It is almost by definition unthinkable to grow old healthily without a healthy microbiome. How far does scientific knowledge about the connections extend?

Healthy ageing is a topic that is receiving more and more attention, because it is becoming clear that attention to fighting diseases alone is insufficient: although we are getting older, we have not yet succeeded in delaying or preventing diseases (1). For this reason, attention is now being paid to the biology behind the aging process itself and the hypothesis is that it can also delay the onset and progression of diseases (1).
In a large number of diseases, age is one of the most important risk factors. Given the suspected role of the microbiome in many of these diseases and conditions, such as obesity, metabolic syndrome, type 2 diabetes, cardiovascular disease, and neurodegenerative diseases such as multiple sclerosis, Parkinson's, and Alzheimer's (see separate sections), abnormalities of the microbiome appear to be one of the underlying processes that plays a role in these age-related diseases.
It is therefore striking that the diversity of the microbiome decreases with age, and that the degree of decline is correlated with the decline in health (2,3). Conversely, there are indications that greater presence of subgroups of bacteria associated with health plays a role in achieving high and very high ages (2,3). There are also indications that the microbiome in the elderly is less stable, while conversely there is a relationship between the composition of the microbiome and the degree of 'brittleness' of the elderly (4). Studies on the use of probiotics in the elderly show cautiously positive results on intestinal complaints, infections, and lung complaints (3,5).