For years, we have done Vitamin A (beta-carotene) pulses as a method of heading off an infection. The reason is experience and this statement from Springboard:
“Vitamin A seems to stimulate the immune system and the production of white blood cells. Often when some kind of infection like the flu or a cold hits an intake of 100,000 to 150,000 IU for not more than three days will effectively turn on the immune system to the extent that the infection is stopped. The implications of this for other health problems should be obvious adequate supplementation of vitamin A on a daily basis will keep the immune system up and ready to fight back, although it is certainly no miracle cure for all the ills and abuses of the body.” springboard4health.com as well as other alternative health sites [evitamins, herbalhut].
I recently appear to start a cold (which the wife appear to have brought home) and could feel something developing — in fact, I noticed a shift in digestion. I did a pulse and things appear to return to normal. This left me with an interesting question because of recent readings…
Studies have found that gut bacteria distribution can be used to reliably predict what infections a person has (at least for the set of infections that the study was tried on). This raises the question — is the impact of vitamin A a disruption of a new invasive (infection related) set of gut bacteria?
As usual, I decided to check out PubMed studies and see what I could find:
- ” a reduction in the intestinal microflora resulted in an increased storage of beta-carotene, alpha-carotene and vitamin A in the liver.” 
- “an association between carotenoid-rich food intakes with a low incidence in chronic diseases” 
- “Retinoic acid (RA), a well-known vitamin A metabolite, mediates inhibition of the IL-6-driven induction of proinflammatory Th17 cells and promotes anti-inflammatory regulatory T cell generation in the presence of TGF-beta…. RA deficiency altered gut microbiome” 
- ” deficiency in retinoic acid-related orphan receptor c(-/-) mice resulted in a complete failure to control the [fungal] infection”
- ” mitochondria participate in innate immunity to viral infection through the pattern recognition receptor retinoic acid inducible gene-I and are involved in inflammasome activation.” 
Translation: Vitamin A increases the body ability to recognize infections…
- ” However, stimulation of dendritic cells via TLR2 increases the expression of host genes associated with generation of the immunoactive form of vitamin A (retinoic acid) while enteric infection has been linked to vitamin A deficiency …. vitamin A has the potential to modulate immune responses through direct interactions with immune cells, or indirectly by modulating the composition of the microbiota.” 
While not finding an ideal study, we do find that Vitamin A has been documented to change the microbiota (gut bacteria) as well as causing the immune system to be more active against infections.
Going through notes (from pubmed articles), I see:
- Availability affected by microfloras
- Decreased in Fibromyalgia patients
- Decreased with Lyme and symptoms worst with lower levels
- Deficiency associated with susceptibility to infection and altered microfloras
- Deficiency increases IL-12 production
- EBV and CMV alters Vitamin A metabolism
- Low intake seen with IBS/IBS patients
- Low level in active Crohn’s disease, but normal when inactive
- Retinoic acid down-regulates colon inflammatory responses in patients with IBD
- Supplements may reduce inflammation
Thus vitamin A levels should be checked, and if not in the top 75%ile, supplementation may be of significant help.