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Characteristics of Energy Metabolism in Males with Mental Retardation

To characterize the energy metabolism in individuals with mental retardation (MRs), we measured energy cost at several physical activity levels (basal, supine, sitting, standing, and walking at 30, 50 and 70m/min), maximal oxygen consumption (Vo2max), and body composi tion in 23 male MRs and the same number of volunteer male controls. Both groups were individually matched for age, body height, and body weight. Energy cost was measured by the Douglas bag technique. The recently developed sulfur hexafluoride (SF6) dilution technique was employed for measuring body composition. In addition, 3-dimensional accelerometry was used for evaluating body movements, and plasma indices of macronutrients were also measured. The energy cost of MRs, when sitting, standing, and walking at 30 and 50m/min, was significantly higher than that of controls (p<0.05), while the basal and resting metabolic rates were similar in both groups. Vo2max was significantly lower (p<0.05) in MRs than controls. Accelerometry demonstrated excessive movement by MRs, which may explain their higher energy cost of exercise. In contrast, no significant difference was observed in percent body fat or lean body mass. Concentrations of plasma total cholesterol, triacylglycerols and albumin were significantly lower in MRs as compared with the controls. Our findings suggest that MRs are burdened with an energy metabolism less economical than non-MRs. Limited physical activity in their daily life may be the cause. These characteristics of MRs' energy metabolism should be considered for planning their proper dietary schedules and physical activity programs.


Summary
To characterize the energy metabolism in individuals with mental retardation (MRs), we measured energy cost at several physical activity levels (basal, supine, sitting, standing, and walking at 30, 50 and 70m/min), maximal oxygen consumption (Vo2max), and body composi tion in 23 male MRs and the same number of volunteer male controls. Both groups were individually matched for age, body height, and body weight. Energy cost was measured by the Douglas bag technique. The recently developed sulfur hexafluoride (SF6) dilution technique was employed for measuring body composition. In addition, 3-dimensional accelerometry was used for evaluating body movements, and plasma indices of macronutrients were also measured. The energy cost of MRs, when sitting, standing, and walking at 30 and 50m/min, was significantly higher than that of controls (p<0.05), while the basal and resting metabolic rates were similar in both groups. Vo2max was significantly lower (p<0.05) in MRs than controls. Accelerometry demonstrated excessive movement by MRs, which may explain their higher energy cost of exercise. In contrast, no significant difference was observed in percent body fat or lean body mass. Concentrations of plasma total cholesterol, triacylglycerols and albumin were significantly lower in MRs as compared with the controls. Our findings suggest that MRs are burdened with an energy metabolism less economical than non-MRs. Limited physical activity in their daily life may be the cause. These characteristics of MRs' energy metabolism should be considered for planning their proper dietary schedules and physical activity programs. Key Words energy cost, mental retardation, Vo2max, body composi tion, sulfur hexafluoride dilution * To whom correspondence should be addressed .
The physical conditions of individuals with mental retardation (MRs) need to be well characterized so that MRs can be provided with effective assistance. Specifically, knowledge about their energy metabolism is indispensable for planning proper dietary schedules and physical activity programs for them. Nevertheless, only few studies have appeared so far which investigate the energy expenditure of MRs.
Physiologists have partitioned energy expenditure into several components; basal metabolic rate (BMR) or resting metabolic rate (RMR), physical activity, and thermogenic effect of food or adaptive thermogenesis (1). In Japan, these components in MRs are usually estimated using the data obtained from healthy persons. This could be misleading because previous data suggest that MRs have low levels of physical fitness (2-6), a higher incidence of obesity (7)(8)(9)(10)(11), and may respond differently to exercise training than persons without mental retardation (12). To examine such possibilities and characterize the energy metabolism in MRs, we measured the energy expended at rest and in some types of physical activity by means of indirect calorimetry with the Douglas bag technique (13), and compared the results of MRs with those of healthy control subjects. In addition, we examined several factors which might be related to energy cost such as maximal oxygen consumption (Vozmax), lean body mass (LBM), body fat, and plasma con centrations of macronutrients. The recently developed SF6 dilution method was employed to attain reliable measurements of body volume in MRs, for whom the underwater weighing method (UWW) is too stressful and effort-requiring to follow. Body movements of MRs were also analyzed using a 3-dimensional accelerometer which helps to evaluate fine movements in a quantified manner. (DC-5A, Shinagawa Factory, Tokyo, Japan) and the measurements were corrected for temperature, pressure, and humidity. The BMR was measured by collecting expired air in two consecutive 7-min samples. The subjects were gathered into a gymnasium 2 h after a light breakfast, and rested, lying for 30min. Then expired air was collected to estimate the energy costs at supine, sitting, and standing positions for 10, 5, and 5min, respectively. Energy cost was then measured as subjects walked at 30, 50, and 70m/min. Each speed was maintained over 4min and expired air was collected for the last 2min of each level. Subjects walked along a track of 60m in circumference , which was equipped with timing lights at 2m intervals which lit in sequence and led subjects at the desired pace (Paceleader, PL-1000, Yagami Co., Nagoya , Japan). To further ensure maintenance of the pace, a trained staff member was assigned to walk with each subject, while another staff member checked the speed with a stopwatch. An additional observer counted the stride frequency of the subjects at each speed. Subjects started at an initial pace of 30m/min. Each speed was maintained for 4min and was increased to the next higher one, with the totally elapsed time of the trial being 12min.

Measurement of movement by accelerometer
Since we had a strong impression that MRs walked clumsily, we recorded the body movements of MRs and controls using an accelerometer (Activetracer, AC-300, GMS Co., Tokyo, Japan), which recorded the averaged magnitudes of 3-dimensional acceleration vectors every 20s. The device was designed so small and light (approximately 100g) that it could be set on the back of a subject at the center of the Jacoby line, without interrupting any movement while he was sitting, standing or walking for calorimetry. Analyzable data were recorded from 17 MRs and 16 controls.

Possible determinants of energy cost
Cardiorespiratory parameters: The stress test was performed by means of submaximal, continuous incremental bicycle ergometry. Since it was extremely difficult for MRs to keep pedaling at a constant rate, a computer-controlled bicycle ergometer (FFS, Bridgestone Cycle Co., Tokyo, Japan) was used. This machine was designed to keep the work rate constant by changing the load continuously according to the pedaling speed. Each subject exercised at three workloads that corresponded to 40, 50 and 60% of his own vo2 max estimated from his age. The average exercise loads actually given were 45, 62, and 86W for MRs, and 64, 85 and 102W for controls. Each loading level continued for 4min (12min in total), and expired air was collected for the last one minute of each level. A regression line was made from the HRs and oxygen consumption at the three loading levels. Then with the assumption that the maximal heart rate (HRmax, beats/min) could be calculated as 220 minus the subject's age in years, the Vo2max was estimated as the oxygen consumption at HRmax.  (19), and LBM was calculated as body weight minus fat mass. In our preliminary study, the correlation coefficient was 0.9995(y=1.0047x-1.5532) between body volume by UWW (y) and that by SF6 dilution (x), and the average of the coefficients of variation was 1% among 12 healthy subjects (unpublished data). Thus, the SF6 dilution method proved to be valid and reproducible.

Plasma concentrations of macronutrients
To assess the plasma concentrations of macronutrients, (i.e., carbohydrate, protein and fat), the following items were measured by the methods noted in the parentheses; plasma glucose (electrode method), insulin (radioimmunoassay second antibody), albumin (brom cresol green), albumin-globulin ratio, total cholesterol (enzymatic assay), high-density lipoprotein cholesterol (dextran-sulfate and phosphotungstic acid with magnesium) and triacylglycerols (enzymatic assay without glycerol blank).

Statistical analysis
Means and standard deviations were calculated for all variables. Paired Student's t-test was used to determine if significant differences existed between the mental retardation and control groups. Analysis of covariance (ANCOVA) was used to adjust the effect of possible confounding variables by adopting these factors as covariates. Values for subscapular skinfold thickness, thigh skinfold thickness, calf skinfold thickness, plasma insulin, plasma total cholesterol and plasma triacyiglycerols were found to follow log-normal distribution approximately . Paired t-test was performed after converting them to logarithms. All the statistical analyses were performed using SPSS version 6.1 (SPSS. Inc. Chicago, II, USA) . Table 2, a statistically significant difference was observed in the energy cost estimated by indirect calorimetry when sitting, standing and walking at the speeds of 30 and 50m/min, being higher in the MRs than in the healthy control subjects. In contrast, no significant difference was observed for estimated BMR or RMR. It should be noted that most MRs could not keep walking at 70m/min and their actual speed fell to between 50 and 70m/min. The energy cost of MRs underestimated in this condition failed to show a significant difference from that of controls who were actually walking at 70m/min.

Energy cost related factors
Whether expressed as per kilogram body weight or per LBM, Vo2max was significantly lower in MRs than in the control subjects (p<0.05, Table 3). The HRs when sitting, standing and walking at the speeds of 30, 50 and 70m/min were significantly higher in MRs than in the control subjects, whereas HRs at the time of BMR and RMR measurements were similar between the two groups (Table 3).
Acceleration, recorded as an averaged magnitude of 3-dimensional acceleration vectors and adjusted for age, body height and weight, and stride frequency was  significantly higher in MRs than in the control subjects when walking at 30, 50 and 70m/min (Table 4). Interestingly, the accelerometer recorded very small movements among MRs in sitting and standing positions, which were not observed for control subjects ( was significantly higher in the MRs than in the controls ( Table 2) . Among indices of fat-free mass, the means of circumferences at forearm and calf, as well as those of muscle areas at the mid-upper arm and calf , were significantly lower in MRs than in the control subjects (Table 5) . LBM measured by SF6 dilution method was similar between both groups. None of the fat mass indices showed a significant difference between MRs and the control subjects . As for the distribution of body fat, a higher waist-thigh ratio was observed in MRs than in the control subjects at the suggestive level (0.05<p<0.1). The means of plasma total cholesterol and triacylglycerols were much lower in MRs than in the control subjects ( Table 6). The plasma concentrations of albumin and albumin-globulin ratio were also lower in MRs than in the control subjects, while glucose and insulin measured after overnight fasting were not different between the MRs and control subjects ( Table 6).

DISCUSSION
This is, to our knowledge, the first systematic study on the energy metabolism of MRs. The experiment was designed in a case-control manner with individual matching for age, height and weight. Technical improvement was pursued by employing a recently developed SF6 dilution method for assessing the body composition and 3-dimensional accelerometry for measuring body movement quantitatively. The results clearly showed an elevated energy cost in MRs under non-resting conditions ( Table 2). Elevation was observed even under mild tasks such as sitting, standing, and walking as slowly as 30m/min, but not for BMR and RMR, which suggests a very limited capacity for MRs when performing physical tasks.
One possible explanation for the elevated energy cost of MRs is a poor ability to coordinate the body parts when performing physical tasks, which was noticed as a "clumsy" walk. This impression was proved in a quantified manner by measurement of stride frequency and acceleration. Since energy cost, when walking at a fixed speed, is minimized at a preferable stride frequency and increases accordingly as the frequency deviates from there (20), more frequent strides by MRs probably elevate the energy cost. Having a significantly smaller calf circumference and calf muscle area than the control subjects, MRs may have difficulty in walking with strides as wide as their control counterparts . In addition, the higher acceleration observed when MRs walk indicates that they need more movement and, therefore, consumed more energy than the control subjects . Also, the elevated energy costs when sitting and standing are probably due to small movements by the MRs, which were not observable to the eyes but were recorded by the accelerometer. MRs might be making a greater effort to keep their posture than the control subjects when sitting and standing . It is unclear how hereditary and environmental factors contribute to such poor coordination . Whatever the mechanism is, poor physical skill with elevated energy costs may discourage MRs from physical exercise, which can lead to the formation of a vicious circle between a sedentary lifestyle and limited physical capacity .
The significantly lower Vo2max in MRs observed in our study (Table 3) is probably a result of such a vicious circle. This finding is in good agreement with previous reports (2)(3)(4)(5)(6). In those studies, the low level of cardiorespiratory fitness in MRs was explained by their physical inactivity in daily life and by their psychological limitations. Several studies reported that aerobic exercise programs improved the cardiorespiratory functions of MRs and brought their Vo2max values close to normal (6,(21)(22)(23). On the other hand , Par-Or et al (2), Reid et al (24), and Seidl et al (25) noted that physical inactivity could be attributed to limitations inherent to MRs such as poor motor skills and lack of coordination . Lack of motivation in MRs due to their poor understanding of the purpose (3 , 24, 26) may also keep them from exercising. In our study , we had to encourage the MRs to make maximal efforts in pedaling the bicycle ergometer . Some of the MRs would discontinue pedaling at moderate intensity (e.g., 60% of the assumed Vo2max) . The difference of Vo2max might partly explain the difference in energy expended at non-rest between the two groups , since the differences were significantly correlated with each other (r=0.253-0.652). However , we found no physiological mechanism for them.
In addition to the lower maximal cardiorespiratory fitness demonstrated by lower Vo2max, HR when walking was higher in MRs than in the control subjects . This might also suggest a poor level of physiological response to workload by MRs . A higher HR for MRs was also exhibited in supine , sitting and standing positions. Again this may be due to the physical inactivity of MRs , because it is reported that physically inactive individuals show higher HR at rest than those who are more active in their daily life (27 Vol 44, No 1, 1998 In summary, this study has clearly shown that MRs have higher energy cost in performing physical tasks, even very mild ones , as compared with normal control subjects. Although the precise mechanism remains unclear , it is most likely that a sedentary lifestyle, observed commonly among MRs , may be causative of their low physical capacity and high energy cost. Also some unknown metabolic abnormality, as is suggested by low levels of cholesterol, triacylglycerols , albumin-globulin ratio, mid-upper arm muscle area and calf muscle area , may be responsible for the high energy cost in MRs. Further study is needed to clarify these mechanisms as well as to investigate MRs with higher obesity or lower IQ .