Oral and intravenous l-[1-13 C]phenylalanine delivery measure similar rates of elimination when gastric emptying and splanchnic extraction are accounted for in adult mixed hounds.

There are few reported estimates of amino acid (AA) kinetics in adult mammals and none exist in adult dogs. The study objectives were to evaluate the use of oral isotope delivery in contrast to the more commonly used intravenous (IV) delivery to estimate AA kinetics in adult dogs and to estimate splanchnic extraction and gastric emptying using a commonly accepted mathematical model. Dogs received 25 × 1/2-hourly meals (13 g/kg BW/day) and either an oral or IV bolus of l-[1-(13) C]Phe (12 mg/kg BW). Blood samples were taken immediately before each feeding. Concentrations of plasma Phe were measured using liquid chromatography-tandem mass spectrometry. There were no differences in baseline plasma Phe concentrations (34 µm ± 0.61), Phe distribution volume, Phe pool size and rate constants between dogs when the tracer was administered IV or orally (p > 0.25). Decay curve for plasma l-[1-(13) C]Phe differed between IV and oral dosing protocols with IV dosing fit best using a two-compartment model. Phe disappeared from plasma at a mean rate of 2.8%/min. Estimates of gastric emptying and splanchnic extraction did not differ based on oral or IV tracer dosing when the decay curves were fit with the two-compartment model (p > 0.40). The half-life for gastric emptying was 18 min, and first-pass Phe extraction by the splanchnic bed was 24% of the dietary Phe. These results suggest that oral isotope dosing can be used as an alternative to IV isotope dosing in studies that utilize a primed, constant dosing approach to measure protein and amino acid kinetics.

A comparison of short-term appetite and energy intakes in normal weight and obese boys following glucose and whey-protein drinks.

OBJECTIVE: To compare the effect of glucose and whey-protein preloads on satiety and food intake (FI) as affected by time to the next meal and body composition in normal weight (NW) and obese (OB) boys. DESIGN: Cross-sectional clinical intervention study of the effect of caloric preloads on FI control in boys. SUBJECTS: Seventeen NW (body mass index (BMI)=18.9+/-0.5 kg/m(2); age=12.2+/-0.3 years) and 17 OB boys (BMI=25.8+/-0.9 kg/m(2); age=11.4+/-0.3 years) (Experiment 1) and 12 NW boys (BMI=18.6+/-0.6 kg/m(2); age=12.1+/-0.3 years) (Experiment 2). MEASUREMENTS: On three separate mornings and in random order each of the boys consumed a noncaloric sweetened preload, glucose (837 kJ) or whey protein (837 kJ) (Experiment 1) or noncaloric preload, glucose (1.0 g/kg) or whey protein (1.0 g/kg) (Experiment 2) made up to 250 ml with water 2 h after the consumption of a fixed breakfast. Food intake from a pizza meal was measured 30 min (Experiment 1) or 60 min (Experiment 2) later. RESULTS: In Experiment 1, glucose suppressed FI (mean kJ+/-s.e.m.) in NW (3126+/-304) and OB boys (3116+/-286) compared with the control (NW, 4015+/-337 and OB, 3791+/-255). Whey protein suppressed FI in NW, but not in OB boys. Body weight, fat-mass and fat-free mass were positively associated with FI after all treatments in NW, but was not related to FI in OB boys. In Experiment 2, FI was suppressed by whey protein (2683+/-367) more than by glucose (3107+/-294) or the control (3585+/-361). CONCLUSION: NW and OB boys respond differently to whey-protein preloads, with time to next meal a factor in the response to both glucose and protein preloads.

Calibration and validation of a carbon oxidation system and determination of the bicarbonate retention factor and the dietary phenylalanine requirement, in the presence of excess tyrosine, of adult, female, mixed-breed dogs,.

Carbon oxidation methods have been used as rapid and sensitive methods to determine whole-body AA requirements in multiple species. The objectives of the current studies were to validate complete CO recovery, determine the bicarbonate retention factor, and estimate the Phe requirement, in the presence of excess Tyr, in adult dogs using the direct oxidation technique. In this series of studies, 2 oxidation chambers were constructed and calibrated to ensure accurate collection of breath CO. First, 104.6 ± 7.1% CO was recovered from chambers and suggests that the chambers were appropriately designed for complete and efficient CO recovery. Second, we determined bicarbonate retention in 5 dogs using repeated oral dosing of a bicarbonate tracer (NaHCO) with small meals. At isotopic and physiological steady state, 102.5 ± 2.6% of the delivered NaHCO was recovered in breath. Third, the Phe requirement, when Tyr was supplied in excess, was determined by the rate of appearance of CO in the breath (CO). Dogs ( = 5) were fed test diets with different concentrations of Phe ranging from deficient to excessive for 2 d prior to conducting the tracer studies. The mean Phe requirement (when Tyr was supplied in excess) was 0.535% of diet (upper 95% confidence interval = 0.645% diet) on an as-fed basis or 0.575% of diet (upper 95% confidence interval = 0.694% of diet) on a DM basis and was based on a calculated (modified Atwater calculation) dietary ME density of 3.73 Mcal/kg DM. These data support the use of carbon oxidation methods and oral dosing of isotope to measure whole-body requirements of indispensable AA in adult dogs and suggest the current recommendations may be low.

Effect of NIDDM on the kinetics of whole-body protein metabolism.

We postulated that dietary protein utilization and body protein metabolism are altered in hyperglycemic individuals with non-insulin-dependent diabetes mellitus (NIDDM). This was tested by estimating the kinetics of protein metabolism in obese NIDDM patients in the hyperglycemic state of isoenergetic feeding and in the normoglycemic state induced by the prolonged use of a very-low-energy diet (VLED) and comparing them with results in obese nondiabetic subjects studied previously. Seven obese subjects with NIDDM (one male, six females, body mass index = 35.8 +/- 2.0 kg/m2) were given a 1.7 MJ (410 kcal) all protein (93 g/day) diet derived from hydrolyzed collagen and supplemented with tryptophan and methionine, which provides 16% of its amino acids as essential, a multivitamin and mineral supplement, and 16 mmol KCl for 42 days. During the seven-day isoenergetic diet and at weeks 4 and 6 of the VLED, amino nitrogen (N) flux rate was calculated from the urine [15N]urea enrichment by using the 60-h oral [15N]glycine method to obtain the integrated feeding-fasting metabolism. Rates of synthesis (S) and breakdown (B) were calculated from N flux. At day 7 of the isoenergetic diet, whole-body N flux, S, B, and resting metabolic rate (RMR) were 12-24% greater (P < 0.05) in the NIDDM subjects than observed in nondiabetic obese subjects. Mean plasma glucose decreased (P < 0.05) from the isoenergetic period (14.9 +/- 2.4 mM) to 7.2 +/- 1.2 mM at week 4 and 6.5 +/- 1.1 mM at week 6 of the VLED. RMR declined progressively by 25% at week 5 of the VLED. Corresponding significant (P < 0.05) decreases from isoenergetic feeding to weeks 4 and 6 of the VLED occurred in whole-body N flux (from 51 +/- 2 to 42 +/- 1 g N/day), in S (from 38 +/- 3 to 24 +/- 1 g N/day), and in B (from 39 +/- 3 to 26 +/- 1 g N/day) resulting in net losses (S-B). S-B was significantly more negative (P < 0.05) in NIDDM than in the nondiabetic obese subjects at week 4 (-1.5 +/- 0.5 vs. 0.9 +/- 0.3 g N/day) but not at week 6 (-1.3 +/- 0.4 vs. -0.9 +/- 4 g N/day). During the VLED, N balance became less negative with time but never reached equilibrium in NIDDM. Thus, abnormal protein metabolism is present in NIDDM in the isoenergetic fed state with moderate hyperglycemia and persists during a VLED that restores glycemia to near normal.

Use of bioelectrical impedance analysis measurements in the clinical management of malnutrition.

It is valuable to be able to measure the body composition of malnourished (undernourished) patients and to monitor their response to refeeding. The use of direct measurements, such as total body water, extracellular water, and total body potassium for clinical monitoring is expensive and cumbersome. We therefore have developed predictive equations for these variables by using fixed-frequency bioelectrical impedance analysis (BIA). Our equations deliberately do not include weight because they are used to predict changes in body composition in patients who are gaining weight during refeeding. BIA could predict changes in body composition with significantly greater precision than anthropometry alone. We conclude that BIA can be used to monitor changes in body composition in patients during refeeding.

Effect of glycemic control on the kinetics of whole-body protein metabolism in obese subjects with non-insulin-dependent diabetes mellitus during iso- and hypoenergetic feeding.

We postulated whether interventions capable of restoring euglycemia would correct whole-body protein metabolism, shown previously to be elevated in hyperglycemic persons with non-insulin-dependent diabetes (NIDDM). The kinetics of protein metabolism were estimated in obese subjects with NIDDM in the hyper- and normoglycemic states during isoenergetic feeding and in the normoglycemic state induced by 4 wk of a very-low-energy diet (VLED) with constant protein intake. Seven NIDDM subjects [three males and four females with a body mass index (in kg/m2) of 39 +/- 2] were given a weight-maintaining, liquid formula providing 95 g protein/d for 15 d, followed in six subjects (two males and four females) for 27 d by a diet providing 1.7 MJ, 93 g protein derived from casein-soy, 13 g carbohydrate, 2 g fat, multivitamins and minerals, and a potassium bicarbonate supplement (32 mmol) per day. Exogenous insulin was given to achieve normoglycemia during the first 8 d of isoenergetic feeding. On days 6-8, 12-14, and 25-27, nitrogen flux rate was calculated from the urine [15N]urea enrichment by using the 60-h oral [15N]glycine method to obtain "integrated" feeding and fasting values. Rates of synthesis and breakdown were calculated from nitrogen flux. During isoenergetic feeding, normoglycemia was associated with more positive nitrogen balance (2.6 +/- 0.5 compared with -0.6 +/- 0.6 g N/d, P < 0.05); 18-23% lower nitrogen flux, and synthesis and breakdown rates (P < 0.05), and a 3% decrease in resting energy expenditure (P < 0.05). During the VLED, euglycemia was achieved but nitrogen balance, although it became less negative with time, never reached equilibrium. This was associated with significant (P < 0.05) decreases in the synthesis rate, resulting in net protein losses. Thus, the altered protein metabolism in moderately hyperglycemic NIDDM subjects was improved with exogenous insulin in doses sufficient to restore normoglycemia in the isoenergetic fed state, but it remained abnormal with a reduced non-protein energy intake. This suggests that protein metabolism is more sensitive to insulinization than was thought previously.

Potassium, magnesium, and calcium balance in obese adolescents on a protein-sparing modified fast.

Ten obese adolescents consumed the protein-sparing modified fast (PSMF), a high-protein, low-carbohydrate diet, for 92 +/- 19 d and lost 14.7 +/- 5.3 kg. The effect of weight loss using the PSMF on potassium, magnesium, and calcium was evaluated using balance method at days 2-4 and 12-20, RBC-Mg, RBC-K and total body K (TBK). The vitamin- and mineral-supplemented PSMF allowed positive Ca and K balance and improved Mg balance (p less than 0.005). TBK decreased significantly, 118.7 +/- 13.7 to 97.4 +/- 1.0 g (3.04 +/- 0.35 to 2.50 +/- 0.03 mol) from baseline to 90 d (p less than 0.001), with no change in the RBC-K concentration. An unexpected finding was a significant decrease in RBC-Mg, 3.2 +/- 1.1 to 1.8 +/- 0.3 mmol/L (p less than 0.001) after 60-90 d on the diet despite maintenance of normal serum Mg level.

Model for determination of 13C substrate oxidation rates in humans in the fed state.

The effect of a test diet of conventional North American foods on breath 13CO2 abundance was determined in 4 healthy males. Subjects consumed a diet containing 41.9% energy as fat and a polyunsaturated:saturated fatty acid ratio of 0.217 for 5 days at a level equal to estimated requirements for total energy. One subject underwent four 5-day feeding periods. Over the feeding period mean change in subjects' body weight was -0.165 +/- 0.64% (means +/- SEM) of initial body weight. On day 5 breath samples were collected hourly from 0745 to 1645 h and analyzed for 13CO2 enrichment relative to the pre-breakfast fasted state (level at 0745 h). Breakfast and lunch were consumed at 0820 and 1300 h respectively. Mean enrichment peaked at 1445 h at 0.00311 atom % excess above fasting level for all subjects and 0.00243 atom % excess for the four repeated trials on one subject. Between subject variation (SEM) in 13CO2 enrichment (0.000618 atom %) was significantly greater than within subject variation (0.000308 atom %). These results indicate that ingestion of normal meals during the breath test period increases breath 13CO2 abundance due to a shift in substrate oxidation. The small within subject variation in repeated 13CO2 enrichment profiles indicates the reliability of 13C breath tests using controlled diets. It is concluded that for tests conducted under identical conditions, an initial background 13CO2 abundance profile determined once for each subject, can be subtracted from the subsequent enrichment profile generated by a labeled test substrate.

Whole body oxidation of dietary fatty acids: implications for energy utilization.

Whole body oxidation of dietary stearic, oleic, and linoleic acid was measured in males consuming a test diet of normal foods at a level commensurate with energy requirements for 16 days. Labeled stearic, oleic or linoleic acid was consumed with the breakfast meal on either day 8, 11, or 14. Breath samples were analyzed for total CO2 content and 13CO2 abundance. Breath enrichment of 13CO2 after ingestion of labeled substrate was calculated over background 13C abundance with diet only and expressed as fraction of substrate dose absorbed. Fecal excretion of labeled and diet fatty acids was determined for pooled stool collections. Stearic, oleic, and linoleic acids were separated from fecal fat extracts and combusted to determine 13CO2 enrichment over background. Both dietary and labeled stearic acid were less well absorbed than either oleic or linoleic acids. At hours 7-9 after ingestion of the labeled breakfast, significant differences in percent of absorbed dose excreted in breath were observed between all three fatty acids. Significant differences were observed in apparent amounts of labeled oleate, linoleate, and stearate oxidized after 3 to 9 h. This difference in fatty acid oxidation challenges the assumption that dietary fat is oxidized at a rate independent of its long chain fatty acid composition.

Comparison of oral and intravenous administration of sodium bromide for extracellular water measurements.

Extracellular water (ECW) is a component of body composition and an indicator of nutritional status. Although bromide has been used to measure ECW, no data regarding serial measurements are available. We attempted to validate giving bromide orally and to determine whether serial measurements of ECW are possible. Seven healthy adult volunteers were studied. Each received 1 cc 3% NaBr solution per kg body weight once intravenously and 3 times orally over 6 wk. Bromide concentrations were determined by neutron activation. ECW did not differ significantly at 2, 3, or 4 h after drug administration although there was a small but significant difference between modes of administration (p = less than 0.05). With repeated oral doses the estimate of ECW was unchanged; thus, oral bromide can be used for serial determination of ECW.

Protein metabolism in obese subjects during a very-low-energy diet.

We postulated that the return to nitrogen equilibrium after 3 wk of a negative balance during a very-low-calorie diet (VLCD) providing low-quality protein in obese subjects was due to availability of endogenously originating amino acids from a "pool" that, when depleted, would result in worsening balance. This should be reflected in altered kinetics of protein metabolism with the requirement for increased breakdown to maintain synthesis constant. Seven female obese subjects [body mass index (BMI) = 34.4 +/- 1.8 kg/m2] were given a 1.7-MJ/d all-protein diet (16.8 g N) derived from hydrolyzed gelatin (supplemented with tryptophan and methionine) that provides 18% of its amino acids as essential, a multivitamin-mineral supplement, and 16 mmol KCl for 42 d. At baseline (7-d isocaloric diet), and weeks 4 and 6 of VLCD, amino nitrogen flux rate was calculated from the 15N abundance in urinary urea using the oral 15N-glycine method and rates of synthesis (S) and breakdown (B) inferred from N flux. Whole-body N flux did not change from baseline to weeks 4 and 6 (39.5 +/- 2.0 vs 37.4 +/- 2.0 vs 39.2 +/- 1.9 g N/d). By contrast, S and B decreased at weeks 4 and 6 with S decreasing more so that net protein synthesis (S-B) was less positive at week 4 than at baseline (2.2 +/- 0.2 and 0.9 +/- 0.3 g N/d; P less than 0.05) and became negative at week 6 (-0.9 +/- 0.2 g N/d; P less than 0.05). Concurrently, N equilibrium was achieved by week 4 but returned to negative balance by week 6.(ABSTRACT TRUNCATED AT 250 WORDS)

Phenylalanine requirements determined by using L-[1-14C]phenylalanine in neonatal piglets receiving total parenteral nutrition supplemented with tyrosine.

The definition of amino acid requirements for neonates receiving total parenteral nutrition (TPN) is critical for the further improvement of this nutritional regimen. In the present study we investigated the kinetics and requirements of phenylalanine and tyrosine in neonatal piglets receiving TPN. Twenty-four 3-d-old male Yorkshire piglets were fitted with external jugular and femoral catheters and maintained on identical TPN formulations for 5 d. Total amino acid, phenylalanine, tyrosine, and energy intakes were 15, 0.61, and 0.51 g. kg-1 . d-1 and 1.1 MJ . kg-1 . d-1, respectively. On day 5, piglets (three per level) were randomly assigned to receive one of eight phenylalanine intakes: 0.2. 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, or 1.2 g. kg-1 . d-1. On day 6, phenylalanine kinetics were measured during a 4-h primed, continuous infusion of L-[1-14C]phenylalanine. Plasma phenylalanine and phenylalanine oxidation were statistically similar for the three lowest phenylalanine intakes and increased thereafter. Crossover regression analysis yielded estimates for the mean requirement and safe phenylalanine intake of 0.45 and 0.48 g . kg-1 . d-1, respectively (equivalent to 30 and 32 mg/g amino acids, respectively), in the presence of excess tyrosine. An inability of piglets to maintain a linear oxidative response at phenylalanine intakes > 0.8 g . kg-1 . d-1 (equivalent to 53 mg/g amino acids) was found. These data represent the first direct estimates of phenylalanine requirements in neonates receiving TPN and demonstrate the use of oxidation techniques for the estimation of amino acid requirements during parenteral nutrition.

Lysine requirement of adult males is not affected by decreasing dietary protein.

Recently, we reported the lysine requirement to be 41.2 mg.kg-1.d-1 using indicator amino acid oxidation (IAAO) in healthy adult males fed a protein intake of 1.0 g.kg-1.d-1. A lower protein intake has been hypothesized to significantly lower the requirement estimate. We tested this hypothesis using IAAO by estimating the lysine requirement at a protein intake of 0.8 g.kg-1.d-1 in five healthy adult males. Lysine requirement was determined from the rate of appearance of 13CO2 in breath (F13CO2) by using a primed 4-h continuous infusion of L-[1-(13)C]phenylalanine. Phenylalanine flux was not affected by graded increases in dietary lysine. Phenylalanine oxidation and F13CO2 decreased linearly (P < 0.05) as lysine intake increased to a breakpoint, after which the rates were not significantly different. Two-phase linear regression determined this breakpoint (mean lysine requirement) to occur at a dietary lysine intake of 45.0 mg.kg-1.d-1. We conclude that no reduction appears in the lysine requirement with a lower protein intake. Thus, lysine requirement appears not to be altered by protein intakes within the lower end of the protein requirement range.

Threonine requirement of young men determined by indicator amino acid oxidation with use of L-[1-(13)C]phenylalanine.

BACKGROUND: Threonine is an indispensable amino acid with a complex degradative pathway. Use of the indicator amino acid oxidation technique should provide an estimate of the threonine requirement that is not affected by its metabolic pathway. OBJECTIVE: Our objective was to determine the requirement for threonine in men by using the indicator amino acid oxidation method and to provide statistical estimates of the population mean and 95% CIs of the threonine requirement. We hypothesized that the current World Health Organization estimate of the threonine requirement, 7 mg*kg(-)(1)*d(-)(1) (based on nitrogen balance studies), is too low. DESIGN: Six healthy men each received 6 different threonine intakes while consuming an energy-sufficient diet with 1.0 g L-amino acid mixture*kg(-)(1)*d(-)(1). The effect of graded alterations in dietary threonine intake on phenylalanine flux and oxidation was studied by using L-[1-(13)C]phenylalanine as the indicator amino acid. RESULTS: The results of two-phase linear regression crossover analysis showed that the mean threonine requirement, based on indicator oxidation, was 19.0 mg*kg(-)(1)*d(-)(1) with an upper safe intake of 26.2 mg*kg(-)(1)*d(-)(1). CONCLUSIONS: This is the first application of the indicator amino acid oxidation technique in humans to study the requirement for an indispensable amino acid with a complex degradative pathway. We found that the upper safe intake for 95% of the population is almost 4-fold higher than the current World Health Organization estimate.

Tryptophan requirement in young adult women as determined by indicator amino acid oxidation with L-[13C]phenylalanine.

To determine tryptophan requirements with use of the indicator amino acid oxidation technique, we studied 5 healthy, premenopausal women who each consumed 6 different intakes of tryptophan. The energy-sufficient diet contained 1.0 g x kg(-1) x d(-1) of an L-amino acid mixture patterned after egg protein, with adequate phenylalanine (14 mg x kg(-1) x d(-1)) and excess tyrosine (40 mg x kg(-1) x d(-1)). The effect of graded alterations in dietary tryptophan on phenylalanine flux and oxidation was studied by using L-[1-(13)C]phenylalanine as the indicator amino acid. Graded increases in tryptophan had no effect on phenylalanine flux. Determined by two-phase linear regression crossover analysis, the mean tryptophan requirement based on indicator (phenylalanine) oxidation was 4.01 mg x kg(-1) x d(-1), with a safe intake at 5.02 mg x kg(-1) x d(-1). These results show that the indicator amino acid oxidation technique can be used to determine the requirement for amino acids such as tryptophan that cannot be successfully studied by direct oxidation because of their characteristic metabolism. Our value of safe intake for 95% of the population was found to be 43% higher than that reported previously for adults.

Lysine requirement of neonatal piglets receiving total parenteral nutrition as determined by oxidation of the indicator amino acid L-[1-14C]phenylalanine.

The indicator amino acid oxidation technique was adapted for use during total parenteral nutrition. Twenty-one male Yorkshire piglets (3 d of age) fitted with jugular and femoral vein catheters, received total parenteral nutrition for 6 d. Total parenteral nutrition included 15 g amino acids .kg-1.d-1 (1.25 g lysine.kg-1.d-1), lipid + glucose (1.1 MJ.kg-1.d-1), and micronutrients. Piglets were randomly assigned to lysine intakes of 0.40, 0.55, 0.70, 0.85, 1.00, 1.15, or 1.30 g.kg-1.d-1 and received a 4-h primed, constant infusion of L-[1-14C]phenylalanine. Phenylalanine oxidation (% of dose) decreased significantly (P < 0.05) from 18.6% at the lowest lysine intake to 3.0% at a lysine intake of 0.85 g.kg-1.d-1. Further increases in lysine intake had no effect on phenylalanine oxidation. Regression analysis yielded a mean lysine requirement of 0.79 g.kg-1.d-1 and a safe intake (upper 95% confidence limit) of 0.84 g.kg-1.d-1. Changes in the plasma lysine concentration and phenylalanine oxidation rates supported this estimate. The indicator amino acid oxidation technique can now be used to determine amino acid requirements during total parenteral nutrition. This means that the ideal amino acid pattern for neonates receiving total parenteral nutrition can now be defined by a sensitive and minimally invasive measurement of amino acid kinetics.

Protein turnover and resting energy expenditure in patients with undernutrition and chronic lung disease.

Whole-body protein metabolism was studied in 11 undernourished cystic fibrosis (CF) patient (7 female), 12 normally nourished CF patients (3 female), 7 anorexia nervosa (AN) patients (all female), and 15 normal control subjects (9 female). Protein turnover was studied by the single dose [15N]glycine method and the cumulative excretion of labeled urinary urea and ammonia. Energy metabolism was studied by open-circuit indirect calorimetry. Contrary to previous reports, no differences were found between the protein turnover of CF groups and the normal control group. However, patients with AN had a negative net protein deposition. Resting energy expenditure was significantly reduced in AN patients and increased in CF patients. The gender of CF patients did not affect protein and energy metabolism but fat mass was higher and fat-free mass was lower in CF females.

Glycine, leucine, and phenylalanine flux in low-birth-weight infants during parenteral and enteral feeding.

Kinetics of three amino acids with different sites and characteristics of metabolic regulation were studied in low-birth-weight infants during enteral and parenteral feeding regimens typical of clinical practice. Primed constant infusions of [15N]glycine, L-[1-13C]leucine, and L-[1-13C]phenylalanine were administered simultaneously by the same route as the feeding, with isotope enrichment measured in urine over 12 h. The effect of feeding regimen was specific to each amino acid (mean +/- SD): glycine flux was lower during parenteral feeding (470 +/- 15 vs 561 +/- 69 mumol.kg-1.h-1, P less than 0.05), leucine flux was unaffected (360 +/- 77 vs 388 +/- 78 mumol.kg-1.h-1), and phenylalanine flux was higher (106 +/- 29 vs 56 +/- 6 mumol.kg-1.h-1, P less than 0.01). Kinetics were influenced by the interaction of several factors, including amino acid intake and routes of feeding and tracer administration. Glycine was most affected by route of feeding and phenylalanine was most affected by intake whereas leucine was little affected. Estimates of whole-body protein turnover calculated from leucine and phenylalanine were different; thus calculations of protein turnover from kinetics of a single amino acid should be interpreted with caution.

Tyrosine requirement of healthy men receiving a fixed phenylalanine intake determined by using indicator amino acid oxidation.

BACKGROUND: The currently accepted total aromatic amino acid requirement for adults is based on nitrogen balance measurements in individuals who received their intake of aromatic amino acids solely as phenylalanine. OBJECTIVE: The objective of this study was to determine the requirement for the amino acid tyrosine in healthy men receiving an adequate, but not excessive, intake of phenylalanine (9 mg x kg(-1) x d(-1)). DESIGN: The effect of a graded intake of tyrosine was determined in 6 healthy men consuming energy-sufficient diets containing 1 g protein x kg(-1) x d(-1). The tyrosine requirement was determined by using indicator amino acid oxidation methodology with L-[1-13C]lysine as the indicator. Subjects were studied at each of 7 tyrosine intakes. RESULTS: A graded intake of tyrosine had no effect on lysine flux. The mean tyrosine requirement was determined from the response of the oxidation of L-[1-13C]lysine to breath 13CO2. A 2-phase linear regression crossover analysis of breath 13CO2 identified the breakpoint and upper 95% confidence limit, which represents the mean and safe intakes, to be 6.0 and 7.0 mg x kg(-1) x d(-1), respectively. CONCLUSIONS: The safe intake of total aromatic amino acids calculated from the present results for tyrosine and our previous estimate for phenylalanine is estimated to be 21 mg x kg(-1) x d(-1). This intake is 1.5 times the currently recommended total aromatic amino acid intake of the FAO/WHO/UNU (1985), 14 mg x kg(-1) x d(-1). Furthermore, the absolute aromatic amino acid requirement may be dependent on the proportional balance of these amino acids in the diet.

One-year follow-up of weight, total body potassium, and total body nitrogen in obese adolescents treated with the protein-sparing modified fast.

Seventeen obese adolescents were treated with the protein-sparing modified fast (PSMF), a high-protein, low-carbohydrate, low-calorie diet, for approximately 3 mo and then followed for a total of 12 mo. The diet provided a mean of 880 kcal.kg-1. d-1 and 2.5 g protein. kg IBW-1. d-1. Body composition was determined using four skinfold measurements, total body potassium (TBK), and total body nitrogen (TBN) at baseline, 3 mo, and 12 mo. Twelve subjects returned for follow-up at 1 y (71%) and 48% had sustained weight loss with the percentage ideal body weight for height decreased from 154 to 125 over the year. Six subjects had TBK and TBN measured and TBK decreased 13.0% and TBN decreased 14.3% over the year. This reflected a change toward normalization of the predicted TBK from 121 to 102% and TBN from 119 to 102%.