Digging into the Bodyrecomposition Mailbag

Since I can’t think of any actual topic this week and think I can answer these questions in a fairly brief manner, I’m just going to clear out some of my questions.  Topics will include women and DEXA, oddities about pregnancy metabolism, endurance nutrition and a bit about health habits when you have bipolar.

Question: Hi Lyle, I recently did a DEXA scan to get an estimate of my body fat % – I’m female, 28 years old, weight train regularly. My DEXA scan results: 5’3, weighing 59kg and an ‘average’ of 20% body fat. I say ‘average’ as the scan showed that the body fat % in my upper body (arms, torso) was 14% while my lower body (hips, thighs) was 27%. Is it fair for me to take the average of these and consider myself to be 20% body fat?

Answer: The above type of question is becoming more and more frequent as more people are using DEXA scans to get an estimate of bodyfat percentage (BF%).   Because of the way DEXA works, it will churn out different BF% for the upper and lower body and this can be confusing.  First and foremost, the above is a completely normal female body fat patterning as women generally carry more body fat in the lower body than upper body (a male would typically have the opposite pattern).  I say generally as there can be exceptions where women have either a very even body fat pattern or more of a male-like upper body fat patterning; the latter typically occurs in situations were testosterone is above normal.  So far as the main question, should the values be averaged to get a usable BF%? The answer is yes.  In any of my books and most discussions of applications of BF%, it’s whole-body BF% that is relevant, not the individual distributions.

Question: I was wondering if you could make sense of this article in terms of what you know about human metabolism:

Answer: The paper in question deals with the issue of resting metabolic rate (RMR) and it’s determinants.  Normally, the largest determinant of RMR is lean body mass (LBM).  Here I’m talking about all LBM, not just muscle.  Organs, brain, etc. actually burn far more calories per day than muscle and LBM predicts a majority (off the top of my head: 75% or so) of RMR.  But this study found that in pregnant women, it was the amount of body fat that predicted RMR, concluding that: