Intestinal bacteria and fiber
In a TV documentary a 15 year old English boy ate junk/fast food for 3 weeks.
During that time, the amount of about 6.000 bacterial species in his intestine collapsed to only about 3.500 species, and he was tired throughout the day.
When the diet was back to normal (a so-called health-conscious family), however, it took 2 months before the amount of bacteria species was on the original level.
It is currently strongly suspected that antibiotics taken in early childhood are sufficient to trigger illnesses such as diabetes-1 and autoimmune diseases, together with some other risk factors that are caused by the typically unintelligible current lifestyle.
The intestinal bacteria have nothing to eat besides fiber. For good intestinal health etc. about 50 g / 1.000 kcal fiber would be needed; that is approximately seven times the current consumption.
When our calories are low, we have to consciously design and implement the addition of fiber and fiber-rich food into our diet if we want to be really healthy, maintain a good immune response and intestinal health, and get from the intestine also serotonin that is important for the brain. According to current knowledge, it is clear that intestinal health / fiber has a lot to do with, for example, memory diseases.
With sensible preventive health policy most of the major intestinal and public health problems could be almost completely eliminated. Unfortunately, the power of the pharmaceutical industry / healthcare cluster is too big and the politicians are too unintelligible and sensitive to do the right thing.