Home parenteral nutrition.

Patients who are unable to sustain their nutritional integrity with foods or intestinal feedings still may be able to thrive in the home setting. A concerted health care team approach, which includes the patient and family, is required to safely carry out long-term home parenteral nutrition.

Methionine kinetics and balance at the 1985 FAO/WHO/UNU intake requirement in adult men studied with L-[2H3-methyl-1-13C]methionine as a tracer.

The upper range of the requirement for methionine plus cystine in healthy adults was proposed in 1985 by FAO/WHO/UNU to be 13 mg.kg body wt-1.d-1. To explore the validity of this estimate, five healthy, young adult men were given for 7 d a diet based on an L-amino acid mixture supplying 13 mg methionine.kg-1.d-1 (87 mumol.kg-1.d-1) without cystine. Constant intravenous infusions of L-[2H3-methyl-1-13C]methionine were given on days 5 and 7 while subjects were in the fed and postabsorptive states, respectively. Estimates were made of methionine oxidation, and daily methionine balance was derived from the intake-oxidation data. For the five subjects, methionine balances were -0.9, +0.7, +3.5, -3.1, and -3.8 mg kg-1.d-1, or -6, +5, +23, -21, and -26 mumol.kg-1.d-1. These findings lead to the conclusion that the upper range of the requirement for methionine plus cystine probably exceeds 13 mg.kg-1.d-1 in healthy young adults. The implications of this conclusion for establishing an appropriate amount of sulfur amino acids in an amino acid requirement pattern for adults is discussed.

Methionine kinetics in adult men: effects of dietary betaine on L-[2H3-methyl-1-13C]methionine.

The effects of a daily 3-g supplement of betaine on kinetic aspects of L-[2H3-methyl-1-13C]methionine (MET) metabolism in healthy young adult men were explored. Four groups of four subjects each were given a control diet, based on an L-amino acid mixture supplying 29.5 and 21.9 mg.kg-1.d-1 of L-methionine and L-cystine for 4 d before the tracer study, conducted on day 5 during the fed state. Two groups received the control diet and two groups received the betaine supplement. Tracer was given intravenously (iv) or orally. The transmethylation rate of MET (TM), homocysteine remethylation (RM), and oxidation of methionine were estimated from plasma methionine labeling and 13C enrichment of expired air. RM tended to increase (P = 0.14) but the TM and methionine oxidation were significantly (P less than 0.05) higher after betaine supplementation when estimated with the oral tracer. No differences were detected with the intravenous tracer. Methionine concentration in plasma obtained from blood taken from subjects in the fed state was higher (P less than 0.01) with betaine supplementation. These results suggest that excess methyl-group intake may increase the dietary requirement for methionine.

[1-13C; methyl-2H3]methionine kinetics in humans: methionine conservation and cystine sparing.

Methionine (Met) conservation in healthy young adult men (4/diet group) was explored by supplying one of the following three L-amino acid based diets: 1) adequate Met but no cystine; 2) neither Met nor cystine; or 3) no Met but cystine supplementation. After 5 days, subjects received a continuous intravenous infusion of L-[1-13C; methyl-2H3]Met for 5 h while the diet was given as small isocaloric isonitrogenous meals. Estimates were made of rates of Met incorporation into protein synthesis (S) and release from body proteins (B), transmethylation (TM), remethylation of homocysteine (RM), and transsulfuration (TS). For the adequate Met diet, the rates were S = 24 +/- 2, B = 18 +/- 1, TM = 12.4 +/- 1.7, RM = 4.7 +/- 1.1, and TS = 7.6 +/- 0.6 (SE) mumol.kg-1.h-1. The sulfur amino acid-devoid diet significantly (P less than 0.05) reduced S, TM, RM, and TS. Supplementation of this diet with cystine reduced Met oxidation (P = 0.05). Therefore, two loci are quantitatively important regulatory points in Met conservation in vivo: 1) the distribution of Met between the pathways of protein anabolism and TM (Met locus) and 2) the distribution of homocysteine between RM and TS (homocysteine locus).

Quantitative study in vivo of methionine cycle in humans using [methyl-2H3]- and [1-13C]methionine.

Kinetic aspects of body methionine (MET) metabolism were examined in healthy young men during the fed and postabsorptive (PA) states. Rates of MET incorporation (S) into and release (B) from body proteins; transmethylation (TM); and remethylation (RM) and transsulfuration (TS) of homocysteine (HCY) were estimated with the aid of a 5-h constant intravenous infusion of [methyl-2H3]- and [1-13C]methionine. The isotopic data (plasma methionine labeling and 13C enrichment of expired air) were submitted to a stochastic model of amino acid metabolism. During the fed state, the subjects (n = 4) received, at 20-min intervals, small isonitrogenous isocaloric meals containing a complete L-amino acid mixture supplying MET at a rate equivalent to 198 mumol.kg body wt-1.day-1. The PA subjects (n = 4) received the isotope after a 10-h overnight fast. For the PA group, the components of MET metabolism were as follows: S, 20 +/- 0.5; B, 24 +/- 0.5; TM, 5.8 +/- 0.6; RM, 1.8 +/- 0.4; and TS, 4.0 +/- 0.4 (+/-SE) mumol.kg-1.h-1. During the fed state the values were S, 26 +/- 2.5; B, 18 +/- 2; TM, 14 +/- 1.3; RM, 5.7 +/- 0.9; and TS 8.3 +/- 0.6 mumol.kg-1.h-1. The meal-induced changes in B, TM, RM, and TS were significant (P less than 0.05). Comparison of the partitioning of MET between S and TM (these two pathways of MET disposal constitute the "methionine locus") in the PA and in the fed states indicates that the MET locus is of regulatory importance in MET homeostasis. A twofold increase in the partitioning of MET to TM was observed in the fed state. The increase in HCY recycling, relative to TS (these two pathways of HCY disposal constitute the "HCY locus"), in the fed state did not reach statistical significance when compared with the PA state. Total daily TM are estimated to be 238 +/- 22 mumol/kg. This is similar to the estimate generated by the methyl balance model of Mudd and Poole (Metabolism 24: 721, 1975) which approximated 241 mumol/kg.