glucose utilization. Therefore, chronic sleep disturbances and sleep apnea in the

older individuals may be associated with alterations in the body metabolism. With

declined physiological functions and sleep patterns, the appetite may be altered in

older adults, resulting in increased susceptibility to GI tract and other metabolic

disorders. Age-related perturbations in gut microbial structure and microbiome

caused by diet and other factors appear to affect the circadian rhythm, promoting

metabolic disorders and obesity.

Ageing-associated alterations in GI physiology influence the amount and types of

nutrients delivered to the small intestine and colon, thereby affecting the intestinal

microbiota composition and functionality of these segments. A number of studies

have compared the fecal microbiota’s composition in elderly versus young adults.

The fecal microbiota was found to be similar in some studies (Biagi et al. 2016),

whereas other investigators reported significant differences in the quality of

microbiomes (Mariat et al. 2009). Generally, the gut microbiota among elderly

was found to be highly variable. Detailed characterization of fecal microbiota

composition was described by Cӑtoi et al. (2020) and He et al. (2020). They found

that in comparison with younger adults, the fecal microbiota in elderly consisted of a

smaller amount of Bifidobacteria, whereas Clostridia, Lactobacillus, Streptococcus,

Enterobacteriaceae species were greater in the elderly than that of younger adults. It

is worth mentioning that the intestinal microbiota compositional changes occurring

during ageing are most likely associated with health status of the elderly and

confounding

factors

such

as

frailty,

comorbidity,

and

living

conditions

(rural vs. urban), dietary intake, hygiene, or antigen exposure. Despite the typical

microbiota profile observed in elderly, it is hard to change the intestinal microbiota

with dietary supplements and functional foods like probiotics, prebiotics, or

synbiotics. However, some studies in elderly have shown changes in fecal

microbiota composition with probiotics, where pronounced changed occurred in

the

amounts

of

Bifidobacteria

spp.

e.g.,

Bifidobacterium

lactis

HN019,

Bifidobacterium longum 46, and Bifidobacterium longum 2C (Salazar et al. 2017).

Although intake of probiotics is generally considered to be safe, extra caution is

warranted in subjects with lactose intolerance and impaired host defense

mechanisms. Popular prebiotics containing galacto-oligosaccharides, inulin, or

fructo-oligosaccharides can cause increase in defecation frequency, thus altering

the bowel habits in constipated elderly (Tiihonen et al. 2010). It seems that gut

microbiota among older adults is affected by a broad range of potentially

confounding factors, such as lifestyle (e.g., excessive drinking and smoking), health

status, sedentary habits, obesity, medical treatment (antibiotics), probiotics, healthy

eating behavior, living conditions, and food insecurity rather than by ageing alone.

The influence of antidepressants on gut microbiomes has been reported recently by a

number of research groups, of course, with varying results (Lukic et al. 2019;

Bastiaanssen et al. 2019; Mikocka-Walus et al. 2019).

While gut microbiomes are essential for maintaining good health and gut-brain

interaction, the dysbiosis can cause chronic non-communicable diseases (NCDs),

including diabetes mellitus, obesity, metabolic syndrome, cardiovascular diseases,

and neurodegenerative disorders. For example, cyanobacteria can secrete neurotoxin

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M. Bhaskar et al.