I was mostly eating vegetables for years, but eventually chronically underweight. At 25 I went to the doctor for severe knee pain, and the doctor told me I just need to eat some god damn food. I went out and gorged myself on korean barbeque. After that I started adding beef, lard, and meat in general to my previously mostly lentils and fried vegetables cooking. I am now no longer underweight and dont experience strange symptoms of deficiency such as the papillae on my tongue dissapearing, and loud painful joints.
FWIW that's not a healthy diet. A vegan diet (assuming you were vegan) needs a lot of variety--carbs/grains, healthy fats, nuts, legumes, etc. You won't meet your daily caloric needs on lentils and vegetables.
I ate yogurt and nuts in the mornings. Breads, lots of potatos. Lots of lentils which are legumes and have about the same protien per dollar and weight as chicken. I also put walnuts, peanuts, lots of other nuts and seeds, oils etc in my cooking. I cooked in bulk in a large pot.
I also ate 5+ bowls a day a day.
I just could not reach my calorie requirements.
I did exercise everyday by going up the nearby mountain. Adding protien powder to my meals helped, but adding meat made it very easy to substantially increase my calorie intake.
