Transport of citrate across the brush border and basolateral membrane of rat small intestine

A Comparison of Sodium Citrate and Sodium Bicarbonate Ingestion: Blood Alkalosis and Gastrointestinal Symptoms

This study compared the recommended dose of sodium citrate (SC, 500 mg/kg body mass) and sodium bicarbonate (SB, 300 mg/kg body mass) for blood alkalosis (blood [HCO3-]) and gastrointestinal symptoms (GIS; number and severity). Sixteen healthy individuals ingested the supplements in a randomized, crossover design. Gelatin capsules were ingested over 15 min alongside a carbohydrate-rich meal, after which participants remained seated for forearm venous blood sample collection and completion of GIS questionnaires every 30 min for 300 min. Time-course and session value (i.e., peak and time to peak) comparisons of SC and SB supplementation were performed using linear mixed models. Peak blood [HCO3-] was similar for SC (mean 34.2, 95% confidence intervals [33.4, 35.0] mmol/L) and SB (mean 33.6, 95% confidence intervals [32.8, 34.5] mmol/L, p = .308), as was delta blood [HCO3-] (SC = 7.9 mmol/L; SB = 7.3 mmol/L, p = .478). Blood [HCO3-] was ≥6 mmol/L above baseline from 180 to 240 min postingestion for SC, significantly later than for SB (120-180 min; p < .001). GIS were mostly minor, and peaked 80-90 min postingestion for SC, and 35-50 min postingestion for SB. There were no significant differences for the number or severity of GIS reported (p > .05 for all parameters). In summary, the recommended doses of SC and SB induce similar blood alkalosis and GIS, but with a different time course.

Factors Influencing Blood Alkalosis and Other Physiological Responses, Gastrointestinal Symptoms, and Exercise Performance Following Sodium Citrate Supplementation: A Review

This review aimed to identify factors associated with (a) physiological responses, (b) gastrointestinal (GI) symptoms, and (c) exercise performance following sodium citrate supplementation. A literature search identified 33 articles. Observations of physiological responses and GI symptoms were categorized by dose (< 500, 500, and > 500 mg/kg body mass [BM]) and by timing of postingestion measurements (in minutes). Exercise performance following sodium citrate supplementation was compared with placebo using statistical significance, percentage change, and effect size. Performance observations were categorized by exercise duration (very short < 60 s, short ≥ 60 and ≤ 420 s, and longer > 420 s) and intensity (very high > 100% VO2max and high 90-100% VO2max). Ingestion of 500 mg/kg BM sodium citrate induced blood alkalosis more frequently than < 500 mg/kg BM, and with similar frequency to >500 mg/kg BM. The GI symptoms were minimized when a 500 mg/kg BM dose was ingested in capsules rather than in solution. Significant improvements in performance following sodium citrate supplementation were reported in all observations of short-duration and very high-intensity exercise with a 500 mg/kg BM dose. However, the efficacy of supplementation for short-duration, high-intensity exercise is less clear, given that only 25% of observations reported significant improvements in performance following sodium citrate supplementation. Based on the current literature, the authors recommend ingestion of 500 mg/kg BM sodium citrate in capsules to induce alkalosis and minimize GI symptoms. Supplementation was of most benefit to performance of short-duration exercise of very high intensity; further investigation is required to determine the importance of ingestion duration and timing.

Sodium-coupled dicarboxylate and citrate transporters from the SLC13 family

The SLC13 family in humans and other mammals consists of sodium-coupled transporters for anionic substrates: three transporters for dicarboxylates/citrate and two transporters for sulfate. This review will focus on the di- and tricarboxylate transporters: NaDC1 (SLC13A2), NaDC3 (SLC13A3), and NaCT (SLC13A5). The substrates of these transporters are metabolic intermediates of the citric acid cycle, including citrate, succinate, and α-ketoglutarate, which can exert signaling effects through specific receptors or can affect metabolic enzymes directly. The SLC13 transporters are important for regulating plasma, urinary and tissue levels of these metabolites. NaDC1, primarily found on the apical membranes of renal proximal tubule and small intestinal cells, is involved in regulating urinary levels of citrate and plays a role in kidney stone development. NaDC3 has a wider tissue distribution and high substrate affinity compared with NaDC1. NaDC3 participates in drug and xenobiotic excretion through interactions with organic anion transporters. NaCT is primarily a citrate transporter located in the liver and brain, and its activity may regulate metabolic processes. The recent crystal structure of the Vibrio cholerae homolog, VcINDY, provides a new framework for understanding the mechanism of transport in this family. This review summarizes current knowledge of the structure, function, and regulation of the di- and tricarboxylate transporters of the SLC13 family.

Organic acid bioavailability from banana and sweet potato using an in vitro digestion and Caco-2 cell model

INTRODUCTION

Organic acids from plant food have been shown to play an important role in the prevention of chronic diseases (osteoporosis, obesity), inherent to western diets, but little is known about their bioavailability in the small intestine, information that needs to be determined in order to quantify likely effects on human health.

METHODS

An in vitro model of human digestion was carried out, comprising simulated oral, gastric and pancreatic digestion followed by an in vitro model of small intestine absorption using Caco-2 cell monolayers. As models for fruits and vegetables, freeze-dried or raw samples of banana and sweet potato were used.

RESULTS

Organic acids have been found to be slowly released from the food matrix during simulated digestion of both banana and sweet potato, either raw or after freeze-drying. In the Caco-2 cell assay, malic and oxalic acids were absorbed more than citric acid. Oxalic and citric acids, but not malic acid, were transported across the cell monolayer. The release and uptake of major organic acids from model fruits and vegetables using established in vitro simulation processes was not quantitative and varied with acid type.

CONCLUSION

Partial uptake is consistent with a dual nutritional role for organic acids as alkalinising agents (fraction which is taken up) and as modulators of large intestinal function (fraction which is not taken up in the small intestine). Studies of in vivo digestive release and uptake are needed in order to identify the contribution of organic acids to the nutritional value of fruits and vegetables.

Organic anions and potassium salts in nutrition and metabolism

The present review examines the importance of dietary organic anions in preventive nutrition. Organic anions are chiefly supplied by plant foods, as partially neutralised K salts such as potassium citrate, potassium malate and, to a lesser extent, oxalate or tartrate salts. Animal products may also supply K anions, essentially as phosphate, but also as lactate as a result of fermentative or maturation processes, but these K salts have little alkalinising significance. Citrate and malate anions are absorbed in the upper digestive tract, while a substantial proportion is probably metabolised in the splanchnic area. Whatever their site of metabolism, these anions finally yield KHCO3which is used by the kidneys to neutralise fixed acidity. This acidity essentially reflects the oxidation of excess S amino acids to sulfate ions, which is mainly related to the dietary protein level. Failure to neutralise acidity leads to low-grade metabolic acidosis, with possible long-term deleterious effects on bone Ca status and on protein status. Furthermore, low-grade acidosis is liable to affect other metabolic processes, such as peroxidation of biological structures. These metabolic disturbances could be connected with the relatively high incidence of osteoporosis and muscle-protein wasting problems observed in ageing individuals in Europe and Northern America. Providing a sufficient supply of K organic anions through fruit and vegetable intake should be recommended, fostering the actual motivational campaigns ('five (or ten) per d') already launched to promote the intake of plant foods rich in complex carbohydrates and various micronutrients.

Effect of potassium salts in rats adapted to an acidogenic high-sulfur amino acid diet

Low-grade metabolic acidosis, consecutive to excessive catabolism of sulfur amino acids and a high dietary Na:K ratio, is a common feature of Western food habits. This metabolic alteration may exert various adverse physiological effects, especially on bone, muscle and kidneys. To assess the actual effects of various K salts, a model of the Westernised diet has been developed in rats: slight protein excess (20 % casein); cations provided as non-alkalinising salts; high Na:K ratio. This diet resulted in acidic urine (pH 5.5) together with a high rate of divalent cation excretion in urine, especially Mg. Compared with controls, K supplementation as KCl accentuated Ca excretion, whereas potassium bicarbonate or malate reduced Mg and Ca excretion and alkalinised urine pH (up to 8). In parallel, citraturia was strongly increased, together with 2-ketoglutarate excretion, by potassium bicarbonate or malate in the diet. Basal sulfate excretion, in the range of 1 mmol/d, was slightly enhanced in rats fed the potassium malate diet. The present model of low-grade metabolic acidosis indicates that potassium malate may be as effective as KHCO3 to counteract urine acidification, to limit divalent cation excretion and to ensure high citrate concentration in urine.

Absorption of α-ketoglutarate by the gastrointestinal tract of pigs

Only a small percentage of alpha-ketoglutarate (AKG) administered lumenally to pigs appears in the portal circulation. This has been attributed to mucosal metabolism, and possibly by limited absorption. Although transporters for di- and tricarboxylic acids, which includes the sodium-dependent transporter NaDC-1, have been detected in the small intestine, correlations with functional assays are lacking. Therefore, intact tissues from three regions of the small intestine, stomach, and colon of weaned pigs were used to measure rates of AKG absorption. Western analysis was used to detect NaDC-1 in the three regions of small intestine. Rates of AKG absorption were highest in the small intestine, lowest in the colon, and intermediate in the stomach. Immunoreactive NaDC-1 was detected in the small intestine and this coincided with a component of AKG absorption that was inhibited by AKG and succinate. In contrast, absorption of AKG was inhibitable by unlabeled AKG, but not succinate, in the stomach, and by neither in the colon. Feeding studies indicated that the amounts of AKG that might be included in practical diets for pigs would not (1) upregulate rates of AKG absorption or (2) exceed estimated capacities of the small intestine to absorb AKG. The present findings indicate that the efficacy of AKG as an alternative metabolic fuel for enterocytes to spare dietary amino acids is not limited by absorption.

Sodium-coupled transporters for Krebs cycle intermediates

Krebs cycle intermediates such as succinate, citrate, and alpha-ketoglutarate are transferred across plasma membranes of cells by secondary active transporters that couple the downhill movement of sodium to the concentrative uptake of substrate. Several transporters have been identified in isolated membrane vesicles and cells based on their functional properties, suggesting the existence of at least three or more Na+/dicarboxylate cotransporter proteins in a given species. Recently, several cDNAs, called NaDC-1, coding for the low-affinity Na+/dicarboxylate cotransporters have been isolated from rabbit, human, and rat kidney. The Na+/dicarboxylate cotransporters are part of a distinct gene family that includes the renal and intestinal Na+/sulfate cotransporters. Other members of this family include a Na(+)- and Li(+)-dependent dicarboxylate transporter from Xenopus intestine and a putative Na+/dicarboxylate cotransporter from rat intestine. The current model of secondary structure in NaDC-1 contains 11 transmembrane domains and an extracellular N-glycosylated carboxy terminus.

Effect of citric acid supplements to a maize-soya diet on the in vitro availability of minerals, trace elements, and heavy metals

The aim of the present study was to evaluate the effect of increasing supplements of citric acid (CTA) on the in vitro availability of mineral (Ca, Mg), trace elements (Mn, Zn) and heavy metals (Pb, Cd). Phytaterich diets (0.8%) based on maize, soybean meal, corn starch and soybean oil (containing 5.0 g Ca, 1.2 g Mg, 50 mg Mn, 22 mg Zn, 10 mg Pb and 5 mg Cd per kg diet) were supplemented with 0, 1, 2, 3 and 4% of CTA. All diets were treated in a multi-enzyme in vitro system followed by equilibrium dialysis after peptic and pancreatic digestion. Under the conditions investigated, dietary addition of CTA enhanced dialysability of calcium, magnesium, manganese and zinc. Furthermore, a dose-dependent increase of lead and cadmium dialysability due to CTA supplementations was evident.