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Department of Nutritional Sciences, University of Connecticut, Storrs, Connecticut
Address reprint requests to: Nancy R. Rodriguez, PhD, RD, Department of Nutritional Sciences, Unit 4017, University of Connecticut, Storrs, CT 06269-4017. E-mail: nancy.rodriguez{at}uconn.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONBACKGROUNDDESCRIPTION OF SUBJECTCONCLUSIONREFERENCES |
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Key words: protein, children, energy, nitrogen balance, amino acids
Abbreviations: branched chain amino acids = BCAA • calorie to nitrogen ratio = C:N • direct amino acid oxidation = DAAO • dietary reference intakes = DRIs • indirect amino acid oxidation = IAAO • nitrogen intake = NI • nitrogen output or excretion = NO
Key teaching points:
• Population specific-data are lacking regarding protein needs of young children. Recommendations for protein intake for growing children have evolved from nitrogen balance studies. Given the additional information garnered from metabolic tracers, it is reasonable to consider an evaluation of current recommendations using contemporary isotope methodology to assess protein utilization in young boys and girls.
• Metabolic reactions involving protein, a nutrient essential to growth, are energy independent. Therefore the relationship between energy balance and protein utilization is an important consideration in pediatric diet design.
• An understanding of the effects of weight management interventions and routine exercise on whole body protein turnover in boys and girls provides a foundation for development of age-appropriate nutritional recommendations for protein intake.
• Nutrient dense foods that are good sources of high quality protein should be part of diet interventions aimed at reducing body weight and improving health in young children.
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONBACKGROUNDDESCRIPTION OF SUBJECTCONCLUSIONREFERENCES |
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BACKGROUND |
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TOPABSTRACTINTRODUCTIONBACKGROUNDDESCRIPTION OF SUBJECTCONCLUSIONREFERENCES |
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For children, protein allowances were traditionally considered to be the amount of protein needed for sustaining positive nitrogen balance and therefore growth [4,6]. As of late, protein requirements have begun to encompass outcomes other than growth (i.e., immune function, behavior) and a more critical evaluation of protein requirements for children have been recommended [3,7,8]. While the exact methodology employed in establishing protein requirements for children is outside the scope of this review, a thorough description of the approach taken to formulate the 1985 recommendations is found in a position paper by Dewey and colleagues [4]. Although nitrogen balance is considered a classic traditional methodology for assessing protein status, modern techniques, that provide additional insight into protein turnover, are more commonly used in research today. The remainder of this review will consider the application of these contemporary approaches to the assessment of protein utilization in children and the potential for this information to impact future recommendations regarding protein intakes, as well as protein sources, for this population.
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DESCRIPTION OF SUBJECT |
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TOPABSTRACTINTRODUCTIONBACKGROUNDDESCRIPTION OF SUBJECTCONCLUSIONREFERENCES |
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Contemporary Approach to Protein Needs of Young Children
Furst and Stehle [5] have suggested that recommended protein intakes be defined by investigations based on the relationship between nitrogen intake, nitrogen balance, and amino acid oxidation, as well as protein deposition and the metabolic needs to maintain body nitrogen or essential amino acid oxidation. Certainly scientific investigations exploring the relationship between nitrogen utilization and amino acid metabolism in the context of growth should be undertaken in children to truly establish protein needs specific to this population. Stable isotope methodology provides an alternative to nitrogen balance for assessing protein and amino acid utilization in healthy children and these techniques can be practically applied to studies in young children [8].
The criticism of nitrogen balance as the basis for establishing protein requirements is not new [14,15]. Currently, there are several techniques that can be employed using stable isotopes for the purpose of evaluating amino acid utilization in the context of protein requirements [8]. Pencharz and Ball [8] recently reviewed data available from a number of published studies in an effort to resolve discrepant findings regarding protein needs based on either nitrogen balance studies or amino acid carbon oxidation findings. According to the authors [8], amino acid requirements should be based on studies for which graded levels of the test amino acid are fed and a distinct change is noted in a related biological factor given a particular research methodology. In theory, similar estimates should be obtained for protein requirements regardless of the method employed. The methods used in the studies cited were nitrogen balance and growth, direct amino acid oxidation (DAAO) or plasma amino acid level, and indicator amino acid oxidation (IAAO) or plasma urea level [8]. All methods were evaluated relative to graded intakes of an essential amino acid and metabolic responses noted.
To summarize their findings [8], nitrogen balance and growth increased progressively with increasing amino acid intake until the point at which the requirement was met for the test amino acid. At this point, there is a plateau in nitrogen balance and growth. With DAAO, there was no change in DAAO as graded levels of the essential amino acid were fed below the requirement, but when the requirement was met a linear increase in amino acid oxidation was noted. The final response was plasma urea or IAAO. These outcome parameters decreased as graded levels of amino acids are fed below the requirement. When the requirement was reached, there was no change as the response leveled off. The primary observation was that the point at which the aforementioned changes occurred in the responses (i.e., nitrogen balance and growth, DAAO, IAAO, and plasma urea), the amino acid requirement was met. For this particular exercise, similar estimates in essential amino acid requirements or recommended intakes resulted regardless of method used when five or more graded levels of a test amino acid was fed [8]. Indeed, investigations employing both traditional and contemporary methodology to evaluate protein-related metabolic responses may be the best approach to delineating protein needs of young children.
While there have been a number of well-designed studies targeted at evaluating current protein requirements in adults, only limited studies have been conducted in children using stable isotope techniques. Mager and coworkers [7] implemented these modern stable isotope techniques (IAAO) to further characterize amino acid requirements in children. In brief, these researchers sought to determine the branched chain amino acid (BCAA) requirements in children using IAAO since nitrogen balance studies, the primary premise for the current Dietary Reference Intakes (DRIs) for children, underestimated BCAA requirements for adults. They found the mean BCAA requirement to be 48% higher than the current DRI recommendations as determined by IAAO. This observation is intriguing and provides an impetus for further exploration of protein requirements for children using noninvasive tracer techniques.
Role of Energy Balance in Optimal Protein Utilization
Protein related metabolic reactions are energy dependent [11]. Indeed, the role of energy balance in the optimal use of protein is well established [4,6,161617]. This relationship is critical to efficient deposition of protein in growing children and should not be overlooked in evaluation of protein utilization by, or protein needs of, young children [4,11]. Since the nation is currently facing a pediatric obesity epidemic, a variety of intervention strategies are being implemented in an effort to improve the health of our nation’s youth [1]. Unfortunately, little is known regarding the impact of weight management and physical activity interventions on protein utilization in young children.
We have employed contemporary, noninvasive stable isotope methodology to evaluate protein utilization in children participating in a variety of diet and exercise interventions in our laboratory [18–22]. Although our studies were not directed at protein requirements or dietary adequacy per se, the interventions did represent current public health initiatives to manage body weight and increase physical activity in young boys and girls. Outcome measures represented both traditional (i.e., nitrogen balance) and contemporary (protein turnover via isotope modeling) assessments of protein utilization in healthy obese and non-obese boys and girls.
Considerations for Weight Management.
We conducted a study in healthy, obese children consuming reduced energy diets [19]. The investigation employed noninvasive stable isotope methodology to evaluate the effect of reduced energy intake on protein turnover in obese boys and girls 8–10 years of age. All children had significant reductions in total body weight and fat mass which were accompanied by a slight decrease in fat free mass. The diet-induced negative energy balance was associated with a downregulation of protein turnover as protein synthesis and nitrogen flux both decreased following the weight loss intervention [19]. From a clinical perspective, a downregulation in protein turnover may have adverse effects in growing children. This downregulation in protein turnover was associated with a less positive nitrogen balance. Theoretically, a reduced rate of protein turnover may indicate decreased sensitivity to changes in cellular and tissue environments that might compromise responsiveness to physiological stimuli associated with growth processes. However, the lack of data concerning the acceptable limits for rates of protein turnover in children, coupled with what is not known regarding protein needs in growing children, limit conclusions that can be made from these data. Since protein intake appeared to be adequate in these children (
1.2 g/kg/d), these findings indicate that caution should be taken in prescribing severe energy restricted diets in growing children [22].
Impact of Routine Exercise.
Because the interventions for obesity management should include a component that focuses on increasing energy expenditure, we sequentially added programmed exercise to the weight management intervention used in the study above. After six weeks on a reduced energy diet, five children maintained the same low energy intake while participating in six weeks of programmed walking (45 to 50 minutes, 5 days/wk). All outcome measures were the same and there was a significant increase, or upregulation, in protein turnover and nitrogen balance was maintained in response to the addition of an exercise component [20].
Because six weeks of programmed exercise significantly impacted protein utilization in healthy obese children, we executed a similar protocol to characterize protein-related metabolic responses in normal weight boys and girls [18]. Unlike their obese counterparts, exercise decreased protein turnover in normal weight boys and girls. We extended this work to include a resistance training component in a similar population of children and again observed a downregulation in protein turnover following six weeks of resistance exercise [21]. While the downregulation following weight loss in obese children was associated with a decreased, or less positive, nitrogen balance, the reduction in protein turnover observed in response to six weeks of either programmed walking or resistance exercise in normal weight children was associated with an increase, or more positive, nitrogen balance. The relevance of these findings to this review is that all interventions (i.e., reduced energy intake, increased physical activity), regardless of study population, impacted protein utilization in children. The question remains as to whether programmed exercise in children has long-term effects on overall growth by acutely impacting protein utilization.
Energy Balance and Protein Utilization.
The significant changes noted in protein metabolism in these studies suggest an energy-based downregulation in whole-body protein turnover. Obese children in the study conducted by Ebbeling [20] were in an established state of negative energy balance prior to initiation of the exercise program. Walking appeared to impart a benefit to protein utilization in obese children on a weight loss regimen by increasing, or upregulating, protein turnover. This response occurred in the presence of no change in energy balance or physical activity-related energy expenditure. However, the fact that a downregulation of protein turnover was noted with six weeks of hypoenergetic therapy (i.e., induction of negative energy balance) [19] provides insight into a possible energy-related mechanism for the decreases observed in parameters of protein utilization in the exercise only studies. That is, one factor common to these studies is the coexistence of negative energy balance. While negative energy balance was established via dietary intervention in the obese children, negative energy balance is likely to have occurred in the exercise studies with normal weight children subsequent to an increase in energy expenditure without a concomitant increase in energy intake. By simple difference, a state of negative energy balance could have resulted from the increase in energy expenditure imparted during the aerobic (walking) or resistance training interventions since energy intake did not change throughout the exercise interventions.
It is important to note that growth took precedence over the short-term or acute energy deficits that might have occurred in response to programmed exercise. Similarly, there was a significant increase in linear height in the obese children following the reduced calorie and exercise intervention studies. Without question, these findings emphasize the importance of adequate energy to efficient protein use by growing children and justify future studies that will significantly contribute to the understanding of optimal energy intake and protein nutriture in young boys and girls.
Protein Sources
Given the prevalence of pediatric obesity it can be difficult to embrace the concept of ensuring sufficient energy in the diets of growing children for optimal protein utilization and deposition. That is why protein source and protein quality become important factors in pediatric diet designed for today’s child. The concept of nutrient density has become increasingly significant in efforts to provide optimal nutrition for children without contributing to excessive calorie consumption [23,24]. Pediatricians, pediatric nurse practitioners, and registered dietitians are promoting healthy eating that does not emphasize highly restrictive diets combined with increased physical activity to manage overweight in children [25]. High biological value protein can be found in animal products which are also significant sources of a variety of essential micronutrients. Dairy products, eggs, and other animal proteins such as beef, fish, and poultry, can contribute significantly to meeting a child’s protein needs for a reasonable amount of energy.
An analysis of beverage intake between 1977 and 2001 across the lifecycle shows that sweetened beverage consumption increased and milk consumption decreased for all age groups. Calories from milk decreased by almost 40% while total energy intake increased almost 300 kcal/d [26]. A presentation of the poor food choices that are currently being made by young children and by parents for their children is not within the purview of this review. However, there is little question that the diets of America’s children could be improved in terms of diet quality and that protein quality is an easy place to start. Table 1 shows the energy to nitrogen ratio (E:N) of several foods and highlights higher quality protein sources, many of which can be easily incorporated into healthy menu options for young children. There are essentially no studies on protein quality or source on protein utilization in young children. Given that there are studies demonstrating a benefit of consuming animal based proteins to protein utilization in adults [27], a similar line of investigation in growing children is justified.
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CONCLUSION |
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0.76–0.77 g protein/kg/d for boys and girls 7–10 yrs of age). Future investigations, simultaneously employing traditional and contemporary protein assessment methodologies in children are warranted to determine the amount of protein needed for optimal growth and development. The value of high quality protein sources in the diets of young children should be evaluated and pediatric diet design should incorporate quality protein from nutrient dense foods, with consideration for appropriate energy intake. Finally, there is likely merit in investigating the value of dietary guidelines specifically directed at children.
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