Blue cats start to gain weight when they are about 8 to 10 months old. At this time, blue cats begin to have gills and look more rounded. Therefore, when blue cats are 4 to 6 months old, they need to supplement nutrition regularly. Feed some egg yolks, chicken breasts, beef, nutritional pastes, canned food and other foods to prevent the nutrition from not keeping up with the body's needs; you also need to allow the cat to exercise properly and have enough rest time, which can help the cat gain weight better.