Though prebiotics may be preferable, probiotics may reduce the risk of upper respiratory tract infections.
Babies delivered via caesarean section "appear to be at increased increase for various allergic diseases.” The thought is that vaginal delivery leads to the first colonization of the gut with maternal vaginal bacteria, while c-section babies are deprived of this natural exposure, and exhibit a different gut flora. This is supported by research noting that a disturbance in maternal vaginal flora during pregnancy may be associated with early asthma in their children. This all suggests our natural gut flora can affect the development of our immune system for better or for worse.
In adulthood, two studies published back in 2001 suggested that probiotics could have systemic immunity enhancing effects. Subjects were given a probiotic regimen between weeks three to six and saw a significant boost in the ability of their white blood cells to chomp down on potential invaders. And what's interesting is that even after the probiotics were stopped there was still enhanced immune function a few weeks later compared to baseline. The same boost was found in the ability of their natural killer cells to kill cancer cells. Similar results were also found using a different probiotic strain.
Improving immune cell function in a petri dish is nice, but does this actually translate into people having fewer infections? For that, we had to wait another 10 years, but now we have randomized double-blind placebo controlled studies showing that those taking probiotics may have significantly fewer colds, fewer sick days, and fewer symptoms.
The latest review of the best studies to date found that probiotics, such as those in yogurt, soy yogurt, or supplements, may indeed reduce one's risk of upper respiratory tract infection, but the totality of evidence is still considered weak, so it's probably too early to make a blanket recommendation.
Unless one has suffered a major disruption of gut flora by antibiotics, or an intestinal infection—unless one is symptomatic—with like diarrhea or bloating, I would suggest focusing on feeding the good bacteria we already have, by eating so-called prebiotics, such as fiber. After all, as we saw before, who knows what you're getting when you buy probiotics, they may not even even be alive by the time you buy them. They have to survive the journey down to the large intestine. Altogether, these points suggest that the advantages of prebiotics—found in plant foods--outweigh those of probiotics. And by eating raw fruits and vegetables we may be getting both. Fruits and vegetables are covered with millions of lactic acid bacteria, some of which are the same type used as probiotics. So when studies show eating more fruits and vegetables boosts immunity, prebiotics and probiotics may be playing a role.
To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video. This is just an approximation of the audio contributed by Ariel Levitsky.
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Probiotics do play an established role in helping to prevent antibiotic-associated diarrhea and may speed recovery from acute gastroenteritis. See my last video, Preventing and Treating Diarrhea with Probiotics. When should they be taken? That's the subject of my next video, Should Probiotics Be Taken Before, During, or After Meals?
How else might we reduce our risk of getting an upper respiratory infection? See:
You can watch a video of white blood cells actually chomping down on foreign invaders in my video Clinical Studies on Acai Berries. A must-see for biology geeks!
For more context, check out my associated blog posts: Probiotics and Diarrhea, Probiotics During Cold Season?, Probiotics During Cold Season?, How Should I Take Probiotics?, How Probiotics Affect Mental Health, and Top 10 Most Popular Videos from 2013
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