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Much has been written about the harmful consequences of runaway oxidation. As part of normal cellular respiration, there is no way to avoid some level of free radical production because for better or worse, oxygen is a very reactive substance yet necessary for life itself.

Oxidation, however, is not merely the addition of oxygen atoms to cellular processes. It applies more generally to a biological reaction whereby electrons are transferred from one atom to another.

At its most basic, oxidation is the removal of electrons. Oxidation always occurs with its opposite reaction, reduction, which involves the addition of electrons. So if one atom gains an electron (is reduced), it gains that electron at the expense of another.

Free radicals are atoms with an odd (unpaired) number of electrons. Because these atoms lack the electrons they need to maintain stability, they collide with other atoms to gain the electrons they require. Doing so creates new free radicals in a chain-reaction manner.

A number of biological free radicals or reactive oxygen species (ROSs) exist. Among these are oxygen itself, superoxide anion, peroxide, hydrogen peroxide, hydroxyl radical and hydroxyl ion.

These ROSs can easily cause damage to important cellular structures like the lipid layers that surround cells and organelles (cellular components) and the organelles themselves like the mitochondrion, Golgi apparatus and nucleus. Damage to the nucleus, the control center of the cell and site of DNA synthesis, increases the risk of cellular mutations that overtime can result in cancerous tumors.


This is an illustration of a typical human cell. The mitochondrion is where cellular energy is produced from the food we eat and where most endogenous free radicals are generated via cellular respiration. Now imagine out-of-control pellets ricocheting every which way damaging these cellular components, and you get a good idea of the havoc reactive oxygen species can cause.

These chain reactions can be stopped by antioxidants that donate electrons to free radicals, stopping the chain reaction dead in its tracks:


Now, not all free-radical generation is bad. You wouldn’t have much of an immune system if various immune cells like neutrophils didn’t produce them to kill infected cells during infections. Nor would many cancer treatments work without them. However, chronic generation of these ROSs by the immune system (inflammation) does present a very big problem and underlies most of the degenerative diseases we are all too sadly familiar with.

The body has a number of endogenous enzymes that are produced to deal with free radicals: superoxide dismutase, catalase, glutathione peroxidase and glutathione reductase. Other endogenic antioxidants include uric acid and bilirubin, the breakdown product of red blood cells.

Vitamins like vitamin A, C and E also act as antioxidants as do polyphenols making diets rich in these antioxidants a useful complement to built-in antioxidant defenses.

Glutathione is the body’s most significant non-enzymatic antioxidant and exists in relatively large amounts if not depleted by combating high levels of oxidation. Not only does it neutralize free radicals, it preserves the antioxidant status of both vitamins C and E. It is also a necessary part of biological processes responsible for DNA synthesis and repair.

Recently, attention has been given to the potential antioxidant role of gut flora. As I wrote in my post Longevity and Gut Flora, one way gut flora may contribute to antioxidant status is via production of polyamines like spermine, putrescine and spermidine. These are all substances that scavenge free radicals. However, gut flora also decreases oxidative stress through other mechanisms that I’ll get to shortly.

In a recently published scientific paper, Italian researchers from the University of Bologna set out to determine the antioxidant properties of various gut flora in both a lab setting (in vitro) and in rats. (1) Let’s first look at what their in vitro results showed.

Seven strains of Bifidobacterium, 11 strains of Lactobacillus, six strains of Lactococcus and ten strains of Streptococcus thermophilus were evaluated for their antioxidant capabilities.

Strains of both Lactococcus and Streptococcus thermophilus are not typically found in commercial probiotics, but are used in the dairy industry to make cheese and yogurt. Both unpasteurized cheese and yogurt would be expected to contain strains from both. Who says maintaining gut health can’t be yummy?

Four tests were utilized to measure antioxidant ability. The first test, TAALA, measures resistance to lipid peroxidation, specifically oxidation of linolenic acid, a constituent of either alpha-linolenic or gamma-linolenic acid, both polyunsaturated fatty acids (PUFAs). Because all cells and organelles are surrounded by lipid membranes, free-radical damage to these structures can lead to cell dysfunction or death.

As our cell membranes tend to reflect the types of fatty acids we eat, the higher the proportion of PUFAs in the diet, the higher the concentration of these fatty acids in cellular lipid membranes and the more susceptible they become to free-radical damage. This is my major concern with dietary advice that encourages high consumption of these fats to the exclusion of saturated and monounsaturated fats that are far less prone to free-radical damage due to their inherent molecular structure.

TAAAA was the second test used. This measures inhibition to ascorbic acid oxidation within cells. Ascorbic acid is one form of vitamin C. Although a good antioxidant under normal conditions, excess levels, especially in the presence of free metal ions like iron, can begin free radical reactions.

A third test conducted was the Trolox equivalent antioxidant capacity test (TEAC). This measures the antioxidant capacity of different substances, in this case gut flora.

The last test measured levels of total glutathione (both oxidized and reduced), as well as levels of the intracellular antioxidant superoxide dismutase (SOD).



Courtesy: Antioxidant properties of potentially probiotic bacteria: in vitro and in vivo activities

Courtesy: Antioxidant properties of potentially probiotic bacteria: in vitro and in vivo activities


This is a graph of the first test measuring inhibition of lipid peroxidation. Please note that it is mislabeled in the research paper and should read TAALA, not TTALA.

Levels ranged between 2% and 37%. Strains showing values exceeding the 75th percentile belonged to Streptococcus thermophilus, Lactobacillus and Lactococcus. Tested strains of Bifidobacterium showed the least antioxidant capacity when it came to lipid peroxidation.

bHere we see illustrated inhibition to ascorbate oxidation. Significant variations between probiotic strains were noted as values ranged from 0% to 82%.

The strains that were in the 75th percentile of anti-ascorbate oxidation were Bifidobacterium breve, Bifidobacterium animalis lactis, Bifidobacterium adolescentis, Lactobacillus reuteri, Lactococcus cremoris, Lactobacillus coryniformis, Streptococcus thermophilus, Lactobacillus helveticus and Lactobacillus acidophilus.



Trolox equivalent antioxidant capacity also varied widely between strains. Strains with the most TEAC capacity were Lactobacillus reuteri, Lactobacillus acidophilusLactococcus lactisLactococcus diacentilactisLactococcus cremoris, Streptococcus thermophilus and Lactobacillus brevis.


With the exception of the Lactococcus group, most strains produced low levels of intracellular glutathione. Of the strains studied, relevant amounts of glutathione production were found in Bifidobacterium lactis, Streptococcus thermophilus, Lactococcus diacentilactis and Lactococcus cremoris.

Only Lactococcus and Streptococcus thermophilus were evaluated for superoxide dismutase activity due to their capacity to produce it. Superoxide dismutase was found to be significantly higher in Lactococcus in contrast to Streptococcus thermophilus.

In light of these results, the researchers selected Bifidobacterium lactis, Lactobacillus acidolphilus and Lactobacillus brevis to prepare a probiotic formula that would now be administered to lab rats to ascertain the antioxidant capabilities of these bacteria in living animals.

After a seven-day acclimatization period, 42 male Wister rats were randomly divided into four groups. Three groups received the daily probiotic formula at different concentrations and one did not. Concentrations for the three groups were as follows: 107, 108 and 109 colony-forming units.

After 18 days of supplementation, 20mg of doxorubicin (DOXO) per kilogram of body weight was administered to all groups receiving probiotics at 107 and 109 concentrations. Seven out of 14 rats receiving the 108 concentration and seven out of 14 rats receiving no probiotics were also given DOXO.

DOXO is a drug used in chemotherapy and increases free radicals in cancer cells to destroy them. It is, to say the least, a very effective generator of oxidative stress.


This is an illustration of free radical plasma levels in these rats. The two bars on the left are the rats that didn’t receive DOXO, NS standing for the non-supplemented group. Under non-oxidative conditions, there was no significant differences seen.

In those rats who were given DOXO (four bars on the right), the higher the concentration of probiotics given, the lower the level of free radicals.




This graphs total antioxidant activity. Again, no significant differences were noted in the group not receiving DOXO. However, in the groups administered the chemotherapy drug, the higher the concentration of probiotics, the higher the level of plasma antioxidants.



Finally, levels of glutathione were significantly lower in the mice administered DOXO and not receiving probiotics in comparison to the rats administered DOXO, but receiving the highest concentration of probiotics. Even in those rats not given DOXO, probiotic supplementation had a noticeably favorable effect on glutathione status.

These researchers make clear that much more research needs to be done to explain the precise mechanisms behind these results. Apart from the production of polyamines that I alluded to earlier, the authors mention other possible reasons for the antioxidant capacity of gut flora.

One involves the well-known ability of healthy gut flora to bind potentially harmful metals and toxins from the gastrointestinal tract and safely remove them via feces. The reduction of heavy-metal toxicity would lessen the chances for free-radical generation.

The authors also note the ability of gut flora to liberate antioxidants from the food we eat. By doing so, this is yet another way healthy gut flora helps contribute to antioxidant status.

The increase in glutathione levels speaks to active involvement in its synthesis by gut flora. These researchers point to two animal studies where gut flora was found to induce the production of glutathione in both intestinal and pancreatic cells. (2) (3) In a third study, rats supplemented with Lactobacillus casei, Lactobacillus acidophilus and Bifidobacterium lactis also showed increases in glutathione levels. (4)

This is an exciting finding as glutathione is, as I mentioned, the body’s major endogenous antioxidant. Higher levels throughout the body would be expected to protect cells against free-radical damage and actively lessen the risk of developing a number of oxidative-based conditions like cardiovascular disease and cancer.

Unmentioned in this paper is the fact that healthy gut flora outcompetes pathogens in the digestive tract and lowers the risk of endotoxemia or leaky gut. Having chronic gut infections caused by the overgrowth of bacterial or yeast pathogens is a recipe for leaky gut, localized and systemic inflammation, free-radical generation and antioxidant depletion.

While research in this area is just beginning, it should be clear that discussing antioxidant status without reference to gut flora is a fool’s game. It appears we can add neutralization of free radicals to the growing list of biological functions gut flora take part in to keep us healthy.



Alberts, B., Johnson, A., Lewis, J., Raff, M., Roberts, K., and Walter, P. (2008) Molecular Biology of the Cell, (5th ed.). New York: Garland Science.

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