Estimating energy expenditure by heart-rate monitoring without individual calibration.

UNLABELLED: Heart rate monitoring has been shown to be a valid method for measuring free-living energy expenditure at the group level, but its use in large-scale studies is limited by the need for an individual calibration of the relationship between heart rate and energy expenditure. PURPOSE: To determine whether energy expenditure can be estimated from heart rate monitoring without individual calibration in epidemiological studies. METHODS: Our previously validated heart rate monitoring method relies on measuring individual calibration parameters obtained from resting energy expenditure and the regression line between energy expenditure and heart rate during exercise. We developed prediction equations for these parameters using easily measured variables in a population-based study of 789 individuals. The predictive ability of these parameters was tested in a separate population-based sample (N = 97). RESULTS: Physical activity level (PAL = total energy expenditure/basal metabolic rate) using the four estimated parameters was correlated with PAL using the measured parameters (r = 0.82, P < 0.01). Comparison of measured and estimated PAL showed that 97.9% of the scores were placed in the same or adjacent quartile. CONCLUSION: A combination of simple measurements and heart rate monitoring produces estimates of energy expenditure that are highly correlated with those obtained using full individual calibration. This simplification of the heart rate monitoring method could extend its use in ranking individuals in epidemiological studies.

Feasibility of heart-rate monitoring to estimate total level and pattern of energy expenditure in a population-based epidemiological study: the Ely Young Cohort Feasibility Study 1994-5.

Increasing the precision of measurements of total energy expenditure in population-based epidemiological studies is important for accurately quantifying the relationship between this exposure and disease. Current questionnaire-based methods cannot accurately quantify total energy expenditure, although they may provide an estimate of the frequency of vigorous activities. Heart rate monitoring with individual calibration has been advocated as a method for assessing energy expenditure in field studies and has been compared with the 'gold standard' techniques of doubly-labelled water and indirect calorimetry. However the method has previously only been used on small and selected populations. This study was, therefore, established to test the feasibility of using heart rate monitoring in a population-based study of adults. A total of 167 individuals aged 30-40 years were randomly selected and underwent 4 d heart-rate monitoring. Only three individuals could not complete the protocol. The mean physical activity level (PAL) measured over 4 d was 1.89 (SD 0.40) in men and 1.76 (sd 0.31) in women. There was no difference between mean PAL on weekend days compared with weekdays (mean paired difference 0.0008, 95% CI -0.06 +0.06). The estimate of mean PAL was not correlated with BMI, percentage body fat or the waist:hip ratio. It was, however, correlated with cardio-respiratory fitness as measured by VO2(max) per kg (Spearman rank correlation coefficient 0.50 in men and 0.42 in women). The pattern of energy expenditure was assessed by calculating the percentage of daytime hours in which PAL was greater than five times basal energy expenditure. This measure was strongly correlated with the mean PAL in both men (Spearman correlation coefficient 0.77) and women (0.71). We conclude that heart-rate monitoring is a feasible method for assessing the pattern and total level of energy expenditure in medium-sized epidemiological studies. It may also prove useful as the reference technique for calibrating questionnaires to estimate energy expenditure in larger scale studies.

A quantitative analysis of the relationship between habitual energy expenditure, fitness and the metabolic cardiovascular syndrome.

Previous epidemiological studies have suggested an association between low levels of physical activity, fitness and the metabolic cardiovascular syndrome. However, many studies have used subjective non-quantitative questionnaire-based methods for assessing physical activity which do not distinguish between the different dimensions of this complex exposure, and in which measurement error in the exposure has not been estimated. These deficiencies in the measurement of this exposure complicate the interpretation of the results of epidemiological studies, and consequently make it difficult to design appropriate interventions and to estimate the expected benefit which would result from intervention. In particular, it is unclear whether public health advice should be to increase total energy expenditure, or to attempt to raise fitness by recommending periods of vigorous activity. To separate the effects of fitness and total energy expenditure in the aetiology of the metabolic cardiovascular syndrome, we measured the physical activity level (PAL), defined as total energy expenditure: BMR, and fitness (maximum O2 consumption (VO2max per kg), measured in a sub-maximal test) in a cross-sectional population-based study of 162 adults aged 30-40 years. Heart-rate monitoring with individual calibration was used to measure total energy expenditure using the HRFlex method (Ceesay et al. 1989) which has been validated previously against doubly-labelled water and whole-body calorimetry. The relationship between a single measure of PAL, VO2max per kg and the usual or habitual level for each exposure was measured in a sub-study of twenty-two subjects who undertook four repeated measures over the course of 1 year. This study design allows the reliability coefficient to be computed, which is used to adjust the observed associations for measurement error in the exposure. Twelve men (16.4%) and sixteen women (18.0%) were defined as having one or more features of the metabolic cardiovascular syndrome. The univariate odds ratio for each increasing quartile for PAL was 0.64 (95% CI 0.43-0.94) and was 0.49 (95% CI 0.32-0.74) for VO2max per kg, suggesting that the association with the metabolic cardiovascular syndrome was stronger for fitness than for PAL. However, after adjustment for obesity and sex, and correction for exposure measurement error, the odds ratio per quartile for PAL was 0.32 (95% CI 0.13-0.83) and 0.44 (95% CI 0.24-0.78) for VO2max per kg. Thus, although univariate analysis would suggest that fitness has a stronger association with the metabolic cardiovascular syndrome than PAL, this conclusion is reversed once confounding and the differences in measurement error are considered. We conclude from the present study that the metabolic cardiovascular syndrome is strongly associated with reduced habitual energy expenditure. The method employed to assess the exposure in the present study demonstrates the utility of assessing a known dimension of physical activity using a physiologically-based and objective measure with repeated estimation to adjust for measurement error. Such quantitative epidemiological data provide the basis for planning and evaluating the expected benefit of population-level interventions.