Evaluation of methods to assess physical activity in free-living conditions

PURPOSE

The purpose of this study was to compare different methods of measuring physical activity (PA) in women by the doubly labeled water method (DLW).

METHODS

Thirteen subjects participated in a 7-d protocol during which total daily energy expenditure (TDEE) was measured with DLW. Body composition, basal metabolic rate (BMR), and peak oxygen consumption were also measured. Physical activity-related energy expenditure (PAEE) was then calculated by subtracting measured BMR and the estimated thermic effect of food from TDEE. Simultaneously, over the 7 d, PA was assessed via a 7-d Physical Activity Recall questionnaire (PAR), and subjects wore secured at the waist, a Tritrac-R3D (Madison, WI), a Computer Science Application Inc. activity monitor (CSA; Shalimar, FL), and a Yamax Digi Walker-500 (Tokyo, Japan). Pearson-product moment correlations were calculated to determine the relationships among the different methods for estimating PAEE. Paired t-tests with appropriate adjustments were used to compare the different methods with DLW-PAEE.

RESULTS

There was no significant difference between PAEE determined from PAR and DLW. The differences between the two methods ranged from -633 to 280 kcal.d(-1). Compared with DLW, PAEE determined from CSA, Tritrac, and Yamax was significantly underestimated by 59% (-495 kcal.d(-1)), 35% (-320 kcal.d(-1)) and 59% (-497 kcal.d(-1)), respectively. VO2peak explained 43% of the variation in DLW-PAEE.

CONCLUSION

Although the group average for PAR-PAEE agreed with DLW-PAEE, there were differences in the methods among the subjects. PAEE determined by Tritrac, CSA, and Yamax significantly underestimate free-living PAEE in women.

Butyric acid is synthesized by piglets

We hypothesized that there is no synthesis of butyric acid within organs or tissues not drained by the portal vein (PV). Two experiments were performed. In six piglets, the colonic vasculature was clamped (n = 4) or the entire colon resected while [1-13C]-butyric acid (99% enriched) was infused into a jejunal vein for 120 min; 13C enrichment of butyric acid was measured in the PV and carotid artery (ART) during the last 30 min of the infusion. In a second experiment, butyric acid tracer and unlabeled disaccharide were infused into the cecum for 120 min, and blood again was sampled from the PV and ART. For the four piglets studied during ligation of the colonic vasculature, the mean (+/- SD) ratio of the butyric acid enrichment in the ART to that in the PV (ART/PV) was 0.80+/-0.05 (ART vs. PV, P = 0.002) and for all six piglets in expt. 1, the ART/PV ratio was 0.74+/-0.1 (ART vs. PV, P = 0.001). The enrichment of butyric acid in the PV averaged 96.0% for the six studies, implying that splanchnic tissues other than the colon did not produce a substantial amount of butyric acid. For the second experiment, the ART/PV ratio was 0.80+/-0.15 (ART vs. PV, P = 0.03). These studies provide the first evidence for endogenous synthesis of butyric acid by piglets.

Fractional oxidation of chylomicron-derived oleate is greater than that of palmitate in healthy adults fed frequent small meals

Differences in oxidation of individual dietary fatty acids could contribute to the effect of dietary fat composition on risk factors for non-insulin-dependent diabetes mellitus and cardiovascular disease. Using a novel stable isotope technique, we compared fractional oxidation of chylomicron-derived oleate and palmitate in 10 healthy adults in a crossover study. 1-(13)C-labeled oleate or palmitate was emulsified into a eucaloric formula diet administered each 20 min for 7 h to produce a plateau in excretion of (13)C label in breath CO(2). Unlabeled oleate and palmitate each provided 16% of dietary energy, and other fatty acids provided 8% of energy. Total dietary fat was 40% of energy, carbohydrate was 46%, and protein was 14%. Diet without tracer was fed for 2 h before beginning tracer administration to establish a baseline fed state. Relative oxidation of oleate versus palmitate was defined as fractional oxidation of oleate divided by fractional oxidation of palmitate. Relative oxidation averaged 1.21 (99.5% confidence interval = 1.03;-1.39), indicating that fractional oxidation of oleate was significantly greater than that of palmitate.

Lactulose feeding in piglets: a model for persistent diarrhea and colitis induced by severe sugar malabsorption

We sought to determine, in a piglet model, whether severe sugar malabsorption causes colonic injury or inflammation. Twenty-four piglets were randomized to receive either control formula (CON) or CON supplemented with lactulose (LAC) (N = 12 each group). After seven days, inflammation, apoptosis, and crypt cell proliferation were assessed in the proximal colon (cecum). Lactulose feeding caused persistent diarrhea. In both groups, breath H2 concentration was low, suggesting no increased fermentation in the LAC group. Weight gain/volume formula intake was identical in the CON and LAC groups (0.09+/-0.13 and 0.09+/-0.11 g/ml) respectively. Injury to the colon did not occur, but inflammation of the colon (scale 0-5) was greater in LAC (score of 1.5+/-1.38) than in CON (0.42+/-0.79; P<0.05). Cell proliferation at the basal 40% of the crypt was 92% increased in CON (labeling index 22.8+/-9.9 vs. 11.9+/-2.8; P<0.05). We conclude that persistent feeding during severe sugar malabsorption permits weight gain but may cause colitis.

Splanchnic uptake of leucine in healthy children and in children with cystic fibrosis

Interpretation of tracer studies of amino acid kinetics in the fed state is dependent on knowledge of splanchnic uptake of diet-derived amino acids. We studied five healthy control children and five children with cystic fibrosis (CF). After an overnight fast, the children ingested, hourly, a formula diet for 11 h. 5,5,5-[2H3]Leucine was added to the feedings during the last 6 h, and an i.v. infusion of 1-[13C]leucine was administered during the last 2 h of the formula feeding. The mean rate of splanchnic uptake of leucine was similar in the CF and control group, 23.8 +/- 24.0 and 21.5 +/- 21.2 mumol.kg-1.h-1, respectively. Fractional splanchnic uptake of leucine was not significantly different in the patients with CF (0.16 +/- 0.112 mean +/- SD) compared with the control children (0.244 +/- 0.256(-1)). The rate of whole body protein breakdown was not significantly different between the groups (CF versus control) with (159 +/- 18 versus 135 +/- 28 mumol.kg-1.h-1) or without (135 +/- 14 versus 114 +/- 20 mumol.kg-1.h-1) correction for splanchnic leucine uptake. However, for the 10 cases combined, protein breakdown corrected for splanchnic leucine uptake (147 +/- 26 mumol.kg-1.h-1) was 18% greater than uncorrected protein breakdown (124 +/- 20 mumol.kg-1.h-1) (p = 0.009). The data suggest that companion studies of splanchnic uptake might enhance the interpretation of leucine kinetics in the fed state.

Differential impact of obesity on glucose metabolism in black and white American adolescents

BACKGROUND

The authors have previously demonstrated abnormalities in glucose and insulin metabolism in nondiabetic black American (BA) adults versus white American (WA) adults. Whether similar glucoregulatory alterations extend to BA adolescents remain unknown. In addition, obesity, a known risk factor for insulin resistance and hyperinsulinemia, occurs in a greater proportion of BA adults and children when compared to WA. The objective of the present study was to examine the differential effects of obesity on glucose homeostasis in BA and WA adolescents.

METHODS

We examined glucose homeostasis in BA and WA adolescents using oral glucose tolerance test (OGTT), intravenous glucose tolerance test (IVGTT), and [6,6-2H2]-glucose infusion. The study consisted of four age-, sex-, and pubertal stage-matched groups: 15 lean BA, 29 lean WA, 7 obese BA, and 9 obese WA.

RESULTS

Both obese groups had significantly increased insulin and C-peptide area under the curve (AUC) during OGTT and IVGTT when compared to their same-race lean counterparts. During OGTT, obese BA demonstrated greater insulin and C-peptide when compared to obese WA. During IVGTT, first- and second-phase insulin were significantly greater in obese BA versus obese WA.

CONCLUSION

In summary, BA adolescents demonstrated insulin resistance which is markedly exaggerated in the face of obesity when compared to WA adolescents, implying a differential impact for obesity on glucose homeostasis that is unique to the obese BA adolescent group. In conclusion, there is a need for early aggressive weight management in obese BA adolescents.

Effects of lactose intake on lactose digestion and colonic fermentation in preterm infants

The purpose of this study was to determine whether doubling the lactose concentration in formula for preterm infants lowers the fraction of lactose digested and/or increases the fraction of lactose fermented. Six preterm infants, 31 to 36 weeks' postconceptional age, were fed a standard preterm formula (carbohydrate is 50% lactose and 50% glucose polymer)(SC) and/or the same volume of formula modified to contain lactose as the sole carbohydrate (LAC). Relative lactose digestion during the LAC formula feeding compared with SC formula feeding was measured by using a stable isotope approach for quantifying the fractional contribution of formula lactose to plasma glucose enrichment. Relative lactose digestion was 0.98 +/- 0.17 (range, 0.70 to 1.19). Fractional fermentation of lactose was estimated from breath H2 excretion (0.52 +/- 0.34 during LAC feeding and 0.23 +/- 0.22 during SC feeding, P = .11). The rate of breath H2 excretion was much higher with LAC (1.34 +/- 0.98 mL/h) than with SC (0.27 +/- 0.29, P = .029). In conclusion, doubling the lactose concentration had only modest effects on fractional lactose digestion. Increased breath H2 excretion with LAC may relate to fermentation of nonlactose sugar or to ill-defined changes in colonic physiology or motility, which could enhance colonic fermentation of malabsorbed sugar by H2-producing bacteria.

Leigh syndrome, cytochrome C oxidase deficiency and hypsarrhythmia with infantile spasms

A rare patient with infantile spasms, hypsarrhythmia, cytochrome c oxidase deficiency and Leigh syndrome is reported. Although rare, infantile spasms and Leigh syndrome may occur simultaneously. Leigh syndrome should be included in the differential diagnosis of infantile spasms.

Measuring energy costs of leisure activity in adolescents using a CO2 breath test

To determine whether a 13C-bicarbonate, isotope dilution technique could be used to estimate relative changes in energy expenditure of leisure activities of short duration, we studied eight adolescents who performed the following activities: watching television (120 min); playing a stringed instrument (60 min plus 60 min of sitting); and walking plus rest during two approximately isocaloric sessions (slow walk at 40% of peak VO2 for 43 min plus 77 min of sitting; fast walk at 73% of peak VO2 for 22 min plus 98 min of sitting). The rate of appearance of CO2 (RaCO2) was determined from the ratio of the oral dose of 13C-bicarbonate and the isotopic enrichment of breath CO2. The net rates of excretion of CO2 (VCO2) and oxygen consumption were measured. VCO2 and RaCO2, were correlated (r = 0.93; P < 0.05). To adjust for the systematic difference in CO2 production between methods, determinations were expressed as a fraction of that during television viewing. For RaCO2, the ratios for instrument playing, walking at 40% peak VCO2, and walking at 73% peak VO2 were respectively 133 +/- 20%, 186 +/- 38%, and 206 +/- 34%; for VCO2, the respective ratios were 129 +/- 19, 210 +/- 50, and 232 +/- 39 (P > 0.05 for methods and interaction, two-way ANOVA). RaCO2 may be a useful method for detecting relative differences in energy expenditure associated with leisure activities of brief duration.

Technical note: pig model for studying nutrient assimilation by the intestine and colon

We have developed a system for chronically catheterizing 10- to 25-d-old pigs that permits stable isotope tracer studies of intestinal or colonic assimilation of nutrients. This model also can be used to ensure constant enteral feeding or to assess the rate of entry into the terminal ileum of carbohydrates, fats, and amino acids. A plastic cannula with a luminal flange can be surgically placed in the stomach for tracer studies of sugar digestion or for controlled infusion of any formula diet. A similar cannula can be placed in the cecum for infusion of tracer and(or) substrates for studies of fermentation. The cannula has been machined so that a washer and nut can be threaded onto it, allowing the entire apparatus to be fixed to the abdominal wall. The distal end protruding above the skin was tapered to fit standard i.v. extension tubing. A carotid arterial catheter was used to sample substrates for isotopic enrichment measurements.

Measurement of the rate of entry of intact colon-derived lactose into the circulation: a model for assessing gut uptake of molecules not endogenously synthesized

BACKGROUND

Results of in vitro studies have documented colonic absorption of lactose in the newborn. A stable isotope model was developed for assessing the entry rate of intact lactose into the portal circulation in newborn piglets.

METHODS

In experiment 1, unlabeled and [D-1-(13C)]-lactose were infused into two separate mesenteric veins, and in experiment 2, labeled lactose was infused into a mesenteric vein and unlabeled lactose was infused into the colon. The 13C-enrichment of plasma lactose was assessed by high performance liquid chromatography gas chromatography combined with mass spectrometry.

RESULTS

The isotopic estimate of the mesenteric venous infusion rate of lactose was 91% of the theoretical. In the second experiment 13% of the unlabeled lactose infused into the colon reached the portal circulation.

CONCLUSIONS

The current study provides the first, direct, in vivo confirmation of colon absorption of intact lactose. The tracer model could be used to evaluate intestinal or colonic absorption of other organic compounds not endogenously synthesized, including vitamins or drugs.

Stable isotope model for assessing production of short chain fatty acids from colon-derived sugar: application in pigs

Sugar reaching the colon because of intestinal maldigestion or malabsorption may be fermented to acetate and other short-chain fatty acids, resulting in stimulation of colonic water absorption and cell proliferation. To explore this phenomenon in more detail, we have developed a stable isotope model for estimating the fraction of colon-derived glucose or lactose that is fermented to acetate, propionate and butyrate. In an initial application of the model, [d3]-acetate and either [1-(13)C]-glucose or [D-1-(13)C]-lactose were infused into the cecum or colon of piglets, and plateau plasma acetate enrichment was monitored in the carotid artery. In acutely anesthetized piglets, the fractions of glucose and lactose fermented to acetate were 17.0 and 20.0%, respectively. In a chronically catheterized piglet, fermentation was higher (34.2%). When conducted in chronically catheterized animals or via a colostomy or ileostomy in infants, this model may be used to determine how age, previous surgery or antibiotic therapy affects the efficiency of colonic assimilation of carbohydrate.

Estimation of CO2 production in enterally fed preterm infants using an isotope dilution stable tracer technique

BACKGROUND

Estimates of the rate of CO2 production may be useful in preterm infants, but assessment of the rate of respiratory excretion of CO2 (VCO2) may not always be practical in infants requiring constant care. We hypothesized that the rate of dilution of 13CO2 (RaCO2) would be a valid index of CO2 production in preterm infants.

METHODS

Twelve studies of RaCO2 and VCO2 were performed in six enterally fed preterm infants. RaCO2 was measured using a 2-hour, primed, constant, orogastric infusion of NaH13CO3 with formula and an assessment of the plateau 13C enrichment of expired CO2. VCO2 was measured over two 10-minute intervals during the infusion using a flow-through system. Energy expenditure was estimated from these data and the food quotient.

RESULTS

Mean (+/- SD) rate of CO2 production using RaCO2 (348 +/- 32 mumol/kg/min) was 114% of that estimated using VCO2 (304 +/- 51 mumol/kg/min). The ratio of VCO2/RaCO2 is equal to the fractional recovery of tracer CO2 in the expired air during the course of the tracer infusion. In studies of short duration, this ratio is generally less than 100% because of isotope exchange. For five pairs of studies performed on consecutive days, each individual value of RaCO2 on day 2 was multiplied by the mean of the individual ratios of VCO2/RaCO2 on day 1 (0.78); corrected RaCO2 was 306 +/- 19 mumol/kg/min compared with 307 +/- 59 mumol/kg/min for VCO2.

CONCLUSIONS

Thus, RaCO2, particularly when corrected for isotope recovery, may be a useful index of group mean CO2 production and energy expenditure in preterm infants.

In vivo lactose digestion in preterm infants

In vitro studies of intestinal lactase activity and breath-hydrogen studies have suggested that the capacity for lactose digestion in preterm infants is less than the usual intake. To explore this question using an in vivo approach, we determined the fraction of dietary lactose hydrolyzed to glucose (and galactose) in 14 preterm infants with a gestational age of 26-31 wk at the time of birth but a postconceptional age of 31-37 wk at the time of study. The percentage of lactose digested was estimated after 6-h, primed, constant gastric infusions of [1-(13)C]glucose and D-[-1-(13)C]lactose on alternate days. A coefficient of lactose fermentation was derived from the rates of pulmonary excretion of hydrogen and carbon dioxide. Mean (+/- SD) lactose digestion was 79 +/- 26%. There was a significant inverse rank (r = -0.799, P < 0.01) and linear (r = -0.587, P < 0.05) correlation between this variable and postconceptional age. The percentage of lactose fermented averaged 35 +/- 27%.

Effects of feeding on protein turnover in healthy children and in children with cystic fibrosis

We hypothesized that there is less suppression of whole-body protein breakdown with feeding in patients with cystic fibrosis (CF) who exhibit decreased insulin secretion after a single meal. Using [1-13C]leucine, we measured rates of nonoxidative leucine disappearance (whole-body protein synthesis) and protein breakdown in nine CF patients (6-11 y of age) and five healthy control subjects (8-10 y of age) during feeding and fasting. In the CF patients, synthesis and breakdown (x +/- SD) were 172 +/- 61 and 157 +/- 67 mumol.kg-1.h-1 during feeding and 140 +/- 24 and 178 +/- 26 mumol.kg-1.h-1 during fasting. The respective control values were 129 +/- 27 and 114 +/- 20 mumol.kg-1.h-1 during feeding and 136 +/- 13 and 173 +/- 18 mumol.kg-1.h-1 during fasting. Leucine balance was nearly identical in the two groups. By analysis of variance, there was a significant effect of feeding on protein breakdown but no difference between the groups. However, when each group was analyzed separately, feeding resulted in a 34% decrease in breakdown in the control subjects (P = 0.001) and a 23% increase in synthesis in the CF group (P = 0.058). Plasma insulin concentrations did not differ in the two groups. Thus, feeding may affect protein turnover differently in children with CF than in control children independently of plasma insulin concentration.

Digestion, absorption, and fermentation of carbohydrates in the newborn

In the newborn, sugars present in human milk and formulas are assimilated by both small intestinal digestion and, especially in the case of lactose, colonic bacterial fermentation. Colonic fermentation of carbohydrate serves three major functions: (1) conservation of a fraction of the metabolizable energy of dietary carbohydrate that is not absorbed in the small intestine; (2) prevention of osmotic diarrhea; and (3) production of short-chain fatty acids that stimulate sodium and water absorption, serve as fuel for colonocytes, and stimulate cell replication in colon and small intestine. Diarrhea produced in association with small bowel malabsorption of sugar may be caused by three, potentially overlapping mechanisms: (1) osmotic effects of unfermented sugar, which may cause secondary disruption of fermentation by purging the bacteria or diluting the bacteria mass; (2) damage to the colon mucosa from excessive fermentation leading to SCFA malabsorption and osmotic diarrhea on this basis; and (3) excessive fermentation leading to lowering of luminal pH and inhibition of bacterial enzymes. Therapy aimed at reducing diarrhea associated with sugar malabsorption might involve either slowing of motility to facilitate fermentation or stimulation of fermentative activity, but such interventions would depend on greater understanding of the mechanisms for colonic dysfunction in this condition.

Galactose-containing carbohydrates are preferentially absorbed in the neonatal pig colon

Previous studies on the piglet colon in newborns cleared of bacterial metabolic activity showed a capacity for lactose absorption. Colonic absorption occurred at a flux rate equal to the assimilation of lactose by the small intestine but by a process that did not involve either glucose-galactose sodium cotransport or simple diffusion. Surprisingly, colonic lactose transport did not require either fermentation or cleavage of the disaccharide for uptake. Experiments were designed to test the selectivity of the colonic transport process for a variety of carbohydrates. Colonic tissues from 4-7-d-old piglets were mounted in Ussing chambers and the mucosal-to-serosal flux of radiolabeled carbohydrates was compared with that of lactose. The results showed a 3-4-fold greater flux of galactose-containing sugars as compared with glucose-containing carbohydrates at concentrations up to 40 mM. Even lactulose, a synthetic disaccharide assumed to require bacterial digestion before assimilation, was transported readily. N-Acetylgalactosamine, a component of colonic mucus, inhibited the flux of lactose, whereas N-acetylglucosamine did not. Similarly, lactosylated BSA inhibited lactose flux, whereas nonlactosylated BSA did not. The capacity of the colon of the newborn to differentiate moieties as similar as glucose and galactose suggests an absorptive process for carbohydrates with a high degree of discrimination.

Energy expenditure in adolescents during low intensity, leisure activities

We sought to determine how a stationary activity such as playing a stringed instrument may affect energy expenditure (EE) in adolescents. Using automated indirect calorimetry, we measured EE in eight adolescents (1 male, 7 females, 14.2 +/- 2.1 yr) while they each performed the following activities: watching television (TV) (60 min), playing a stringed instrument (60 min), and walking at 40% of peak oxygen uptake (43 min). Measurements were made during three, 6- to 7-min steady state periods of each activity. EE (mean +/- SD) was lower during TV (1.0 +/- 0.2 kcal.min-1) and instrument playing (1.4 +/- 0.2 kcal.min-1) than during walking (3.4 +/- 0.4 kcal.min-1) (P < 0.05). EE during instrument playing was 41% greater than during television viewing (P < 0.05). We conclude that relatively sedentary activities such as playing a stringed instrument can elevate EE. Conceivably, other stationary, leisure activities performed by adolescents may increase EE and have substantial, cumulative effects on long-term energy balance and fat accretion.

Elevated hepatic glucose production in children with cystic fibrosis

We hypothesized that elevated hepatic glucose output (HGO) may occur in children with cystic fibrosis (CF) as an early sign of declining insulin secretion and that tolbutamide therapy would correct the defect. We studied eight glucose-tolerant CF patients (mean +/- SD, 9.1 +/- 1.9 y) and five healthy controls (9.0 +/- 1.6 y). Fasting glucose, insulin, and insulin-connecting peptide concentrations were not different in the CF and control subjects; however, meal stimulation tests in the CF patients suggested that insulin secretion was defective in the fed state. HGO (mg.kg-1 body weight.min-1) was 26% higher in the CF patients (4.2 +/- 0.7 versus 3.1 +/- 0.6 in HC) (p = 0.016). When normalized for fat-free mass (mg.kg fat-free mass-1.min-1), HGO was 27% higher in CF (4.9 +/- 0.8 versus 3.8 +/- 0.5) (p = 0.015). However, when expressed as a function of resting energy expenditure (mg.kcal-1), HGO was not significantly different in CF (121 +/- 22) versus healthy controls (116 +/- 30). In seven of the CF group, HGO was re-assessed after a 2-h glucose infusion at a rate of 0.90 +/- 0.02 mg.kg-1.min-1. HGO was suppressed (p < 0.05) by an amount equal to 103 +/- 18% of the glucose infusion rate. Finally, in five CF patients, HGO was re-measured after 2 wk of oral therapy with tolbutamide (750 mg/d). Tolbutamide did not affect HGO (fasting or during the glucose infusion). In conclusion, fasting HGO was elevated in the CF patients in proportion to energy expenditure.

Effect of endurance training on hepatic glycogenolysis and gluconeogenesis during prolonged exercise in men

In humans, endurance training markedly reduces the rate of hepatic glucose production during exercise. To determine whether this is due to a reduction in glycogenolysis, in gluconeogenesis, or in both processes, six men were studied at rest and during 2 h of cycle ergometer exercise at 60% pretraining peak O2 consumption (VO2peak), both before and after completion of a strenuous endurance training program (cycling at 75-100% VO2peak for 45-90 min/day, 6 days/wk for 12 wk). The overall rate of glucose appearance (Ra) was determined using a primed continuous infusion of [6,6-2H]glucose, whereas the rate of gluconeogenesis (Rgng) was estimated from the incorporation of 13C into glucose (via pyruvate carboxylase) from simultaneously infused [13C]bicarbonate. Training did not affect glucose kinetics at rest but reduced the average Ra during exercise by 42% [from 36.8 +/- 3.8 to 21.5 +/- 3.6 (SE) mumol.min-1.kg-1; P < 0.001]. This decrease appeared to be mostly due to a reduction in hepatic glycogenolysis. However, the estimated Rgng during exercise also decreased significantly (P < 0.001) with training, falling from 7.5 +/- 1.6 mumol.min-1.kg-1 (23 +/- 3% of total Ra) before training to 3.1 +/- 0.6 mumol.min-1.kg-1 (14 +/- 3% of total Ra) after training. These training-induced adaptations in hepatic glucose metabolism were associated with an attenuated hormonal response to exercise (i.e., higher insulin and lower glucagon, norepinephrine, and epinephrine concentrations) as well as a reduced availability of gluconeogenic precursors (i.e., lower lactate and glycerol concentrations). We conclude that endurance training reduces both hepatic glycogenolysis and gluconeogenesis during prolonged exercise in men.

A portable device for estimating the rate of CO2 production in preterm infants

We developed a simple flow-through system for measuring the net rate of pulmonary excretion of CO2 (VCO2). Its components, connected in series by plastic tubing, are the following: plastic oxygen hood, suction flask used both as a mixing and sampling chamber, Brooks mass flow controller, and wall suction. Gas aliquots withdrawn from the sampling flask are analyzed for CO2 concentration in the laboratory. The system was validated by measuring over a 60-min period the evolution of CO2 from the reaction of sodium carbonate with phosphoric acid. In 9 trials, the mean recovery of CO2 was 99.8%. In 14 studies in 11 preterm infants, we evaluated the reproducibility of duplicate measurements of CO2 production on consecutive days; in 9 of the studies, the coefficient of variation was < 5%, but in other 5 studies, it ranged from 7.7 to 35.8%. We believe that this device might have both research and clinical applicability.

Comparison of methods for estimating fecal carbohydrate excretion in premature infants

Our previous studies have suggested that there is minimal fecal excretion of carbohydrate-derived energy in premature infants fed lactose or glucose-polymer as the source of carbohydrate. In these studies, carbohydrate energy excretion was equated with the non-nitrogenous, nonfat energy. Although these studies provided consistent results, this methodological approach cannot distinguish actual excretion of dietary carbohydrate from carbohydrate in the form of large molecular compounds derived from infant or bacterial cells (which in fact comprise approximately 90% of the measured value). Therefore, in this study we compared the absorption of carbohydrate energy to the fractional absorption of 13C derived from [D-1-13C]-lactose in 10 premature infants 30-32 weeks gestational age fed either a commercial premature infant formula containing a mixture of carbohydrates (50% lactose: 50% glucose polymer) or the same formula in which lactose was the sole carbohydrate. The two methods provided comparable estimates of carbohydrate absorption (96 and 94%, respectively), although there were significant discrepancies in two infants. These studies provide evidence of external validity for the two methods.

Isotopic estimation of CO2 production during exercise before and after endurance training

Endurance training reduces the rate of CO2 release (i.e., VCO2) during submaximal exercise, which has been interpreted to indicate a reduction in carbohydrate oxidation. However, decreased ventilation, decreased buffering of lactate, and/or increased fixation of CO2 could also account for a lower VCO2 after training. We therefore used a primed continuous infusion of NaH13CO3 to determine the whole body rate of appearance of CO2 (RaCO2) in seven men during 2 h of cycle ergometer exercise at 60% of pretraining peak O2 uptake (VO2peak) before and after endurance training. RaCO2 is independent of the above-described factors affecting VCO2 but may overestimate net CO2 production due to pyruvate carboxylation and subsequent isotopic exchange in the tricarboxylic acid cycle. Training consisted of cycling at 75-100% VO2peak for 45-90 min/day, 6 days/wk, for 12 wk and increased VO2peak by 28% (P < 0.001). VCO2 during submaximal exercise was reduced from 86.8 +/- 3.7 to 76.2 +/- 4.2 mmol/min, whereas RaCO2 fell from 88.9 +/- 4.0 to 76.4 +/- 4.4 mmol/min (both P < 0.001). VCO2 and RaCO2 were highly correlated in the untrained (r = 0.98, P < 0.001) and trained (r = 0.99, P < 0.001) states, as were individual changes in VCO2 and RaCO2 with training (r = 0.88, P < 0.01). These results support the hypothesis that endurance training decreases CO2 production during exercise. The magnitude and direction of this change cannot be explained by reported training-induced alterations in amino acid oxidation, indicating that it must be the result of a decrease in carbohydrate oxidation and an increase in fat oxidation.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo estimation of lactose hydrolysis in premature infants using a dual stable tracer technique

To investigate their putative capacity for lactose digestion, primed continuous orogastric infusions of [1-13C]glucose and D-[1-13C]lactose were administered on consecutive days to five premature infants (30-31 wk gestation, 15-32 days of age), who were fed by orogastric infusions of human milk or formula. By monitoring the plateau isotopic enrichment of plasma glucose using isotopomers containing the entire derivatized glucose molecule or C-2 through C-6, we were able to distinguish label appearing in the peripheral circulation deriving from unmetabolized glucose from that arising from recycled or fermented glucose (or lactose). Isotopic enrichment of the C-1 of glucose, corrected for recycling, was then calculated during each tracer infusion, and the fraction of dietary lactose subjected to in vivo hydrolysis was estimated from these values and the respective tracer infusion rates, assuming similar absorptive and metabolic fates of labeled glucose arising from either tracer. This fraction averaged 1.02 +/- 0.16 (SD), suggesting that lactose digestion is efficient by 34-wk postconceptional age.

Efficient assimilation of lactose carbon in premature infants

Previous studies have shown that there is minimal fecal excretion of carbohydrate energy in premature infants with high breath H2 concentrations, implying that the colon plays an important role in salvaging lactose energy. The present study extends this work by using a stable isotope method for serial assessment of lactose carbon excretion. Thus, in nine premature infants (30-32 weeks of gestation and 11-36 days of age), we conducted 23 longitudinal studies of breath hydrogen concentration and the cumulative fecal excretion of 13C derived from a single dose of [D-1-13C]lactose. The ranges of values (mean +/- SD) for breath hydrogen concentration and lactose absorption were respectively 4-440 ppm/5% CO2 (92 +/- 100) and 82-100% tracer dose (98.4 +/- 3.9). In 21 studies, lactose carbon excretion was less than 2% of the dose. There was a significant correlation between postnatal age and breath hydrogen concentration (R = 0.48; p = 0.02). These studies suggest that colonic fermentation activity is adequate for efficient colonic salvage of lactose even during the second week of life.

Stable isotope model for estimating colonic acetate production in premature infants

In premature infants, a nutritionally significant proportion of lactose is apparently fermented in the colon to acetate. To estimate the rate of entry of acetate into the peripheral circulation, a model that takes into account extraction of gut-derived acetate by splanchnic and hepatic tissues was developed. Using a [1-13C]acetate orogastric infusion technique, six studies were carried out on five premature infants during constant orogastric feeding. Ranges in gestational age, postnatal age, and breath H2 concentration (corrected for CO2 content) were 28-32 weeks, 16-29 days, and 45-252 microL/L, respectively. The estimated rate of entry of acetic acid (mean +/- SD) was 63.7 +/- 33.8 mumol.kg-1.min-1 (range, 22.9-123.2 mumol.kg-1.min-1), which corresponded to 64.3% +/- 38.6% (24%-136%) of the potential two carbon units from dietary lactose. Thus, a substantial fraction of dietary lactose in premature infants may be converted to acetic acid; this conversion could have a significant effect on protein as well as energy requirements.

Effects of glucose infusion on leucine transamination and oxidation in the ovine fetus

During fasting of the ewe, the rate of amino acid oxidation by the ovine fetus increases substantially. We hypothesized that the increase in amino acid oxidation derived mainly from reduced protein synthesis. We further hypothesized that fetal glucose supplementation would result in diminished amino acid oxidation. To test these hypotheses, nine ovine fetuses were infused with [15N,1-13C]leucine to determine the rates of leucine appearance and disposal. Simultaneously, the fetal uptake of leucine was determined. Animals were studied in the fed and fasted state. After baseline measurements, glucose was infused into the fetal inferior vena cava at a rate estimated to match the fetal glucose uptake. Results of these studies indicate that leucine nitrogen flux, leucine carbon flux and fetal leucine uptake were constant. Leucine oxidation was increased by 50% in the fasted state (6.3 versus 13.4 mumol/min); glucose infusion resulted in a 25% decline in oxidation (to 10.4 mumol/min) in the fasted state, but had no effect in the fed state. Mean leucine umbilical uptake during fasting was 9.3 mumol/min, 4.1 mumol/min less than leucine oxidation. These data suggest that leucine (and potentially other amino acids) may be in negative balance during maternal fasting, and can be spared by supplementation of the fetus with exogenous glucose.

Effects of tolbutamide on growth and body composition of nondiabetic children with cystic fibrosis

Previously, we reported that nondiabetic children with cystic fibrosis show a blunted insulin response to a meal stimulus. In the study presented here, using tolbutamide, we determined the effects of augmented insulin secretion/action on height and lean body mass of children with cystic fibrosis. Twelve subjects (mean +/- SEM age, 11.0 +/- 0.5 y) were studied for three 4-mo periods: 1) pretreatment, 2) treatment, consisting of 750 mg/d of tolbutamide, and 3) posttreatment. Before the pretreatment period, insulin response to a meal stimulus was evaluated in relation to three doses of tolbutamide: 0, 250, and 500 mg. Growth was monitored during each period, and incremental changes in lean body mass were calculated from height data. To validate the change in lean body mass based on height measurements, we determined lean body mass in seven subjects during the treatment period by using a criterion method (H218O). Growth velocity (cm/4 mo) significantly increased (p less than 0.05) during the treatment (2.58 +/- 0.31) compared with the pretreatment period (0.88 +/- 0.20). The increase in lean body mass calculated from height was greater during the treatment (1.61 +/- 0.29 kg/4 mo) than during the pretreatment period (0.44 +/- 0.18 kg/4 mo) (p less than 0.05). There was also a significant increase (p less than 0.05) in lean body mass during the treatment as measured with H218O (1.91 +/- 0.65 kg/4 mo). Acute administration of either 250 or 500 mg of tolbutamide reduced (p less than 0.05) the area under the glucose concentration curve in response to a meal compared with the control condition of no tolbutamide.(ABSTRACT TRUNCATED AT 250 WORDS)

Measurement of stable isotopic enrichment of underivatized acetate by gas chromatography/mass spectrometry: application to in vivo estimation of acetate production

In order to assess endogenous and colonic production of acetate, we have developed an assay for determining the isotopic enrichment of plasma acetate using gas chromatography/mass spectrometry (GC/MS). Acidified, deproteinized plasma (200 microliters) was extracted into ethyl ether, and the ether phase was then injected into a gas chromatograph/mass spectrometer fitted with a 30 m x 0.252 mm i.d. capillary column (temperature program 50-245 degrees C at 10 degrees C min-1). Using electron impact GC/MS and selected ion monitoring, peak areas of ions with m/z 60 and 61 (M + 1) were determined. Triplicate extractions of enriched plasma samples (mol.% excess 1.38-1.5%) resulted in a coefficient of variation of 1.6-5.9%. Unenriched plasma samples were found to have an enrichment close to theoretical natural abundance, and analysis of our (1-13C)acetate tracer (99 at.% excess) revealed an ion at m/z 61 and no ion at m/z 60. To verify accuracy, we conducted an in vivo isotope dilution study. In a 1-month-old piglet, fasted for 24 h, changing the rate of a 4 h infusion (mmol h-1) of (1-13C)acetate from 0.141 to 0.282 doubled the isotope enrichment (2.08 x) of the second plateau. The rate of appearance of acetate was 26.0 mumols kg-1 min-1, which is comparable to that reported in fasting sheep.

Contribution of low-molecular-weight compounds to the fecal excretion of carbohydrate energy in premature infants

It was hypothesized that low-molecular-weight products of carbohydrate fermentation would contribute only a small percentage to the total fecal excretion of nonfat, nonnitrogenous energy (carbohydrate energy) in premature infants. Infants born at 28-32 weeks' gestation who were 2-4 weeks of age were randomized to receive a formula with lactose as the sole carbohydrate (n = 7) or the same formula with 50% of the carbohydrate as glucose polymer (n = 8). The percent contribution (X +/- SD) to total carbohydrate energy of sugars (glucose, galactose, lactose, glucose polymer), short-chain fatty acids (acetate, propionate, butyrate, isobutyrate, valerate, and isovalerate), and D- and L-lactate was 9.4% +/- 2.9% for the 15 subjects and was not significantly different between groups. The percent contribution of all four sugars was 5.8% +/- 1.7% and did not differ between the two groups. Doubling the lactose intake resulted in significant increases in fecal excretion (kilocalories per kilogram per day) of acetate (77% increase; P = 0.03), total short-chain fatty acids (54%; P = 0.04), and galactose (188%; P = 0.03). These data suggest that as much as 90% of fecal carbohydrate energy may be in the form of large-molecular-weight compounds, presumably bacterial in origin.

A metabolic bed for collecting excreta in premature infants

A mattress and plastic bed for collection of urine and feces in premature infants housed in isolettes is described. The dimensions of the bed are 65.5 x 34.4 x 13.5 cm. A drainage hole is located 18 cm from the foot of the bed and midway (16.5 cm) from either side. The angle of the depression in the bed for the drainage hole is approximately 12.5 degrees. The bed is covered with a plastic-backed foam rubber mattress. The plastic surface on which the infant lies is impervious to water. The method of excreta collection using the bed does not involve the need for restraints; urine and/or feces are washed off the undiapered infant and the bed into a container beneath the drainage hole. The bed has been well tolerated in over 30 studies of 4 days duration, and laboratory experiments with urine and feces applied to the mattress for 15-60 min have indicated excellent recovery of total nitrogen.

Isotopic dilution of CO2 as an estimate of CO2 production during substrate oxidation studies

The rate of oxidation of a substrate is often divided by the numerical estimate of a correction factor for the proportion of labeled CO2 that is excreted during the course of the experiment. This factor, derived from tracer studies of bicarbonate kinetics, equals the product of the net rate of respiratory CO2 excretion and the ratio of the plateau isotopic enrichment of CO2 to the rate of infusion of labeled bicarbonate. The inverse of this ratio may be equated to the rate of appearance of unlabeled CO2. When one substitutes the expression for the correction factor into a typical equation for the rate of substrate oxidation, the rate of appearance of CO2 substitutes for both net CO2 excretion and the correction factor in the original oxidation equation. Thus the rate of appearance of CO2 is an index of CO2 production that takes into account both net fixation of CO2 and isotopic exchange of labeled CO2; it also may have application as an index of net CO2 production.

Digestion, absorption, and fermentation of carbohydrates

Newborn infants born after a term gestation appear to have an adequate capacity for small intestinal digestion and absorption of carbohydrate. Premature infants do not have a mature level of intestinal lactase activity. Apparently, in the premature infant, colonic fermentation serves an important, if not the only major route, for lactose carbon absorption, and it also appears that signs of fermentation activity such as increased BH2 are not indicative of disease. Important questions remaining in the premature infant relate to developmental aspects of colonic fermentative activity, effects of systemic antibiotic treatment on colonic salvage, the effects of various fermentation pathways on energy balance, the capacity for absorption of sugars, SCFA, and electrolytes by colonic epithelia, and the effects of fermentation products on metabolism and on the mucosal cells of the intestine or colon. Research addressing these questions may have relevance in the following areas: further development of formula diets for healthy premature infants and for patients of all ages with intestinal or colon dysfunction; the effects of colonic disease or surgical resection; the nutritional or clinical effects of dietary fiber; and the effects of antibiotic therapy on intestinal and colon function.

Effect of pH on fecal recovery of energy derived from volatile fatty acids

We assessed the effect of pH on volatilization of short-chain fatty acids during lyophilization. Acetic, propionic, valeric, and butyric acids were added to a fecal homogenate in amounts sufficient to raise the energy density by 18-27%. Fecal homogenate samples were either acidified (pH 2.8-3.2), alkalinized (pH 7.9-8.7), or left unchanged (4.0-4.8) prior to lyophilization and subsequent bomb calorimetry. Alkalinizing the fecal samples prevented the 20% loss of energy derived from each of these volatile fatty acids observed in samples either acidified or without pH adjustment. These data suggest that in energy balance studies involving subjects with active colonic fermentation, fecal samples should be alkalinized prior to lyophilization and bomb calorimetry.

Dietary carbohydrate assimilation in the premature infant: evidence for a nutritionally significant bacterial ecosystem in the colon

Carbohydrate energy absorption and breath hydrogen concentration were measured in 12 premature infants 28-32 wk gestational age and 2-4 wk postnatal age. Each of two groups of six infants were randomly assigned to receive one of two formulas that differed only in carbohydrate source: 100% lactose (LAC) or 50% lactose: 50% glucose polymer (LAC + GP). In 11 infants the peak breath hydrogen concentration suggested extensive colonic fermentation (range 44-239 ppm/5% CO2 or 44-239 microL/L per 50 mL/L CO2). An approximate 100% increase in lactose intake in the LAC group was associated with a similar increase in breath hydrogen concentration at 30, 60, and 120 min. None of the infants exhibited diarrhea or vomiting or developed delayed gastric emptying. Carbohydrate energy absorption (mean +/- SD) was, respectively, 86 +/- 5% and 91 +/- 3% in the LAC and the LAC + GP groups (p greater than 0.05). Thus, colonic bacterial fermentation may be critical to energy balance and to the prevention of osmotic diarrhea in premature infants fed lactose.

Close association of accelerated rates of whole body protein turnover (synthesis and breakdown) and energy expenditure in children with newly diagnosed acute lymphocytic leukemia

Using a single-dose [15N]glycine turnover technique, we studied protein metabolism in 15 children with newly diagnosed acute lymphocytic leukemia (ALL). As in our previous study, we observed increased rates of whole body protein synthesis (S) and breakdown (B) in comparison to healthy controls. In eight patients, we assessed basal metabolic rate (BMR). There was a significant linear regression between BMR (kcal/d) (Y) and S (g protein/d) (X): y = 3.7 X + 850 (R = 0.925, p less than 0.001). There was also a significant linear correlation between BMR, expressed as a percentage of the normal value, and S expressed as a percentage of the mean value in the healthy children (r = 0.79, p less than 0.05). There were also significant positive correlations between BMR and body weight (r = 0.75, p less than 0.05) or age (r = 0.83, p less than 0.05) and between S and weight or age (both, r = 0.86, p less than 0.01). BMR (kcal/d) also correlated with B (g protein/d) (r = 0.91, p less than 0.01). Multiple regression analysis revealed that BMR was much more highly related to S than to weight. These data suggest that increased rates of S are closely related to increased energy requirements in patients with ALL. Furthermore, these data provide evidence for the biological relevance of whole body protein kinetics.

Effects in premature infants of normalizing breath H2 concentrations with CO2: increased H2 concentration and reduced interaliquot variation

Interval sampling of expired breath samples from the anterior nares is a feasible and noninvasive method for detecting elevated breath H2 and thus colonic carbohydrate fermentation, especially in nontherapeutic research studies of healthy premature infants. However, there may be a risk of falsely low breath H2 concentration and an unacceptable experimental error due to contamination with room air as well as with dead space air. We studied ten premature infants (28-32 week gestational age and 2-4 weeks postnatal age) who were receiving either a proprietary formula containing equal proportions of lactose and glucose polymer, or a similar formula in which lactose was the sole carbohydrate. In 70 breath samples (obtained 30-180 min after feeding) we assessed the coefficient of variation in breath hydrogen concentration among three aliquots obtained over a 3-5 min interval. Breath was collected from the anterior nares. The interaliquot coefficient of variation averaged 11% when expressed as parts per million per 5% CO2, compared to 19% when expressed as parts per million (Wilcoxon, p less than 0.001). Mean breath hydrogen concentration at each time period using the former method (parts per million per 5% CO2) was about 100% higher than when using the latter method. Although ventilation rate can alter alveolar CO2, normalizing for CO2 concentration reduces a major source of experimental error.

Mild peripheral neuropathy but biochemical chromium sufficiency during 16 months of "chromium-free" total parenteral nutrition

A 6-yr, 4-month-old boy was started on total parenteral nutrition (TPN) because of chronic diarrhea. The TPN regimen (3 liter/day) initially included supplemented Cr (3 micrograms/day) in addition to standard components (including FreAmine III). At age 8 yr, 8 months, the serum Cr level was elevated: 3.7 ng/ml (normal 0.03-0.85). A repeat level at the same time by another commercial laboratory was also high (7.0). Cr supplementation was stopped. At age 10 yr, he was noted to have mild peripheral neuropathy although glucose tolerance was excellent (alpha-linolenic acid was undetectable in the plasma). Cr status was reevaluated in a research lab. The serum level was 1.4 ng/ml (normal 0.05-0.4). The urine chromium excretion was 1.27 micrograms/day (normal 0.22). The TPN regimen (unsupplemented with Cr) provided 4 micrograms/day. Normal Cr intake is about 60 micrograms/day with 0.4% absorption (net 0.24 microgram/day). We conclude that Cr contamination of standard PN fluid may prevent biochemical evidence of low Cr status. In addition, alpha-linolenic acid-free parenteral nutrition for 46 months was not associated with clinically significant neurological dysfunction.

Vitamin E-deficient spinocerebellar syndrome due to intestinal lymphangiectasia

A 23-year-old woman with congenital intestinal lymphangiectasia developed a progressive spinocerebellar syndrome and neuropathy. The clinical findings were typical of vitamin E deficiency, which was documented in the patient. Intestinal lymphangiectasia is an additional vitamin E-deficient disease that may cause this neurologic syndrome.

Dressing changes by specialized personnel reduce infection rates in patients receiving central venous parenteral nutrition

In 60 pediatric patients, 75 central venous catheters representing 1866 patient days were placed to provide parenteral nutrition. During the 21-month study period, surgical residents were responsible for dressing changes during the first 16 months and a specially trained nurse was responsible during the final 5 months. Twenty percent of the lines became infected. The infection rate was significantly higher in the lines cared for by residents, 28.8% compared to 3.3% in the lines cared for by the nurse. This occurred in spite of the lines being in place significantly longer in the nurse group (31.7 vs 20.3 patient days/line). These data indicate that a specially trained person using aseptic techniques can reduce infection rates in patients receiving central venous parenteral nutrition.

Tissue nitrogen-sparing effect of high protein diet in mice with or without ascites tumor treated with Acinetobacter glutaminase-asparaginase

Forty-eight tumor-free mice and 32 mice bearing Ehrlich ascites tumor were randomized into 2 treatments, Acinetobacter glutaminase-asparaginase (AGA) (600 IU/kg/day for 7 days) and 0.9% NaCl controls, and into 2 or 3 isocaloric diets, normal protein (NP) (20 g protein/100 g diet), high protein (HP) (58 g protein/100 g diet), and zero protein (ZP) (tumor-free mice only). In tumor-free, NP-fed mice, AGA caused percentage reductions (P less than 0.01) in the nitrogen content of liver (50%), intestine (42%), thymus (89%), spleen (75%), and carcass (20%), but HP prevented this effect on intestine and carcass and caused percentage increases in the nitrogen content of liver (53%), intestine (36%), thymus (122%), and carcass (25%). In Ehrlich ascites tumor mice (NP or HP fed) AGA caused markedly lower (P less than 0.01) tumor burdens and increased nitrogen content of intestine (HP), kidney (NP and HP), and spleen (NP and HP). Ehrlich ascites tumor, AGA-treated, HP-fed mice ate 31% less food (P less than 0.01) (compared to NP) but HP resulted in percentage increases in the nitrogen content of liver (18%; P = 0.05), intestine (25%; P less than 0.05), and thymus (164%; P less than 0.01). In the Ehrlich ascites tumor, AGA group the HP diet caused higher hematocrit and serum total protein (both, P less than 0.05). Adverse nutritional effects of AGA seen in normal mice were markedly diminished in tumor-bearing animals. The observed nitrogen-sparing effects of the high protein: energy ratio may be relevant to humans and to other forms of neoplasia and chemotherapy.