Comparisation of different methods of categorization for physical activity on coronary heart disease risk factors

Abstract

Objective: There is a general agreement that physical activity (PA) has a beneficial effect on health and those who are more active have a reduced risk of developing many chronic diseases, such as coronary heart disease (CHD). However, the amount, type and intensity of PA deemed to be sufficient to achieve good health remains unclear. Different methods have been used to categorise activity behaviour, but the level of agreement, consistency and coherence between methods and how this might influence their relationship with CHD risk factors and estimated CHD (eCHD) risk are poorly understood. This uncertainty is reflected in many different messages communicated to the public as to how active they should be to prevent chronic diseases. The primary objective of this thesis was to determine whether the methods used to categorise PA (as either inactive/active or level of PA) influence the extent to which PA is associated with CHD risk factors and eCHD risk.
Methods: This thesis was divided into two parts. The first part was to conduct a secondary analysis of data on activity and CHD risk factors (blood pressure and lipid profile) obtained from the 2004 UK National Diet and Nutrition Survey (NDNS) in 1658 adults aged 19-64 years. Using the information obtained from the NDNS 7-day diary, it was possible to extend the original observations and to re-categorise individuals according to measures of PA in terms of number of days and minutes of at least moderate PA, total activity expressed as metabolic-equivalents (METs) and self-perception of PA. Each of these methods was then used to examine the proportion of the variance in CHD risk factors and the eCHD risk attributable to differences in PA using General Linear Modelling with adjustment for BMI, age and smoking. Partial eta squared a “proportion of variance due to physical activity plus error that is attributed to physical activity alone” was used. In the second part, the concurrent validity of measures of PA derived from the NDNS 7-day diary, using different systems for coding and classifying of different physical activities, was compared against those measures of PA obtained from the International Physical Activity Questionnaire (IPAQ) in a group of medical students (n = 26).
Results: Taken together, this thesis revealed: 1) poor agreement across different methods of categorisation of PA level, 2) no support to justify a curvilinear dose-response relationship between PA level and CHD risk factors and eCHD risk and that a linear model was sufficient, 3) the differences in CHD risk factors or eCHD risk that could be directly attributable to differences in PA in men was modest (generally < 5%) although no associations evident in the women, 4) effect was most obviously demonstrable as improvements in lipid profile, no demonstrable effect on blood pressure, 5) a potential problem might arise when using one system and applying its results to different guidelines established by different systems.
Conclusion: These findings support the view that being physically active is associated with markers of better health and lower CHD risk; a small but consistent effect that was the same irrespective of which method of categorizing PA was used and even after adjustment for differences in age, BMI and smoking. The effects were most evident in men and largely attributable to improvements in lipid metabolism.

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