You need to increase the surface area of your gut bacteria in order for you to absorb more food. When you have leaky gut, you have holes in your intestinal walls. When this occurs you don’t absorb nutrients like catlady said. In order to heal the intestines, you need to improve digestion and use a variety of supplements to heal it. One supplement that is both popular in weight gain and healing your intestines is L-glutamine. Another one worth noting is SF722 which slowly destroys the candida walls. Then there are digestive aids such as digestive enzymes (helps repair lining and break down food), HCL (helps for those who are deficient in stomach acid break down food), etc.
Then there are a variety of prebiotic and probiotic sources of foods that help nurture these bacteria…examples include teff, artichoke, onions, kefir, buckwheat, etc.
If you were to do smoothies, this should give your digestion a break while helping build up your beneficial flora.
Your lymphs also will benefit from sf722 because it will prevent candida from attaching to your mucousal membranes which is basically your lymph system.