I'm surprised that absolutely no one here has mentioned (possibly even conceived of) the possibility that perhaps Reina just has a REALLY fast metabolism. For all we know, she could be one of those people who can eat as much as they want and they don't gain an ounce. I've known a few people (some of them girls) like that, and yes, they were sometimes as skinny as Reina was. I was in a friend's cotillion in high school and her one cousin was like that, skinny/boney arms and all. Did she look like she wasn't eating? Yep. Were we concerned after she fainted during practice when she was in the middle of a twirl for our cha-cha number? Of course. But it turned out that I was just jumping the gun. Her cousin has a metabolism so fast, that she HAS to eat something at least once an hour so she doesn't get lightheaded. And man, I swear that girl could eat circles around me when it came to actual "meals".
As far as the whole Reina "wishing she was thinner" thing, seriously, it's a pretty common thing for young women to say or think about themselves, even if they're perfectly healthy. As a previous post pointed out, Reina's able to last through the pair of 2 hour concerts that MM/H!P performs per day on their tours. In the end, my point is that unless we hear reports of Reina constantly fainting or being hospitalized due to lack of strength or exhaustion (not unlike what happened to Maki before she left H!P), then in all likelihood we shouldn't be worrying so much and trying to make a mountain out of a molehill.
Yeah, if Reina was actually starving herself she probably wouldn't have been able to make it through those tours, not to mention the musical. I've seen a lot of people worry about Reina's weight for a while but it seemed to blow up after Reina said "中1に 戻りたぁーい(->∀<!!! それか・・・お腹のくびれを ください。。。(恥;)" in a Pocket Morning Weekly Q&A. H!O translated this as "If it's just one thing, I want to go back to the first year of middle school (->∀<!!!That, or... getting thinner please... ( ashamed; )".
This is a wrong translation - it should have said "I want to go back to the first year of middle school(->∀<!!! That, or...(I want) a flatter stomach please...(*embarrassed*)".
In short, Reina actually answered that she wants to lose weight from a specific area of her body, rather than her whole body. お腹のくびれ can also refer to an hourglass figure. (And I don't know where the doink H!O got the "If it's just one thing" part from...)
Even though some people were able to see that it was mistranslated, many more people didn't and so there were these "OMG Reina must have an eating disorder, and she must hate being in H!P because she entered H!P in her first year of middle school" comments, especially on H!O.
Reina has always been rather skinny, and is also pretty short. Like JFC said, I don't think that people should be so quick to say "Reina has an eating disorder" when we don't really know what is going on and when she appears to be doing fine otherwise.