. ▁These ▁instruments ▁are ▁generally ▁regarded ▁as ▁being ▁less ▁accurate ▁than ▁those ▁used ▁clin ically ▁or ▁in ▁nut r itional ▁and ▁medical ▁practice . ▁They ▁tend ▁to ▁under - read ▁body ▁fat ▁percentage . ▁ ▁De h yd r ation ▁is ▁a ▁recognized ▁factor ▁affect ing ▁B IA ▁measurements ▁as ▁it ▁causes ▁an ▁increase ▁in ▁the ▁body ' s ▁elect rical ▁resistance , ▁so ▁has ▁been ▁measured ▁to ▁cause ▁a ▁ 5   kg ▁under est imation ▁of ▁fat - free ▁mass ▁i . e . ▁an ▁over est imation ▁of ▁body ▁fat . ▁ ▁Body ▁fat ▁measurements ▁are ▁lower ▁when ▁measurements ▁are ▁taken ▁shortly ▁after ▁consumption ▁of ▁a ▁me al , ▁causing ▁a ▁variation ▁between ▁highest ▁and ▁lowest ▁read ings ▁of ▁body ▁fat ▁percentage ▁taken ▁throughout ▁the ▁day ▁of ▁up ▁to ▁ 4 . 2 % ▁of ▁body ▁fat . ▁ ▁Mod er ate ▁exercise ▁before ▁B IA ▁measurements ▁lead ▁to ▁an ▁over est imation ▁of ▁fat - free ▁mass ▁and ▁an ▁under est imation ▁of ▁body ▁fat ▁percentage ▁due ▁to ▁reduced ▁imped ance . ▁For ▁example , ▁moder ate ▁intensity ▁exercise ▁for ▁ 9 0 – 1 2 0 ▁minutes ▁before ▁B IA ▁measurements ▁causes ▁nearly ▁a ▁ 1 2   kg ▁over est imation ▁of ▁fat - free ▁mass , ▁i . e . ▁body ▁fat ▁is ▁significantly ▁under est im ated . ▁Therefore , ▁it ▁is ▁recommended ▁not ▁to ▁perform ▁B IA ▁for ▁several ▁hours ▁after ▁moder ate ▁or ▁high ▁intensity ▁exercise . ▁ ▁B IA ▁is ▁considered ▁reason