Departments of 1 Nutritional Sciences and 2 Paediatrics, University of Toronto, Toronto M5S 3E2; 3 The Research Institute, The Hospital for Sick Children, Toronto, Ontario M5G 1X8; and 4 Department of Agricultural, Food and Nutritional Services, University of Alberta, Edmonton, Alberta, Canada T6G 2P5
![]() |
ABSTRACT |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Dietary restriction of
phenylalanine is the main treatment for phenylketonuria (PKU), and
current estimates of requirements are based on plasma phenylalanine
concentration and growth. The present study aimed to determine more
precisely the phenylalanine requirements in patients with the disease
by use of indicator amino acid oxidation, with
L-[1-13C]lysine as the indicator. Breath
13CO2 production
(F13CO2) was used as the end point.
Finger-prick blood samples were also collected for measurement of
phenylalanine to relate phenylalanine intake to blood phenylalanine
levels. The mean phenylalanine requirement, estimated using a two-phase
linear regression crossover analysis, was 14 mg · kg1 · day
1,
and the safe population intake (upper 95% confidence interval of the
mean) was found to be 19.5 mg · kg
1 · day
1.
A balance between phenylalanine intake and the difference between fed
and fasted blood phenylalanine concentration was observed at an intake
of 20 mg · kg
1 · day
1.
The similarity between these two values (19.5 and 20 mg · kg
1 · day
1)
suggests that the maximal phenylalanine intake for children with PKU
should be no higher than 20 mg · kg
1 · day
1.
phenylketonuria; amino acid requirements; lysine
![]() |
INTRODUCTION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
DIETARY PHENYLALANINE RESTRICTION has been the mainstay of treatment of phenylketonuria (PKU) for over 40 years (8). Its main aim is to maintain phenylalanine intakes that will allow optimum growth and brain development by supplying adequate energy, protein, and other nutrients while restricting phenylalanine. Implementation soon after birth usually prevents most of the overt clinical manifestations of PKU. Nevertheless, there is a considerable body of evidence suggesting that neuropsychological and cognitive functions are not entirely normalized in individuals with PKU receiving present treatment regimens (28, 30-32).
Although dietary phenylalanine restriction remains the main treatment in PKU, present estimates of phenylalanine requirements are based on plasma phenylalanine levels and growth rate in relation to dietary intake and not on direct and sensitive measurements of amino acid metabolism. This study is the second in a two-part series, from the same laboratory, estimating more sensitively and accurately the aromatic amino acid (phenylalanine and tyrosine) requirements in children with PKU by use of isotope tracer methods (20, 38, 40).
The indicator amino acid oxidation technique, which is used to
determine amino acid requirements, involves feeding the subjects at
levels above and below the predicted requirement break point (20,
40). Because patients with classical PKU have a negligible or
very minimal capacity to oxidize phenylalanine (33, 34), we reasoned that their dietary phenylalanine requirements would be
lower than those of healthy children by an amount equal to the
obligatory losses of phenylalanine. Dietary requirements for phenylalanine in children have yet to be defined. Therefore, we turned
to our previous study (3) using indicator amino acid oxidation, which showed that the mean tyrosine requirement of children
with PKU was 19.2 mg · kg1 · day
1.
Next, we used a ratio between phenylalanine and tyrosine in the tissues
of humans and animals of 55:45 (16, 24, 37, 38), which,
when multiplied by the tyrosine mean requirements, predicts a mean
phenylalanine requirement for healthy children of ~23.5
mg · kg
1 · day
1.
The requirement for phenylalanine or any other indispensable amino acid
is the sum total of that needed for protein synthesis plus irreversible
losses (10). Because there are no data in children, we had
to turn to a study of phenylalanine requirement in adult males
(38), in which obligatory oxidation was estimated to be
~26%. By use of the predicted phenylalanine requirement for children
and the estimated obligatory oxidation for phenylalanine, the
obligatory loss was calculated to be ~6.1
mg · kg
1 · day
1.
When the estimate of obligatory loss from the predicted mean requirement of 23.5 mg · kg
1 · day
1
was subtracted, the resulting value of 17.4 mg · kg
1 · day
1
was the mean phenylalanine requirement predicted for children with PKU.
The objectives of this study were to determine the phenylalanine requirement of children with classical PKU by use of the technique of indicator amino acid oxidation and to compare the results with our previously estimated tyrosine requirement obtained using the same technique.
![]() |
METHODS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Study subjects.
Five children (mean ± SE age, 10.2 ± 1.2 yr) with classical
PKU, treated by the PKU Clinic at the Hospital for Sick Children (Toronto, ON, Canada), participated in this study. All subjects were
studied on an outpatient basis. Each subject was selected for study on
the basis of the following criteria: having a plasma phenylalanine
concentration of 1,200 µM at diagnosis, being prepubertal males or
females 5-13 yr old and in good health, being treated by dietary
phenylalanine restriction from early infancy, and being willing to
participate in the study. Subjects were excluded if they were taking
medication that might alter protein or energy metabolism or if they had
recent illness, IQ <80, history of endocrine disease or any other
medical condition that might alter protein and energy metabolism, or
significant weight changes (>10% body wt) in the 3 mo before the
study. The standard dietary management of subjects consists of a
phenylalanine-free medical food fed at a level that provides the
age-specific recommended dietary protein for healthy children
(9). In addition, low-protein foods are fed that provide
additional protein in an amount that is 50% of the age-specific
recommended protein intake for healthy children (9).
Dietary compliance is monitored regularly (on average about every 8 wk)
by an experienced clinical dietitian and by measuring blood
phenylalanine by use of the same technique as that used in the present
study (see Blood collection and analysis).
|
Study design.
Each subject was studied on six nonconsecutive days over a 3-mo period,
at each of six phenylalanine intake levels, in random order, to allow
an estimation of each individual's needs as well as the population
requirement. The levels were 0, 10, 15, 20, 25, and 35 mg · kg1 · day
1.
A total of 30 oxidation studies were conducted.
Experimental diet. A flavored liquid formula (Protein-Free Powder, Product 80056, Mead Johnson, Evansville, IN) and protein-free cookies (39) developed for use in amino acid kinetic studies supplied the energy in the diet. The diet was prepared and weighed (Sartorius Balance model BP110 S; Sartorius, Mississauga, ON, Canada) in the research kitchen at The Hospital for Sick Children. The diet was administered on the study day as eight isocaloric, isonitrogenous, hourly meals. Each meal represented one-eighth of the subject's total daily requirements. The macronutrient composition of the experimental diet, expressed as a percentage of dietary energy, was ~37% fat, 52% carbohydrate, and 11% protein (39).
The nitrogen content of the diet was provided as a crystalline amino acid mixture and was based on the amino acid composition of egg protein. The amino acid mixture (protein) was provided to each child on each study day at a level of 1.5 g · kgBody composition measurements. Body composition [fat and fat-free mass (FFM)] was determined using bioelectrical impedance analysis (BIA) and multiple skinfold thickness measurements. BIA was performed at the beginning of each study day before meal ingestion. Resistance and reactance measurements were done using a four-terminal bioimpedance analyzer (model 101A, RJL Systems, Detroit, MI) while the subject lay in a supine position on a hospital bed with all four limbs apart. Two detector electrodes were placed on the dorsal surfaces of the right hand and foot proximal to the metacarpal-phalangeal and metatarsal-phalangeal joints, respectively. Two detector electrodes were placed at the right pisiform prominence of the wrist, with the proximal edge bisecting the ulnar tubercle and between the medial and lateral malleoli, with the proximal edges dissecting the medial malleolus. An excitation current of 800 µA at a fixed frequency of 50 kHz was introduced into the subject at the distal electrodes of the hand and foot, and the voltage drop was detected by the proximal electrodes. Three readings for both reactance and resistance were taken for each subject, and the mean of the three readings was used to determine FFM.
Multiple skinfold thickness measurements were taken from four different sites on the subject's nondominant side: triceps, biceps, subscapula, and suprailiac. Each measurement was taken by the same individual. A total of three measurements from each site were taken using a skinfold caliper (British Indicators, St. Albans, UK), and the average value was used in the equation. Body density was derived from the sum of the four skinfolds by use of an age-specific equation (2, 7), and another equation was used to predict the percentage of body fat from body density (27).Oral isotope infusion studies.
L-[1-13C]lysine · HCL,
with an enrichment of 99% (MassTrace, Woburn, MA), was used in this
study. Quality control tests were performed by the manufacturer.
Chemical purity, isotope enrichment, and position were confirmed by
GC-MS, and a second confirmation was performed by nuclear magnetic
resonance (NMR). Optical isomer purity (<0.2% D-isomer)
was confirmed by chiral HPLC. A stock solution of 10 mg/ml was prepared
using sterile water. From the stock solution, the priming and
continuous bolus doses were dispensed into multiple vials. Each subject
was given a priming oral dose of
L-[1-13C]lysine · HCl
in the amount of 2.5 mg/kg (13.6 µM/kg) and eight subsequent oral
bolus doses of equal amounts: 1.4 mg · kg1 · h
1
(7.62 µM · kg
1 · h
1)
(3).
Blood collection and analysis.
Capillary blood samples were obtained from a finger-prick incision made
to the index finger of the nondominant hand (Softlick Bloodletting
Device; Boehringer, Laval, QC, Canada). This was done to relate
phenylalanine intake to blood phenylalanine levels, because diet
therapy in PKU is managed mainly by monitoring blood phenylalanine in
response to dietary intake. To ensure arterialized blood, the hand was
heated inside a thermostatic chamber maintained at 60°C for 15 min
before the blood was sampled (41). While the finger was
held, about five drops (<1 ml) of blood were gently spotted onto
Guthrie filter paper (Newborn Screening Program, Ministry of Health,
Toronto, ON, Canada) to totally saturate an area of ~1 cm in
diameter. Blood was collected at the beginning of each study day,
before meal ingestion, with subjects still in the fasted state, and
again at the end of the study day after each subject had received and
consumed all eight meals. At each time, two blood spots were collected.
The blood spots were left to air-dry overnight and were then analyzed
for phenylalanine concentration according to the method of Dooley
(4). This method is used for the quantitative
determination of blood phenylalanine and is based on the NAD-dependent
oxidative deamination of phenylalanine in the presence of excess
phenylalanine dehydrogenase. Phenylalanine reacts with the enzyme
phenylalanine dehydrogenase and in the process converts NAD to NADH.
The NADH was measured by reaction with indonitrotetrazolium
chloride catalyzed by diaphorase, which forms a colored formazan product.
Analytical procedures. The 13CO2 enrichment in expired CO2 was measured with a continuous-flow isotopic ratio mass spectrometer (model ANCA GSL; Europa Scientific, Crewe, UK). Each set of eight samples was separated by two reference samples (5% CO2), which were previously calibrated to an international reference standard (NBS-20; National Institute for Standards and Technology, Gaithersburg, MD). The results represent the absolute 13C enrichment present in that sample. Plateau enrichment was calculated as the difference in isotopic abundance at plateau and natural (baseline) isotopic abundance and was expressed as atom percent excess (APE).
The amino acids in 500 µl of urine were derivatized to their N-heptafluorobutyryl-n-propyl esters by the method of Patterson et al. (23). Isotopic enrichment for urinary free lysine was measured by GC-MS [Hewlett-Packard model 5890 Series II GC (Mississauga, ON, Canada) VG Trio-2 quadrupole mass spectrometer system]. Details of the method have been previously described by Bross et al. (3).Data analysis. A stochastic model was used to calculate lysine kinetics (36). Isotopic steady-state values of lysine in urine and CO2 breath were defined as a coefficient of variation of <5% between sampling time points and the absence of a significant slope. The difference between the mean breath CO2 isotope enrichment values of the three baseline and five plateau samples was expressed as APE above baseline at isotopic steady state. Also, the difference between the mean ratio of the enriched peak (m + 1) to the unenriched (m) peak of lysine in urine for baseline and plateau samples was expressed as mole percent excess (MPE). Typical 13CO2 and [13C]lysine enrichments in breath and urine, respectively, at baseline and plateau for an individual study have been previously presented by Bross et al. (3).
Lysine kinetics were estimated from breath and urine enrichment data by use of standard equations (22). The equations used to calculate flux, oxidation, and rate of release of 13CO2 in breath in response to lysine oxidation have been described previously by Bross et al. (3). Briefly, apparent lysine flux (Q) was calculated using the equation
![]() |
(1) |
![]() |
(2) |
![]() |
RESULTS |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Subject characteristics are summarized in Table 1. Their body
composition, determined by skinfold measurements, gave group values
that were very similar to those obtained by BIA. The group FFM was 81%
and fat mass 19% of total body weight. Energy intake on the six study
days was established as resting metabolic rate times 1.5, since the
children's activity was limited during the 8-9 h of the study
days, and the individual values are also shown in Table 1, ranging from
1,505 to 2,139 kcal · kg1 · day
1.
Phenylalanine intake had no effect on apparent lysine flux
(P = 0.79; Table 2).
However, significant differences were observed among apparent lysine
flux values for the individual subjects (P = 0.02).
|
Breath CO2 rates were constant within
each subject across the six test intakes of phenyalanine (data not
shown) but ranged from 189 to 235 ml/min among the five subjects
(P < 0.001). The effect of phenylalanine intake on
individual rate of 13CO2
(F13CO2) release is shown in Table
3. For every subject,
F13CO2 decreased with increasing
phenylalanine intakes up to a specific phenylalanine intake, after
which F13CO2 increased. The
individual subject (P = 0.008) as well as phenylalanine intake (P = 0.004) had a significant effect on
F13CO2. Figure
1 shows the mean break point in the
F13CO2 data. With the use of a
two-phase linear regression crossover model, a break point of 14 mg
phenylalanine · kg
1 · day
1
was found. The upper 95% confidence limit of the break point, which
represents the safe population intake, was determined to be 19.5 mg
phenylalanine · kg
1 · day
1.
The individual F13CO2 data are
shown in Fig. 2. From these data,
individual phenylalanine requirement estimates can be obtained by
visual inspection, and the data ranged from 13 to 20 mg · kg
1 · day
1,
with an average of 15.2 mg · kg
1 · day
1.
Despite the approximate nature of the visual estimates, the average
value and range are comparable to those obtained by two-phase linear
crossover regression analysis. The pattern of lysine oxidation mirrored
the F13CO2 data but was not
significantly affected by phenylalanine intake (Table 2).
|
|
|
Mean fasted- and fed-state blood phenylalanine concentrations are
presented in Fig. 3. In the fasted state,
there was no difference in the mean blood phenylalanine concentration
at any of the intake levels. In the fed state, there was no difference
in mean blood phenylalanine concentration at phenylalanine intakes from
0 to 25 mg
phenylalanine · kg1 · day
1.
However, blood phenylalanine concentration at an intake of 35 mg of
phenylalanine · kg
1 · day
1
was increased and was significantly higher than at all other phenylalanine intake levels.
|
Figure 4 presents the relation among the
mean differences between fed and fasted blood phenylalanine
concentrations in response to varying phenylalanine intake levels.
Correlation analysis showed that there was a direct relationship
between phenylalanine intake and difference in (fed minus fasted) blood
phenylalanine concentration, with 98% of the difference in blood
phenylalanine concentration between fed and fasted states being
accounted for by phenylalanine intake (r2 = 0.98, P < 0.0001). The individual data sets for each
subject share a similar pattern to that of the group as a whole. From Fig. 4, at intakes of 20 mg of phenylalanine
kg1 · day
1 or less,
blood phenylalanine concentration was less than or similar to the
fasted concentration, whereas at intakes of 25 and 35 mg of
phenylalanine · kg
1 · day
1,
the fed blood phenylalanine concentration was higher than the fasted concentration. At an intake of ~20 mg of
phenylalanine · kg
1 · day
1,
there was a balance between phenylalanine intake and the difference in
(fed minus fasted) blood phenylalanine concentration.
|
![]() |
DISCUSSION |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The results of the present indicator amino acid oxidation study
show that the mean phenylalanine requirement estimated by a two-phase
linear regression crossover model is 14 mg · kg1 · day
1
for prepubertal children between the ages of 6 and 13 yr with classical
PKU (Fig. 1). Holt and Snyderman (15), using growth and
nitrogen balance, estimated the phenylalanine requirement (in the
presence of dietary tyrosine) for 27 children with PKU. Their
requirement estimates covered a broad range: 55-90
mg · kg
1 · day
1
at 2 mo (mean, 70 mg · kg
1 · day
1)
to 25-80
mg · kg
1 · day
1
at 12 mo (mean, 35 mg · kg
1 · day
1).
On the basis of those results, Holt and Snyderman concluded that the
phenylalanine requirement of children with PKU is no different from
that of normal children. However, these authors made no attempt to
differentiate between classical and variant PKU in their study. In
addition, when the requirement estimates from Holt and Snyderman were
used to treat children with PKU from birth, the children had prolonged
periods of high plasma phenylalanine (1).
Conversely, our present data are consistent with those of a retrospective study that examined phenylalanine requirements in children with PKU who had normal rates of growth (18). Those children were followed from birth to 2 yr of age. Plasma phenylalanine over the 2-yr period averaged 345 ± 285 µM; therefore, those children were fairly well controlled. The authors found that between the ages of 0 and 4, 4 and 12, and 12 and 24 mo, the phenylalanine intake needed to maintain a normal or moderately elevated plasma phenylalanine was ~68, 58, and 50%, respectively, of the requirement of normal children (18). There was a separation of the children into classical and variant PKU groups. When the authors compared their data with those of Holt and Snyderman (15), they found that the lowest estimate corresponded to the intakes of the classical PKU children in their study, whereas the higher estimates from Holt and Snyderman corresponded to the intakes of the variant PKU children.
In addition to the mean phenylalanine requirement being defined in the
present study, a safe level of intake necessary to meet the requirement
of 95% of the population (the 95% confidence limit) was estimated at
19.5 mg · kg1 · day
1.
This was necessary because, by definition, a requirement should be
broad enough to cover the needs of almost all individuals (95%) within
a given population. However, in a disease state like PKU, the
implications of setting a phenylalanine requirement at the 95%
confidence level are unclear. All individuals were studied at each of
the six phenylalanine intake levels; therefore, estimation of
individual requirement was possible and ranged from 13 to 20 mg · kg
1 · day
1.
The 95% confidence limit of 19.5 mg · kg
1 · day
1
is 4.5-6.5
mg · kg
1 · day
1
higher than the individual estimate for four of the five children studied. In the present study, however, there was no change in mean
blood phenylalanine concentration between phenylalanine intakes of
0-25
mg · kg
1 · day
1
(Fig. 3). This is evidence that the indicator method is more sensitive
than measurement of blood phenylalanine and suggests that, at such
small differences in intake above the mean requirement (4.5-6.5
mg · kg
1 · day
1),
no significant changes would be detected in blood phenylalanine concentration. On the other hand, there are many studies in which the
consequences of inadequate intakes of phenylalanine in a PKU population
have been described (11, 13, 14). Severe mental retardation and growth retardation have been reported. Because the
impact on blood phenylalanine is negligible with intakes in such a
small excess of requirements, and because we have evidence of the
consequences of intakes that are inadequate to meet requirements in
children, we propose that the estimated 95% confidence limit of 19.5 mg · kg
1 · day
1
be accepted as the recommended phenylalanine intake in prepubertal children with classical PKU between the ages of 6 and 13 yr.
There was a very high degree of interindividual as well as
intraindividual variability between the study days in the baseline blood phenylalanine concentrations in the present study (data not
shown). However, when this was controlled for, by subtracting the
fasting levels from the fed levels, a very clear picture emerged (Fig.
4). These data showed that 98% of the change in blood phenylalanine concentrations with feeding was accounted for by phenylalanine intake
alone. These data also lend support for the mean and 95% confidence
requirement estimate determined from the indicator tracer studies. The
least difference in blood phenylalanine occurred at phenylalanine
intakes between 15 and 20 mg · kg1 · day
1
(Fig. 4). The fact that no change from baseline was detected in the
mean blood data at phenylalanine intakes from 0 to 25 mg · kg
1 · day
1
(Fig. 3) is evidence that blood levels of an amino acid (even in a
disease, such as PKU, with the absence of catabolic enzyme activity)
are a relatively insensitive measure compared with oxidation, measured
by F13CO2, and should not be used
as a sole measure of requirement.
In every indicator study that we have previously performed (3, 6, 20, 40) to determine amino acid requirements, the expected pattern of indicator amino acid oxidation has been observed, namely that a decrease in the oxidation of the indicator amino acid as the level of the test amino acid is increased in the diet until the mean requirement level (break point) is reached, after which increase in the test amino acid has no further effect on the oxidation of the indicator amino acid. This pattern was also observed by Bross et al. (3) in a study on tyrosine requirement in a similar population of children with PKU. In the present study, however, an increase in the oxidation of the indicator was observed beyond the F13CO2 break point (mean requirement; Fig. 1). A similar pattern was observed for all individuals studied (Table 3 and Fig. 2). Because indicator amino acid oxidation is a reflection of the partitioning of the essential amino acids between incorporation into protein (synthesis) and oxidation, this suggests that, beyond the mean requirement, a further increase in the intake of phenylalanine in PKU results in a decrease in whole body protein synthesis.
The present data do not permit an explanation for the decreased whole body protein synthesis when phenylalanine intakes increase above the break point. There are, however, data in the literature that show that elevated plasma phenylalanine levels interfere with the metabolism of other essential amino acids. Wapnir and Lifshitz (35) have shown that plasma tryptophan levels in PKU are lower than in controls, even after the implementation of a low-phenylalanine diet. Lipovac et al. (21) have shown that amino acid catabolism was increased in the tissues of rats in which hyperphenylalaninemia was induced. Optimal protein synthesis is dependent on an ideal balance of amino acids being present together with sufficient nonprotein energy (5).
An alternative explanation of the
F13CO2 pattern is that the
indicator amino acid oxidation model failed, in part, in the present study. Against this is the fact that the indicator model worked satisfactorily in a similar group of children with PKU, who were being
studied to determine their tyrosine needs (3).
Furthermore, the data on the change in blood phenylalanine (Fig. 4) in
response to increasing intakes of phenylalanine are supportive of the
value for the upper limit of phenylanine intake, defined by the
indicator model. Although this issue needs further investigation, we
believe that the balance of the evidence supports our interpretation
that protein synthesis is adversely affected above a phenylalanine intake of 20 mg · kg1 · day
1.
Although the lysine oxidation data (Table 2) followed the same pattern as the F13CO2 data (Table 3), there was too much variance in the oxidation data to show a significant effect of phenylalanine intake on lysine oxidation (by analysis of variance). Furthermore, we were unable to define a break point when performing two-phase linear crossover analysis of the lysine oxidation data in response to graded intakes of lysine (data not shown). We have made similar observations in studies of tryptophan requirement in women (20) and lysine requirement in men (6). In those studies, we were able to define a break point in 13C-label oxidation (F13CO2), although we were not able to define a break point in the oxidation of the indicator amino acid. The latter is calculated from plasma [1-13C]lysine enrichments in addition to F13CO2. We believe that the failure to be able to show a break point with oxidation of the indicator amino acid is due to the fact that plasma may not always be representative of the intracellular pool(s) from which amino acid oxidation takes place.
In the present study, we chose to use an orally administered lysine tracer to make the study minimally invasive and thereby suitable for studies in children (3). A consideration when an oral tracer is used is that it may be taken up and oxidized in the gut; indeed, it has been shown that 35-53% of dietary lysine is taken up and oxidized during first pass in the gut (29). This uptake of tracer within the gut results in a lower plasma enrichment and, hence, accounts for the higher estimates of apparent lysine flux in the present study compared with earlier work in children when the tracer was given intravenously (17). We (19) have recently shown that gut uptake of the tracer does not alter the break point and, hence, does not alter the requirement estimate of the test amino acid. (19)
What is important is that changes in the test amino acid intake (phenylalanine) do not affect the flux of the indicator (lysine) (see Table 2). This condition is necessary because it means that indicator label flux is being partitioned between oxidation and whole body protein synthesis in response to changes in the test amino acid intake (40, 42).
This study provides further support for the suitability of the indicator amino acid technique for the estimation of amino acid requirement in vulnerable groups of individuals. It also demonstrates the suitability of lysine as an indicator. This is underlined by the fact that phenylalanine intake had no effect on apparent lysine flux, a critical condition for any indicator study.
In conclusion, in light of the similarity of the results from the upper
95% confidence interval of the indicator study and the change in blood
phenylalanine levels (fed minus fasting), children with PKU should not
be fed more than 20 mg · kg1 · day
1
of phenylalanine in their diet.
![]() |
ACKNOWLEDGEMENTS |
---|
This research was supported by Grant MT 10321 from the Canadian Institutes of Health Research. Mead Johnson Canada generously provided the protein-free powder.
![]() |
FOOTNOTES |
---|
Address for reprint requests and other correspondence: P. B. Pencharz, Div. of Gastroenterology and Nutrition, The Hospital for Sick Children, 555 University Ave., Toronto, ON, Canada M5G 1X8 (E-mail: paul.pencharz{at}sickkids.on.ca).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
August 27, 2002;10.1152/ajpendo.00319.2001
Received 24 July 2001; accepted in final form 14 August 2002.
![]() |
REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Berry, HK,
Hunt MM,
and
Sutherland BK.
Amino acid balance in the treatment of phenylketonuria.
J Am Diet Assoc
58:
210-214,
1971[ISI][Medline].
2.
Brook, CG.
Determination of body composition of children from skinfold measurements.
Arch Dis Child
46:
182-184,
1971[ISI][Medline].
3.
Bross, R,
Ball RO,
Clarke JTR,
and
Pencharz PB.
Tyrosine requirements in children with classical phenylketonuria determined by indicator amino acid oxidation.
Am J Physiol Endocrinol Metab
278:
E195-E201,
2000
4.
Dooley, KC.
Enzymatic method for phenylketonuria screening using phenylalanine dehydrogenase.
Clin Biochem
25:
271-275,
1992[ISI][Medline].
5.
Duffy, B,
Gunn T,
Collinge J,
and
Pencharz PB.
The effect of varying protein quality and energy intake on the nitrogen metabolism of parenterally fed very low birthweight (<l600 g) infants.
Pediatr Res
15:
1040-1044,
1981[Abstract].
6.
Duncan, AM,
Ball RO,
and
Pencharz PB.
Lysine requirement of adult males is not affected by decreasing dietary protein.
Am J Clin Nutr
64:
718-725,
1996[Abstract].
7.
Durnin, JV,
and
Rahaman MM.
The assessment of the amount of fat in the human body from measurements of skinfold thickness.
Br J Nutr
21:
681-689,
1967[ISI][Medline].
8.
Elsas, LJ,
and
Acosta PB.
Nutrition support of inherited metabolic disease.
In: Modern Nutrition in Health and Disease, edited by Shils ME,
and Young VR.. Malvern, PA: Lea and Febiger, 1988, p. 1337-1379.
9.
FAO/WHO/UNU Expert Consultation.
Energy and Protein Requirements. Geneva: World Health Organization, 1985.
10.
Fuller, MF,
and
Garlick PJ.
Human amino acid requirements: can the controversy be resolved?
Annu Rev Nutr
14:
217-241,
1994[ISI][Medline].
11.
Hanley, WB,
Linsao L,
Davidson W,
and
Moes CAF
Malnutrition with early treatment of phenylketonuria.
Pediatr Res
4:
318-327,
1970[ISI][Medline].
12.
Health and Welfare Canada.
Nutrition Recommendations. Ottowa, ON, Canada: Ministry of National Health and Welfare, 1990.
13.
Holm, VA,
and
Knox E.
Physical growth in phenylketonuria. I. A retrospective study.
Pediatrics
63:
694-699,
1979[Abstract].
14.
Holm, VA,
Kronmal RA,
Williamson MR,
and
Alex F.
Physical growth in phenylketonuria. II. Growth of treated children in the PKU collaborative study from birth to 4 years of age.
Pediatrics
63:
700-707,
1979[Abstract].
15.
Holt, EL,
and
Snyderman SE.
The Amino Acid Requirements of Children. New York: University Press, 1967, p. 381-389.
16.
House, JD,
Pencharz PB,
and
Ball RO.
Phenylalanine requirements determined by using L-[1-14C]phenylalanine in neonatal piglets receiving total parenteral nutrition supplemented with tyrosine.
Am J Clin Nutr
65:
984-993,
1997[Abstract].
17.
Jahoor, F,
Desai M,
Herdon DN,
and
Wolfe RR.
Dynamics of the protein metabolic response to burn injury.
Metabolism
37:
330-337,
1988[ISI][Medline].
18.
Kindt, E,
and
Halvorsen S.
The need of essential amino acids in children. An evaluation based on the intake of phenylalanine, tyrosine, leucine, isoleucine, and valine in children with phenylketonuria, tyrosine amino transferase defect, and maple syrup urine disease.
Am J Clin Nutr
33:
279-286,
1980[Abstract].
19.
Kriengsinyos, W,
Wykes LJ,
Ball RO,
and
Pencharz PB.
Oral and intravenous tracer protocols of the indicator amino acid oxidation method provide the same estimate of lysine requirement in healthy men.
J Nutr
132:
2251-2257,
2002
20.
Lazarus-Brunner, G,
Rafii M,
Ball RO,
and
Pencharz PB.
Tryptophan requirement in young adult women as determined by indicator amino acid oxidation with L-[13C]phenylalanine.
Am J Clin Nutr
68:
303-310,
1998[Abstract].
21.
Lipovac, K,
Zanic-Grubisic T,
Juretic D,
Mihaljevic I,
and
Fuks Z.
Disturbances of energy metabolism in rats with experimentally induced hyperphenylalaninemia.
In: Models for the Study of Inborn Errors of Metabolism, edited by Hommes FA.. Amsterdam: North-Holland Biomedical, 1979, p. 133-140.
22.
Matthews, DE,
Motil KJ,
Rohrbaugh DK,
Burke JF,
Young VR,
and
Bier DM.
Measurement of leucine metabolism in man from a primed, continuous infusion of L-[113C]leucine.
Am J Physiol Endocrinol Metab
238:
E473-E479,
1980
23.
Patterson, BW,
Hachey DL,
Cook GL,
Amann JM,
and
Klein PD.
Incorporation of a stable isotopically labeled amino acid into multiple human apolipoproteins.
J Lipid Res
32:
1063-1072,
1991[Abstract].
24.
Roberts, SA,
Thorpe JM,
Ball RO,
and
Pencharz PB.
Tyrosine requirement of healthy adult males receiving a fixed phenylalanine intake determined using indicator amino acid oxidation.
Am J Clin Nutr
73:
276-282,
2001
25.
Ruch, T,
and
Kerr D.
Decreased essential amino acid requirements without catabolism in phenylketonuria and maple syrup urine disease.
Am J Clin Nutr
35:
217-228,
1982[Abstract].
26.
Seber, GAF
Linear Regression Analysis. New York: Wiley, 1977.
27.
Siri, WE.
Body composition from fluid spaces and density: analysis of methods. 1961.
Nutrition
9:
480-491,
1993[ISI][Medline].
28.
Smith, I,
Beasley MG,
and
Ades AE.
Intelligence and quality of dietary treatment in phenylketonuria.
Arch Dis Child
65:
472-478,
1990[Abstract].
29.
Stoll, B,
Henry J,
Reeds PJ,
Yu H,
Jahoor F,
and
Burrin DG.
Catabolism dominates the first-pass intestinal metabolism of dietary essential amino acids in milk protein-fed piglets.
J Nutr
128:
606-614,
1998
30.
Thompson, A,
Smith I,
Brenton D,
Youl BD,
Rylance G,
Davidson DC,
Kendall B,
and
Lees AJ.
Neurological deteriorations in young adults with phenylketonuria.
Lancet
336:
602-605,
1990[ISI][Medline].
31.
Thompson, AJ,
Smith I,
Kendall BE,
Youl BD,
and
Brenton D.
Magnetic resonance imaging changes in early treated patients with phenylketonuria.
Lancet
337:
1224,
1991.
32.
Thompson, AJ,
Trittotson S,
Smith I,
Kendall B,
Moore SG,
and
Brenton DP.
Brain MRI changes in phenylketonuria.
Brain
116:
811-82,
1993[Abstract].
33.
Treacy, EP,
Delente JJ,
Elkas G,
Carter K,
Lambert M,
Waters PJ,
and
Scriver CR.
Analysis of phenylalanine hydroxylase genotypes and hyperphenylalaninemia phenotypes using L-[1-13C]phenylalanine.
Pediatr Res
42:
430-435,
1997[Abstract].
34.
Van Spronsen, FJ,
Reijngoud DJ,
Smit GP,
Nagel GT,
Stellard F,
Berger R,
and
Heymans H.
Phenylketonuria. The in vivo hydroxylation rate of phenylalanine into tyrosine is decreased.
J Clin Invest
101:
2875-2880,
1998
35.
Wapnir, RA,
and
Lifshitz F.
Intestinal transport of aromatic amino acids, glucose and electrolytes in a patient with phenylketonuria.
Clin Chim Acta
54:
349-356,
1974[ISI][Medline].
36.
Waterlow, JC,
Golden MHN,
and
Garlick PJ.
Protein turnover in man measured with 15N: comparison of end products and dose regimes.
Am J Physiol Endocrinol Metab Gastrointest Physiol
235:
E165-E174,
1978
37.
Widdowson, EM.
Changes in body composition during growth.
In: Scientific Foundation of Paediatrics, edited by David J,
and Dobbing J.. London: Heinemann, 1981, p. 330-342.
38.
Zello, GA,
Pencharz PB,
and
Ball RO.
Phenylalanine flux, oxidation, and conversion to tyrosine in humans studied with L-[1-13C]phenylalanine.
Am J Physiol Endocrinol Metab
259:
E835-E843,
1990
39.
Zello, GA,
Pencharz PB,
and
Ball RO.
The design and validation of a diet for studies of amino acid metabolism in adult humans.
Nutr Res
10:
1353-1365,
1990[ISI].
40.
Zello, GA,
Pencharz PB,
and
Ball RO.
Dietary lysine requirement of young adult males determined by oxidation of L-[1-13C]phenylalanine.
Am J Physiol Endocrinol Metab
264:
E677-E685,
1993
41.
Zello, GA,
Smith JM,
Pencharz PB,
and
Ball RO.
Development of a heating device for sampling arterialized venous blood from a hand vein.
Ann Clin Biochem
27:
366-372,
1990[ISI][Medline].
42.
Zello, GA,
Wykes LJ,
Ball RO,
and
Pencharz PB.
Recent advances in method of assessing amino acid requirements for adult humans.
J Nutr
125:
2907-2915,
1995[ISI][Medline].