~면서 means "while' or "during" But most importantly, korean learners often forget about the context in which the words are used. The sentence you are wondering is the answer about this question "I take in some supplements. How would you do that?" so you can also use "과밀을 먹고 영먕보충했어." of course "과일을 먹으면서 영양보충했어" is also natural. the difference between the two sentences is the former is a little late in tense meaning. so you ate some fruit today, when somebody ask above question of you, the latter sentence is more natural. but you ate fruit yesterday, or the day before yesterday, the former is more natural.