How do acute changes in nutrition and exercise affect body composition data?

Dual-energy X-ray absorptiometry (DXA-DEXA|DXA) is an increasingly common means to assess body composition, particularly among elite athletes competing at the international level,^[1] and it is considered the standard by many in the field of sports science.^[2] The ability of DXA to quickly provide detailed body composition estimates is a notable advantage over other body composition methods, but it is not without limitations.

A variety of factors can significantly affect the accuracy of DXA measurements. For example, recent food and fluid intake, supplemental creatine, physical activity, and cold water immersion can all affect DXA estimates of fat mass and fat-free mass (FFM).^{[3][4][5][6][7]}

To illustrate the magnitude of effect that the acute manipulation of some of these variables can have on body composition data, consider the reported changes in FFM over a 3-day period in a combat sport athlete making weight for competition.^[8] Body composition was assessed after a 12-hour overnight fast on each day.

On the day before their weigh-in (day 1), the individual ingested one meal containing 300 kcal alongside 500 mL of fluid, and they utilized a sauna to promote further body water loss. On day 2, they weighed in for their competition. On the day after their weigh-in (day 3), they consumed food and fluid ad libitum. These practices resulted in a reported FFM of 53.0 kilograms on day 1, 50.9 kilograms on day 2, and 56 kilograms on day 3. This corresponds to approximately a 4% change in FFM between days 1 and 2 and a 10% change between days 2 and 3. Given the fact that it’s physiologically impossible for skeletal muscle and organ mass to undergo significant changes over such a short timespan, these data highlight how influential changes in glycogen and body water can be on DXA measurements.

As such, a strict standardized protocol for the use of DXA is necessary to make the most of the data it provides.^[2] Scans should be performed:

• By the same skilled technician, on the same scanner, using the same software and reference database
• In minimal, lightweight clothing
• In a glycogen replete and hydrated state
• After abstaining from intense exercise and alcohol for at least 24 hours
• In an overnight-fasted and rested state, including abstaining from fluid intake upon waking
• After voiding the bladder

To track changes in body composition over time, scans should be taken at least 8 weeks apart.^[2] Moreover, similar nutrition and training practices should be followed in the 24 hours prior to each scan. For women, scans should occur in the same phase or on the same day of the menstrual cycle.