Longitudinal Study of Total Body Potassium in Healthy Men

Transcription

1 Original Research Longitudinal Study of Total Body Potassium in Healthy Men Angela Andreoli, MD, PhD, Stella L. Volpe, PhD, Sarah J. Ratcliffe, PhD, Nicola Di Daniele, MD, Antonio Imparato, PhD, Luigi Gabriel, PhD, Graziano Parente, PhD, Silvio Possemato, PhD, Antonino De Lorenzo, MD Human Physiology and Nutrition Unit (A.A., A.D.L.), Internal Medicine (N.D.D.), University of Rome Tor Vergata, Rome, Nuclear Central (A.I., L.G., G.P., S.P.), Borgo Sabotino, ITALY, Division of Biobehavioral and Health Sciences, School of Nursing (S.L.V.), Center for Clinical Epidemiology and Biostatistics, School of Medicine (S.J.R.), University of Pennsylvania, Philadelphia, Pennsylvania Key words: aging, body cell mass, body mass index, 40 K, total body potassium Objectives: Total body potassium (TBK) is an index of fat-free mass and an accurate measure of body cell mass (BCM). To date, however, no longitudinal studies have evaluated body composition using TBK in aging healthy adults. The purpose of this study was to assess TBK and BCM in healthy men over a 21-year period, and to evaluate changes with age. Design: A 21-year longitudinal, prospective, observational study evaluating TBK and BCM and their relationship to changes with aging and body mass index (BMI). Subjects: Body composition of 133 healthy Italian men, 20 to 66 years of age, was measured by whole-body counting of 40 K. TBK was calculated by using this formula: 40 K BCM was calculated from TBK by using this formula: BCM (kg) TBK (mmol). Results: The prevalence of overweight and obesity increased significantly from baseline to end of study (p, 0.01), with no change in TBK observed over the 21-year period. BCM summaries were tested to determine if any was a good predictor of BMI after age 30. Participants with a low maximum BCM (,27 kg) at an early age tended to exhibit decreased BMI as they aged, whereas those with a high maximum BCM (.27 kg) at an early age tended to show increased BMI as they aged. Conclusion: Despite an overall increase in the incidence of overweight and obesity over the 21-year period, a high maximum BCM at an early age was a predictor of an increase in BMI as men got older. INTRODUCTION Total body potassium (TBK), an index of fat-free mass and a more accurate measure of body cell mass (BCM), has been the subject of intensive research during the past 50 years. Most of the research, however, has been cross-sectional [1 3]; longitudinal studies conducted to evaluate the body composition of aging healthy adults using TBK are uncommon [4,5]. An accurate but less commonly used method by which to evaluate body composition is to measure BCM. Body cell mass is defined as the total mass of oxygen-exchanging, potassium-rich, glucose-oxidizing, work-performing cells of the body [6]. BCM is considered the actively metabolizing portion of the body, which is known as protoplasm [6]. Presently, the gold standard for quantifying BCM is via measurement of the naturally occurring isotope 40 K [7]; however, assessment of 42 K and measurement of the difference between extracellular water (ECW) and total body water (TBW) using multiple isotopes are also acceptable [8]. Total body potassium concentration is linearly correlated with the size of the BCM [7]. Whole-body counting of 40 K enables, by noninvasive nuclear technique, in vivo determination of the naturally occurring radioactive isotope of 40 K in the human body [6]. The whole-body counter is a reference method used for research, but not for clinical purposes. Because 40 K is 0.012% Address reprint requests to: Angela Andreoli, MD, PhD, Human Physiology and Nutrition Unit, Via Montpellier 1, University of Rome Tor Vergata, 00173, Rome, ITALY. angela.andreoli@uniroma2.it Abbreviations: BCM 5 body cell mass, BMI 5 body mass index, ECW 5 extracellular water, TBK 5 total body potassium, TBW 5 total body water. Journal of the American College of Nutrition, Vol. 29, No. 4, (2010) Published by the American College of Nutrition 352

2 of natural potassium, it provides an accurate assessment of TBK [9]. The naturally occurring 40 K has a half-life of log year and is assumed, by virtue of its natural occurrence and long halflife, to be equally distributed in all living biological material [9]. Potassium is almost exclusively an intracellular cation (95%) [7,10,11], found chiefly in muscle and viscera (hence, essentially not found in fat, bone, or ECW). The prevalence of obesity has increased to epidemic proportions throughout the world. Although body mass index (BMI) is used to provide an estimate of obesity, it fails to detect altered nutritional status in the presence of overweight or obesity because malnutrition can be masked by an abnormal amount of fat. Measuring BCM for the evaluation of muscle mass and protein tissue would allow for a better indication of nutritional status across the BMI spectrum. An accurate measure of BCM would prove extremely useful for establishing an individual s state of health or disease over time. Therefore, the aims of this 21-year longitudinal study were (1) to evaluate TBK and BCM in healthy men, and (2) to examine changes in TBK and BCM with aging and BMI. SUBJECTS AND METHODS The study was first approved by the Ethical Committee of the University of Rome Tor Vergata. Each participant gave verbal and written informed consent before participating. Participants were 133 healthy Italian men, 20 to 66 years of age, who were employees at a nuclear power plant in Borgo Sabotino (Latina, Italy). Whole-body counting is used at this nuclear plant to monitor the ingestion of radioactivity by employees who work with radiation, when unsealed sources are used in biomedical or industrial applications. Each year, the participants were weighed, had 40 K analyzed by wholebody counting, received a physical examination by a physician, and reported changes in diet, exercise, medications, medical diagnosis and treatment, drug prescriptions, and lifestyle. Body Weight, Height, Body Mass Index All body composition techniques were administered to each participant on the same morning. On the test day, participants were in a postabsorptive state (fasted for at least 10 to 12 hours) and were euhydrated. Body weight, using a standard balance beam scale (Invernizzi, Rome, Italy), and height (stadiometer; Invernizzi) were measured to the nearest 1 kg and 0.5 cm, respectively. BMI was calculated using this formula: body weight (kg)/height (m 2 ). 40 K, Total Body Potassium, Body Cell Mass Measurement of 40 K was assessed using a whole-body counter, surrounded by a cell, 2.5 cm wide and 3 m high, of 10-cm thick lead bricks, with a door whose entrance was formed by a 22-cm thick iron slab [7]. The room was continuously ventilated. A single cm thalliumactivated sodium iodine crystal was positioned above the participant, who was measured in a sitting position for 20 minutes. During the measurements, participants were dressed in paper pajamas. 40 K was measured once a year for 21 years in all 133 participants. TBK was calculated by using this formula: 40 K [9]. BCM was calculated from TBK by using this formula: BCM (kg) TBK (mmol) [6]. Statistical Analyses Descriptive statistics were calculated for the final cleaned data. We also developed plots of the simple mean, standard deviation, maximum, and minimum for all outcome variables versus age. Mixed models (also known as random effects models) were used to calculate the effects of the independent variables on the outcome. By using mixed models, we are able to adjust for the multiple measurements on a given participant, plus use all available data for any particular participant, that is, the participants did not need to have the same number of measurements or be measured at the same time. Both unadjusted and adjusted results are given. Correlations were conducted between BCM and BMI, between age and BMI, and between BCM and BMI at #30 years of age ( early age ) and.30 years of age. All statistical analyses were conducted using Statistical Analysis Software (SAS), version 9.1 for windows (Copyright 2002, SAS Institute Inc, Cary, NC). RESULTS The median length of time for participation in the study was 21 years. The interquartile range was 8 years (16 to 24 years). The minimum and maximum times of participation in the study were 6 and 28 years. Entry into the study was defined as the age at which the first 40 K measurement was recorded. Exit from the study was defined as the age at which the final 40 K was measured. Participant characteristics at entry and exit from the study are listed in Table 1. Table 2 lists the correlation coefficients for BCM, BMI, and age, comparing baseline versus end of study (using t-tests). Participants were divided by BMI into normal (18.5 to 24.9 kg/m 2 ), overweight (.25 to 29.9 kg/m 2 ), and obese (.30 kg/m 2 ). At study entry, 54.7%, 38.7%, and 6.6% of participants were measured as normal, overweight, and obese, respectively, on the basis of BMI. At study exit, 31.2%, 54.1%, and 14.7% were categorized as normal, overweight, and obese, respectively, on the basis of BMI. A significant increase in overweight and obese participants occurred during the course of the study (p, 0.001). JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION 353

3 Table 1. Descriptive Characteristics for All Participants at Baseline and End of Study 1 Baseline End of Study Age (yr) * Height (m) Body weight (kg) * Height (m) Body mass index (kg/m 2 ) * 40 K (g) * Body cell mass (kg) Values represent means 6 standard deviation. * p, 0.01 for t-test comparing baseline versus end of study. Fig. 1 shows 40 K versus age, and Fig. 2 illustrates BCM versus age. Age is significantly related to 40 K in grams in an unadjusted model. For every year of age, the grams of 40 K value increased by approximately 0.1. For example, a 20-yearold would have a predicted 40 K-gram-value of 147.6, and a 30- year-old would have a predicted 40 K-gram-value of Adjusting for BMI does not significantly influence the age effect with 40 K because BMI is not directly related to 40 K. Height is significantly related to the 40 K-gram-value. With adjustment for height, the age effect increases from the unadjusted model to a 0.12-gram increase in 40 K-gram-value for every year of age. Sixty-five (49%) of the participants in the sample had at least one BCM measurement during their early age. These early-age BCM measures were summarized for each participant via minimums, means, medians, and maximums. The BCM summaries were then tested to determine whether any was a good predictor of BMI at 30 years of age. Random effects models were used to model the relationship between BCM summaries and BMI, with adjustments for age and the repeated measures. For this subsample, ages ranged from 31 to 55 years, with an average age across all participants of 40 years. The average BMI was 26.2 (62.8) kg/m 2. No BCM summary measure was significantly predictive of the BMI scores, except for the maximum BCM measurement. A significant relationship was noted between age, maximum BCM, and BMI. Participants with a low maximum BCM (,27 kg) at early age tended to Fig K versus age. Solid black line 5 simple mean at each age across participants with a value at that age; dashed red lines 5 95% confidence interval for the mean; dashed blue line 5 maximum and minimum values. have a decrease in BMI score as they aged. Participants with a high maximum BCM (.27 kg) at an early age tended to have an increase in BMI as they aged (Fig. 3). DISCUSSION The use of a whole-body counter to evaluate 40 K, and subsequently to calculate TBK and BCM, is a valuable tool that could accurately assess cellular changes in body composition with aging [5]. The aims of this study were to assess TBK and BCM longitudinally, and to evaluate their relationship with BMI over time. This study involved serial Table 2. Pearson Partial Correlation Coefficients BCM BMI Age BCM (kg) * BMI (kg/m 2 ) , Age (yr) * * , BCM 5 body cell mass, BMI 5 body mass index. * Significant correlation between 2 measures. Fig. 2. Body cell mass versus age. Solid black line 5 simple mean at each age across participants with a value at that age; dashed red lines 5 95% confidence interval for the mean; dashed blue line 5 maximum and minimum values. 354 VOL. 29, NO. 4

4 Moreover, our results reveal a significant relationship between age, maximum BCM, and BMI. Participants with a low maximum BCM at an early age tended to have a decrease in BMI score as they aged. Participants with a high maximum BCM at an early age tended to have an increase in BMI as they aged. Our data are consistent with those of Wang et al. [13], who conducted a cross-sectional study on the relationship between BMI and BCM. In their healthy adult population, BMI and BCM were positively related when adjustments were made for age, sex, and race. BCM increased with increases in BMI in all subgroups at any given age. Males had higher slopes than females, and age did not affect the slope in any subgroup. We also found that age was significantly related to 40 K in grams in an unadjusted model; however, adjusting for BMI not significantly influence the age effect with 40 K. Fig. 3. Predicted body mass index scores by age, adjusted for maximum body cell mass (Max. BCM) before 30 years of age. Predicted scores are plots for varying maximum BCM values. 40 K measurements in 133 males who were employees at a nuclear power plant in Italy. Both the elegant 40 K measurements and the extended observation period make this a rare and valuable data set. Furthermore, the data set is unique and provides important information regarding changes in BCM as men age. The results of our study are not consistent with those of others who reported decreases in TBK and BCM with age [1,5,11,12]. For example, Kyle et al. [11] reported significant declines in BCM in individuals older than 80 years of age compared with those in individuals 70 to 79 years of age. Because our participants were younger, we cannot directly compare our results with those of Kyle et al. [11]; however, their study provides invaluable information for BCM changes in the later years of life, and our data provide information for BCM changes until 66 years of age. Kehayias et al. [12] stated that, potassium depletion with age can be explained by loss of cell mass because of insufficient replacement of cells, or because of a change in intracellular potassium concentration. In a longitudinal study, Flynn et al. [4] reported that males showed a decrease in TBK from 41 years of age onward, with the most rapid rate of loss between 41 and 60 years of age. They preferred to relate TBK to height rather than to body weight because fat, which is low in potassium, can be bypassed in comparisons of male and female participants. We did find an effect of 40 K on height in our male participants as they aged. Although participants in the study conducted by Flynn et al. [4] were not overweight or obese, our participants showed a significant weight gain over time. The focus of our study was to evaluate changes in BMI over time to verify the effect on BCM. These data confirmed the findings of our previous crosssectional study [3], where we reported that BCM was greater in obese than in normal-weight subjects. CONCLUSION This 21-year longitudinal study provides new and unique data on the relationship between BCM and BMI changes as men age. We found that participants with a low maximum BCM (,27 kg) at an early age tended to have a decrease in BMI as they aged, whereas those with a high maximum BCM (.27 kg) at an early age tended to have an increase in BMI as they aged. The relationship between BMI and BCM could provide useful information in studies related to obesity and nutrition assessment. Future research should focus on the effect that weight loss has on BCM in obese participants. In addition, because all of our participants had an increase in body weight over time, another future study could compare the success of weight loss between those who started with a low BCM at a young age versus those who started with a high BCM at a young age. REFERENCES 1. Kyle UG, Genton L, Hans D, Karsegard VL, Michel JP, Slosman DO, Pichard C: Total body mass, fat mass, fat-free mass, and skeletal muscle in older people: cross-sectional differences in 60- year-old persons. J Am Geriatr Soc 49: , De Lorenzo A, Andreoli A, Serrano P, D Orazio N, Cervelli V, Volpe SL: Body cell mass measured by total body potassium in normal-weight and obese men and women. J Am Coll Nutr 22: , De Lorenzo A, Andreoli A, Battisti P, Candeloro N, Volpe SL, Di Daniele N: Assessment of total body potassium in healthy Italian men. Ann Human Biol 31: , Flynn MA, Nolph GB, Baker AS, Martin WM, Krause G: Total body potassium in aging humans: a longitudinal study. Am J Clin Nutr 50: , Gallagher D, Ruts E, Visser M, Heshka S, Baumgartner RN, Wang J, Pierson RN, Pi-Sunyer FX, Heymsfield SB: Weight stability JOURNAL OF THE AMERICAN COLLEGE OF NUTRITION 355

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