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1 From the Department of Cellular and Molecular Medicine—Infectious Diseases, St George’s Hospital Medical School, London (AS, GEG, and DCM); HIV Medicine, Coleridge Unit, North Middlesex University Hospital NHS Trust, London (AS); the Departments of Clinical Dietetics (LH), Chest Medicine (CFJR), and Clinical Radiology (SG), St George’s Hospital NHS Trust, London; the Elsie Widdowson Laboratory, MRC Human Nutrition Research, Cambridge, United Kingdom (AW); and the Department of Biochemistry, University of Queensland, St Lucia, Australia (LCW).
2 Supported by the Wellcome Trust (International Research Fellowship to AS), the Medical Research Council, and Serono International SA (to DCM) 3 Address reprint requests to A Schwenk, Consultant in HIV Medicine-Coleridge Unit, North Middlesex University Hospital, Sterling Way, London N18 1QX, United Kingdom. E-mail: a.schwenk{at}doctors.org.uk.
ABSTRACT
Background: Tuberculosis is an important cause of wasting. The functional consequences of wasting and recovery may depend on the distribution of lost and gained nutrient stores between protein and fat masses.
Objective: The goal was to study nutrient partitioning, ie, the proportion of weight change attributable to changes in fat mass (FM) versus protein mass (PM), during antimycobacterial treatment.
Design: Body-composition measures were made of 21 men and 9 women with pulmonary tuberculosis at baseline and after 1 and 6 mo of treatment. All subjects underwent dual-energy X-ray absorptiometry and deuterium bromide dilution tests, and a four-compartment model of FM, total body water (TBW), bone minerals (BM), and PM was derived. The ratio of PM to FM at any time was expressed as the energy content (p-ratio). Changes in the p-ratio were related to disease severity as measured by radiologic criteria.
Results: Patients gained 10% in body weight (P < 0.001) from baseline to month 6. This was mainly due to a 44% gain in FM (P < 0.001); PM, BM, and TBW did not change significantly. Results were similar in men and women. The p-ratio decreased from baseline to month 1 and then fell further by month 6. Radiologic disease severity was not correlated with changes in the p-ratio.
Conclusions: Microbiological cure of tuberculosis does not restore PM within 6 mo, despite a strong anabolic response. Change in the p-ratio is a suitable parameter for use in studying the effect of disease on body composition because it allows transformation of such effects into a normal distribution across a wide range of baseline proportion between fat and protein mass.
Key Words: Body composition • bromides • densitometry • X-ray • deuterium oxide • longitudinal studies • nutrient partitioning • nutritional status • tuberculosis • wasting syndrome
INTRODUCTION
The term consumption has been virtually synonymous with tuberculosis throughout history (1). In the 21st century, tuberculosis is still the most frequent underlying cause of wasting worldwide. However, the pathophysiology of wasting in tuberculosis remains poorly understood (2), in contrast with the wealth of data related to HIV-associated wasting (3, 4). Although antimycobacterial treatment of tuberculosis is highly successful (5), many patients remain underweight after 6 mo of treatment (6).
Body compartments differ in their contribution to weight gain and its clinical benefit. Fat-free mass (FFM) is more closely correlated with quality of life and physical functioning than are fat mass (FM) and body weight (7, 8). However, FM is predominantly gained during recovery from wasting, as documented for AIDS-related opportunistic infections (9) and bacterial sepsis (10).
The relative distribution of nutrient loss or gain between fat and protein stores is called nutrient partitioning. During starvation and refeeding, nutrient partitioning varies widely between individuals but is a stable intraindividual trait (11-13). The contribution of protein stores to the total energy stored or mobilized during a given time period has been defined as the p-ratio. This ratio has been proposed as an internal set point that guides the restoration of an individual’s usual body composition after periods of starvation and refeeding (11). This dynamic definition of the p-ratio as the ratio of changes in compartments produces erratic results when the denominator is small. An alternative approach is to derive the static p-value, which represents the energy equivalent of body composition at a given point in time, and to then estimate changes in this ratio with time. Using this novel approach, we investigated nutrient partitioning during recovery from disease-related wasting. Tuberculosis is a suitable disease model for such investigation because it causes severe wasting but is curable by standardized treatment.
We studied changes in protein mass (PM) and FM in a cohort of patients with pulmonary tuberculosis receiving standard antimycobacterial treatment. Data relating to energy metabolism, food intake, proinflammatory cytokines, and plasma leptin concentrations from this study were previously published (14). Our primary hypothesis in this study was that FM but not PM would increase during the first month of treatment, followed by equal accumulation of both PM and FM toward the end of 6 mo of treatment. Thus, the p-ratio would decrease during the first month and increase between months 1 and 6. Potential determinants of partitioning between PM and FM were investigated, such as disease severity, clinical response to treatment, sex, and age.
SUBJECTS AND METHODS
In this prospective observational study, variables of body composition and immune response were assessed at 3 time points: within 3 d of the initiation of antimycobacterial treatment, after 1 mo of treatment, and after 6 mo of treatment. Patients with a clinical diagnosis of pulmonary tuberculosis were recruited from 2 London hospitals. Diagnosis was based on standard microbiological or clinical criteria, specifically, either acid-fast bacilli in sputum microscopy, Mycobacterium tuberculosis in sputum culture, or a combination of typical signs on chest X-ray, typical clinical symptoms, and clinical response to empirical antimycobacterial treatment. Patients were aged =" BORDER="0">18 y and were able to give informed consent.
Patients were excluded if HIV infection was documented but were allowed to enter the study if their HIV status was unknown. Surgery, pregnancy, or childbirth within 2 mo of the study were exclusions, as were severe renal, hepatic, or cardiac insufficiency; diabetes mellitus; or intake of corticosteroids at baseline. Participants with a final primary diagnosis other than tuberculosis and those later diagnosed with HIV infection were excluded from the data analysis.
Patients received antimycobacterial treatment according to the guidelines of the British Thoracic Society (5): rifampicin, isoniazid, pyrazinamide, and ethambutol for 2 mo then rifampicin and isoniazid alone for 4 mo. This treatment was modified for clinical indications in 4 patients as described in the Results.
The local research ethics committee approved the study, which was conducted in accordance with the rules of the Helsinki declaration (1983 revision). All subjects gave written informed consent.
Measurement methods
Body composition was assessed by a combination of weighing, dual-energy X-ray absorptiometry (DXA), and dilution methods. Body weight was measured on a calibrated scale to the nearest 0.1 kg while the patient wore only light clothes. The weight of the patient’s clothes, which was based on the average of 10 measurements of each standard clothing item, was subtracted. Height was measured on a calibrated wall-mounted scale to the nearest 0.1 cm. The patients were asked about the perceived duration and severity of weight loss. Patient notes and reports were searched for evidence of prior body weight.
FM and bone mineral ash were measured by DXA (Lunar DPX; GE Medical Systems Lunar, Bedford, United Kingdom) with analysis software version 3.65 in the advanced research analysis mode. Regions of interest were defined by a single operator (AS) with the use of standardized criteria as suggested by the manufacturer. Bone mineral ash was multiplied by 1.2741 to estimate bone mineral mass (BM) (15, 16). FFM was calculated as body weight minus FM.
Deuterium (2H) and bromide dilution methods were used to measure total body water (TBW) and extracellular water (ECW). A mixture of 2H2O (150 mg/kg body wt) and sodium bromide (NaBr, 77 mg/kg body wt) was given orally to the subjects after they had fasted overnight; the dose ingested was calculated by weighing to the nearest 0.001 g before and after ingestion. The patients were then allowed to drink water freely but remained otherwise fasting for 4 h. Blood samples were taken at baseline and after 4 h into a heparin-coated syringe and were centrifuged at 2380 x g and 10 °C for 15 min, and plasma was immediately frozen at –80 °C.
2H enrichment was measured by isotope ratio mass spectrometry with using a Sira 10 instrument (Micromass, Cheshire, United Kingdom) with minor modifications to the method previously described (17). Predose and postdose plasma (0.1 mL) samples were equilibrated for 6 h with 6 bar mL hydrogen gas in the presence of a platinum catalyst, and 2H enrichment of the equilibrated gas was measured by isotope ratio mass spectrometry. Plasma samples, internal laboratory standards, and a dilution of the ingested labeled dose were measured in duplicate, and the values were expressed relative to standard mean ocean water (SMOW). Precision of analysis was 0.47/152.06 (analytic SD/mean in ppm) for the predose samples and 0.69/377.44 for the postdose samples. The estimated error in the measurement of deuterium dilution space was 0.4%. TBW space was calculated as
RESULTS
Baseline characteristics
Between June 1999 and November 2000, 102 patients with possible pulmonary tuberculosis were screened for eligibility. Pulmonary tuberculosis was clinically diagnosed in 80 of these patients, but 24 had concomitant diseases that excluded them from the study (5 had diabetes, 11 had HIV co-infection, and 8 had other diseases). A further 16 patients did not give informed consent. Therefore, 40 patients entered the study. One patient died of a cause unrelated to tuberculosis, and 7 patients withdrew from the study for personal reasons. Two of the remaining 32 patients were excluded from the present analysis because their deuterium dilution test results could not be interpreted because of mislabeled samples; other data from these patients were included in another report from this study (14). The present analysis was therefore based on 30 patients, 21 of whom (70%) were male. The patients’ mean (±SD) age was 42.3 ± 21.3 y and ranged from 18 to 84 y. The patients’ ethnic backgrounds were South Asian (n = 16), white (n = 9), African (n = 4), and Hispanic (n = 1).
Diagnosis of tuberculosis and clinical response
Tuberculosis was diagnosed on the basis of sputum microscopy and culture, culture alone, histology alone, and clinical criteria in 9, 14, 6, and 1 patients, respectively. Chest X-ray films showed parenchymal infiltrates in 1, 2, or >2 lung fields in 5, 10, and 12 patients, respectively. Three further patients had no evidence of parenchymal lung disease but extensive mediastinal lymphadenopathy and pleural effusions. Cavities were identified in the initial chest X-ray of 11 patients (37%). Four patients had manifestations of extrapulmonary disease in addition to pulmonary lesions; one of these patients had disseminated miliary tuberculosis.
At the end of 6 mo, all patients but one had responded satisfactorily to the antituberculous chemotherapy. One patient developed new pulmonary infiltrates in month 4 of antimycobacterial treatment but recovered during an extended 9-mo course of treatment. Three further patients received 9 mo of treatment because of prior episodes of drug-sensitive tuberculosis (n = 2) or isoniazid resistance (n = 1) but showed clinical and radiologic signs of response to treatment after 6 mo. Ten patients (30%) received corticosteroids during the study for clinical indications [severe pleurisy (n = 6) and cutaneous drug reactions (n = 4)].
Body weight and body composition
The patients’ mean body weight and body mass index at baseline are given in Table 1. Four patients (1 woman, 3 men) could not give a history of recent weight changes, but interviews with relatives suggested weight stability in the 6 mo preceding the tuberculosis diagnosis. The remaining 26 patients reported a mean recent weight loss of 10.1 ± 6.8% that had first been noted 151 ± 70 d before baseline (range: 16–724 d).
View this table:
TABLE 1. Changes in body compartments during treatment of tuberculosis
Changes in body composition are described in Table 1. From baseline to month 6, the patients gained 9.5 ± 8.9% in body weight. Such weight gain was mainly due to a 43.6 ± 66.0% gain in FM, in contrast with a lack of significant change in PM during the study period (Table 1). Similarly, no significant change in TBW, ECW, or ICW was observed (Table 1). In terms of percentage of body weight, FM increased and TBW decreased from baseline to month 6 (Table 1). The women had significantly higher FM but lower FFM, TBW, ICW, bone mineral, and PM than did the men at all time points. However, changes in these parameters did not differ significantly between the sexes (Table 1).
Four-compartment body-composition model
The apparent hydration of FFM (TBW/FFM) in the four-compartment model was 0.69 ± 0.03 in men (range: 0.61-0.74) and 0.73 ± 0.06 in women (range: 0.63-0.82) at baseline. Thus, the overall results were compatible with the expected range of hydration from 0.69 to 0.77 (19), although TBW/FFM was below 0.69 in 7 men and 2 women and higher than 0.77 in 2 women. TBW/FFM was correlated with FM in percentage of body weight, increasing by 0.003 with each percent (P < 0.001). This may signify a small systematic overestimation of body fat in obese persons and an underestimation in lean persons by DXA. Reanalysis of the data with a newer software version from the manufacturer did not eliminate this bias (data not shown).
The p-ratio at the 3 time points is shown in Figure 1. It was higher in the men than in the women and decreased significantly over time. The decrease was not restricted to the first month of tuberculosis treatment but continued until month 6.
FIGURE 1.. P-ratio during 6 mo of treatment of tuberculosis. The p-ratio is the proportion of energy stored as protein mass to total stored energy, which is based on energy equivalents of 38.9 MJ/kg fat and 18.6 MJ/kg protein. P = 0.007 for time (months 0, 1, and 6) and P < 0.001 for sex (repeated-measures general linear models procedure). No significant interaction between time and sex was found. The boxes indicate the median and interquartile range (IQR), the error bars indicate the range without the outliers, and the dots indicate the outliers, which were defined as values >2.5 x IQR away from the mean.
Changes in weight from baseline to month 6 were strongly correlated with changes in FM but not with changes in PM, and no correlation between changes in FM and changes in PM was found (Figure 2). The patients achieved a positive energy balance over 6 mo of tuberculosis treatment of 159.6 ± 146.6 MJ in women and 145.9 ± 118.4 MJ in men. The primary determinant of the energy balance was change in FM (adjusted r2 = 0.92, P < 0.001) rather than change in PM (adjusted r2 = 0.18, P < 0.05). P-ratios at months 0, 1, and 6 were statistically independent from p between these time points.
FIGURE 2.. Correlation between changes in fat mass, protein mass, and body weight plotted as the kilogram change from baseline to month 6 of tuberculosis treatment in men (; n = 21) and women (•; n = 9). The mean linear regression line with its 95% CI is shown in each panel. Adjusted r2 = 0.81, P < 0.001 for panel A; r2 = 0.05, P = 0.11 for panel B; and r2 = 0.001, P = 0.31 for panel C.
Disease severity as measured by radiological criteria was not correlated with p (Figure 3). The p-ratio and p were also independent of age, ethnicity, glucocorticoid prescription and cumulative dose, and presence of extrapulmonary disease. None of the proinflammatory cytokines reported previously (14) correlated with the p-ratio or p (data not shown).
FIGURE 3.. Influence of disease severity on changes in the p-ratio (p) plotted against the radiologic severity of disease, as measured by the number of lung fields involved and the presence () or absence () of cavities. Neither variable was predictive of p (P = 0.43 for cavities and P = 0.33 for lung fields).
DISCUSSION
The present longitudinal observation of body composition in patients with pulmonary tuberculosis shows that clinical recovery does not guarantee the restoration of body protein, despite marked weight gain in most patients. Most of this weight gain was attributable to the accumulation of FM. The extent of PM repletion was independent of gain in weight and FM. At the end of 6 mo of treatment, despite substantial accumulation of FM, there was no significant accrual of PM.
This finding confirms and extends prior reports about preferential fat storage during recovery from catabolic illness, such as after critical care (10) and in HIV infection (9). In a recent micronutrient intervention trial of Indonesian patients with pulmonary tuberculosis, body fat percentage was assessed by anthropometry at baseline and month 6. Mean FM increased from 11.5% to 14.2% and from 13.0% to 16.0% in the micronutrient and placebo arms, respectively, over the 6-mo period (P < 0.001 between time points, NS between intervention groups; 24).
In the present report, we investigated the distribution of body mass repletion between fat and protein under the assumption that each individual has an inherent set point for this distribution called the p-ratio (11, 12). We hypothesized that during the first month of tuberculosis treatment, the p-ratio would decrease as the result of preferential fat deposition, followed by an increase in the p-ratio from month 1 to month 6 as the result of protein accretion. In fact, the p-ratio continued to decrease until the end of tuberculosis treatment, reflecting a lack of protein accretion despite a strongly positive energy balance.
Protein metabolism was found to be abnormal in stable-isotope studies of patients with active tuberculosis. In the first study from our group (25), Indian patients with pulmonary tuberculosis were compared with malnourished and normally nourished healthy subjects. Whereas protein synthesis and breakdown in the fasting state were not significantly different between groups, patients with tuberculosis used a larger proportion of protein from oral feeding for oxidation and hence for energy production than did either control group. Such failure to channel food protein into endogenous protein synthesis has been termed anabolic block (25).
A recent study compared the metabolic response to oral feeding in Singaporean patients with tuberculosis, HIV infection, or dual infection (26). Tuberculosis patients with or without HIV co-infection had higher protein turnover than did HIV-infected patients without tuberculosis, but no evidence of an anabolic block was found in either group (26). Such apparently conflicting data may result from diverging methods and populations. Stable-isotope methods measure short-term alterations in protein metabolism very precisely in small patient samples. By contrast, longitudinal body-composition studies such as the present study measure the long-term consequences of such metabolic changes in larger patient groups. Our data are consistent with the presence of an anabolic block (25).
In this study, we used a novel approach to the concept of the p-ratio, using a change in the static ratio (p) rather than comparing changes to derive a dynamic ratio. This approach avoids error propagation and large artifacts when the denominator of a change is small. We propose that such analysis has wide potential application for the investigation of nutrient partitioning in wasting disease. Although p may be a less intuitively understood variable than FM or PM in kg or % body wt, its advantage over the latter variables is its near normal distribution and independence from sex and initial body fatness. Thus, it should be easier to detect the influence of disease on nutrient partitioning.
The absence of a correlation between radiologic evidence of disease severity and body-composition changes may partly be explained by methodologic limitations. Chest X-ray findings in pulmonary tuberculosis, although widely accepted as clinical criteria for severity, represent a mixture of both active inflammatory infiltration and chronic scarring and tissue repair (27). They are therefore unlikely to accurately reflect current disease activity. The absence of an effect of corticosteroid use on body composition may be due to covariance between disease severity and corticosteroid use, masking the well-documented body-composition changes resulting from corticosteroid treatment.
The four-compartment body-composition model has been used previously in the investigation of adults (18) and children (16). Air-displacement plethysmography (28) combined with DXA (for bone mass) and deuterium dilution (for TBW) is superior to a model in which FM is determined by DXA (16) but was not available in the present study. Analysis of the apparent hydration of FFM suggested a small systematic bias in measurement of percentage fat by DXA. In most four-compartment models, the protein compartment suffers from error propagation, being calculated by subtraction of all measured compartments from body weight. Despite these limitations, the four-compartment model is still more precise than any two-compartment model with FM and FFM, as used in most previous investigations of the p-ratio (12, 29). FFM overhydration is a feature of severe infection and critical illness (30) and prohibits the calculation of PM as a constant fraction of FFM.
What are the clinical implications of our findings? Tuberculosis has become a curable disease since the introduction of rifampicin and isoniazid 30 y ago (31). However, clinical and functional recovery often lags behind microbiological cure and patients may develop fever or malaise during treatment (32). Most patients have difficulties regaining their usual body weight within the 6 mo of treatment (6). Decreased muscle mass and weight loss are associated with fatigue and physical inactivity in other diseases (33-35). A two-way interaction between physical inactivity and loss of muscle mass is likely. Further investigations into changes in body composition in tuberculosis should include measurement of physical activity and disease-related quality of life.
Although we anticipated an early effect of tuberculosis on fat-lean partitioning, the persistence of preferential fat repletion at 6 mo was striking. This may indicate ongoing metabolic stress in patients who appear recovered from tuberculosis by clinical criteria. Alternatively, the internal regulation of nutrient partitioning may need more time to reset to the individual’s internal set-point (p-ratio) after the cause of metabolic stress has been removed. The mediators of nutrient partitioning remain unknown. We showed in a previous publication from the study presented here that leptin does not play a role beyond its well-established correlation with body fat percentage and sex (14).
In conclusion, patients recovering from pulmonary tuberculosis achieved a strongly positive energy balance and impressive weight gain but failed to restore body protein within 6 mo. Predominant fat accumulation persisted until the end of antituberculosis treatment at 6 mo, consistent with an ongoing anabolic block.
ACKNOWLEDGMENTS
We are grateful to the nurses and physicians in the Departments of Infectious Diseases and Chest Medicine, St George’s Hospital, and the Department of Chest Medicine, St Helier Hospital (Dr N Cooke), who were of great help in recruiting patients to this study. Marinos Elia, Institute of Human Nutrition, University of Southampton, advised us on study design and body-composition methods. Sanjeev Patel, rheumatologist at St George’s Hospital, provided us with access to the DXA scanner. We are also very grateful to all of the patients for contributing their time and effort to the study.
AS, GEG, and DCM designed the study protocol. AS recruited patients, documented data, analyzed DXA scans, performed the statistical analysis, wrote the paper, and coordinated the trial under the supervision of GEG and DCM. LH assisted in data collection and documentation. AW and LCW analyzed the deuterium and bromide samples, respectively. CFJR recruited patients and contributed clinical data. SG analyzed the chest X-rays. All authors contributed to the critical review and interpretation of data and revised the final paper.
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