Susie Hennings

Validity and repeatability of a simple index derived from the short physical activity questionnaire used in the European Prospective Investigation into Cancer and Nutrition (EPIC) study.

OBJECTIVE To assess the validity and repeatability of a simple index designed to rank participants according to their energy expenditure estimated by self-report, by comparison with objectively measured energy expenditure assessed by heart-rate monitoring with individual calibration. DESIGN Energy expenditure was assessed over one year by four separate episodes of 4-day heart-rate monitoring, a method previously validated against whole-body calorimetry and doubly labelled water. Cardio-respiratory fitness was assessed by four repeated measures of sub-maximum oxygen uptake. At the end of the 12-month period, participants completed a physical activity questionnaire that assessed past-year activity. A simple four-level physical activity index was derived by combining occupational physical activity together with time participating in cycling and other physical exercise (such as keep fit, aerobics, swimming and jogging). SUBJECTS One hundred and seventy-three randomly selected men and women aged 40 to 65 years. RESULTS The repeatability of the physical activity index was high (weighted kappa=0.6, ). There were positive associations between the physical activity index from the questionnaire and the objective measures of the ratio of daytime energy expenditure to resting metabolic rate and cardio-respiratory fitness As an indirect test of validity, there was a positive association between the physical activity index and the ratio of energy intake, assessed by 7-day food diaries, to predicted basal metabolic rate. CONCLUSIONS The summary index of physical activity derived from the questions used in the European Prospective Investigation into Cancer and Nutrition (EPIC) study suggest it is useful for ranking participants in terms of their physical activity in large epidemiological studies. The index is simple and easy to comprehend, which may make it suitable for situations that require a concise, global index of activity.

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.

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.

Non-esterified fatty acid levels and physical inactivity: the relative importance of low habitual energy expenditure and cardio-respiratory fitness

The fasting concentration of non-esterified fatty acids (NEFA) and the degree to which it declines during an oral glucose tolerance test are closely associated with insulin resistance and glucose intolerance. However, relatively few studies have described possible environmental determinants of NEFA concentrations. Physical activity is likely to be related to NEFA levels, but habitual activity level is difficult to quantify in epidemiological studies. In particular, it is unclear whether NEFA is more closely related to cardio-respiratory fitness or to habitual energy expenditure. In order to quantify these relationships, we analysed data from the Ely prospective population-based study in which 931 subjects underwent a glucose tolerance test with measurements of cardio-respiratory fitness and 4 d energy expenditure by heart-rate monitoring, a technique previously validated against whole-body calorimetry and doubly-labelled water. In order to estimate the latent variables of usual fitness and energy expenditure, a subset of 190 subjects underwent repeat testing on three further occasions over 1 year. In analyses adjusting only for age and sex, energy expenditure and cardio-respiratory fitness were both negatively correlated with the total area under the NEFA curve following the oral glucose load (standardised beta coefficients -0.030 and -0.039 respectively; both P<0.001) However, further adjustment for degree of obesity and bivariate measurement error suggested that the effect of energy expenditure was significantly greater than that for fitness (-0.047 and -0.005 respectively). These results suggest that the area under the NEFA curve in the oral glucose tolerance test, a measure of insulin sensitivity, is strongly associated with the habitual level of physical activity.