The validity of using BIA to measure changes over time
A further consideration for the use of BIA is the validity of its use in measuring changes in fat mass and fat-free mass over time, as this may indicate the efficacy of a nutritional or training intervention looking to manipulate body composition. To revisit the study by Ritz et al., , BIA was unable to accurately assess changes in body composition when compared to the four-compartment model. Fat mass was underestimated by 1.6kg, whereas fat-free mass was overestimated by 1.8kg. Individual error rates were greater than at baseline, with BIA underestimating fat mass by 7.5kg in some subjects.
A further study on obese populations  showed that individual disagreement in body fat measurement between BIA and the four-compartment model was high. Individual measures of body fat ranged from -3.6% to 4.8% of the four-compartment value, highlighting the potential for significant discrepancies when measuring individual body composition over time. BIA is likely to misrepresent changes over time, potentially missing significant changes in body composition, or suggesting changes that haven’t occurred.
There are a limited amount of comparisons between BIA and the reference four-compartment model in athletic populations. There is disagreement amongst the limited research available, with only one study suggesting that BIA is suitable for assessing body composition in athletes , whereas other research suggests that body fat estimates are much higher in athletes when using the BIA method .
The discrepancies between the studies may be due to various issues including differences in methodology, equations, and athletic population. There are currently no BIA equations for athletes that have been derived from the criterion four-compartment method (fat mass, total body water, bone mineral mass, residual mass). This makes the application of BIA in this population difficult, as athletes are likely to possess substantially different quantities of fat and fat-free mass when compared to the general population or diseased populations that current equations are based on.
The reliability of BIA
The reliability of BIA (the reproducibility of the observed value when the measurement is repeated) is also important to determine single-measurement precision, as well as the ability to track changes over time. A plethora of research has indicated the importance – and potentially the inability – of standardising BIA measures to sufficiently account for various confounders.
The mean coefficient of variation for within-day, intra-individual measurements, has ranged from 0.3% to 2.8%, with daily or weekly variability ranging from 0.9% to 3.6%, respectively [2, 17]. Standard measurement conditions may vary depending on the machine type (e.g. hand-to-hand, leg-to-leg, supine vs. standing, etc.). Other factors which may impact the BIA measurement and should therefore also be standardised are :
- Room temperature
- Placement of electrodes
- Preparation of the skin
- Hydration status
- The analyser itself
The standardisation of hydration status is clearly of importance for BIA, as the method is reliant on estimations of total body water to ascertain fat-free mass. For female athletes, difference in hydration status during menses may significantly alter impedance , and should be a consideration when assessing female athletes with BIA. Saunders et al.,  showed that BIA was not a suitable method of body composition assessment in athletes with abnormal hydration status (e.g. hypohydrated), indicating that even small changes in fluid balance that occur with endurance training may be interpreted as change in body fat content.
In addition, eating and strenuous exercise 2-4 hours prior to assessment has also previously been shown to decrease impedance; ultimately affecting the accuracy of the measurement . The need to standardise eating, exercise, and both acute and chronic hydration changes are clearly important to provide valid body composition estimations.