Eating ultra processed foods with added sugar / loads of calories. A huge reason why they're so destructive is because they lack fibre (which keep our microbiomes healthy) and lack compounds which are good for us. Try keeping weight on while eating minimally-processed food that is more vegan in nature: the amount you'll have to consume to keep the weight on is significant. When you start this diet, you won't like it that much and you will feel hungry since your microbiome is used to your old diet -- but try doing it for a month, and you'll see a huge shift in the way you feel and the foods which you crave (the above will start transforming your microbiome quite significantly). Do it for 3 - 6 months you'll wonder how in the world you ever lived without it.