Related Experiment Video
Updated: Dec 23, 2025
Probiotic Studies in Neonatal Mice Using Gavage
Published on: January 27, 2019
Effect of probiotics on thymus size and markers of infection in late infancy: a randomized controlled trial
Anni Larnkjær1, Kim F Michaelsen2, Maren J H Rytter2,3
1Department of Nutrition, Exercise and Sports, Faculty of Science, University of Copenhagen, Copenhagen, Denmark. ala@nexs.ku.dk.
Insights
Probiotics did not significantly alter thymus size or CRP levels in Danish infants starting daycare. Thymus size showed inverse associations with CRP and certain infections, warranting further investigation.
Area of Science:
- Immunology
- Pediatrics
- Microbiology
Background:
- Probiotics are known immune system modulators.
- The impact of probiotics on thymus size in late infancy remains under-explored.
- Infants starting daycare face increased infection exposure during immune development.
Purpose of the Study:
- To investigate the effect of a specific probiotic combination on thymus size and C-reactive protein (CRP) in healthy Danish infants.
- To explore associations between thymus size, CRP, and recent infections in this age group.
Main Methods:
- 186 healthy Danish infants were randomized to receive a probiotic (Lactobacillus rhamnosus, LGG® and Bifidobacterium animalis spp. lactis, BB-12®) or placebo for 6 months.
- Thymus size was assessed using ultrasound (thymus index and thymus weight index).
- C-reactive protein (CRP) levels and parent-reported infections were recorded.
Main Results:
- No significant differences in thymus size or CRP were observed between the probiotic and placebo groups.
- A trend suggested a 5% higher thymus weight index in the probiotic group (p=0.068).
- Thymus size was inversely associated with CRP, diarrhea, and absence from daycare due to infections.
Conclusions:
- Probiotic supplementation did not impact thymus size or CRP in Danish infants starting daycare.
- While no direct effect was found, thymus size correlated with infection markers, suggesting a link between immune status and thymus size in early childhood.
Background:
Probiotics are known to stimulate the immune system but the effect on thymus size in late infancy is unknown. We examined the effect of probiotics on thymus size and C-reactive protein (CRP) in healthy Danish infants starting daycare. We further examined associations between thymus size, CRP and recent infections.
Methods:
The study included 186 children randomized to a combination of Lactobacillus rhamnosus, LGG® and Bifidobacterium animalis spp. lactis, BB-12® or placebo for 6 months. Thymus size, assessed as thymus index (TI) and thymus weight index (TWI), was measured by ultrasound at baseline and at endpoint. Blood samples were drawn to measure CRP. Infections were parent-reported.
Results:
There was no significant difference in thymus size between the probiotic group and placebo (p ≥ 0.248) but TWI tended to be higher in the probiotic group corresponding to 5% higher than placebo (p = 0.068) in an adjusted model. There was no effect of probiotics on CRP (p = 0.331). At the endpoint, thymus size was inversely associated with CRP (p ≤ 0.040), diarrhea (p ≤ 0.050), and TI was also associated with the absence from daycare due to respiratory or gastrointestinal infections (p = 0.010).
Conclusion:
The probiotic intervention had no effect on thymus size or CRP in Danish children at the age of starting daycare.
Impact:
Overall there was no effect on thymus size of a combination of Lactobacillus rhamnosus, LGG® and Bifidobacterium animalis spp. lactis, BB-12® administered to Danish children starting daycare. This study examines the effect of probiotics on thymus size in healthy children when they start daycare thus exposed for infections while their immune system is still developing. This has to our knowledge not been described before. We found no significant difference in thymus size between the probiotic and placebo groups, but for thymus weight index, there was a trend. This should be investigated further in studies designed for this as primary outcome.
