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One of the most obvious indications of the safety of stevia is that there have never been any reports of ill effects in over 1500 years of continuous use by Paraguayans. A similar indication of safety is the observaion that despite over ten years of widespread use of stevioside as a sweetening agent in Japan, years in which literally scores of tons of stevioside were ingested, not a single report of side effects of any kind has been reported. Compare that record to the track record of aspartame, which is the number one source of consumer food complaints made to the FDA.
In spite of the record of safety, however the official laboratory tests must take place. The first official investigation of possible toxicity from stevia was performed in 1931 by Pomaret and coworkers in South America. Their tests were negative.33 They observed that stevioside passes through the human alimentary canal without being altered by digestive processes. That is, it goes out in exactly the same form that it goes in. In the decades since that observation there has raged a minor debate over this issue, but so far nobody has been able to prove Pomaret wrong. The issue is important because some of the metabolites of stevioside, as opposed to whole leaf, have been shown to be toxic (see below), and researchers have cautioned against the use of stevioside for human consumption until it is known for certain that stevioside is not metabolized in the human body. A typical statement is this from a report published in 1974:" . . . the long-term effects of ingestion of stevioside would have to be investigated carefully before it could be considered for human use as a sweetener in the United States . . . It remains to be proved that stevioside does not split to form any steviol in the human digestive tract." (italics theirs).34 This challenge is, of course, tantamount to proving a negative. Perhaps that is why the United States resists all efforts to seriously explore the possible use of stevia as a sweetener. No further progress on the issue has been made since 1974. It appears that Pomaret's observations still hold.
More elaborate safety tests were performed by the Japanese during their evaluations of stevia as a possible sweetening agent. Few substances have ever yielded such consistently negative results in toxicity trials as has stevia. Almost every toxicity test imaginable has been performed on stevia extract or stevioside at one time or another The results are always negative. No abnormalities in weight change, food intake, cell or membrane characteristics, enzyme and substrate utilization, or chromosome characteristics. No cancer,no birth defects, no acute and no chronic untoward effects. Nothing.35-39
The only related effect ever observed was the inhibition cell respiration (oxidative phosphorylation) in certain isolated cell components, but never in whole cells. The only observable result of this action, even after prolonged observation, was a reduction in toxicity due to a substance known as atractylignin, a poison that attacks cells of the liver. This result suggests that stevia could be used as an antidote to rare cases of poisoning by that chemical. The overall result of this action of stevia, then, turns out to be positive.40
An example of a good toxicology trial was one performed in 1985 by Yamada and coworkers. They administered stevioside and rebaudioside A to rats for two years at the rate of 0.3 - 1% of their diet. The animals were then sacrificed, and the researchers conducted bio-chemical, anatomic, pathological and carcinogenic tests on 41 organs following autopsy. In addition they performed ongoing hematologic and urine tests on the same animals. Each of the animals was matched to a control animal that experienced exactly the same treatment except for the stevia. In the end, the symptoms and alterations noted by the research staff did not vary at all between the groups, and no dose-response effects were noted, even at the highest dose (1%), which is equivalent to 125 times the average daily dose of sweeteners that a normal human would require.41
Similar batteries of tests carried out by the National Ministry of Health and Welfare in Japan also failed to find any form of toxicity whatsoever.42
But there is a fly in the ointment, so to speak. As mentioned earlier, there has been a fear that metabolites of stevioside and rebaudioside A might be doing serious harm to the body. As one author put it: "In spite of the fact that acute oral administration of large doses of stevioside and/or Stevia rebaudiana extracts and long-term studies with feeding either of these materials to laboratory animals have shown them to be virtually devoid of toxic effects, one must consider the limited data available on metabolites (italics mine) of the major sweet principles of this plant."43 Now this comment was made in full knowledge of the fact that stevioside and the other glycosides of stevia are remarkable for their chemical stability; that is, due to their peculiar chemical or molecular shape, stevia glycosides are extremely resistant to acid and enzymatic degradation. They simply cannot be broken down into their metabolites under normal gastric conditions. Gastric acids and enzymes, as found in humans, are incapable of degrading these extremely stable molecules. This is in line with Pomaret's study that found that steviosides passed unchanged through the human gastrointestinal tract.
Apparently the situation is different in the rat. In 1980 R.E. Wingard and associates reported that stevioside and rebaudioside A were both degraded to steviol by rat intestinal microflora in a test tube.44 Steviol is one of the nasty metabolites that could, maybe, perhaps, do humans serious harm. Wingard incubated the stevioside for 2-4 days in a specially prepared solution containing the contents of the rat cecum. Under these conditions, conversion was almost 100%. However, as Kinghorn and Soejarto have pointed out, there are just two things wrong with extrapolating these results to humans.45 First, humans do not have a cecum, as does the rat; therefore, a critical step in the conversion process has no equivalent physical location in which to occur. And second, there is no good reason to believe that the microflora of the human intestinal tract contains the same microorganisms as does the rat cecum.
One would think, in light of the seriousness of the theoretical charge posed by Wingard, that scientists would be clamoring to settle the issue through appropriate experimental measures. Not so. It's as if no one really takes the threat seriously. After all, it is unlikely that some kind of observable consequence of steviol (the metabolite) intoxication would not have been reported during decades of stevia use if, in fact, a real problem existed. Since no reports have been forthcoming, we can daringly conclude (apparently along with the rest of the scientific community) that humans are different from rats.
Conclusions from Safety Data.
One might reasonably ask, based on these toxicological data, why efforts to make stevia the sugar substitute of choice in the United States and Europe have failed so miserably, and why, in fact, individuals who have attempted to produce high quality stevia liquid extracts in the United States have been threatened with prosecution. Here we have a plant, totally innocuous, posing no threat to human life and health, holding out in fact great hope for the production of a non-caloric sweetener with health benefits, that is being systematically suppressed.
Perhaps, in view of the numerous health benefits discussed in this booklet (and the dozens of anecdotal uses not discussed, such as the ability to reduce the craving for sweets and fatty foods, and as a stop-smoking and/or stop drinking aide, the time has arrived for consumers to begin insisting on their right to freely use this fine, delectable plant from Paraguay. This author is certainly growing impatient with ongoing regulatory actions that appear to be deliberately designed to keep stevia out of his diet--a sad fate for a wonderful food like stevia.
Note: I have been eating Sunrider concentrated stevia since 1992 with nothing but positive results. I eat it daily, using it in all my herbal beverages, my Calli tea, my NuPlus and even on my oatmeal in the mornings. Sometimes I believe it is the one thing I would never be without. Stevia is such an important food, I choose to eat it daily for the rest of my life.