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Editorial

Referring to the article published on pp. 931

939 of this issue

Why Epidemiological Studies of Physical Activity in Prostate

Cancer Often Underestimate its Benefits

Mary Kathryn Downer

*

Harvard T.H. Chan School of Public Health, Boston, MA, USA

Wang et al

[1]

conducted a rigorous analysis investigating

physical activity and survival after prostate cancer diagno-

sis. Major strengths of this study include a large sample size,

almost 20 yr of follow-up, and information on physical

activity both before and after prostate cancer diagnosis.

Among men later diagnosed with nonmetastatic (Mx/M0)

prostate cancer, those with higher levels of prediagnostic

physical activity (equivalent to 4.4 h/wk of brisk walking)

had a 37% lower risk of progression to prostate cancer

specific mortality compared to sedentary men. Postdiag-

nostic recreational physical activity was associated with a

lower risk of progression to prostate cancer

specific

mortality, regardless of tumor stage.

These findings contribute valuable evidence to the

growing body of literature supporting the benefits of

physical activity on prostate cancer progression. Moreover,

because of inevitable methodological challenges inherent to

epidemiological studies, it seems likely that these remark-

ably strong inverse associations between recreational

physical activity and prostate cancer are underestimated,

for several reasons.

First, there is a substantial measurement error for

physical activity assessment, causing an underestimate of

the magnitude of the inverse associations. Self-reported

(and objectively measured) exposures are always subject to

measurement error, but physical activity is particularly

difficult to report accurately, as it occurs throughout the day

with large within- and between-person variations in type,

duration, and intensity. Most of the validated physical

activity questionnaires list various types with duration

categories for each. Each physical activity item is typically

assigned a single standardized metabolic equivalent of task

(MET) value to represent the typical intensity and metabolic

expenditure per hour. This minimizes both participant and

investigator burden and allows for comparison across

studies. It is often assumed for physical activity and other

exposures alike that a measurement error exists but is

nondifferential with respect to the outcome of interest. If

this is the case, associations are almost always biased to the

null hypothesis. However, in analyses such as the current

one, the measurement error is probably differential with

respect to the outcome. It is likely that the true intensity and

metabolic expenditure for each physical activity item are

higher for healthier men and lower for men who are

becoming sicker because of their progressing disease. Thus,

in epidemiological studies of physical activity, total activity

is probably underestimated for healthy men and over-

estimated for unhealthy men, including those with more

aggressive prostate cancer. This underestimation for

healthy men and overestimation for men more likely to

die of prostate cancer will bias the protective association

still further to the null hypothesis. Furthermore, this

measurement will not reflect a decrease in physical activity

intensity, only a decrease in duration. As a consequence, this

may not sufficiently rank people; those with more intensity

but shorter duration may be assigned fewer MET-h/wk than

those with lower intensity but longer duration. This is a

common problem with physical activity assessments in

epidemiological studies, particularly when physical activity

is self-reported. The problem can be mitigated by asking for

further details on intensity within each item (eg, pace for

walking, jogging, and running). However, these estimates

may be imprecise and increase participant burden on

questionnaires that are often already long. Adjusting for

E U R O P E A N U R O L O GY 7 2 ( 2 0 17 ) 9 4 0 9 41

available at

www.scienced irect.com

journal homepage:

www.europeanurology.com

DOI of original article:

http://dx.doi.org/10.1016/j.eururo.2017.06.037 .

* Harvard T.H. Chan School of Public Health, Epidemiology, 677 Huntington Avenue, Boston, MA 02115, USA. Tel. +1 206 2288067;

Fax: +1 617 5667805.

E-mail addresses:

mkd690@mail.harvard.edu , downerma@gmail.com

.

http://dx.doi.org/10.1016/j.eururo.2017.07.027

0302-2838/© 2017 European Association of Urology. Published by Elsevier B.V. All rights reserved.