Thank you in advance for your efforts. I've been a vegetarian for ten years and followed a vegan diet for about a year now. I put weight on at the drop of a hat. And don't think about it so much. The harmful nitrogenous waste product of this conversion is uric acid, which is broken down into harmless urea and excreted in urine.
Protein is made up of amino acids.
We need twenty of them, but the body can manufacture twelve as long as it has a source of nitrogen. Nitrogen is found any food with a protein content, ie. This is where, as vegans, we get our protein from. And you have to eat a helluva lot of them to maintain muscle growth. If you want to put on weight, then muscle is a grand place to start, and for that, you need protein.