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To examine the effects of acute preassessment diets on body composition estimates obtained by dual-energy x-ray absorptiometry (DXA) and bioelectrical impedance analysis (BIA).In a counterbalanced design, 48 males and females were provided with two 1-d diets: high-carbohydrate diet (9 g CHO·kg−1) and very low-carbohydrate diet (1 to 1.5 g CHO·kg−1). For each condition, body composition was assessed in the morning after an overnight fast, in the afternoon after feeding, and the following morning after a second overnight fast.Acute food ingestion, regardless of macronutrient content, altered DXA and BIA body composition estimates, and both sexes responded similarly. DXA total and regional lean soft tissue estimates increased up to 1.7% and 3% on average in response to feeding, with individual increases of over 4.5% and 9%. DXA total and trunk fat mass estimates decreased by up to 3% on average. All DXA-derived measures of body composition returned to baseline values after the second overnight fast. Impedance measured by BIA decreased by 4.4% in response to feeding, leading to a 2% increase in total body water and fat-free mass, with individual increases up to 4.5%. BIA fat mass estimates decreased 1.4% to 2.4%, with individual decreases of up to 10%. Unlike DXA, most BIA-derived estimates did not return to baseline values after a second overnight fast.Acute food and fluid intake can artificially influence body composition estimates, regardless of macronutrient content. An overnight fast is likely sufficient preassessment dietary control for DXA and possibly sufficient for BIA.