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Body Composition, Weight, and BMI: What they Mean for an Athlete.

Athletes like numbers – personal bests, max reps, power output, anaerobic threshold, VO2 max…you name it we like to measure it.  And goals. We like setting goals, too.  So it’s not surprising I get lots of questions regarding body composition, weight, and BMI. Typically it’s something like, “What’s my ideal number?” Seems a simple enough question yet it’s one of the most complex to answer. And here’s why…

I don’t know the answer. And frankly you’ll be hard-pressed to find someone that does without a little investigation and trial and error.   Generic, cookie-cutter numbers based on age, gender, and sport do exist, but I like to consider these just a starting point before really getting down to the business of identifying an athlete’s individualized numbers (and how frequently, if at all, they need to pay attention to them).  So let’s break ‘em down.


This is pretty much as basic as it gets.  It literally refers to the downward force created by the mass of the body.   Oddly enough the definition of ‘weight’ as a verb is to attach importance or value to something.

Freaky. We love to attach importance to the downward force created by our body mass.

But it’s soooo one dimensional.  It provides no picture of fat mass versus fat-free mass (we’ll talk about why that’s important later).

Yes, weight is associated with health and performance, and certain sports even have weight classes (wrestling, boxing, rowing, etc.).  But an ideal weight can be different for different people, and a reduction in power or endurance can come at the expense of picking an arbitrary number. Plus weight can shift several pounds in a day due to hydration status and glycogen stores.

So before you cut out an entire daily meal to lose 5 more pounds let’s talk about if the weight loss will be a benefit or hindrance to you.


This is where we take one basic number (weight), put it into a mathematical formula with another basic number (height) and come up with a value that defines us as:

                Underweight (< 18.5)

                Normal (18.5-24.9)

                Overweight (25.0-29.9)

                Moderately obese (30.0-34.9)

                Severely obese (35.0-39.9)

                Very severely obese (>40.0)

The formula was first developed in the 19th century (it’s called the Quetelet Index) and it was meant to provide an indicator of weight status (and thus disease risk) for general populations. It was never meant to provide feedback for individual athletes who typically carry more muscle – thus skewing the results.  So for athletes it’s really like the end link of a dangling chain – it can hang around but if it went away we wouldn’t really pay much attention to it.

Body Composition

So now we’re getting a little more specific. Assessing the composition of the body refers to identifying the proportionate amounts of various tissues such as bone, muscle, connective, adipose, etc.   (Note:  Not all methods of assessing body composition provide all of this data, nor are all methods equally accurate – but this is an issue for a whole other post.)  While I call weight one dimensional, I consider body composition to be the 3D experience (minus the nauseating visual effects).

Most folks can break it down into fat mass (adipose tissue) and fat-free mass (everything else). There are certainly other medical uses such as identifying bone density, but for this discussion we’ll keep it simple.

Now don’t freak out on me because I wrote the word “fat”. We all have it; we all need it.  But excess fat is linked to both an increased risk of disease and decreased performance, in particular in sports that require moving the body horizontally or vertically (think running or jumping). It doesn’t do much for us, unlike muscle.  Excess fat is like your 14-year old brother on a car trip – he can’t really drive so he’s just along for the ride and pestering you along the way.

But we also need to look at the opposite effect. Can we have too little body fat? Absolutely.  In which case we may also see increased health problems and decreased performance. 

There are general ranges for body fat in different sports floating around out there (I’m not even going to write them here as human nature dictates they would probably permanently imprint on your brain), but in truth specific body composition recommendations haven’t really been identified. And furthermore, a body composition that works fantastically well for one person could be abysmal for another.

So consider trying this. Look at your training. Look at your nutrition. Look at your performance. Aim to make goals and changes in those areas and use your changes in weight and body composition as points of reference along the way to hone in on your target numbers.  Working with a dietitian, nutritionist, or exercise physiologist can do wonders for helping you identify an ideal weight and body composition range.


Ackland TR, Lohman TG, Sundgot-Borgen J, et al. Current Status of Body Composition Assessment in Sport: Review and Position Statement on Behalf of the Ad Hoc Research Working Group on Body Composition Health and Performance, Under the Auspices of the IOC Medical Commission. Sports Med. 2012; 42(3): 227-249.

Burke L, Deakin V. Clinical Sports Nutrition. 4th Edition. North Ryde, Australia. Mcgraw-Hill Australia Pty Ltd. 2010.

Eknoyan G. Adolphe Quetelet (1796-1874)-the average man and indices of obesity. Nephrol Dial Transplant. 2008; 23: 47-51.

Rosenbloom CA, Coleman EJ. Sports Nutrition: A Practice Manual for Professionals. 5th Edition. 2012.

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