Hello, i'm 403yo & i want to share the DIY with the world :
1) Dont eat "Dead products" i mean mainly meat ofc, but you should also avoid eggs, cheese (it's made with dead calf stomach enzymes nowaday) & fish. Also dont eat alive things (shellfishes, moving squidtentacles )
2) Check the PRAL index, it gives the acid/base value of numerous foods, u should know that acid deteriorates almost everything, most of our bodily functions are depending on having the correct acid/base equilibrium(blood, lymph, stomach secretions...). Basically try to avoid acid food, your body will correct it as long as u get a good daily salt & water intake.
3) Keep good mental & physical activities, try always learning & discovering new stuff & places, brain needs stimulation as much as body. B positive, u prolly heard of psychosomatism,how feelings can cause physical illnesses, so fight that depressing mood that float around most of the time, it counts.
4) Dont go around saying you are immortal to everyone, either they'll laugh @ you, either they'll try to prove you the contrary ;P
Some plants & trees are aged over 2000 years, many animals live longer than most humans (geoducks, tuataras, giant tortoise, some parrots...) so start putting some savings in the bank, that's gonna pay up in some centuries !