I’ve been using different forms of stevia for several years now. I always wondered if stevia is safe in normal and even in higher doses.
I did a search in the scientific literature. I went back to 1968 for individual studies and I found compilation studies as well. Before I tell you what I discovered, let’s first talk about stevia and its different forms.
Stevia comes from the Stevia Rebaudiana plant in the form of a complex sweetening compound under the name Steviol glycosides. It includes the two most common forms of commercial stevia sweeteners – Stevioside and Rebaudioside A. Steviol glycosides are 250-300 times sweeter than sucrose (table sugar). So, stevia extracts are know as a high-intensity sweeteners. They are also 0-Calorie or non-nutritive sweeteners.
Steviosides are usually sold as drops or in powder form and are labeled “supplements”. Rebaudioside A was approved by the FDA in 2008 as a food additive, thus it can be used as a sweetener in commercial applications (baking, beverages, etc.).
In humans, steviol glycosides are metabolized in the liver and filtered out in the kidneys and excreted in the urine in the form of steviol glucoronide.
But is stevia safe? Are its sweetening compounds – stevioside and rebaudioside A (or Reb-A) safe?
There are several health-related properties that are commonly studied when a new food is introduced to the food supply. These are toxicity, genotoxicity (does it cause mutations in the DNA, cancer), carcinogenicity (does it cause cancer), reproduction and developmental safety, effects on glucose and blood pressure.
Scientists have studied extensively the potential of toxicity of steviosides and rebaudioside A for more than 20 years. Studies, mainly in rats demonstrated that periods of 13 to 56 weeks and doses of up to 2.5 g/kg bw/day steviosides and up to 11.7 g/kg bw/day of rebaudioside A do not exhibit toxicity in these doses (1, 2, 3, 4, 5).
Multiple tests have shown that steviols do not possess genotoxicity in doses upwards of 2 g/kg bw/day (6,7,8). Just for reference this dose is huge. I weight 70 kg (154 lbs), so this means I can ingest as much as 140 g of steviols a day. This is poison! I can tell you that I use 3 g of Reb-a in conjunction with erythritol to sweeten a whole batch of 0-sugar protein cookies or protein muffins.
One study in 2007 showed DNA-induced damage in the blood and several sites in the bodies of rats. The study concluded that steviosides have possible mutagenic properties. However, several toxicology scientists have reviewed this particular study and its conclusions and have determined that the study had been compromised by serious flaws in methodology, data interpretation and lack of adequate controls.
In 1997 Japanese researchers, based on the doses determined in previous 13-weeks studies, conducted a 104-week long study in rats, which is pretty much how long rats live, as far as I know. The researchers concluded that “there was no significantly altered development of neoplastic or non-neoplastic lesions attributable to the stevioside treatment in any organ or tissue”. Similar results were shown to be valid for rebaudioside A in 2008 (1).
Reproduction and developmental safety
One study on the contraceptive properties of the stevia plant on female rats, carried out in 1968 (scientists used ground up leaves of the stevia plant) concluded that 10 mg/day of the stevia “weed” decoction significantly reduces the fertility in female rats. Researchers, who interpreted this study considered it seriously limited due to the fact that the test material was a concoction of ground up leaves, which is almost impossible to establish what it exactly is as far as the doses of the active compounds.
Subsequent studies, done on rodents and chickens, have demonstrated that steviols (steviosides and rebaudioside A) there are no grounds for concerns as to the fertility affecting properties of purified stevia extracts in doses up to 80 times higher than the suggested daily dose of steviosides for humans (9, 10, 11, 12).
Effects of stevia on blood glucose and blood pressure
Effects of stevia on blood pressure
A study from 2000 demonstrated that both systolic and diastolic blood pressure decreased in hypertensive human subjects. This raised a concern that stevia and its extracts may have a strong therapeutic (blood pressure-lowering) effect that may negatively affect those who are not hypertensive (with normal blood pressure).
A subsequent study in 2008 examined the potential blood pressure altering effects of rebaudioside A in subjects with normal and low blood pressure. Doses of 1000 mg/day did not cause significant change in the blood pressure of these subjects.
Yet, another more recent study showed that stevioside “may also offer therapeutic benefits, as they have anti-hyperglycemic, anti-hypertensive, anti-inflammatory, anti-tumor, anti-diarrheal, diuretic, and immunomodulatory actions”. Also from the same study: “It is of interest to note that their [stevioside] effects on plasma glucose level and blood pressure are only observed when these parameters are higher than normal.”
Effects of stevia on blood glucose
Numerous studies to date have demonstrated that steviols do not exhibit adverse effects on the glucose levels in subjects (both humans  and animals) with diabetes mellitus. If anything, some studies show that steviols may be able to positively affect blood sugar levels (antihyperglycaemic, insulinotropic, and glucagonostatic actions) via many different biological mechanisms at least in laboratory animals (13, 14, 15, 16, 17).
I have to admit that I’m an amateur in interpreting scientific literature. For that reason alone I constantly strive to account for what’s known as “confirmation bias“. I am not drawing any definitive conclusions from the above text and the literature I’ve found – at least not for you. I will leave it up to you to decide whether stevia and its compounds are safe for you or not.
But, logic tells me that if stevia has been in use in some form or another for many many years in several countries in Central and Latin America, as well as in Japan for almost 40 years now (since saccharin was banned there). I haven’t seen and couldn’t find any serous and non-flawed evidence of substantial negative effects of stevia consumption.
I may have missed something – this article is by no means conclusive. If you find anything that is inaccurate or you know of something that should be added to this article, please let me know. I’d also like to hear your comments about the experience you have with stevia in any of its forms.
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