Comparison 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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