A few episodes back on Fox's (fairly) new show 'House,' there was a young actress who played an obese girl. In the episode, she had a heart attack while in Gym class. House's medical team discovered later that she had the heart attack because she had shoplifted some Diet Pills that she had taken without adult supervision. I didn't get to see the rest, but that should be enough info.

Does anybody know who this actress is?

I'm not sure, without a little more information, but you can check IMDb for the House, MD guest stars. If you knew the episode name, I could get you closer. On March 29, they aired the episode called "Heavy." This might be the one you are looking for. Click that link above and scroll down a bit and you will see each episode's guests.

I believe the young actress's name is Jennifer Stone.

Thank you very much.