The purpose of this investigation was to examine the effects of acute pre-assessment diets on body composition estimates obtained by dual-energy x-ray absorptiometry (DXA) and bioelectrical impedance analysis (BIA) in active adults. In a counterbalanced design, 48 male and female participants completed two one-day dietary conditions in random order: a very low-carbohydrate diet (1 – 1.5 g CHO/kg) and a high-carbohydrate diet (9 g CHO/kg). For each condition, measures of body composition were taken in the morning after an overnight fast, in the afternoon after feeding, and the following morning after an overnight fast. Three-factor (time, gender, and dietary condition) repeated measures analysis of variance was performed for measures of total and regional body composition, and appropriate post-hoc procedures were followed. Acute food ingestion, regardless of macronutrient content, altered body composition estimates obtained from DXA and BIA, and both genders responded similarly. DXA total and regional lean soft tissue estimates increased up to 1.7% and 3% on average in response to feeding, with some participants demonstrating an increase of over 4.5% and 9%, respectively. DXA total and trunk fat mass estimates decreased by up to 3% on average in response to feeding, with individuals demonstrating both decreases and increases of greater magnitude. All DXA-derived measures of body composition returned to baseline values after a second overnight fast. Impedance measured by tetrapolar BIA decreased by 4.4% in response to feeding, and this subsequently increased measures of total body water and fat-free mass by up to 2% on average, with individuals exhibiting increases as large as 4.5%. BIA fat mass estimates decreased 1.4 to 2.4%, with individuals exhibiting decreases of as much as 10%. Unlike DXA, the BIA-derived measures of body composition did not return to baseline values after a second overnight fast. The magnitude of changes seen in present study is deemed sufficient to potentially obscure true changes in body composition or produce artificial changes. It is recommended that body composition assessment take place after an overnight fast in order to minimize error due to acute food and fluid ingestion, although additional dietary control may be necessary when BIA is utilized.