To investigate whether splanchnic uptake of the indicator amino acid ([1-(13)C] phenylalanine) during the fed state alters the estimate of lysine requirement, five healthy men were studied at graded levels of lysine intake, with either an oral or intravenous (IV) tracer protocol, in a randomized, crossover design. Splanchnic extraction of the oral tracer was expressed as the difference between the ratio of the enrichments in urinary phenylalanine between tracer protocols. The rate of release of (13)CO(2) from (13)C-phenylalanine oxidation (F(13)CO(2)) was measured and a two-phase linear regression crossover model was applied to determine the lysine requirement. Mean splanchnic extraction of the oral tracer was approximately 19%. Although actual F(13)CO(2) was higher during oral tracer infusion (P < 0.001), the breakpoint was not different from that determined with IV infusion (P = 0.98), with both yielding a mean lysine requirement of 36.6 mg/(kg. d). The upper 95% confidence intervals were 52.5 and 53.3 mg/(kg. d) for the oral and IV isotope infusions, respectively. These results demonstrate that routes of isotope administration using the indicator amino acid oxidation technique do not affect the estimated amino acid requirement. Therefore, the indicator amino acid oxidation method using the oral route, which is less invasive and allows for studies in vulnerable groups such as infants and children, should be the preferred method for studying amino acid requirements.
. Phase of menstrual cycle affects lysine requirement in healthy women. Am J Physiol Endocrinol Metab 287: E489 -E496, 2004; 10.1152/ ajpendo.00262.2003.-The aim of this study was to investigate whether the phases of the menstrual cycle affect lysine requirement in healthy adult females, as determined by the indicator amino acid oxidation (IAAO) method. Five healthy females with regular menstrual cycles were studied at seven graded levels of lysine intake, in random order, with an oral [13 C]phenylalanine tracer protocol in both the follicular and luteal phases. A total of 14 studies were conducted for each subject. Breath and plasma samples were collected according to the standard IAAO protocol. Serum 17-estradiol and progesterone concentrations were measured on each IAAO study day. The rate of release of 13 CO2 from [ 13 C]phenylalanine oxidation (F 13 CO2) was measured, and a two-phase linear regression crossover model was applied to determine lysine requirement. F 13 CO2 was higher during the luteal phase (P Ͻ 0.001) and was positively associated with serum concentrations of 17-estradiol and progesterone. The F 13 CO2 data were adjusted for subjects and sex hormones and used to define breakpoints for lysine requirements. The lysine requirement of healthy females in the luteal phase was 37.7 mg ⅐ kg Ϫ1 ⅐ day Ϫ1 and higher (P ϭ 0.025) than that of females in the follicular phase (35.0 mg ⅐ kg Ϫ1 ⅐ day Ϫ1 ). At all lysine intake levels, plasma amino acids were lower and phenylalanine oxidation was higher in the luteal relative to the follicular phase. Therefore, we reason that the higher lysine requirement observed in the luteal phase is probably due to higher amino acid catabolism.indicator amino acid oxidation STUDIES ON THE REQUIREMENT of indispensable amino acids (IDAAs) in women, especially using the advanced carbon oxidation technique, are scanty. In fact, there has been only one study conducted specifically on women by use of the indicator amino acid oxidation (IAAO) technique to determine tryptophan requirement in women during the follicular phase (23). A few studies looked at IDAA requirements using both male and female subjects together in the experiments (2,12,27,28,35). Two of the five studies (2, 12) were conducted in female subjects during the 7-10 days after the onset of menstrual bleeding, whereas the other three studies (27,28,35) did not control for menstrual phases of the female subjects. The lack of female studies is possibly due to the complexity imposed on the experimental design by including the menstrual cycle.Some evidence exists regarding metabolic changes associated with the menstrual cycle. Increased basal metabolic rate (37) and 24-h energy expenditure (40), higher nitrogen excretion (7), and lower plasma amino acid concentrations (29) have been reported in women in the luteal vs. the follicular phase of the menstrual cycle. The use of tracer techniques has produced conflicting results regarding protein or amino acid turnover during the different phases of the menstrual cycle. Us...
Water is essential for survival, but one in three individuals worldwide (2.2 billion people) lacks access to safe drinking water. Water intake requirements largely reflect water turnover (WT), the water used by the body each day. We investigated the determinants of human WT in 5604 people from the ages of 8 days to 96 years from 23 countries using isotope-tracking ( 2 H) methods. Age, body size, and composition were significantly associated with WT, as were physical activity, athletic status, pregnancy, socioeconomic status, and environmental characteristics (latitude, altitude, air temperature, and humidity). People who lived in countries with a low human development index (HDI) had higher WT than people in high-HDI countries. On the basis of this extensive dataset, we provide equations to predict human WT in relation to anthropometric, economic, and environmental factors.
The essentiality of histidine in healthy adults is a controversial topic. To study the potential metabolic effects of a lack of exogenous histidine, four healthy adults consumed a histidine-free diet, with adequate energy and 1.0 g/(kg. d) of an L-amino acid mixture for 48 d. Protein metabolism was monitored every 4 d by using indicator amino acid (L-[1-(13)C]phenylalanine) oxidation (in four subjects) and [(15)N]glycine (in one subject). Urine samples (24-h) were collected for measurement of urea, total nitrogen, creatinine, 3-methylhistidine (3-MH), histidine and beta-alanine. Albumin, transferrin and hematologic concentrations were measured on d 0, 24 and 48. Urinary excretion of nitrogen, urea, creatinine and 3-MH were not affected by the histidine-free diet. However, there was a significant (P < 0.001) linear decline (24-28%) in whole-body protein turnover. Significant (P < 0.05) decreases in albumin (12%), transferrin (17%) and hemoglobin (Hb) (11%) concentrations occurred slowly over the histidine depletion period. The urinary excretion of beta-alanine (an index of carnosine catabolism) generally increased in the smallest subject during the consumption of histidine-free diet. This study demonstrates that a lack of histidine in the diet for a prolonged period resulted in an accommodation of protein turnover and phenylalanine oxidation, measured by the (13)C-phenylalanine indicator amino acid. The extensive metabolic accommodation, together with decreases in Hb, albumin and transferrin during histidine depletion, leaves unresolved the issue of whether histidine is a dietary essential amino acid in healthy adults.
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