EDITORIAL Indirect Calorimetry: The Search for Clinical Relevance Nutrition support is directed toward treating malnutrition, yet we still have not satisfactorily answered the question, &dquo;What is malnutrition?&dquo; in the hospital patient. Weight loss is the most common image of malnutrition in the mind of the physician as well as the lay public. Weight loss can occur from inadequate intake alone or from an inadequate intake of food with a catabolic drive associated with disease or injury. In the former case, the intake of food is curative, whereas in the latter case, food intake can only be supportive. In strongly catabolic patients, the tissue loss usually continues despite food intake, although the food can minimize the rate of depletion. The article by McClave and Snider is a useful overview of indirect calorimetry that provides an important perspective for all health professionals interested in clinical nutrition as well as for those directly involved in measuring indirect calorimetry at the bedside. This article provides 83 references, nearly all of them published since 1980. This selection of references reflects the relatively recent growth of interest in bedside calorimetry. The level of hypermetabolism is often considered to be a measure of the degree of catabolic influence. Yet, the proper role of indirect calorimetry in nutrition support has not been fully defined. The history of shifting attitudes toward calorimetry can be of help in understanding our present level of knowledge and uncertainty concerning the nature of energy expenditure and the significance of the resting energy expenditure (REE) in clinical practice. CALORIMETRY-BACKGROUND
Some authors have identified the beginning ofnutrition as a scientific discipline with the development of calorimetry in the second half of the 18th century. Yet, it was a century later when the quantitative measurement of energy expenditure was developed in the German laboratories of Voit and Rubner. Calorimetry in the United States was the direct descendant of this German leadership, with the work ofAtwater, Benedict, Lusk, and DuBois. Their work from 1890 to 1930 represented a golden age of calorimetry in relation to nutrition. Thereafter, the science of nutrition was dominated by the biochemistry of enzymes and vitamins, and moved away from an interest in calorimetry. Dubois focused interest on the basal metabolic rate (BMR), particularly on the influence of fever on the basal metabolic rate (BMR).2 The major clinical result of these studies was the application of the BMR to the management of thyroid disease. Most hospitals maintained special facilities for BMR measurements until 1950, when they were abandoned in favor of chemical measurements for evaluating thyroid function. During the period from 1950 to 1980, it was not possible to obtain any measurement of energy expenditure in most hospitals. This situation was discussed in a conference in 1979, when a commercial device had just been introduced for the bedside measurement of gas exchange.3 There was much curiosity and uncertainty about the relevance of such measurements in clinical care. Since 1980, an increasing number of commercial instruments have been introduced for the bedside measurement of gas exchange, with a correassociated with sponding rise in reports of the REE 4 different clinical conditions.4 many
The
history of nutrition is interwoven with our understanding of both energy and heat. It is no accident that food intake is spoken of as the intake of caloriesa unit of heat. General interest in this subject can be traced back to early humans, who recognized that the living body was warm and began to cool at the time of
MEASUREMENT OF INDIRECT CALORIMETRY
McClave and Snider have properly emphasized the
importance of attention to details, particularly instrument calibration, when performing the measurement of gas
exchange. One may generalize that accurate measurements in the gas phase are usually more difficult to achieve than are comparable measurements in the liquid phase. A variety of indirect methods have been used to assess energy expenditure without the usual measurements involved in calorimetry. These have been based upon physiologic measurements such as ventilation or heart rate, a measurement of REE by gas exchange plus the energy cost of activity estimated by a diary, a measurement of REE plus the output of an activity
death. There, life became defined as animal or &dquo;vital&dquo; heat. This concept of vital heat came to involve vital &dquo;humors,&dquo; which dominated medical thought for 1500 years, perhaps one of the most influential and misleading doctrines in medical history.’
203
Downloaded from ncp.sagepub.com at UNIV OF NEW ORLEANS on June 6, 2015
204
meter, measurement
over
24 hours in
a
respiration
chamber, or the use of doubly labeled water for calculating the average co2 output over a period of 5 to 10 days. The assumptions and problems with each of these methods have been compared with those of conventional indirect calorimetry by Jequier et a1.5 Some of these methods are not applicable to the acutely ill or injured patient who might require nutrition support. Important information in certain clinical conditions may become evident from studies in respiration chambers or by the use of doubly labeled water. A respiration chamber is a room-sized enclosure where the gas exchange can be measured on a 24-hour basis while the subject or patient is sleeping, awake at rest, during and after meals, or during period of activity an a treadmill or cycle ergometer. The 24-hour energy expenditure of any particular normal individual is surprisingly constant; however, interpersonal variations are considerable. The isolation required for such measurements prevents their widespread use in hospitalized patients, and the expense of design and operation limits their availability to a few institutions. Information from chamber studies on the thermic effect of food and the energy demands of light exercise may shed new light on selected clinical situations during the next few yearns.6 The use of doubly labeled water for measuring energy expenditure in free-living animals was extended by Schoeller et al’to the study of human subjects in various conditions of normal life. The method involves administering a loading dose of water labeled with deuterium and l8o to a subject or patient and following the rate of excretion of both isotopes in urinary water. The difference in isotope excretion over 5 to 10 days represents the average co2 output per day, from which the total energy expenditure is estimated on the basis of an average respiratory quotient (RQ). The advantages are the use of a nonradioactive isotope, which allows the study of infants and pregnant women, and a simple and convenient method that also provides information on total body water and allows the simultaneous study of many individuals. The disadvantages are the expense and availability of the isotope and mass spectrometry. The method only provides information on co2 output with its greater potential error than when measuring o2 2uptake.’ Furthermore, it cannot indicate variations in energy expenditure from day to day. The method has been used to study parenteral nutrition,8,9 sports medicine&dquo; and bum patients.&dquo; Studies in various other forms of disease and injury are currently
underway.
ing the principles of indirect calorimetry and having a clear understanding of what is represented by the REE. The total energy expenditure (TEE) is usually exas kilocalories per unit of time, which is the sum of the following factors:
pressed
TEE BMR + TEF + ACT where TEF is the thermic effect of food. =
Hospital measurements of indirect calorimetry represent the BMR only if the measurement is carefully performed under postabsorptive conditions with the patient at rest in a thermoneutral environment. Hospital measurements of indirect calorimetry for nutrition purposes have commonly been made as the resting metabolic rate. This may differ from a true BMR measurement because it is made at any time during the waking day, after the individual has been at bedrest for perhaps 30 minutes. Another difference may be that the individual is not postabsorptive (no food since the previous evening) but is receiving nutrition such as total parenteral nutrition. The TEF and diurnal variations in basal energy expenditure are generally considered to represent less than 10% of the total energy expenditure, but information on TEF in clinical states is still very limited. Therefore, a resting measurement of energy expenditure should include information regarding the time of measurement and whether nutrients were being administered. McClave and Snider have reviewed the ranges of increase in REE found with various clinical conditions, including the extreme hypermetabolism of the severely burned patient. Clinical studies in the early 1970s showed a range of REE of up to +100% above the predicted normal in very large burns. A growing number of reports in the past decade indicate that the extent of bum hypermetabolism has substantially decreased. Modern burn care has made great strides that have reduced mortality and morbidity, alongwith less marked increases in REE. Burned patients were commonly treated during the 1960s and early 1970s with open exposure to normal ambient temperatures with surface antibacterial ointments for prolonged periods. The rationale was to allow the maximum opportunity for islands of deep second degree damage to regrow spontaneously if bacterial infection could be controlled. The radiative as well as the evaporative cooling of these patients was continuously increased to a variable degree. The severe hypermetabolism in such patients appeared to be some combination of obligatory increases in heat loss by uncontrolled evaporative losses through the burn surface together with increased heat
REE
production on internal organs as a result of strong and
The decade of the 1980s has produced both new instruments for the bedside measurement of gas exchange and many new products for nutrition support. The increasing availability of gas exchange measurements has placed additional emphasis on understand-
continued catabolic stimuli. In the past 20 years burn mortality has seen dramatic improvement. Resuscitation is usually prompt, pulmonary injury is recognized and treated, the environment is warmed, and excision of the burn surface is started soon after resuscitation, together with increased
Downloaded from ncp.sagepub.com at UNIV OF NEW ORLEANS on June 6, 2015
205
of closed dressing that sharply reduced evaporative cooling. With such modern techniques, the burned patient seldom has elevations of the REE of more than 70% above normal, and more often shows increases of only 30% to 50% above normal values.ll The REE of every patient represents a balance between increases due to catabolic influences and decreases due to whatever tissue depletion may have occurred. Just as improvements in burn care have reduced the extent of hypermetabolism currently seen, the improvement in nutrition support has reduced the incidence of extreme hypometabolism seen with advanced cachexia. The reduction in the overall range of REE seen in clinical conditions has led some physicians to question whether measuring energy expenditure continues to be as important as in previous years. Much remains to be learned about the proper interpretation of REE measurements. Yet any significant increase or decrease in the REE can be expected to be important as a &dquo;metabolic marker.&dquo; An example of this is related to the studies of DuBois and colleagues on fever. Measurement of the BMR in a variety of febrile conditions yielded an average increase in BMR of 13% for every degree centigrade of fever. Yet inspection of the original data reveals that the patients with chronic pulmonary tuberculosis had less increase in BMR per degree of fever (presumably secondary to weight loss and depletion) whereas the patients with acute typhoid fever had greater increases in BMR per degree of fever. The typhoid patients were found to be excreting unusually large amounts of nitrogen in the urine and were very difficult to put into positive nitrogen balance by increasing the nitrogen intake.l2
use
THE NONPROTEIN
RQ
Standard textbooks present the range of the nonprotein RQ as being from 0.71 when fatty acids are the tissue fuel and rising to 1.0 when carbohydrate is the tissue fuel. Values of up to 1.3 represent the conversion of additional carbohydrate to fatty acids. Some metabolic studies are tending to use the RQ to indicate the fuel being oxidized by a particular tissue or organ. It is important to remember that gas exchange is a whole body measurement, and thus an RQ of 1.0 does not necessarily mean that all tissues are burning carbohydrate. There is reason to believe that fat oxidation may be continuing to some degree in a tissue such as muscle at a time that lipogenesis is occurring elsewhere in the body and that the net RQ of these processes happens to be 1.0. Some clinicians have proposed that because carbohydrate has many assets as a metabolic fuel, total parenteral nutrition should be given in amounts sufficient to insure an RQ of 1.0 or higher. It is important to realize that sustained RQs of 1.2 to 1.3 can only be achieved with significant calorie excesses provided by a high carbohydrate, lipid-free intake. Until further
knowledge is available, it seems reasonable to provide nutrition support that strives at some balance between carbohydrates and lipid intake that will provide an RQ between 0.8 and 0.9. RELATIONSHIP OF ENERGY INTAKE TO NITROGEN BALANCE
An important aspect of energy balance is the mainsatisfactory amount and function of body protein. The sensitivity of nitrogen balance to the intake of energy as well as the intake of nitrogen has been recognized for many years, yet the mechanisms involved are poorly understood. These relationships have recently been reviewed by Young et al.s 6 (ppl39,439) There is reason to believe that the influence of energy intake on nitrogen balance is more evident when the energy is considered as energy balance rather than energy intake. The combination of data from various studies suggests how energy balance affects nitrogen balance.l3 The nitrogen retention produced by an increasingly positive calorie balance appears to vary from significant amounts in the depleted patient to essentially no nitrogen retention with an increasingly positive calorie balance in the acutely catabolic patent. 14 tenance of a
CLINICAL SIGNIFICANCE OF INDIRECT CALORIMETRY
Indirect calorimetry is a measurement of clinical physiology that is struggling to find its proper role in patient care. There is an intuitive feeling that a knowledge of energy expenditure should be important for managing individual patients, yet as McClave and Snider have pointed out, there is often a sense of frustration regarding just how to use such a measurement in treating an individual patient. When indirect calorimetry measurements are made in a heterogeneous population of hospital patients, the range ofREE will have a much wider distribution above and below the mean value than in a normal population. Even in patients with the same diagnosis, the range of values is much larger than with normal subjects. The clinical importance of this lies in the fact that even experienced clinicians may be unable to identify the
patients who are hypermetabolic versus those who are hypometabolic on the basis of the conventional bedside examination. There has been considerable difference of opinion as to whether normal values for REE should be presented in reference to some aspect of body composition such as fat-free mass. (The ideal segment of body composition should be some measure of the body cell mass because that is the energetically active portion of the body.) It seems reasonable to expect that future clinical measurements of REE will be expressed more commonly in relation to some aspect of body composition. This should be of greater importance in narrowing the distribution
Downloaded from ncp.sagepub.com at UNIV OF NEW ORLEANS on June 6, 2015
206
being reported in patient populations than in improving the already narrow distribution in normal subjects. At the present time, measurements of indirect calorimetry can help to define the metabolic behavior of the patient. However, a given measurement must be combined with other information to determine the degree of and need for nutrition support and the composition of nutrients that should be provided.33 A patient with an REE in the hypermetabolic range can be expected to have an accelerated loss of fat and lean tissue. A level of REE below normal suggests that the patient has already undergone significant tissue depletion or has some endocrine dysfunction. Nutrition support should be designed to achieve either calorie equilibrium for daily maintenance or a positive calorie balance in order to restore tissue. In either case, knowledge of the actual REE can avoid inappropriate calorie
administration. John Kinney, MDS Rockefeller University New York REFERENCES 1.
Kinney JM. Energy metabolism: heat, fuel and life. In: Kinney, JM, Jeejeebhoy KN, Hill GL, et al, eds. Nutrition and metabolism in patient care. Philadelphia, WB Saunders, 1988:3.
2. DuBois EF. Basal metabolism in health and disease. Philadelphia: Lea & Febiger, 1924. 3. Kinney JM (ed). Assessment of energy metabolism in health and disease. Columbus, OH: Ross Laboratories, 1980. 4. Kinney JM. Indirect calorimetry in malnutrition: nutritional assessment or therapeutic reference? JPEN 1987;11:90S-94S. 5. Jequier E, Acheson K, Schutz Y. Assessment of energy expenditure and fuel utilization in man. Am Rev Nutr 1987;7:187. 6. Kinney JM, Tucker HN, eds. Energy Metabolism: tissue determinants and cellular corollaries. New York: Raven Press, 1991:187. 7. Schoeller DA, Ravussin E, Schutz Y, et al. Energy expenditure by doubly labeled water: validations in humans and proposed
calculation. Am J Physiol 1988;250:R223. 8. Schoeller DA, Kushner RF, Jones PJH. Validation of doubly labeled water for measuring energy expenditure during parenteral nutrition. Am J Clin Nutr 1988;44:291. 9. Riumallo JA, Schoeller D, Barrera G, et al. Energy expenditure in underweight free-living adults: impact of energy supplementation as determined by doubly labeled water and indirect calorimetry. Am J Clin Nutr 1989;49:239. 10. Westerterp KR, De Boer JO, Saris WHM, et al. Measurements of energy expenditure using doubly labeled water. Int J Sports
Med 1984;5 (Suppl):74. 11. Goran MI, Peters EJ, Herndon DN, et al. Total energy expenditure in burned children using doubly labeled water technique. Am J Physiol 1980;259:E576-E585. 12. ColemanJ, DuBois RF:Clin Cal 7, Arch Intern Med 1915;15:887. 13. Goldstein SA, Elwyn DH. The effects of injury and sepsis on fuel utilization. Am Rev Nutr 1989;9:445-473. 14. Bursztein S, Elwyn DH, Askanazi J, et al. Energy metabolism, indirect calorimetry, and nutrition. Baltimore: Williams &
Wilkins, 1989:108.
Downloaded from ncp.sagepub.com at UNIV OF NEW ORLEANS on June 6, 2015
Some authors have identified the beginning ofnutrition as a scientific discipline with the development of calorimetry in the second half of the 18th century. Yet, it was a century later when the quantitative measurement of energy expenditure was developed in the German laboratories of Voit and Rubner. Calorimetry in the United States was the direct descendant of this German leadership, with the work ofAtwater, Benedict, Lusk, and DuBois. Their work from 1890 to 1930 represented a golden age of calorimetry in relation to nutrition. Thereafter, the science of nutrition was dominated by the biochemistry of enzymes and vitamins, and moved away from an interest in calorimetry. Dubois focused interest on the basal metabolic rate (BMR), particularly on the influence of fever on the basal metabolic rate (BMR).2 The major clinical result of these studies was the application of the BMR to the management of thyroid disease. Most hospitals maintained special facilities for BMR measurements until 1950, when they were abandoned in favor of chemical measurements for evaluating thyroid function. During the period from 1950 to 1980, it was not possible to obtain any measurement of energy expenditure in most hospitals. This situation was discussed in a conference in 1979, when a commercial device had just been introduced for the bedside measurement of gas exchange.3 There was much curiosity and uncertainty about the relevance of such measurements in clinical care. Since 1980, an increasing number of commercial instruments have been introduced for the bedside measurement of gas exchange, with a correassociated with sponding rise in reports of the REE 4 different clinical conditions.4 many
The
history of nutrition is interwoven with our understanding of both energy and heat. It is no accident that food intake is spoken of as the intake of caloriesa unit of heat. General interest in this subject can be traced back to early humans, who recognized that the living body was warm and began to cool at the time of
MEASUREMENT OF INDIRECT CALORIMETRY
McClave and Snider have properly emphasized the
importance of attention to details, particularly instrument calibration, when performing the measurement of gas
exchange. One may generalize that accurate measurements in the gas phase are usually more difficult to achieve than are comparable measurements in the liquid phase. A variety of indirect methods have been used to assess energy expenditure without the usual measurements involved in calorimetry. These have been based upon physiologic measurements such as ventilation or heart rate, a measurement of REE by gas exchange plus the energy cost of activity estimated by a diary, a measurement of REE plus the output of an activity
death. There, life became defined as animal or &dquo;vital&dquo; heat. This concept of vital heat came to involve vital &dquo;humors,&dquo; which dominated medical thought for 1500 years, perhaps one of the most influential and misleading doctrines in medical history.’
203
Downloaded from ncp.sagepub.com at UNIV OF NEW ORLEANS on June 6, 2015
204
meter, measurement
over
24 hours in
a
respiration
chamber, or the use of doubly labeled water for calculating the average co2 output over a period of 5 to 10 days. The assumptions and problems with each of these methods have been compared with those of conventional indirect calorimetry by Jequier et a1.5 Some of these methods are not applicable to the acutely ill or injured patient who might require nutrition support. Important information in certain clinical conditions may become evident from studies in respiration chambers or by the use of doubly labeled water. A respiration chamber is a room-sized enclosure where the gas exchange can be measured on a 24-hour basis while the subject or patient is sleeping, awake at rest, during and after meals, or during period of activity an a treadmill or cycle ergometer. The 24-hour energy expenditure of any particular normal individual is surprisingly constant; however, interpersonal variations are considerable. The isolation required for such measurements prevents their widespread use in hospitalized patients, and the expense of design and operation limits their availability to a few institutions. Information from chamber studies on the thermic effect of food and the energy demands of light exercise may shed new light on selected clinical situations during the next few yearns.6 The use of doubly labeled water for measuring energy expenditure in free-living animals was extended by Schoeller et al’to the study of human subjects in various conditions of normal life. The method involves administering a loading dose of water labeled with deuterium and l8o to a subject or patient and following the rate of excretion of both isotopes in urinary water. The difference in isotope excretion over 5 to 10 days represents the average co2 output per day, from which the total energy expenditure is estimated on the basis of an average respiratory quotient (RQ). The advantages are the use of a nonradioactive isotope, which allows the study of infants and pregnant women, and a simple and convenient method that also provides information on total body water and allows the simultaneous study of many individuals. The disadvantages are the expense and availability of the isotope and mass spectrometry. The method only provides information on co2 output with its greater potential error than when measuring o2 2uptake.’ Furthermore, it cannot indicate variations in energy expenditure from day to day. The method has been used to study parenteral nutrition,8,9 sports medicine&dquo; and bum patients.&dquo; Studies in various other forms of disease and injury are currently
underway.
ing the principles of indirect calorimetry and having a clear understanding of what is represented by the REE. The total energy expenditure (TEE) is usually exas kilocalories per unit of time, which is the sum of the following factors:
pressed
TEE BMR + TEF + ACT where TEF is the thermic effect of food. =
Hospital measurements of indirect calorimetry represent the BMR only if the measurement is carefully performed under postabsorptive conditions with the patient at rest in a thermoneutral environment. Hospital measurements of indirect calorimetry for nutrition purposes have commonly been made as the resting metabolic rate. This may differ from a true BMR measurement because it is made at any time during the waking day, after the individual has been at bedrest for perhaps 30 minutes. Another difference may be that the individual is not postabsorptive (no food since the previous evening) but is receiving nutrition such as total parenteral nutrition. The TEF and diurnal variations in basal energy expenditure are generally considered to represent less than 10% of the total energy expenditure, but information on TEF in clinical states is still very limited. Therefore, a resting measurement of energy expenditure should include information regarding the time of measurement and whether nutrients were being administered. McClave and Snider have reviewed the ranges of increase in REE found with various clinical conditions, including the extreme hypermetabolism of the severely burned patient. Clinical studies in the early 1970s showed a range of REE of up to +100% above the predicted normal in very large burns. A growing number of reports in the past decade indicate that the extent of bum hypermetabolism has substantially decreased. Modern burn care has made great strides that have reduced mortality and morbidity, alongwith less marked increases in REE. Burned patients were commonly treated during the 1960s and early 1970s with open exposure to normal ambient temperatures with surface antibacterial ointments for prolonged periods. The rationale was to allow the maximum opportunity for islands of deep second degree damage to regrow spontaneously if bacterial infection could be controlled. The radiative as well as the evaporative cooling of these patients was continuously increased to a variable degree. The severe hypermetabolism in such patients appeared to be some combination of obligatory increases in heat loss by uncontrolled evaporative losses through the burn surface together with increased heat
REE
production on internal organs as a result of strong and
The decade of the 1980s has produced both new instruments for the bedside measurement of gas exchange and many new products for nutrition support. The increasing availability of gas exchange measurements has placed additional emphasis on understand-
continued catabolic stimuli. In the past 20 years burn mortality has seen dramatic improvement. Resuscitation is usually prompt, pulmonary injury is recognized and treated, the environment is warmed, and excision of the burn surface is started soon after resuscitation, together with increased
Downloaded from ncp.sagepub.com at UNIV OF NEW ORLEANS on June 6, 2015
205
of closed dressing that sharply reduced evaporative cooling. With such modern techniques, the burned patient seldom has elevations of the REE of more than 70% above normal, and more often shows increases of only 30% to 50% above normal values.ll The REE of every patient represents a balance between increases due to catabolic influences and decreases due to whatever tissue depletion may have occurred. Just as improvements in burn care have reduced the extent of hypermetabolism currently seen, the improvement in nutrition support has reduced the incidence of extreme hypometabolism seen with advanced cachexia. The reduction in the overall range of REE seen in clinical conditions has led some physicians to question whether measuring energy expenditure continues to be as important as in previous years. Much remains to be learned about the proper interpretation of REE measurements. Yet any significant increase or decrease in the REE can be expected to be important as a &dquo;metabolic marker.&dquo; An example of this is related to the studies of DuBois and colleagues on fever. Measurement of the BMR in a variety of febrile conditions yielded an average increase in BMR of 13% for every degree centigrade of fever. Yet inspection of the original data reveals that the patients with chronic pulmonary tuberculosis had less increase in BMR per degree of fever (presumably secondary to weight loss and depletion) whereas the patients with acute typhoid fever had greater increases in BMR per degree of fever. The typhoid patients were found to be excreting unusually large amounts of nitrogen in the urine and were very difficult to put into positive nitrogen balance by increasing the nitrogen intake.l2
use
THE NONPROTEIN
RQ
Standard textbooks present the range of the nonprotein RQ as being from 0.71 when fatty acids are the tissue fuel and rising to 1.0 when carbohydrate is the tissue fuel. Values of up to 1.3 represent the conversion of additional carbohydrate to fatty acids. Some metabolic studies are tending to use the RQ to indicate the fuel being oxidized by a particular tissue or organ. It is important to remember that gas exchange is a whole body measurement, and thus an RQ of 1.0 does not necessarily mean that all tissues are burning carbohydrate. There is reason to believe that fat oxidation may be continuing to some degree in a tissue such as muscle at a time that lipogenesis is occurring elsewhere in the body and that the net RQ of these processes happens to be 1.0. Some clinicians have proposed that because carbohydrate has many assets as a metabolic fuel, total parenteral nutrition should be given in amounts sufficient to insure an RQ of 1.0 or higher. It is important to realize that sustained RQs of 1.2 to 1.3 can only be achieved with significant calorie excesses provided by a high carbohydrate, lipid-free intake. Until further
knowledge is available, it seems reasonable to provide nutrition support that strives at some balance between carbohydrates and lipid intake that will provide an RQ between 0.8 and 0.9. RELATIONSHIP OF ENERGY INTAKE TO NITROGEN BALANCE
An important aspect of energy balance is the mainsatisfactory amount and function of body protein. The sensitivity of nitrogen balance to the intake of energy as well as the intake of nitrogen has been recognized for many years, yet the mechanisms involved are poorly understood. These relationships have recently been reviewed by Young et al.s 6 (ppl39,439) There is reason to believe that the influence of energy intake on nitrogen balance is more evident when the energy is considered as energy balance rather than energy intake. The combination of data from various studies suggests how energy balance affects nitrogen balance.l3 The nitrogen retention produced by an increasingly positive calorie balance appears to vary from significant amounts in the depleted patient to essentially no nitrogen retention with an increasingly positive calorie balance in the acutely catabolic patent. 14 tenance of a
CLINICAL SIGNIFICANCE OF INDIRECT CALORIMETRY
Indirect calorimetry is a measurement of clinical physiology that is struggling to find its proper role in patient care. There is an intuitive feeling that a knowledge of energy expenditure should be important for managing individual patients, yet as McClave and Snider have pointed out, there is often a sense of frustration regarding just how to use such a measurement in treating an individual patient. When indirect calorimetry measurements are made in a heterogeneous population of hospital patients, the range ofREE will have a much wider distribution above and below the mean value than in a normal population. Even in patients with the same diagnosis, the range of values is much larger than with normal subjects. The clinical importance of this lies in the fact that even experienced clinicians may be unable to identify the
patients who are hypermetabolic versus those who are hypometabolic on the basis of the conventional bedside examination. There has been considerable difference of opinion as to whether normal values for REE should be presented in reference to some aspect of body composition such as fat-free mass. (The ideal segment of body composition should be some measure of the body cell mass because that is the energetically active portion of the body.) It seems reasonable to expect that future clinical measurements of REE will be expressed more commonly in relation to some aspect of body composition. This should be of greater importance in narrowing the distribution
Downloaded from ncp.sagepub.com at UNIV OF NEW ORLEANS on June 6, 2015
206
being reported in patient populations than in improving the already narrow distribution in normal subjects. At the present time, measurements of indirect calorimetry can help to define the metabolic behavior of the patient. However, a given measurement must be combined with other information to determine the degree of and need for nutrition support and the composition of nutrients that should be provided.33 A patient with an REE in the hypermetabolic range can be expected to have an accelerated loss of fat and lean tissue. A level of REE below normal suggests that the patient has already undergone significant tissue depletion or has some endocrine dysfunction. Nutrition support should be designed to achieve either calorie equilibrium for daily maintenance or a positive calorie balance in order to restore tissue. In either case, knowledge of the actual REE can avoid inappropriate calorie
administration. John Kinney, MDS Rockefeller University New York REFERENCES 1.
Kinney JM. Energy metabolism: heat, fuel and life. In: Kinney, JM, Jeejeebhoy KN, Hill GL, et al, eds. Nutrition and metabolism in patient care. Philadelphia, WB Saunders, 1988:3.
2. DuBois EF. Basal metabolism in health and disease. Philadelphia: Lea & Febiger, 1924. 3. Kinney JM (ed). Assessment of energy metabolism in health and disease. Columbus, OH: Ross Laboratories, 1980. 4. Kinney JM. Indirect calorimetry in malnutrition: nutritional assessment or therapeutic reference? JPEN 1987;11:90S-94S. 5. Jequier E, Acheson K, Schutz Y. Assessment of energy expenditure and fuel utilization in man. Am Rev Nutr 1987;7:187. 6. Kinney JM, Tucker HN, eds. Energy Metabolism: tissue determinants and cellular corollaries. New York: Raven Press, 1991:187. 7. Schoeller DA, Ravussin E, Schutz Y, et al. Energy expenditure by doubly labeled water: validations in humans and proposed
calculation. Am J Physiol 1988;250:R223. 8. Schoeller DA, Kushner RF, Jones PJH. Validation of doubly labeled water for measuring energy expenditure during parenteral nutrition. Am J Clin Nutr 1988;44:291. 9. Riumallo JA, Schoeller D, Barrera G, et al. Energy expenditure in underweight free-living adults: impact of energy supplementation as determined by doubly labeled water and indirect calorimetry. Am J Clin Nutr 1989;49:239. 10. Westerterp KR, De Boer JO, Saris WHM, et al. Measurements of energy expenditure using doubly labeled water. Int J Sports
Med 1984;5 (Suppl):74. 11. Goran MI, Peters EJ, Herndon DN, et al. Total energy expenditure in burned children using doubly labeled water technique. Am J Physiol 1980;259:E576-E585. 12. ColemanJ, DuBois RF:Clin Cal 7, Arch Intern Med 1915;15:887. 13. Goldstein SA, Elwyn DH. The effects of injury and sepsis on fuel utilization. Am Rev Nutr 1989;9:445-473. 14. Bursztein S, Elwyn DH, Askanazi J, et al. Energy metabolism, indirect calorimetry, and nutrition. Baltimore: Williams &
Wilkins, 1989:108.
Downloaded from ncp.sagepub.com at UNIV OF NEW ORLEANS on June 6, 2015
