The 1987 Borden Award lecture. Protein metabolism in premature human infants

Our studies have focused on the regulation of whole body and skeletal muscle protein metabolism in premature infants. Net deposition of protein is the result of a positive balance between protein synthesis and breakdown. To measure protein metabolism we have employed end-product studies with [15N]glycine and 13[C]leucine. Myofibrillar protein degradation was estimated by measuring the excretion of N-t-methylhistidine in urine. Energy expenditure and substrate utilization were also measured. Premature infants have high rates of protein synthesis (12 g.kg-1.d-1), twice those measured in children and four times those found in adults. Intrauterine malnourished babies have increased rates of protein turnover. Very low birth weight infants (less than 1500 g) have higher myofibrillar protein turnover than larger babies. Intravenous feeding decreases whole body protein turnover, and we estimate visceral protein synthesis to be approximately 4 g.kg-1.d-1. Suboptimal energy intake worsens nitrogen utilization by reducing the reutilization of endogenous amino acids for protein synthesis. We have also examined the effects of varying the source of nonprotein energy (i.e., glucose only versus glucose plus lipid) at requirement levels and have shown there is no effect on protein metabolism. Recent improvements in technology have opened the way to detailed study of individual amino acid metabolism in neonates in the future.     

A reappraisal of protein turnover values in neonates fed human milk or formula

We investigated the effect of human milk feeding on the nitrogen metabolism of appropriate-for-gestational age infants of birth weight 1.5-2.0 kg. Eight infants received pooled mature human milk. The remaining 20 were divided into two equal groups, who received one of two low-protein, milk-based formulae. The formulae were identical in composition except for the protein source, which was either casein- or whey-predominant. The three diet groups received similar total nitrogen (390 mg N.kg-1.d-1) and energy (500 kJ.kg-1.d-1) intakes. The human-milk-fed group, however, received a significantly higher intake of nonprotein and urea nitrogen and a significantly lower true protein nitrogen. Nitrogen metabolism was studied using a modified constant infusion of [15N]glycine, mixed with the feeding every 2-3 h. Urine was collected in approximately 3-h aliquots and analysed for total ammonia and urea nitrogen. Excretion of the 15N label was measured in urinary urea and ammonia. No differences were seen between the three diet groups in total [15N]urea or [15N]ammonia urinary excretion. However, the concentration of 15N in urinary urea in the human-milk-fed group was lower than in the two formula-fed groups. This reduction in concentration appeared due to a higher dietary intake of urea among the human-milk-fed group, and the consequent dilution of the label in the urine. As a result, protein turnover rates calculated from the [15N]urea end product were artificially raised in the milk-fed group, and were significantly higher than those in the formula groups.(ABSTRACT TRUNCATED AT 250 WORDS)     

Energy requirements of captive cotton-top tamarins (Saguinus oedipus oedipus)

Food intake of captive cotton-top tamarins was measured at various stages of the life cycle and energy intake was calculated from the results. The intake of adults for maintenance was found to vary around 500 kJ X kg-3/4 X day-1 and to decrease with age. During the last 8 weeks of pregnancy, females increased in weight by about 2 g X day-1 but energy intake did not show a marked rise above the maintenance range. During lactation, their energy intake appeared to double. Measurements of intake at various stages of growth were compared with a model of the energy requirements for growth which predicted intake quite closely. Compared with non-primate mammals of similar size, the growth rate of this species is slow, and because of this, the energy intake during growth and lactation is also comparatively low.     

Energy intake and weight gain of very low birthweight babies fed raw expressed breast milk.

The energy intake and weight gain of low birthweight infants (under 1500 g) fed expressed breast milk were measured. Between the second and fourth weeks of life the mean energy intake was 577 kJ (138 kcals)/kg/day and the mean weekly increase in weight 119 g/week. Feeding energy-rich hind milk to two babies increased their energy intake but had little effect on their rate of weight gain. There appeared to be no correlation between energy intake and weight gain, probably owing to variation in the absorption of nutrients from expressed breast milk. This study forms a basis for a comparison of weight gain in babies fed alternative regimens of artificial milks.     

Nutritional management of multiple pregnancies.

Good nutrition is central to achieving optimal fetal growth and development, and to maintaining maternal health and wellbeing. For multiple pregnancies, there is a growing body of evidence linking good maternal nutrition and weight gain to positive perinatal outcomes including reduced incidence of low birthweight and very low birthweight infants. Nutrition intervention appears to have important clinical implications for infant morbidity and mortality. Nutritional counselling can help women improve and maximise their intake of "at risk nutrients" such as iron, folate, essential fatty acids, calcium and ensure adequate energy intake to meet weight gain targets. Recommended optimal weight gains range from 16-20 kg for twin pregnancies. Expert dietetic guidance is often warranted, especially for women expecting triplets or quadruplets.     

Protein Intake and Plasma Amino-acids of Infants of Low Birth Weight

The plasma amino-acid levels in infants of low birth weight fed on expressed human milk and on a proprietary breast-milk substitute, S26, with a protein intake of not more than 4·5 g/kg/day were compared with those in infants fed on an evaporated milk formula whose protein intake ranged from 6·15 to 12·3 g/kg/day, as well as with normal infants on normal feeds and protein intake. In general, there was little difference between the levels in infants of low birth weight and in normal infants on the same protein intake. The five infants of low birth weight on high protein intake had generally higher levels of plasma amino-acids compared with the group on the lower protein intake, and in particular the levels of tyrosine, phenylalanine, methionine, and cystathionine could be extremely high. Apart from methionine these high levels may be the result both of a reduction in activity of the enzymes involved in the metabolism of these amino-acids, due to the immaturity of the infant, and of the increased stress of a high protein intake. In view of a possible long-term effect of abnormally high plasma amino-acid levels it is suggested that the protein intake of infants of low birth weight should not exceed 6 g/kg/day.     

A comparison of the estimates of whole-body protein turnover in parenterally fed neonates obtained using three different end products

Protein turnover rates in neonates have been calculated largely by measuring urinary [15N]urea enrichment following administration of [15N]glycine. Although ammonia has been increasingly recognized as an end product of nitrogen metabolism, in neonates it yields a different estimate of protein turnover than does urea. Comparisons of ammonia and urea end products in parenterally fed neonates have not previously been reported. A third and independent way of estimating protein turnover, developed for adults, is to use breath 13CO2 as an end product following administration of [1-13C]leucine. We therefore carried out simultaneous measurements of protein turnover in 10 parenterally fed neonates, using the three end products. The infants were clinically stable, weighed 2.6 +/- 0.2 kg, and received 3.1 +/- 0.2 g.kg-1.d-1 of amino acid, 2.2 +/- 0.1 g.kg-1.d-1 of lipids, and an energy intake of 90 +/- 4 kcal.kg-1.d-1 (1 kcal = 4.186 kJ). The turnover estimates derived from the 13CO2 and [15N]urea end products were very similar. The [15N]ammonia end product produced values approximately 66% (p less than 0.01) of the other two. We conclude that the ammonia and urea end products probably originate in different precursor pools. The similarity of the urea and breath carbon dioxide results helps validate the use of the urea end product in studying the nitrogen metabolism of parenterally fed neonates. Ideally in future studies two or more end products should be used, since they provide information about different aspects of the neonates' protein metabolism.     

New gestational weight gain guidelines: an observational study of pregnancy outcomes in obese women

In 2009, the Institute of Medicine (IOM) revised their pregnancy weight gain guidelines, recommending gestational weight gain of 11–20 pounds for women with prepregnancy BMI >30 kg/m². We investigated the potential influence of the new guidelines on perinatal outcomes using a retrospective analysis (n = 691), comparing obese women who gained weight during pregnancy according to the new guidelines to those who gained weight according to traditional recommendations (25–35 pounds). We found no statistical difference between the two weight gain groups in infant birth weight, cesarean delivery rate, pregnancy‐related hypertension, low birth weight infants, macrosomia, neonatal intensive care unit admissions, or total nursery days. Despite showing no evidence of other benefits, our data suggest that obese women who gain weight according to new IOM guidelines are no more likely to have low birth weight infants. In the absence of national consensus on appropriate gestational weight gain guidelines, our data provide useful data for clinicians when providing evidence‐based weight gain goals for their obese patients.     

Production and composition of Iberian sow's milk and use of milk nutrients by the suckling Iberian piglet

Sixteen purebred Iberian (IB) sows were used in two consecutive trials to determine the efficiency of conversion of sow's milk into piglet body weight (BW) gain and the relationship between milk protein and body protein retention and between milk energy yield and body energy retention in the nursing IB piglet. In each trial, four sows were selected in order to evaluate their milk production, litter growth and nutrient balance measurements, together with four additional sows for milk sampling. Litter size was equalized to six piglets. Daily milk yield (MY) was determined weekly by the weigh-suckle-weigh technique over a 34-day lactation period. Piglets were weighed individually at birth and then weekly from day 5 of lactation. Milk samples were collected on days 5, 12, 19, 26 and 34 post partum. The comparative slaughter procedure was used to determine piglet nutrient and energy retention. One piglet from each litter was slaughtered at birth and four on the morning of day 35. Total MY was on average 5.175 ± 0.157 kg/day. The average chemical composition (g/kg) of the milk was 179 ± 4 dry matter, 53.4 ± 1.0 CP, 58.5 ± 3.8 fat, 10.4 ± 0.3 ash and 56.9 ± 2.3 lactose. Milk gross energy (GE) was 4.626 ± 0.145 MJ/kg. Milk intake per piglet tended to increase in trial 2 (832 v. 893 g/day; P = 0.066). Piglet BW gain contained (g/kg) 172.1 ± 1.3 protein, 151.5 ± 3.5 fat, 41.4 ± 0.6 ash and 635 ± 3 water and 10.127 ± 0.126 MJ GE/kg. Throughout the 34-day nursing period, the piglets grew at an average rate of 168 ± 3 g/day. The ratio of daily piglet BW gain to daily MY was 0.195 ± 0.002 g/g and the gain per MJ milk GE intake was 41.9 ± 0.5 g/MJ. The overall efficiency of protein accretion (g CP gain/g CP milk intake) was low and declined in trial 2 (0.619 v. 0.571; P = 0.016). Nutrient and energy deposition between birth and weaning were 27.4 ± 0.5 g/day protein, 24.2 ± 0.8 g/day fat and 1615 ± 40 kJ/day energy. Piglet energy requirements for maintenance were 404 kJ metabolizable energy (ME)/kg BW0.75. ME was used for growth with a net efficiency of 0.584. These results suggest that poor efficiency in the use of sow's milk nutrients rather than a shortage in milk nutrient supply might explain the low growth rate of the suckling IB piglet.     

Effect of short-term energy intake level and exercise on oxygen consumption in men

The influence of short-term energy intake and cycle exercise on oxygen consumption in response to a 1.5 MJ test meal was investigated in ten young, adult men. On the morning after a previous day's "low-energy" intake (LE regimen) of 4.5 MJ, the mean resting oxygen consumption increased by 0.7 ml X kg-1 X min-1 after the test meal (P less than 0.025). After a "high-energy" intake (HE regimen) of 18.1 MJ, the resting measurement was unchanged (+0.4 ml X kg-1 X min-1) after the meal (n.s.). These trends are the reverse of what would be expected if oxygen consumption in response to feeding is a factor in the acute control of body weight. The mean fasting oxygen consumption during cycle exercise at 56% of VO2max (constant work) for both LE and HE prior intakes was not different at 31.1 ml X kg-1 X min-1. Oxygen consumption during exercise increased after feeding by 0.5 ml X kg-1 X min-1 on the LE regimen (n.s.) and decreased by 1.2 ml X kg-1 X min-1 on the HE regimen (n.s.). These results are also the reverse of what would be expected if oxygen consumption in response to exercise is related to short-term energy intake.     

Maternal care and infant development in <Emphasis Type="Italic">Callimico goeldii</Emphasis> and <Emphasis Type="Italic">Callithrix jacchus</Emphasis>

Callimico goeldii gives birth to single offspring, whereas other callitrichids, including Callithrix jacchus, twin. This study compares maternal effort and infant development in C. goeldii and C. jacchus; it is the first study to look at nursing frequency. Infants were observed from birth for 7 weeks in two captive groups each of C. goeldii and C. jacchus. C. goeldii mothers physiologically invested the same or less than C. jacchus mothers. C. goeldii mothers gained the same amount of weight during pregnancy in absolute terms as did the smaller C. jacchus. This results in a smaller gain in proportion to maternal weight but an equivalent proportional gain on a per fetus basis. C. goeldii mothers nursed their infants less based on duration of nursing bouts compared with C. jacchus mothers. C. goeldii mothers transported their infants exclusively through the first 2 weeks of life, which is longer than C. jacchus mothers, who exclusively transported infants only during the first week of life. As maternal infant carriage declined, other group members transported offspring in both species. C. goeldii infants engaged in independent locomotive sequences later in development and tasted solid foods less frequently than C. jacchus infants when compared at equivalent ages. A single, opportunistic milk sample obtained from a C. goeldii mother when her infant was 48 days old indicates that C. goeldii milk contains gross energy from crude protein within the range of variation observed in Callithrix milk. Despite the similarities in milk quality and prenatal effort in individual fetuses, C. goeldii infants gain weight faster from 0 to 18 months than do C. jacchus infants. A reduction in litter size allows C. goeldii mothers to spend more time carrying their infant and to delay weaning, thereby allowing accelerated infant and juvenile growth rates compared with C. jacchus.     

Glucose polymer supplementation of feeds for very low birthweight infants.

The feeds of 14 very low birthweight infants (birth weight less than 1500 g) were supplemented with a glucose polymer (Caloreen) at the rate of 6 g/kg body weight daily. Seven day periods of supplementation were alternated with seven day periods of normal feeding. Adding the glucose polymer significantly increased the rate of weight gain in these infants from 105 g/week to 140 g/week; growth rates in terms of length and head circumference were not affected. No adverse effects were noted. Glucose polymer is a useful energy supplement for very low birthweight infants.     

Rapid Postnatal Weight Gain and Visceral Adiposity in Adulthood: The Fels Longitudinal Study

Rapid infant weight gain is associated with increased abdominal adiposity, but there is no published report of the relationship of early infant growth to differences in specific adipose tissue depots in the abdomen, including visceral adipose tissue (VAT). In this study, we tested the associations of birth weight, infant weight gain, and other early life traits with VAT, abdominal subcutaneous adipose tissue (ASAT), and other body composition measures using magnetic resonance imaging (MRI) and dual‐energy X‐ray absorptiometry in middle adulthood (mean age = 46.5 years). The sample included 233 appropriate for gestational age singleton white children (114 males) enrolled in the Fels Longitudinal Study. Multivariate‐adjusted general linear models were used to test the association of infant weight gain (from 0 to 2 years), maternal BMI, gestational age, parity, maternal age, and other covariates with adulthood body composition. Compared to infants with slow weight gain, rapid weight gain was associated with elevated risk of obesity (adjusted odds ratio = 4.1, 95% confidence interval = 1.4, 11.1), higher total body fat (+7 kg, P = 0.0002), percent body fat (+5%, P = 0.0006), logVAT mass (+0.43 kg, P = 0.02), logASAT mass (+0.47 kg, P = 0.001), and percent abdominal fat (+5%, P = 0.03). There was no evidence that the increased abdominal adipose tissue was due to a preferential deposition of VAT. In conclusion, rapid infant weight gain is associated with increases in both VAT and ASAT, as well as total adiposity and the risk of obesity in middle adulthood.     

Adrenalectomy increases norepinephrine turnover in brown adipose tissue of obese (ob/ob) mice

The hyperphagia and rapid body weight gain normally observed in young obese (ob/ob) mice were abolished by removal of their adrenal glands, whereas food intake and weight gain of lean mice were not significantly affected by adrenalectomy. Adrenalectomy lowered body energy density (kcal/g carcass) in obese mice more than could be attributed to reduced food intake per se, suggesting that their energy expenditure was also increased. In control obese mice, low stimulation of brown adipose tissue by the sympathetic nervous system, as indicated by the low fractional rates of norepinephrine (NE) turnover in their brown adipose tissue may have contributed to the reduced energy expenditure in these animals. Adrenalectomy increased the rates of NE turnover in brown adipose tissue of obese mice to rates nearly equal to those observed in lean mice without affecting NE turnover in this tissue of lean mice. Likewise, removal of the adrenals normalized the low rates of NE turnover in hearts of obese mice without affecting lean mice. Rates of NE turnover in two other organs, white adipose tissue and pancreas, of control and adrenalectomized obese mice were similar to rates observed in lean counterparts. The adrenal may thus contribute to both the hyperphagia and the low energy expenditure by brown adipose tissue that together cause gross obesity in ob/ob mice.     

Relationship between milk energy intake and growth rate in suckling mammalian young at peak lactation: an updated meta-analysis

The milk energy intakes and growth rates of suckling young at peak lactation of 62 mammalian species and subspecies, all measured using either the weigh–suckle–weigh, the isotope dilution or the isotope transfer method, were evaluated. The mean daily gross energy intakes (GEI) were as low as 12 kJ in a small rodent (mouse) to as high as 249 MJ in a phocid seal (hooded seal Cystophora cristata ), while the daily growth rates at peak lactation ranged from 0.4 to 5.9 kg. Several allometric equations were calculated to explore the relationships between gross energy intake via milk and body weight (BW), growth rate and BW, as well as between gross energy intake via milk and growth rate. The results suggest that both GEI via milk and growth rates are proportional to BW to the power of 0.82. Accordingly, the metabolic weight of suckling mammalian young should be expressed as BW^0.82. The predictive values of the calculated equations indicate that suckling young at peak lactation consume c . 883 kJ day⁻¹ kg⁻¹ BW^0.82 and have growth rates of 32 g day⁻¹ kg⁻¹ BW^0.82. However, large deviations for some species and few outliers were found. These equations could be used to predict values for species that have not been studied, provided that the BW falls within the range of weights used to derive the equations.     

Size at birth, postnatal growth and risk of obesity

Epidemiological studies over the last 15 years have shown that size at birth, early postnatal catch-up growth and excess childhood weight gain are associated with an increased risk of adult cardiovascular disease and type 2 diabetes. At the same time, rising rates of obesity and overweight in children, even at pre-school ages, have shifted efforts towards the identification of very early factors that predict risk of subsequent obesity, which may allow early targeted interventions. Overall, higher birth weight is positively associated with subsequent greater body mass index in childhood and later life; however, the relationship is complex. Higher birth weight is associated with greater subsequent lean mass, rather than fat mass. In contrast, lower birth weight is associated with a subsequent higher ratio of fat mass to lean mass, and greater central fat and insulin resistance. This paradoxical effect of lower birth weight is at least partly explained by the observation that infants who have been growth restrained in utero tend to gain weight more rapidly, or 'catch up', during the early postnatal period, which leads to increased central fat deposition. There is still debate as to whether there are critical early periods for obesity: does excess weight gain during infancy, childhood or even very early neonatal life have a greater impact on long-term fat deposition and insulin resistance? Early identification of childhood obesity risk will be aided by identification of maternal and fetal genes that regulate fetal nutrition and growth, and postnatal genes that regulate appetite, energy expenditure and the partitioning of energy intake into fat or lean tissue growth.     

Structural abnormalities of the Y chromosome and abnormal external genitals

Summary Three infants with different types of Y-chromosome abnomalies, including short- and/or long-arm deletion and mosaicism, are reported. The karyotypes of these patients were: 45,X/46,X,del(Y)/47,X,del(Y), del(Y) on peripheral lymphocytes and 45,X/46,X, del(Y) on gonadal tissue (case 1), 45,X/46,X,del(Y) (case 2), and 45,X/46,X,r(Y) (case 3). In case 1 the euchromatic segment on the deleted Y was distinctly larger than that of the father's Y.The three infants had no gross phenotypic anomalies except ambiguous genitals and low birth weight, and they were small for date. The histologic diagnosis in two of them was mixed gonodal dysgenesis (cases 1 and 2).The relationship between structural abnormalities of the Y chromosome and ambiguous genitals as well as male-determining factors is discussed.     

Albumin synthesis in preterm infants on the first day of life studied with [1-13C]leucine

Albumin is the major binding protein in the human neonate. Low production of albumin will lower its transport and binding capacity. This is especially important in preterm infants, in whom albumin binds to potentially toxic products such as bilirubin and antibiotics. To study the metabolism of plasma albumin in preterm infants, we administered a 24-h constant infusion of [1-(13)C]leucine to 24 very low birth weight (VLBW) infants (28.4 +/- 0.4 wk, 1,080 +/- 75 g) on the first day of life. The caloric intake consisted of glucose only, and therefore amino acids for albumin synthesis were derived from proteolysis. The fractional synthesis rate (FSR) of plasma albumin was 13.9 +/- 1.5%/day, and the absolute synthesis rate was 148 +/- 17 mg x kg(-1) x day(-1). Synthesis rates were significantly lower (P<0.03) in infants showing intrauterine growth retardation. Albumin synthesis increased with increasing SD scores for gestation and weight (P<0.05). The FSR of albumin tended to increase by 37% after administration of antenatal corticosteroids to improve postnatal lung function (P=0.09). We conclude that liver synthetic capacity is well developed in VLBW infants and that prenatal corticosteroids tend to increase albumin synthesis. Decreased weight gain rates in utero have effects on protein synthesis postnatally.     

The influence of birth timing upon infant growth and survival in Captive Rhesus Macaques (Macaca mulatto)

Weights, growth rates, and mortality data of 815 captive-born Macaca mulattainfants were studied to determine if date of birth influences infant growth and survival. The six groups studied displayed a unimodal spring-summer birth season that has become systematically more restricted since 1977. Males exhibited higher rates of stillbirth and neonatal death and were more frequently born outside the normal birth season, when infant mortality was more common. Within the normal birth season, infant weight increased linearly with birth date, and infant growth rate declined linearly with birth date. Female infants with weights and growth rates near the developmental norm, especially those born in the middle of the birth season, have the greatest probability of survival. Males are more likely to survive if their weights and growth rates exceed the developmental norm, and thus male infants might be initially more costly to produce than female infants. These results are inconsistent with the hypothesis that offspring of high-ranking males, which conceived predominantly in the first third of the breeding season, enjoy a selective advantage.     

Three months energy restricted diet does not reduce peripheral insulin resistance in newly diagnosed non insulin dependent diabetics

Sixteen newly diagnosed non insulin dependent diabetic patients were treated for 3 months with an individual energy restricted diet. The effect on weight, hyperglycaemia and insulin response to oral glucose was measured in all subjects, and in 7, peripheral insulin resistance was estimated using a hyperinsulinaemic glucose clamp at two insulin infusion rates (40 and 400 mU m-2 X min-1). After diet, fasting plasma glucose fell from 12.0 +/- 0.7 mmol/l (mean +/- SEM) to 7.4 +/- 0.5 mmol/l (P less than 0.001) and weight fell from 92.9 +/- 4.2 kg to 85.0 +/- 3.1 kg (P less than 0.001). The plasma insulin response to oral glucose was unchanged after diet therapy. Insulin induced glucose disposal (M) was also unaffected by diet at insulin infusion rates of 40 mU m-2 X min-1 (12.5 +/- 1.5 mumol X kg-1 X min-1 vs 15.7 +/- 1.6 mumol X kg-1 X min-1) and 400 mU m-2 X min-1 (49.5 +/- 2.7 mumol X kg-1 X min-1 vs 55.1 +/- 2.5 mumol X kg-1 X min-1). These results show that 3 months reduction of energy consumption with weight loss in newly diagnosed non insulin dependent diabetics improves B-cell responsiveness to glucose but has no effect on liver glucose output or on peripheral insulin action.