I am 6 months post surgery. I struggle to hit all the targets for my post-op diet - I am getting around 900 calories at most - I prioritize protein and water - and taking my vitamins. I still have problems with nausea.
Getting the recommended amount of protein and exercise is the best way to prevent muscle loss. Given that I am having a hard time hitting my nutritional targets - I am tracking key body measurements (waist, hips etc) on the Baritastic app in addition to monitoring weight loss.
I saw some BMI scales online for around $100. They can compute body composition - I will see how it goes and maybe invest in one of those if I continue to struggle with eating.