Not Just a Man's World

After a few weeks of studies about sedentary women, the impact resistance training can have on their weight and how good new trainees are at adhering to their training programmes, I have decided to move the topic sideways.

The main reason I took up weight training several years ago was because I wanted to be able to continue being as active as I am now into my old age.  Specifically I phrased it as wanting to be able to ride my bike as an octogenarian or nonagenarian without worrying about falling off, fracturing every bone in my body due to frail bones and then losing all my health and fitness while I waited long months for the bones to heal.

Next week I’ll be looking at another bone density study, just to really finalise the resistance training for bone density debate, but I felt that I needed to start at the beginning of the story with this female-specific study when they were still establishing easy ways to measure and analyse bone density.

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Can the effects of exercise on bone quality be detected using the CUBA clinical ultrasound system?

Messenger N, Scott S, McNaught-Davis P.  British Journal of Sports Medicine 1998; 32:162-166.  (Free copy of the study here.)

The purpose of the study

It has been well known for some time that weight-bearing activity, especially something as weight-bearing as full resistance training, builds bone density but it was very difficult to measure as bone mineral content and bone mineral density can only be directly measured using a bone biopsy sample.

Since retrieving a bone biopsy requires an intrusive surgical procedure it wasn’t really practical for studies looking at bone density.  However the 1990s saw the development of the CUBA, a clinical quantitative ultrasound bone analyser that measures the velocity of sound (VOS) and broadband ultrasound attenuation (BUA) through the heel of the foot.

The purpose behind this study was to “determine whether the CUBA […] was able to distinguish variations in bone quality between groups categorised according to activity level.”

What are VOS and BOA?

Let’s get to grips with some basic terminology before we get underway with this study, since VOS and BUA are the two things we’ll be measuring.

The VOS measurement will tell us the elasticity and density of the heel bone.  These factors affect the movement of the sound waves through the heel, changing the type of sound wave (longitudinal and transverse) and the speed of the wave depending on its elasticity and density.

The BUA measurement is affected by the density and structure of the bone.  Various studies over the 1980s and 1990s showed that BUA measurements were directly correlated with osteoporosis measures.

The study

This study was incredibly simple, after all the complexities of the studies I’ve looked at in the previous weeks.  The authors contacted 81 white women aged 30-89 in East Sussex in the south of England.  The women were drawn from university staff, a local leisure centre that was running a general practitioner (doctor) referral scheme, some warden assisted retirement homes and sport and leisure clubs in the area.  Once the women understood what was required of them they all agreed to participate, so there was a really big group of participants for this study.

Very simply, each participant completed a questionnaire that provided the authors with detailed information on everything ranging from their age, weight, height and activity levels through to more detailed information, such as expected osteoporosis risk factors like oral contraception, family history of osteoporosis and calcium intake.

VOS and BOA measurements were then taken for each of the women using the CUBA and the results were analysed and compared alongside the information that had been gathered through the questionnaires.

The authors wanted to see if there was a clear correlation between the measurements and activity levels.  If there was then the authors could conclude that, in future studies, these measurements could be usefully used with populations to confirm whether the change being studied affected bone density.

Activity levels assessment

The assessment of activity levels was based on time spent and participant’s self perception of their activity levels.  Four measurements of activity were taken the responses were then divided into three classes and given a score of 0, 1 or 2:

• Number of hours of weight bearing leisure activity per week (0=0-2 hours; 1=2-5 hours; 2=5+ hours).
• Activity levels in the home (0=inactive/relatively inactive; 1=moderately active/active; 2=very active).
• Activity levels at work (0=sedentary; 1=moderate or semimanual sometimes resulting in breathlessness or perspiration; 2=manual often causing breathlessness or perspiration).
• Hours spent working (0=<15; 1=16-25; 2=26+).

By giving each measure a score the authors were able to look at not only the correlation between bone density and each activity measure, but could also calculate a “combined exercise score” based on the mean score of the four measures and reclassify the participants into a more general activity category of below average, average or above average.

Results

I’m not going to reproduce all the results here.  Needless to say that, as far as the authors were concerned they proved their hypothesis that these two measurements would suffice as proxy measures for bone density, specifically when participants are classified by activity levels.  For my write up though I’d like to focus on where the differences appeared.

Data tables have been copied from the original paper and I’ve left them with the original table references in case you want to refer back to the original text.

Age-related bone density decline

To start with let’s look at the results when the population was divided into pre and postmenopausal women.  While this isn’t an exact age-split it does provide a reasonably safe division between younger and older women and there is a very clear difference in the measurements, especially with BUA which, if you recall from earlier, tells us about the structure (eg. the measurement shown below tells us that the bone in postmenopausal women is more porous than in premenopausal women).

This is not entirely surprising, since the general expectation is that bone density declines as women get older unless they actively try to prevent this deterioration.  Something does bother me about this though – have we always seen that bone declines with increased age because the natural human condition in the Western world is to become less active the older we get, or is it genuinely age that I causing the decline?  Even if I maintained my activity levels would I still see a decline in bone density simply because time was passing?  Let’s see if we can learn more by looking on to the activity levels section.

Activity levels and bone density

Here’s the bit many of you were probably interested in.  Some empirical evidence that all that activity you do is really helping you to prevent osteoporosis.

As expected, bone density by the BUA measurement increases noticeably with the increase in activity levels.  While VOS also increases, it doesn’t seem to be a significant movement so perhaps activity levels only really impact the structure of the bone rather than its elasticity and exact mineral content?

In the discussion of results the authors note that “although the mean age of the above average activity level group was lower than the average and below average groups, this was not found to be statistically significant (p>0.05).”  In other words, you might at first glance think that we can’t get any further down the consideration of whether the age or actually activity levels are the driving factor in what is believed to be age-related bone density decline since the most active group are also younger.

But there’s not a statistical difference, suggesting that there is a fairly even division of older people throughout all three categories.  If this is the case then it looks like we might have to seriously consider that age-related bone density decline is a problem we need to be prepared for.  However, it’s worth noting that it would take something significant to have a statistically significant age difference, so perhaps we can’t really draw this conclusion until we’ve got an even larger study population to look at (and looked at a few more studies).

Osteoporosis risk factors and bone density

There is just one set of results I’d like to pull out here – the ones for oral contraceptive pill use.

At a first glance here we’ve got major cause to think hard about our oral contraception – should we be taking oral contraceptives as our preferential way to deal with contraception given the apparent beneficial bone density preservation that seems to be bestowed by it.  One theory put forward in various places (most notably by companies encouraging the use of oral contraceptives) is that the longer elevation of oestrogen levels provided by the contraceptive protects bone density and this could, at first glance appear to be plausible so should we believe them?

Well, it turns out that this study is not the one to rely on for this evidence.  The authors state that “only four women aged over 60 had taken the pill, and, when this age group was removed from the study, no significant differences were obtained.”  In other words, this study doesn’t support the idea that oral contraceptives help prevent loss of bone density.  That doesn’t mean that there isn’t a possibility of this being the case, we just can’t prove it here.  I suspect a larger population of women would need to be studied to look at this question.  They would all need to be in a younger age bracket and have therefore had oral contraceptives available to them as a realistic contraceptive option throughout their fertile life to date.  In this study our data was skewed by a population who were going through some of their most formative bone density years before oral contraceptives were widely available or approved of.

Calcium intake and bone density

I’ve previously noted on this blog that the ‘increase your calcium to improve bone density’ argument is a fallacy since the additional calcium is not used to create additional bone unless something tells your body that it needs that extra bone.  In this I am supported by the study where the authors advise “there were no significant differences in BUA or VOS between low, medium, and high calcium intake groups (p>0.05)”.

Height and weight

Finally I would just like to quote something that the authors put in their final discussion:

“Height and weight were highly significant variables affecting bone quality. An investigation into the ponderal index indicated that no subject was underweight for her height, a risk identified in a study by Falch et al.45 Height and weight not only increases the loading on the bones, but, according to Sowers et al,46 more fat tissue results in an increased production of oestrogen which slows bone loss. None of these factors correlated significantly with age.”

Well I’m average height and until I started weight training I was at the bottom end of healthy weight, since I’m 5’6” and was always 50kg.  These days I weigh a much more respectable 56kg so perhaps just by putting on extra muscle mass I’ve done something to improve my bone density.  It certainly makes sense that walking about with additional weight would increase loading on bones and the bones would subsequently remain denser to handle the extra load.  I’m not sold on that final comment about the additional fat tissue increasing oestrogen production and this slowing bone loss.  This seems like one connection too many for me – I’ll have to go and find some studies to convince me.

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So there you have it.  This week’s study tells us quite a few things we already knew about bone density but gives us some empirical data to support it.  It also suggests we may need to cast our nets wider to look at bone density and oral contraceptive use and hopefully I’ll be able to find something about that in due course.  In the meantime, next week’s study will get a whole lot more specific about that activity.  I’m going to be looking at a study that compared bone density in competing powerlifters to a control group of non-powerlifters.  Unfortunately it is a study on male subjects, but it seems a valuable companion to this piece.

Were you surprised by any of the results in this week’s study?  Did you think calcium intake or oral contraceptives would create a more statistical difference?  What are your thoughts on the age vs. activity debate.  Do you agree that activity is a confounding factor in any comparison of age groups?

Related posts:

1. Blog-watch: recent bone density research
2. Blog-watch: acid-base and bone density
3. Blog-watch: Bret’s female strength levels
4. Blog-watch: the female athlete triad