Depending on the audience, I say
"I'm on an elimination diet for health reasons. I am not eating known allergens like grains, soy, peanuts, dairy, and lupins plus I'm avoiding sugar because of my hypoglycemia."
or "I'm trying this cool Primal Blue Print 30 Day Fitness Challenge" or "For homeschool PE, we are working on earning the Presidential Fitness Challenge and the 5-A-Day Scout Patch"
"We're doing a unit study on the Little House series, and they had to do without food luxuries for a while, we are trying the same thing so we can understand their hardship"
"I'm avoiding sugar and artifical stuff and increasing my protein to help with my ADHD." (Sometimes I have to explain that I can't use the meds because of my tic)
"I'm fasting in conjunction with prayer and gave up anything not God-made for 490 days while I work on breaking through a forgiveness issue."
"Beans and dairy make me fart"