Probiotics and nutritional supplements: Do they really "boost" the immune system? (Proceedings)


Probiotics and nutritional supplements: Do they really "boost" the immune system? (Proceedings)

Nov 01, 2009

Considering the large impact of immunology on companion animal practice (e.g. vaccines, allergies, infectious and immune-mediated diseases), it's natural to wonder what role nutrition plays. Commercial diets, supplements, special foods, home recipes, nutraceuticals – is there any scientific evidence behind the promotional claims that they "strengthen" or "boost" immunity or ''modulate the immune system"? Many studies have been done looking at the immune system in people and laboratory animals, but there is far less research available in dogs and cats. The following overview will attempt to sort out what we know and don't know about nutrition and immunology.


One of the oldest scientific pursuits is how to extend lifespan, and postpone aging as long as possible. As the immune system plays a major (but not exclusive) role in this process, research has looked into how to measure changes in immunity over time and whether diet can help. The concept of "immunosenescence", which is a decreased immune response to internal and external stress (e.g. cancer, infections, degeneration) is believed to be a factor in aging and death. Certain measures of immune function do change over time, but the significance is often unknown.

The best study to date is a longitudinal (lifelong) research project involving Labrador Retrievers, who were housed, fed, and managed at the Nestlé Purina Pet Care Center (Gray Summit, Missouri) from birth until natural death (1987-2001). A number of investigations were done on 48 dogs who were pair-matched into well-fed and caloric restricted groups (25% less food intake than controls). Blood samples were taken three times a year from ages 4-11 for evaluation of the status of the immune system (total leukocytes and lymphocytes, lymphocyte proliferation, natural killer cell activity, and neutrophil phagocytosis).

Some tests of immune status declined or remained the same in both the control and calorie-restricted groups. Other effects were positive in the restricted group, with significant differences in age-related declines in cell numbers and lymphoproliferative responses. The study's conclusion was that calorie-restriction retarded (or postponed) immunosenescence as well as enhanced several immune markers predictive of survival. Overall, calorie restriction to 75% of control dogs' diets extended the median life span by 15% (from 11.2 years to 13 years).

These findings are not as dramatic as those seen in laboratory rodents. However, in many species studied to date, calorie restriction appears to be a major factor in prolonging life span. Cats have not yet been studied in this manner. Whether this effect is due to slowing or modifying immunosenescence is an attractive hypothesis but more research is needed. The practical relevance of this lifelong study, which practitioners may communicate to their clients, is that feeding dogs less (fewer calories) will likely improve immunity and promote a longer, healthier life.


Probiotics are live bacteria administered orally that may result in a health benefit. Live microbial food supplements have been investigated in lab animals, livestock, and humans for improving microbial balance and modulating the immune system. Few scientific studies have been conducted in dogs and cats. Some strains have effects on nonspecific immune responses (increased phagocytic and natural killer cell activity) as well as humoral and cell-mediated immunity (increased antibody and cytokine levels).

Some of the medical conditions that have been studied using probiotics include allergies and asthma, inflammatory bowel disease, diabetes, cancer, rheumatoid arthritis, and gastrointestinal viral infections. Different types of probiotics (yogurt, powdered supplements, different strains and doses) used in various studies makes interpretation of studies difficult.

Products marketed for veterinary use in dogs and cats include FortiFlora (Nestlé Purina), Prostora Max Iams), and Proviable (Nutramax). Many other companies produce various types of probiotics for animals and humans. Probiotics are not regulated by the FDA and the quality of commercial products varies widely. One study of products sold for human use found that 8 out of 26 contained 1% or less of the amount of live bacteria claimed on the labels. An evaluation by found that 5 of 13 human probiotics did not contain an adequate amount of organisms and 2 of 3 products marketed for pets did not contain enough organisms (and one was contaminated with mold). Commercial pet diets claiming to contain probiotics were recently evaluated. Of 19 products, only 10 contained at least one of the live bacteria claimed on the label. No growth was present in 5 diets, and no product contained all the listed organisms. The conclusion was that the diet contents were not accurately represented by the labels.

One study compared the responses to FVRCP vaccination in 18 kittens, where half were given E. faecium and half were controls. A positive effect was seen at certain time intervals where IgA and IgG antibody levels were higher in the treated group. The overall immune modulating effects were not significantly different, however. In puppies, an enhanced response to distemper vaccine was seen when E. faecium was given. Another study using laboratory mice showed a significant reduction in Giardia shedding and increased intestinal IgA with E. faecium. However, in a kennel of Beagles with chronic giardiasis, E. faecium had no benefit.

Probiotics appear to be safe when given to dogs and cats, and practitioners may wish to use them for gastrointestinal disorders or other conditions when "immune system enhancement" is desired. Medical claims for probiotics cannot be made by manufacturers, as the FDA would then consider them to be unapproved drugs. Before selecting products to recommend to clients, ask if there is proof of stability (are the organisms alive in stable form? can they be recovered from the GI tract?), safety (any side effects or problems with unbalancing the GI normal flora?), and efficacy (studies in healthy animals or those with disease where a product benefit was seen). Keep in mind that concurrent antibiotic therapy may destroy probiotic organisms in the GI tract.