Eat beans.

In this study from the UK, galactooligosaccharides (GOSs) were found to improve the make-up of gut bacteria, improve immune response, and decrease inflammation:

Modulation Of The Fecal Microflora Profile And Immune Function By A Novel Trans-galactooligosaccharide Mixture (B-GOS) In Healthy Elderly Volunteers, American Journal of Clinical Nutrition, November 2008

Background: Aging is associated with reduced numbers of beneficial colonic bifidobacteria and impaired immunity. Galactooligosaccharides (GOSs) stimulate the growth of bifidobacteria in younger adults, but little is known about their effects in the elderly and their immunomodulatory capacity.

Objective: We assessed the effect of a prebiotic GOS mixture (B-GOS) on immune function and fecal microflora composition in healthy elderly subjects.

Design: In a double-blind, placebo-controlled, crossover study, 44 elderly subjects were randomly assigned to receive either a placebo or the B-GOS treatment (5.5 g/d). Subjects consumed the treatments for 10 wk, and then went through a 4-wk washout period, before switching to the other treatment for the final 10 wk. Blood and fecal samples were collected at the beginning, middle (5 wk), and end of the test period. Predominant bacterial groups were quantified, and phagocytosis, natural killer (NK) cell activity, cytokine production, plasma cholesterol, and HDL cholesterol were measured.

Results: B-GOS significantly increased the numbers of beneficial bacteria, especially bifidobacteria, at the expense of less beneficial groups compared with the baseline and placebo. Significant increases in phagocytosis, NK cell activity, and the production of antiinflammatory cytokine interleukin-10 (IL-10) and significant reduction in the production of proinflammatory cytokines (IL-6, IL-1β, and tumor necrosis factor-α) were also observed. B-GOS exerted no effects on total cholesterol or HDL-cholesterol production, however.

Conclusions: B-GOS administration to healthy elderly persons resulted in positive effects on both the microflora composition and the immune response. Therefore, B-GOS may be a useful dietary candidate for the enhancement of gastrointestinal health and immune function in elderly persons.

Again, they used a galactooligosaccharide (GOS) supplement (see study in last post). I don’t know if you have to take this pill to get this benefit. But GOSs are found in foods all over the plant kingdom, especially in beans. Unfortunately, or perhaps fortunately, the reason studies like this get done is because they provide support and documentation for the marketing of a pill.

There was a lot of good reading in this study. Some bits:

• “The intestinal microflora can be considered as an organ composed of a large diversity of bacterial cells that can perform different functions for the host.”
• “The composition of the elderly intestinal microflora differs from that of younger adults.”
• “The aging process also leads to a marked decline in immune function (immunosenescence), which can promote hyporesponsiveness to vaccination and a predisposition to infectious and noninfectious diseases.”
• “Age-related changes in GIT [gastrointestinal tract] physiology and function, such as greater permeability of mucosal membrane, reduced transit times, and secretion of acids by the gastric mucosa, result in a significant change in the composition of the intestinal microflora, marked by a decline in bifidobacterial numbers and an increase in putatively detrimental populations such as clostridia and enterobacteria. These bacteriologic and other physiologic changes may result in increased putrefaction in the colon and greater susceptibility to disease.” Transit time is important. Don’t let things putrefy.
• This appears to be a mechanism for how beneficial bacteria boost immunity: “Probiotic bacteria, such as lactobacilli and bifidobacteria, were shown in vivo to interact with mouse epithelial cells of the small intestine and be internalized with the use of distinct pathways into the epithelial cells of the large intestine by the follicle-associated epithelium cells (32). They make contact with underlying immune tissues, and, through these interactions, they were suggested to bring about strain-specific immunomodulation (33), such as differentially induced cytokine production by macrophages in a concentration-dependent manner.” … “Some prebiotics are also reported to bind to specific receptors on cells of the immune system, suggesting that a direct interaction between prebiotics and the host is feasible.”
• “Probiotics were also shown to regulate the balance between necessary and excessive immune response.” So, beneficial bacteria balance pro- and anti-inflammatory responses.