Graduation Year


Document Type




Degree Name

Doctor of Philosophy (Ph.D.)

Degree Granting Department

School of Aging Studies

Major Professor

Brent J. Small, Ph.D.

Committee Member

Cathy McEvoy, Ph.D.

Committee Member

Ross Andel, Ph.D.

Committee Member

Roland Douglas Shytle, Ph.D.

Committee Member

Paula Bickford, Ph.D.


aging, gastrointestinal, gut, nutrition, dysbiosis, cognitive performance


As we age, the risk for gut issues, such as smooth muscle tone, may be an underlying indirect or direct cause or risk factor for many age-related issues, such as frailty. Consequences of decreased motility and depleted epithelial barrier may result in nutrient deficiencies that may increase the risk for malnutrition (Brownie, 2006). Further, there is increasing evidence that there is a gut-brain-axis relationship that may influence cognition and mental health issues, such as depression and anxiety. While there are relationships established, the interconnections of these factors have yet to be fully understood.

This dissertation examined several relationships specific to nutrient intake, physical function, and depression in older adults while probing for a moderating effect of gut health. Looking further at this theory of the gut-brain bi-directional relationship, an additional gut health assessment was further examined to investigate the relationship with cognitive performance.

Participants were from two separate but complementary data sets. The first data set from the National Health and Nutritional Examination study included a depression outcome analytic sample and a physical function analytic sample who had valid data on nutrient intake, bowel measures, demographic characteristics, depression scores, physical function measurements, and total BMI. The depression analytic sample had a total of 1918 participants with a mean age of 73.76 years, and 1864 participants with a mean age of 73.28 years in the physical function analytic sample. The available nutrients within the data set were further broken down into several different components by a component factor analysis and each component used as a predictor. Two separate bowel measures were examined with one as a fecal incontinence measure and the other, the Bristol Stool Form Scale, as categorical (normal, constipation, or diarrhea). The second data set, the Nutraceutical Blueberry Study, had a total of 108 participants with a mean age of 73.42 years who had valid data on cognitive measures and a complete gut assessment.

Among the depression analytic sample, there were significant moderating effects of fecal incontinence between several nutrient components and depression after accounting for the control variables. An additional moderated multivariate regression with only the significant components was carried out and resulted in only Component 9 (carbohydrates, sugar, beta-cryptoxanthin, and vitamin C) and Component 12 (alcohol) having the fecal incontinence measure as a significant moderator with depression as the outcome.

Within the physical function analytical sample, the Bristol Stool Form Scale categorical measure was a significant moderator among Component 6 (MFA22_1, PFA18_4, PFA20_5, PFA22_5, and PFA 22_6) and physical function. Both the constipation and diarrhea categories were related to worse physical function, while in all groups, increase in nutrients from Component 6 resulted in better physical function.

Within the second data set, AVLT and AVLT Delay had a significant quadratic relationship with bowel function. Within the four different groups in the bowel measure (gastric function, gastrointestinal inflammation, small intestine and pancreas, and colon), gastrointestinal inflammation with a negative association and the colon category with a positive association were significant. Among the AVLT Delay, gastrointestinal inflammation was also negatively associated significant predictor.

Outcomes from the current study suggest that fecal incontinence was indicative as a moderator among the first data set, as well as significant predictor for AVLT and AVLT Delayed in relation to cognition in older adults. Although there were many relationships not found with bowel function as a moderator, the current findings suggest that more thorough measures in additional to microbiota measures could further provide possible directions for new therapeutics in psychological and cognitive therapy, as well as improving physical function in older adults.