Now that's a good question.
In the original study, the participants drank shakes that provided nutritional requirements and 510 calories. The remaining 90 calories came from vegetables. I'm going to use some shakes and vegetables but probably throw in the odd egg or handful of nuts too. I may even (gasp!) go as high as 700 calories a day occasionally. I'll be drinking lots of water and zero-calorie drinks too. I'll also be supplementing with vitamins and minerals.