Tuesday, October 20, 2009

Enhancing Immunity with Probiotics: Can They Ward Off the Flu?

Kefir is a natural source of probiotics.
Kefir is a natural source of probiotics. (Photo by uncle andrey)

I don't know if it's the swine flu or a normal influenza virus going around, but seems like everybody's been getting sick during the past few months. This particular virus is particulary resilient, with people being ill for weeks at a time.

I succesfully managed to avoid the flu for about a month, while just about everyone else was complaining about the cold that just won't go away. Feeling very pleased with myself, I put my exceptional immunity down to the optimal vitamin D levels I've achieved after supplementing with 5,000 IU per day. This really has reduced the frequency of colds, but it wasn't enough to save me from catching the same thing as everyone else.

So, having nothing better do to than to lie in bed feeling crappy, I thought I'd educate myself on the subject of curing and preventing viral infections such as the common cold. Given that a prescription for antibiotics (to treat an ear infection) oddly coincided with my falling ill, I was interested in whether probiotics might be of use. In this post, we'll take a look at the evidence behind probiotics and immunity.

What are probiotics?

Our bodies are host to trillions of microorganisms, most of which are good and some of which are harmful. Probiotics is the name given to some strains of bacteria (a type of microorganism) that are thought to promote overall health.

These bacteria either naturally exist in foods, or are used to fortify foods, or sold as supplements. Yoghurt, for example, may naturally contain live strains or bacteria, but there are also lots of yoghurts with added bacteria. The idea is that by consuming these bacteria, their proportion in the gastrointestinal tract is increased, which in turn will have benefits for health.

Probiotic bacteria play several roles in maintaining overall health. We probably don't know all the ways probiotics effect health, but here's a list of functions commonly cited in the literature:

  • Boosting the immune system
  • Limiting the growth of pathogenic organisms
  • Food digestion
  • Nutrient absorption
  • Protection from cancer
  • Allergy resistance

The function we're interested here is immune enhancement. Probiotic bacteria play a key part in immune function through a variety of mechanisms (link). For example, they increase the number of circulating white blood cells or leukocytes, which defend the body against infectious disease. They also increase levels of antibodies or immunoglobulins, which are used to identify and neutralize foreign objects like viruses and bacteria.

Can probiotics prevent the flu?

While the bacteria in the gastrointestinal tract clearly play an important role in immune function, it's far less clear whether probiotics consumed with foods or as supplements have any effect. Some product reviews have revealed that up to a half of the probiotic products studied may contain significantly less live bacteria than advertised. And even if they do contain the bacteria, will eating them enhance the immune function?

Well, at least pharmaceutical grade probiotic products seem to do the trick. In one study, healthy Swedish workers were randomised to receive either Lactobacillus reuteri or placebo for 80 days (link). In the placebo group, 26.4% reported being sick during this period, compared with only 10.6% in the probiotic group. These figures included only illnesses related to the respiratory and/or gastrointestinal tracts, so the probiotic treatment clearly improved their immune system. It was especially effective in those participants who worked the night shift.

In a 6-month study on children, probiotic consumption reduced both the duration and incidence of fever and cough compared to placebo (link). The probiotic strains used were Lactobacillus acidophilus and Bifidobacterium animalis. The combination of the two was better than L. acidophilus alone, reducing fever incidence by 72.7%.

Another study found that the cellular immunity of the elderly improved when the probiotic Bifidobacterium lactis HN019 was added into their milk (link). The authors note that the improvements were greatest in those participants who had poor immune systems before the treatment. Instead of measuring how often they got sick, however, the authors measured the proportions of T lymphocytes and natural killer cells, so it's not clear how this translated into overall health.

Indeed, not all studies have shown that probiotics can prevent you from catching a cold. One large study found that supplementing healthy adults with probiotic lactobacilli and bifidobacteria had no effect on the incidence of the common cold (link). It did, however, reduce the severity of symptoms and shorten their duration by almost 2 days.


Probiotic bacteria are known to play an important role in immune function. Less is known, however, about whether taking probiotic supplements can enhance the immune system and reduce the incidence of viral infections such as the common cold.

Based on the few studies that have actually looked into this, it seems that at the very least, long-term supplementation with probiotics reduces the duration and severity of the symptoms flu. Some of the studies also found that probiotics dramatically reduced the incidence of flu. Thus, there is a basis for the claim that probiotics can help prevent you from getting sick.

There's no evidence that taking probiotic supplements after you've gotten sick will do anything, however. It also appears that the effect of probiotic is greatest in those people whose immune systems are not working properly or are under a lot of stress.

Keep in mind that some of the foods with added probiotics may not actually contain any live bacteria, despite what it says on the label, since the bacteria die quickly unless the product is kept refridgerated. The same is true of supplements in capsule form, unless it specifically states that the product is stable in room temperatures. With powdered supplements, moisture is a potential problem.

For more information on supplements and immunity, see these posts:

L-Carnitine, Exercise Performance & Oxidative Stress
The Role of Coenzyme Q10 in Oral Health
Swine Flu and Avoiding the Cytokine Storm: What to Eat and What Not to Eat?
Examining Possible Causes for Slower Wound Healing

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3 kommenttia:

In October 26, 2009 at 4:22 PM  

Nice analysis, thanks for the info. How do the studies look for the other purported benefits?

I know I could look them up myself, and you may not have looked at them. I was just wondering if you have any preliminary thoughts.

New Rose June 25, 2012 at 7:58 AM  

I have been taking 1 probiotic capsule every morning for the past 2 months and so far have not had a single cold or flu symptom. My health in general is much imporved since I started taking probiotics.

Anonymous July 13, 2012 at 1:43 AM  

The measured impacts are enormous, but the sample sizes in the studies mentioned here are fairly small... Which means the results are promising and suggestive but not conclusive.

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