“My pants are feeling tighter, is this good or bad?” Have you ever had a similar thought?
Sometimes it’s hard to know if we’re making progress on our body composition (levels of lean mass and fat mass in the body). The scale often doesn’t tell the truth, as we may be getting heavier but leaner (by increasing muscle mass while losing or maintaining fat mass), or lighter but fatter (by losing muscle mass while increasing or maintaining fat mass).
So how do we measure our body composition?
The gold standard is called DEXA scan, it’s as precise as it gets and tells us how much muscle, fat and bone mass we have. It requires a specific type of X-Ray machine and it takes about 30min. To our knowledge, there’s one company(1) in San Diego that provides it, and it costs $45-75 per assessment. It’s important to keep in mind that this type of scan emits a small dose of radiation, so some people (like pregnant women) shouldn’t do it.
Other lab tests that are also very precise are the hydrostatic weighing and the air displacement plethysmography. The former requires you to wear a swimsuit and submerge in a water tank for a few seconds. There’s a company in SD (2) that provides it and it costs $35-50. The latter requires you to sit inside a pod for a couple minutes, wearing compression shorts, swim cap and a sports bra. Another company here in SD offers it for $85-95 (3). They’re both quick, about 30min.
Alternatively, we have tests that don’t have the same level of precision, but don’t require you to go anywhere. The most common ones are the Bioelectrical Impedance (BIA) and skinfold measurements.
BIA requires a specific scale in which a harmless electrical current runs through the body. It only takes a few seconds. Those scales can be as cheap as $25 and as expensive as $10k. The ones that involve the upper and lower body are more precise. Measurement error is about 3-6% and specific procedures should precede the test (No previous exercise for 12h, no alcohol for 48h, no eating or drinking for 4h. Pee 30 min or less before the test, and women should avoid measuring prior to menstruation).
Skinfold measurements can be made by your trainer, it’s a bit invasive as several skinfolds need to be pinched, to measure the fat directly below the skin. The measurement error is around 3.5%, assuming proper technique is used by the evaluator and appropriate regression equations are used to estimate the body fat. (4)
And finally, we have the most common methods, the scale and the photos. As mentioned before, the scale’s big limitation is the lack of differentiation between lean mass and fat mass, so we only recommend its use as an additional tool. It doesn’t make sense to measure your weight daily, as day-to-day variations are mostly due to water retention (we don’t build/lose enough muscle or fat in a day so that the scale can detect it). Once a week or once a month makes more sense.
Progress photos are easy and cheap, and there are many free apps that can help make the comparisons. Taking the photos in the same place, with the same lightning and at the same time of the day is recommended. Combining the weight on the scale with the progress photos is great, as that combination doesn’t lie.
Body composition is one of the most important factors for health and longevity, not just for aesthetics. Hopefully this post gives you enough insight regarding how to track your body comp.
See you at the gym!
Willian Alba
ACSM’s resources for the Exercise Physiologist
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