Body fat measurement: getting to the truth

Athlete testing

Because of this, amateur athletes and their coaches often rely on simpler and cheaper body composition analyzers such as those using bioelectrical impedance analysis (BIA – devices that send a small electrical current through the body to measure water content then deduce muscle fat mass using equations) or simple skinfold measurements (which involves measuring skinfold thickness at a number of specific sites using special skinfold calipers then applying standardized equations to calculate fat and lean body mass).

But while these simpler devices can be useful to track body composition changes over time, there is a problem; BIA devices they have been developed for the general (largely sedentary) population rather than for use by endurance athletes such as runners, cyclists and triathletes^(3,4). Athletes tend to have much greater variations in hydration status and in skeletal muscle tissue. Meanwhile, skinfold measurements assume a standardized regional distribution of body fat stores tissue, but the distribution in athletes is likely to be significantly different^(5,6). Also, skinfold measurements can produce variable results depending on the number and location of sites sampled, the skill of the technician and the equations used⁽⁷⁾.

This means that when athletes use BIA and skinfold measurement to assess body composition, there are likely to be significant biases and estimation errors due to differences in body density, water content, and muscle mass. In plain English, body composition analyzers developed for the general public are already subject to a degree of error, but in athletes, these errors may be significantly larger due to differences in hydration, muscle mass, body fat distribution and bone density, which are typically induced about when an athlete undergoes a period of serious training! What would be useful is some data on how BIA and skinfold measurement stack up against each other and how they compare to the gold standard (DXA) when used in trained athletes. Fortunately, we can turn to some new research on triathletes for some answers.

New research

Published in the journal ‘Nutricion Hospitalaria’, this research by a team of Mexican sports scientists sought to discover which of these two commonly used body composition measurements (BIA/skinfold measurement) most closely aligns with the ‘gold standard’ DXA results when used for well-trained triathletes⁽⁸⁾ In particular, they wanted to find out what happens when body composition measurements are taken by expert technicians under rigorously controlled conditions – ie with as much user error eliminated as possible.

To do this, the researchers recruited 26 triathletes (18 men and 8 women) with an average age of approximately 34 years. All the triathletes were keen amateurs, training at least five times per week, with at least six months triathlon training background. In a random order, all of the 26 triathletes underwent three types of body composition assessments on the same day. These were as follows:

·        DXA scanning – this entailed a full-body scan using a ‘GE Lunar Prodigy densitometer’. This provided accurate data on three types of tissue in the body: bone mineral content, lean mass, and fat mass. The results from this DXA were then used as the reference point.

·        BIA – this was carried out using an ‘In-Body 720 multi-frequency analyzer’, which estimated the triathletes’ body water content based on electrical current resistance, which was then used to calculate body fat percentage.

·        Skinfold measurements – the researchers measured eight different skinfold sites (triceps, subscapular (just below the shoulder blade), biceps, iliac crest (top of the hip bone, roughly where people place their hands when ‘putting hands on hips)’, supraspinale (side of the torso, in the lower abdominal/upper hip region, slightly above the hip bone), abdominal, front thigh, and the calf - see figure 1. These measurements were taken using a strict protocol set out by the International Society for the Advancement of Kinanthropometry (ISAK) and then used a well-validated mathematical formula known as the ‘Withers equation’, which is frequently used for determining body fat percentage in athletic populations.

To ensure consistency and a fair comparison, all participants observed strict dietary and hydration protocols prior to measurements being taken⁽⁹⁾. This involved:

·        A minimum of eight hours of fasting prior to testing.

·        At least 24 hours without performing physical exercise.

·        A minimum of 48 hours without consuming any of alcoholic beverages or any medication that might affect hydration.

·        For the female triathletes, the measurements were timed between the follicular phase, ovulatory phase, and up to 7 days before the end of the luteal phase. This was to ensure no fluid-related alterations associated with the menstrual cycle.

Figure 1: Common skinfold sampling sites

All the triathletes were assessed in the morning using all three methods before breakfast and to make doubly sure that hydration levels were in the reference range, the triathletes’ urine specific gravity was tested before any body composition measurements, with values between 1010 and 1020 used to indicate proper and complete hydration⁽¹⁰⁾.

What they found

The first finding was that the three different methods of measuring body composition showed significant differences in the results they produced, highlighting the fact that they are not interchangeable. This means that to track changes in body composition in an athlete, the same method should be used throughout. Just as important were the findings comparing BIA and skinfold measurements to DXA scans (the gold standard reference). On average, BIA method showed a tendency to underestimate body fat percentage, especially in the male triathletes.

To make matters worse, BIA measurements had ‘wide range of error’. In short, there was a lot of ‘spread’ of body fat estimation around the actual fat percentage as determined by DXA. This means that for any individual athlete, while there’s a tendency on average to underestimate body fat, BIA might still report a fat percentage that is significantly higher or lower than the true value, thereby increasing the uncertainty of the result. As the researchers pointed out, this is likely due to the fact that BIA is very sensitive to hydration levels, and the fact that the typical physiological traits of athletes (eg higher total body water and different glycogen storage characteristics) are different to those in sedentary adults.

The skinfold measurements also tended to underestimate body fat percentage compared to DXA, but unlike BIA, there was no significant discrepancy in the results between males and females. More importantly, the skinfold results were more tightly correlated to the DXA results, with a correlation coefficient of 0.876 compared to 0.697 for the BIA results. [NB: the closer a correlation coefficient is to 1.0, the stronger the linear relationship between two variables] This indicates that the skinfold measurements produced around 57% less variance from the true DXA result compared to BIA, showing a much tighter relationship and less likelihood of error compared to DXA. In short, skinfold measurements provided a more reliable and predictable estimate of body fat in triathletes – a finding that aligns with previous research suggesting that manual measurements of subcutaneous fat (ie skinfolds) are often more reliable for lean athletes than BIA⁽¹¹⁾.

Implications for athletes

What are the main takeaway points from this new research? The first thing to say is that relying on BIA measurements to get accurate body fat percentage measurements is fraught with problems. It’s not just that BIA tends to underestimate fat levels, it’s also that there’s quite a lot of variability, which is likely related to changes in hydration. This variability means that tracking body composition changes over time may also be unreliable due to the inconsistent measurements. This makes it harder for athletes and coaches to monitor small body composition changes through the season – for example when making dietary adjustments designed to reduce body fat. That said, so long as you understand its limitations, BIA still has a place, especially if you are an amateur athlete without access to a coach or technician who can perform skinfold measurements.

The second takeaway is manual skinfold measurements provide a much more consistent reflection of an athlete’s actual fatness (adiposity), and should therefore be considered a superior choice to BIA for coaches tracking their athletes. Skinfold callipers are not fooled by the hydration changes common in endurance athletes! Ideally, the coach or assistant taking skinfold measurements should be well-trained (ideally ISAK certified) to minimize human error. The Withers equation is recommended for calculating actual body fat percentage from skinfolds; however, even without using an equation to get a body fat %, simply adding up the sum of skinfold measurements in millimetres is an excellent way to track body fat gains/losses over a period of time, and avoids any possibility of mathematical errors that can occur when manipulating equations.

Thirdly, whatever method is used, consistency is essential because different methods are likely to produce different numbers. An athlete or his/her coach should never compare a BIA result to a skinfold result or a DXA result from a clinic. If you are tracking changes over time, pick one method and stick to it. If you are an amateur and need to self measure, you will likely still need to use BIA – unless you can get your partner or a friend you trust to undergo some training and take skinfold measurements for you! When using BIA, remember that hydration is critical; try to take the readings at the same time of day (and time of month for women) when fully rested and hydrated. Finally, if you really do want to know your actual body fat levels and body composition, the best option of all is stump up some cash and seek out a DXA scan!

References

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