My husband is having his surgery tomorrow and has been on a liquid diet with Protein shakes 3 times a day. He is doing really well so far. I am having my surgery June 22. To be supportive of my hubby, I started having Protein Shakes 2 times a day and a small sensible, low in fat, sugar and calories, meal for dinner. I am having a problem with headaches myself. I have found that it is due to low blood sugar. I have found that a real life saver, not sugar free, relieves the headache right away. Good luck to you!