 2.2±1.1 μg (geometric mean 1.90 μg). This compares with amounts of 0.9–10 μg of extra lead per day estimated to be released into blood from the skeleton during pregnancy and postpartum. There were no differences in excretion rates during the trimesters of pregnancy and between pregnancy and postpartum time periods. The renal clearance relative to blood ranged from 0.8 to 10 g/h (arithmetic mean 3.2±1.9; geometric mean 2.7). Renal clearance relative to blood was somewhat higher in trimesters 2 and 3 compared with postpartum 150–180 days ( P=0.004, 0.006, respectively). Reassessment of earlier published blood and dietary data for Australian pregnant controls indicates there is no increased gastrointestinal absorption of lead during pregnancy and postpartum. This differs from calcium, which shows increased absorption during late pregnancy. In light of the inconvenience of sampling and potential contamination at the low levels of lead found in most of these subjects, we do not consider the 24 h urines to provide sufficient useful information.

Trophic accumulation and depuration of mercury by blue crabs ( Callinectes sapidus) and pink shrimp ( Penaeus duorarum)

Mercury concentrations in blue crabs ( Callinectes sapidus) collected from an area of mercury-contaminated sediments in Lavaca Bay, TX, USA, are more than an order of magnitude greater than concentrations in penaeid shrimp from the same area. Laboratory feeding experiments using mercury-contaminated fish as food showed that both blue crabs and pink shrimp ( Penaeus duorarum) could accumulate mercury concentrations comparable to those in their food in 28 days. Calculated mercury assimilation efficiencies averaged 76% for blue crabs and 72% for pink shrimp. Significant depuration of mercury by blue crabs was not observed during a subsequent 28-day period, but pink shrimp lost mercury at a rate of about 0.012 day −1. Model calculations predict biomagnification factors of mercury of about two to three at steady state for both species. The large difference in observed concentrations of mercury in field-collected blue crabs and penaeid shrimp does not result from differences in efficiency of mercury assimilation from their food or from differences in excretion rates. It is more likely the result of differences in residence times in the contaminated area and of differences in feeding habits.

Norfloxacin and Salmonella Excretion in Acute Gastroenteritis – a 6-Month Follow-up Study

In a controlled, randomized, double-blind study, 100 patients (66 women, 34 men; age 18-69 years) with acute salmonellosis were treated with norfloxacin (400 mg) or placebo tablets twice daily for 10 days to compare the effects on the excretion time of salmonella bacteria. In all patients salmonellae were detected in the stools before the start of treatment. The follow-up included 6 stool cultures after the start of treatment: day 3-4, day 12-14, and 4 times during 1-6 months. At 3-4 days there were 98% non-excretors in the norfloxacin group (46/47 patients) compared to 38% (17/45) in the placebo group (p < 0.001). The cumulative 6-month elimination rate in norfloxacin patients at 3-4 days was 72%, which was significantly (p = 0.0001) greater than the 31% in the placebo patients. However, there was no significant difference in the proportion of non-excretors or the elimination rate between the 2 groups at the following visits. Only one patient had an adverse event resulting in discontinuation of the treatment. We conclude that norfloxacin treatment for 10 days decreased the excretion of salmonella bacteria during the first week, but there was no difference in excretion rates 1-6 months after treatment initiation in the treatment versus placebo group.

Potential use of ketoconazole in a dynamic endocrine test to differentiate between biological outliers and testosterone use by athletes

Determination of the ratio of testosterone to epitestosterone (T/E) in urine is used to detect testosterone administration in athletes, with a ratio > 6 considered as evidence of an offense. We show that administration of ketoconazole, which inhibits testosterone biosynthesis, may be useful for differentiating between an athlete who is using testosterone and one who naturally gives a ratio > 6. In a control subject pretreated with testosterone, ketoconazole caused the ratio to increase; conversely, it caused a decrease in the ratio in an athlete under investigation. Repeated administration of ketoconazole to two normal men caused a decrease in the ratio due to a large decrease in the urinary excretion rate of testosterone relative to epitestosterone. Stimulation with human chorionic gonadotropin exacerbated the differences in excretion rates. A single administration of ketoconazole to six normal men caused the T/E ratios to decrease significantly within 8 h, a suitable time scale for use in a dynamic test.

A Radioimmunoassay for Human Urinary Prealbumin

We describe a validated radioimmunoassay for prealbumin in urine. Using timed overnight urine samples, the normal reference range was less than 10-148 micrograms/L; or less than 1.8-9.6 micrograms/mmol creatinine, excretion in women being significantly greater than in men (P < 0.05); or less than 7.3-114 ng/min with no significant difference in excretion rate between the sexes. Urines exhibited loss of immunoreactivity after storage at -20 degrees C and thawing. No such loss occurred after storage for 4 weeks at 4 degrees C or room temperature. The urinary excretion of prealbumin was highly correlated with that of albumin (r = 0.85), and clearance relative to creatinine was 2 x 10(-6), the same order as that of albumin.

Ammonia Excretion by Fingerling Walleyes Fed Two Formulated Diets

Abstract We investigated the effect of dietary protein level (41 or 61%) on the total ammonia nitrogen (TAN) excretion by fingerling walleyes (Stizostedion vitreum) at 21°C. Total ammonia nitrogen concentrations ranged from 0.06 to 0.75 mg/L and were positively correlated with tank biomass (41% protein, r = 0.72, P < 0.01; 61% protein, r = 0.83, P < 0.01). The overall between-treatment difference in excretion rates, 28.1 versus 29.2 mg TAN/kg fish per hour for 41 and 61 protein diets, respectively, was not significant. Fingerling walleyes fed 41 or 61% protein feed produced an estimated 14.3 and 14.4 g TAN!kg feed per day.

Depletion of Plasma Glycine and Effect of Glycine by Mouth on Salicylate Metabolism During Aspirin Overdose

1. The metabolism of aspirin was investigated in 45 patients who had taken self-administered overdose of aspirin and were treated with fluids only, glycine, N-glycylglycine by mouth, or by sodium bicarbonate i.v. 2. The major metabolite recovered in the urine of patients treated with oral fluids, glycine or N-glycylglycine was salicyluric acid, which accounted for means of 51%, 47% and 38% of the total, respectively; salicylic acid comprised 19%, 29% and 29%. In contrast, salicylic acid (42%) was the major urinary metabolite recovered from patients treated with sodium bicarbonate. 3. Plasma glycine concentrations in healthy volunteers who had taken no aspirin remained constant through the day and were not affected by a therapeutic dose (500 mg) of aspirin. Plasma glycine was consistently lower in patients with aspirin overdose than in these healthy volunteers, suggesting depletion of available glycine. 4. Orally administered glycine and N-glycylglycine increased plasma glycine. While the fraction of total salicylate recovered as salicyluric acid was not altered, the maximum rate of excretion of salicyluric acid was higher in patients who received glycine than in the control group; there was no significant difference in the maximum rate of excretion of salicyluric acid between the group that received glycine and the group that received N-glycylglycine. 5. The data suggest that exogenous glycine increases the rate of formation of salicyluric acid in salicylate overdose.

Glutathione conjugation of the α-bromoisovaleric acid enantiomers in the rat in vivo and its stereoselectivity. Pharmacokinetics of biliary and urinary excretion of the glutathione conjugate and the mercapturate

The glutathione (GSH) conjugation of ( R)- and ( S)- α-bromoisovaleric acid (BI) in the rat in vivo, and its stereoselectivity, have been characyerized. After administration of racemic [1- 14C]BI two radioactive metabolites were found in bile: only one of the possible diastereomeric BI-GSH conjugates, ( R)-I-S-G (35 ± 2% of the dose), and an unidentified metabolite “X” (6 ± 1%). In urine, only one of the possible BI-mercapturates, ( R)-I-S-MA (14 ± 1%), minor unidentified polar metabolites (5 ± 1%) and unchanged BI (13 ± 2%) were excreted. When ( R) or ( S)-BI were administered separately, the same metabolites were found. However, a ten-fold difference in excretion half lives of the biliary metabolites was observed following ( S)- and ( R)-BI administration, ( S)-BI being more rapidly excreted. The excretion of the mercapturate in urine shows the same difference in excretion rate: its half life after administration of ( R)-BI was more than 10 times longer than after a dose of ( S)-BI. More of the dose of ( S)-BI was excreted after 5 hr in bile and urine: 58% and 23% respectively for ( S)- and ( R)-BI. Therefore, a pronounced stereoselectivity in GSH conjugation exists for the ( R) and ( S) enantiomers of BI in the rat in vivo, which is a major determinant of their pharmacokinetics. The results suggest that (slow) inversion of the chiral centre of BI occurred in the rat in vivo.

6 beta-hydroxycortisol in serum and urine as determined by enzyme immunoassay on microtitre plates.

We modified our previous enzyme immunoassay (Clin Chim Acta 1986;157:267-76) for estimating 6 beta-hydroxycortisol (6 beta-OHF) to provide a more routine procedure for measuring it in urine and to increase sensitivity for its measurement in serum. Precision, analytical recovery, specificity, and detection limit are reported. 6 beta-OHF and 17-hydroxycorticosteroids in 24-h and 4-h (08:00-12:00 h) urine collections from healthy volunteers showed no significant day-to-day differences in excretion rate for either collection period. Mean values for 6 beta-OHF in serum of men, women, and women taking oral contraceptives showed no significant differences among these groups.

Physiological energetics of the intertidal sea anemone Anthopleura elegantissima

Energy budgets were calculated for individuals of the sea anemone Anthopleura elegantissima (Brandt), collected in 1981 and 1982 from Bodega Harbor, California, USA. Rates of ammonium excretion were measured in high-and low-intertidal, symbiotic and aposymbiotic sea anemones within 24 h of collection. Among symbiotic and aposymbiotic individuals, no differences in excretion rate were found on the basis of intertidal height. However, rates of ammonium excretion in aposymbiotic anemones (2.14 μmol NH + 4 g-1 h-1) were significantly higher than in symbiotic ones (0.288 μmol NH + 4 g-1 h-1). Rates of excretion were used with estimated rates of oxygen uptake to calculate nitrogen quotients (NQ). NQ and RQ values from the literature were used to calculate an oxyenthalpic equivalent [501 kJ (mol O2)-1 for R+U], and mass proportions of protein (54%), carbohydrate (44%) and lipid (2%) catabolized during routine metabolism in this species 24 h after feeding. Integrated energy budgets of these experimental anemones were calculated from data on ingestion, absorption and growth, and estimates of translocated energy from the symbiotic algae. Contribution of zooxanthellae to animal respiration based on translocation=90% and RQ=0.97 are 41 and 79% in high-and low-intertidal anemones, respectively. Calculated scope for growth is greater than directly measured growth in both high-and low-intertidal individuals. The deficit, estimated as 30% of assimilated energy in high-intertidal anemones, is attributed to unmeasured costs (specific dynamic effect) or production (mucus). Low-intertidal anemones lost mass during the experiment, implying that the magnitude of the deficit was greater in these anemones than in upper intertidal individuals. Anemones from both shore levels lost zooxanthellae during the experiment, which contributed to energy loss since the contribution of the zooxanthellae is greater in low-intertidal anemones. Scope for growth is preserved in high-intertidal anemones (29% of assimilated energy) because metabolic demands are lower due to aerial exposure, and prey capture rate is higher compared to lowshore anemones. Although possibly underestimated, lower scope for growth in low-shore anemones may result from continuous feeding and digestion processes that are less efficient than those of periodically feeding high-intertidal anemones.

The effect of breed of sire on the urinary excretion of phosphorus and magnesium in lambs

ABSTRACTSeventy-eight lambs, 9 weeks of age, the offspring of sires of the Scottish Blackface, East Friesland, Finnish Landrace, Suffolk and Texel breeds mated to Scottish Blackface females, as a common maternal breed, were given a barley-fish meal diet containing 1-6 g magnesium and 5·9 g phosphorus per kg dry matter for 13 weeks. Urinary excretion of Mg and P was estimated from the ratio of Mg or P to creatinine in spot samples of urine. A relationship between creatinine excretion and live weight was obtained using total urine excretion from a sample of 20 lambs, two of each sex from each sire breed group. Good agreement was found between the actual and predicted urinary excretion of minerals by the 20 lambs on three occasions.The effect of sire breed on urinary excretion of P was significant (P &lt; 0·05) and the breed types ranked, in descending order of excretion rate, Texel, Blackface, East Friesland, Finnish Landrace and Suffolk, with a four-fold difference in excretion rate between Texel and Suffolk. This difference in urinary excretion of P was taken as evidence of breed differences in efficiency of absorption of dietary P.Urinary excretion of Mg was correlated with the intake of Mg. The mean estimated efficiency of absorption of dietary Mg varied, over the three occasions, from 0·42 to 0·50 and from 0·39 to 0·45 for total collections and for spot samples of urine respectively. Individual variation in the estimated efficiency of absorption within sires was not significant whereas that between sires was significant (P &lt; 0·05). Heritability was estimated to be 0·62 (s.e. 0·30).