1) Gut microbiome - the flora and fauna can be affected by antibiotics and probiotics, and also when you travel. For example kids given antibiotics early on become more overweight throughout life.
2) Meat and factory farms - they overuse antibiotics, and the older the animals you eat, the more effect it has on your own aging process.
3) The brain’s ability to burn calories AND regulate the gut brain too, through neuron pathways, is still poorly understood. But it might be a factor of mind over matter: https://nypost.com/2022/09/01/the-secret-to-weight-loss-coul...
4) Cutting out processed sugars, bread/starches and a lot of other artificial bullshit. Check out Jordan Peterson opening up about his all-meat diet. He and his famoly had many things clear up that were considered incurable, including arthritis and depression.