Steve

This sounds like a great paper.

Glad you can cope with the concept of VO2max and find it more comprehensible
than hyperbilirubinaemia (note that most doctors would probably see things
the other way round).

What I tell my patients is that they should be doing an hour of gentle
activity five times a week, and your question prompted me to look out some
papers in my files to see if that was evidence-based.  

As William Haskell said (see Med Sci Sports Ex June 1994; 649-660), there
needs to be a paradigm shift from "training to produce physical fitness" to
"physical activity to promote health".   For exercise levels, such as
walking, the general thinking these days is that "20 minutes of
sweat-provoking exercise three times a week" is somewhat old hat. It's
generally thought that exercise should be sustained for longer, and take
place on more days, than the 20 minutes three times a week we used to be
taught, but that it needn't be at a really high pace, nor need it be taken
in one continuous bout. This is because (as I understand it) effective
exercise works not by "strengthening the heart" (how did we all fall for
THAT in the first place?) but by reducing the propensity for the blood to
clot in the coronary vessels - through a combination of effects on lipids,
fibrinogen etc.  

Haskell's paper broadly supports that view, and gives some figures that may
be useful:

1.  Exercise that occurs at less than 60 percent maximum theoretical heart
rate (220 minus age) produces no health benefit.

2.  The difference in mean energy expenditure per day between the 'lowest
activity level' study participants and the next group up which had a
significantly better CHD mortality, was between 150 and 400 calories per
day, depending on the study.

3.  Haskell produced a 'dose response curve' of physical activity level
against the odds ratio for CHD mortality, based on 6 studies available at
the time.  I don't quite understand his units, but it looks like people with
four times the level of energy expenditure of the least active participants
(presumably that's energy expended over and above BMR) have an odds ratio of
between 0.4 and 0.7 for CHD mortality, and those with eight times 'slob'
level activity have an odds ratio of  between 0.15 and 0.6, depending on the
study.  But I can't find in his paper how much energy the slobs expended!


Anecdotally, as an ex-serious athlete, a change in my VO2max of 5% comprised
the difference between winning a major race and finishing halfway down the
field.  And it was damn hard to achieve.  Not sure how far that might
extrapolate to couch potatoes.

For energy intake, you surely want to express desired change in terms of
what it would mean for weight control - for example, a weight loss of 1Kg
per week (or if their energy intake was that much too high, preventing a
weight gain of that amount per week!).  In clinical practice I reckon that
1Kg per week is about the 'right' pace for weight loss in moderate obesity,
though I have no evidence to support that at all!

Finally, you might like to look at a paper 'The validity of single item,
self assessment questions as measures of adult physical activity'.  T Weiss
et al.  J Clinical Epidemiology 1990, which looks at how much store you can
set by people saying they take this or that amount of exercise (answer:
some, but not much).  

Sorry these refs are a bit outdated but they may be  better than nothing! 

I'd be most interested in a copy of any teaching notes you produce on the
latest JAMA paper, which sounds like it could be GTM (good teaching
material) for a multidisciplinary audience.

trish


At 09:02 24/02/99 -0600, you wrote:
>I'm going to give a talk tomorrow about confidence intervals, and I will be
>using examples from a paper on exercise
>
>Dunn AL et al (1999) Comparison of Lifestyle and Structured Interventions to
>Increase Physical Activity and Cardiorespiratory Fitness, JAMA 281(4)
>327-334.
>
>I chose this paper because it doesn't have words like "hyperbilirubinemia"
>that I always mispronounce and barely understand.
>
>The basic conclusion of this paper is that both a traditional program of
>structured exercise and a newer program that also includes lifestyle changes
>are effective in increasing physical activity and fitness, but that there is
>no substantive difference between the two interventions.
>
>I want to get a discussion going about whether the size of the changes
>indicated by confidence intervals in this paper are clinically relevant.
>Perhaps some members of this list would be willing to share their expertise
>on what size difference they would hope to see and why.
>
>The primary outcome variable is energy expenditure (kcal/kg per day). In the
>methods section, the authors imply that an improvement of 2 units would be
>considered relevant. Although both interventions showed an improvement, the
>confidence limits were well below this target (0.42 to 1.25 and 0.25 to
>1.12). In the discussion section, the authors nicely point out that "even
>though the mean increases were statistically significant, some may not
>consider them to be practically significant."
>
>They then mention that changes which may seem small from an individual
>viewpoint may still be important from a public health context.
>
>So the question becomes, is it worthwhile to find a change of about 0.5 to 1
>kcal/kg per day over 24 months for a targeted intervention of structured
>exercise and/or lifestyle changes?
>
>Some of the secondary endpoints are interesting also. How much of an
>improvement would be considered clinically relevant for the following
>outcomes:
>
>Walking (min/day)
>
>VO2peak (ml/kg per minute)
>
>Submaximal heart rate (beats/min)
>
>Body fat (percentage)
>
>Total cholesterol (mg/dL)
>
>Systolic and Diastolic blood pressure (mm Hg)
>
>I'm very interested in HOW you arrived at the decision about what you would
>consider clinically relevant for these endpoints.
>
>Any comments received today (Wednesday, February 24) would be greatly
>appreciated, but late comments will also be helpful as I hope to repeat this
>talk for other groups.
>
>Steve Simon, [log in to unmask], Standard Disclaimer.
>STATS - Steve's Attempt to Teach Statistics: http://www.cmh.edu/stats
>
>
Dr Trish Greenhalgh

Senior lecturer in primary health care
Unit for Evidence-Based Practice and Policy
Department of Primary Care and Population Sciences 
University College London and Royal Free School of Medicine
Whittington Campus
London N19 5NF

Personal Assistant and Unit Administrator (Marcia Rigby):  + 44 (0) 171 288 3246
Fax: + 44 (0) 171 281 8004
email [log in to unmask]

Websites

Unit for Evidence Based Practice:
http://www.ucl.ac.uk/primcare-popsci/uebpp/uebpp.htm

MSc in primary care:  http://www.ucl.ac.uk/primcare-popsci/msc/index.html





%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%