The bottom line: Stevia extract, at the miniscule dose used to sweeten food and beverages, when used in moderation does not appear to exert any negative effect on human fertility at all. The doses used in animal studies of doubtful scientific methodology were enormous in comparison, and the negative effects where they occurred, were unable to be reproduced by other scientists. Both studies showing impaired fertility used an extraction of the whole herb. Exhaustive toxicity studies on compounds isolated from stevia, such as stevioside and rebaudioside A have not found any negative effects whatsoever.
What is Stevia?
Stevia is a herb in the sunflower or Asteacea family the leaves of which contain a variety of substances which taste very sweet, but have no calories or carbohydrates when used to sweeten food and drinks. It has been used by the inhabitants of its native Paraguay, where it was known as kaa-he-e, for over 1,500 years, but was only described to the western world in 1899 by Moises S. Bertoni, an Italian botanist.
Stevia is a popular non-caloric, non-glycaemic, non-insulinaemic sweetener, meaning it has no calories and affects neither blood glucose levels nor insulin levels after consuming it.
It has been estimated that stevia extracts like rebaudioside A are about 300 times as sweet as sucrose, or white table sugar. The ground whole leaf of the stevia plant is 30-40 times as sweet as table sugar. The two most abundant chemical compounds in stevia are stevioside and rebaudioside A and these are extracted and refined for sale as a sweetener in many countries. Rebaudioside A is considered to provide the best taste with no bitter aftertaste.
Some people find stevia has an unusual taste at first. It may help to mix it with other sweeteners at first, such as raw sugar, muscovado sugar or xylitol.
In addition to being non-caloric, the studiesdone to date on Stevia indicate that it could have quite a raft of beneficial effects -antioxidant properties, improving glucose metabolism, revitalising the pancreatic beta cells which produce insulin,improving lipid profiles (cholesterol and triglycerides), stimulating the immune system, anti-tuberculosis activity,improving the solubility of other drugs, and even potentially inhibiting cancer development.
A 2004 study published in the journal Metabolism found that when type II diabetics were given 1 gram of stevioside with a test meal, compared to control subjects who were given a gram of corn starch instead, stevioside reduced the incremental area under the glucose response curve by 18%, meaning that the total amount of glucose in the blood over the 4 hours after the meal was reduced by 18%. The insulinogenic index, which is the total amount of insulin produced divided by the total amount of blood glucose (AUC,insulin/AUC,glucose) was increased by approximately 40% by stevioside compared to control, indicating that the diabetics were producing more insulin in response to the stevioside and meal.
The FDA controversy
Throughout the 1980′s the popularity of stevia as a sweetener, particularly in teas was growing. Even large companies like Lipton and Celestial Seasonings included it in their blends. The herb had been used throughout Asia for decades with no adverse effects and was the most popular sweetener in Japan, taking up half the non-caloric sweetener market, where artificial sweeteners like aspartame have been banned for over 30 years due to concerns over their carcinogenicity. In 1991, the US FDA banned the import of stevia and stevia products into the US. An article in the September 28-October 4, 2000 issue of the Sonoma County Independent quoted numerous business owners whose businesses had been stormed by FDA officials, confiscating herbal teas containing stevia, after allegedly being pressured by NutraSweet, now a Monsanto subsidiary. In 1995, the FDA was forced to repeal its ban on stevia and allowed it to be sold as a supplement, though it could not make claims about its extraordinary sweetening properties. Coca-cola and Pepsi, under fire for the role their beverages play in the obesity epidemic, became interested in the potential of stevia to lower the calories in their beverages without reducing the sweetness. In 2007, researchers working for Coca-Cola filed a patent for purifying Reb. A from stevia and in 2008, the FDA approved the use of the isolated rebaudioside A compound from stevia as a sweetener in these companies’ iconic beverages.
Truvia, Coca-cola’s brand of stevia extract and PureVia, Pepsi’s version, are manufactured using an extraction and purification process which involves the use of toxic and dangerous solvents like acetone and methanol. Other methods, using only water or water and alcohol are available.
The fertility debate
Anecdotes in online forums
There are some posts by women in online forums that indicate that stevia has adversely affected their menstrual cycle, although these posts are generally by women who have pre-existing hormonal conditions, such as PCOS that are equally as likely to be the cause of cycle irregularity. It is impossible to rule out coincidence when dealing with the experiences of a single individual. You can never reproduce the exact set of circumstances in which the ‘reaction’ occurred so it is impossible to know whether these menstrual cycle irregularities can really be attributable to stevia use or whether they would have occurred anyway.
Reports of traditional use by Paraguayan women of stevia as a contraceptive
It was reported in the scientific literature soon after stevia was first described to the western world that women in Paraguay traditionally drank a cup of stevia tea per day as a contraceptive. There is no report of how effective this was, or of the dose used. It is likely that around 15 grams of dried herb per cup of water would be used if the use of stevia was in line with the use of other traditional herbal medicines in the area.
Scientific Studies: Kuc, Planas & Melis
The scientific basis for the questions around stevia’s effects on fertility centre around two studies. In 1968, a young biochemist and plant pathologist by the name of Joseph Kuc who was working at the time out of Purdue University published a study in collaboration with Gladys Planas from the University of the Republic in Paraguay on the effects of very high doses of stevia on rats.
Some factors which may negatively affect the value of this study:
- Firstly, rats are not humans and we may not experience the same results that the rats in this study did.
- Secondly, the dose of whole stevia extract given was very, very high – 10 ml of a 5% decoction of the whole dried plant which had been boiled for 10 minutes, then cooled and strained. This amount equates to 0.5 grams per rat or 2 grams per kilogram of bodyweight. The equivalent dose for a woman weighing 60 kg would be 120 grams of dried stevia or 24 teaspoons of the dried herb.
- Thirdly, other scientists have not been able replicate the findings from this experiment, indicating that the results were due to some other confounding factor which was not realised at the time.
- Fourthly, the study was carried out on a relatively small number of animals, just three groups of 14 female rats from a colony of albino rats that had been inbred for 28 years from the offspring of a single pair of rats.
The study concluded that:
“Fertility was reduced 57 to 79 percent in female rats drinking the decoction as compared to rats drinking water. A reduction of 50 to 57 percent in fertility was still evident 50 to 60 days after intake of the decoction had ceased.”
In 1999, Melis of the Biology Department of the University of Sao Paulo in Brazil published a study which claimed that very high doses of stevia for 60 days reduced the weight of the testicles and some attached structures of male rats, as well as decreasing sperm count and testosterone levels, which he hypothesized occurred due to the glycosides in the extract being attracted to testosterone receptors in the rat.
Scientific studies refuting Kuc, Planas & Melis’ findings
The results of these studies have, however, since been refuted by numerous researchers, using better methodologies and larger numbers of animals. As Geuns et al. wrote in their exhaustive analysis of the literature in 2003
“The results of a decrease of live birth rate in rats (Planas and Kuæ, 1968) by Stevia decoctions were refuted by Shiotsu (1996) who did more reliable experiments with many more animals using methods as similar as possible to the methods used by Planas and Kuc. No effect on general condition, body weight, water consumption, live birth rate or litter size was found. No effects of stevioside were found on fertility or reproduction in mice (Akashi and Yokoyama, 1975), rats ( Mori et al., 1981, Xili et al., 1992 and Sinchomi and Marcorities, 1989) or hamsters (Yodyingyuad and Bunyawong, 1991).
No significant effect was found on spermatogenesis, nor on the interstitial cell proliferation and tumor formation in the testes of F344 rats fed a ration containing up to 1% stevioside (95.2% purity) for 22 months (Yamada et al., 1985).
Whereas Melis (1999) suggested a possible decrease of the fertility of male rats by a very high dose of Stevia extract, Oliveira-Filho et al. (1989) who administered extracts with similar stevioside content stated that there is certainly not an effect on male fertility. It is not sure that the observed effects were due to the stevioside present in the extract. It should also be mentioned that the used extract concentrations were extremely high, at the start of the experiments even 5.34% of the body weight (or around 5.3 g stevioside/kg bw). For an adult person of 65 kg this means 3.47 kg of dry Stevia leaves or about 34.7 kg fresh leaves/day, i.e. more than 50% of the body weight! The significance of such experiments where only one extremely high concentration was tested, should be questioned. Melis’ results are also in contradiction with the above and below cited studies that could not reveal any effect on fertility of male or female animals.”
The newly published American Herbal Products Association Botanical Safety Manual, 2nd edition states “although two animal studies reported reductions in male or female fertility after large doses of stevia (Mazzei Planas and Kuc 1968; Melis 1999), no adverse effects on fertility were observed in four other studies (Akashi and Yoyoyama 1975; Mori et al 1981; Sinchomi and Marcorities 1989; Yodyingyuad and Bunyawong 1991). Animal studies have shown no adverse effects of large doses (up to 3g/kg) of the compound stevioside on pregnancy or fetal and maternal health (Mori et al 1991, Usami et al 1995, Yodyingyuad and Bunyawong 1991).
In a 1996 article written by Linda Bonvie, she quotes Dr Kuc as saying “that while he still “stands by the results we had” – that is, a finding of toxicity and a marked reduction in the number of young born and “restrictions in circulation to the extremities”-those results should not be interpreted as applying to humans consuming either stevia tea or its extract, stevioside.
For one thing, Kuc noted, the study involved “a very high concentration” given to the rats instead of drinking water, and consisted of the whole plant, dried as a powder, and not just the leaves. Asked if his study should be a basis for keeping stevia off the market in this country, Kuc replied: “That in itself, no.”
Professor Mauro Alvarez, who was a lead researcher or collaborator on many of the studies into stevia’s safety has said “as a scientist with over 15 years researching the safety of Stevia and of many other plants used as food or food ingredients, I can assure that our conclusions in these various studies indicate that Stevia is safe for human consumption as per intended usage, that is, as a sweetener.”
Akashi and Yokoma (1975) observed absolutely no difference in the fertility rates of rats fed with up to 100 mg/kg of various stevia extracts for 21 days, nor did they find any abnormality in the foetuses conceived during the study.
Mori et al (1981) of the Stevia Safety Committee of Japan fed a group of 20 male and 20 female rats extracts of the whole stevia plant or purified steviosides at up to 525 mg/kg, roughly 75 times more than a human would ingest and in some experimental groups the dose was of purified stevioside was 500 times greater than a human would encounter in using stevia as a sweetener. The male rats were fed this stupendous amount for 60 days, the females for 14 days. Again, no effects on fertility were noted, nor any abnormalities detected which could be attributed to the stevia.
How much stevia is safe?
The Joint FAO/WHO Expert Committee on Food Additives (JECFA) is an international scientific expert committee that is administered jointly by the Food and Agriculture Organization of the United Nations (FAO) and the World Health Organization (WHO). In 2009, JECFA established an acceptable daily intake (ADI) for steviol glycosides of between 0 and 4 mg per kilogram of bodyweight. For a 60 kg woman this would be up to 240 mg per day. The JECFA committee took into consideration the results of studies showing no adverse effects on diabetics and people with low-normal blood pressure who took up to 4 mg/kg bw/day of steviol for up to 16 weeks.
If stevia were an effective and safe contraceptive, wouldn’t we all be using it?
As stevia has been confirmed to be non-toxic by numerous toxicological studies, it is likely that if the contraceptive effect of stevia were real, researchers would have long ago discovered that fact and isolated the compounds responsible. That no studies have been able to reliably and repeatably confirm this, is a strong indication that the contraceptive properties of stevia are just unfounded folklore. The first study to imply a contraceptive effect from stevia was published in 1968, in the early days of pharmaceutical contraceptives and the flower power revolution. Contraception was a hot topic then. If there was substance to these claims they would very likely have been thoroughly researched by more than one team of scientists with the objective of adding to the growing arsenal of chemical contraception options.
So, what’s the bottom line?
There have been a few studies which have found a negative effect on fertility in rats with extremely high doses, however, these studies have been refuted and are considered to be either of poor methodology or simply unrepeatable by other researchers, indicating that the results were due to a confounding factor that was not discovered at the time.
Dozens of studies have determined that stevia, particularly isolated sweetening compounds stevioside and Reb. A are safe to use in food and drink as a sweetener and have no adverse effect on human health or fertility.
As with anything, however, the bottom line is “all things in moderation”. Use stevia in small amounts, and all should be well. Go overboard and use heaped teaspoonfuls every few hours and you may experience problems. Women with PCOS generally already have disordered hormones and may be more sensitive than the general population to compounds which may further influence those hormones. Be observant towards your body and how you react to stevia. If you are trying to conceive, it may not be worth the infinitesimal risk. If you are not trying to conceive, even the studies which found a reduction in fertility, found that two months after ceasing to take stevia fertility had significantly, but not completely, recovered.
At this point, there is no evidence to support the assertion that stevia causes a decrease in fertility in animals or humans, male or female at normal doses used to sweeten food. Less than 4 milligram grams per kilogram of bodyweight per day, or around 240 mg for a 60 kg woman is considered safe by the FDA. There is some evidence that it may even be beneficial, particularly in relation to blood sugar metabolism. As a non-caloric, non-glycaemic and non-insulinaemic sweetener, stevia may be very useful for women who have PCOS and need to limit the amount of sugar in their diet and do not wish to use artificial sweeteners due to concerns over their safety.
There is a great body of toxicological research on the compounds stevioside and rebaudioside A which are responsible for the sweetening properties of stevia. If there is an anti-fertility compound in stevia, which is considered at this point to be unlikely, it would be present in the whole stevia plant, but not in a purified extract containing only steviosides or rebaudioside A.
A personal anecdote
I had been using stevia extract powder by Nirvana Organics, which I believe is predominantly rebaudioside A, for many years to replace sugar in all sorts of things. When I was asked about the basis for the anti-fertility claims in relation to stevia, I stopped using it for a few months until I had time to research and write this article. Based on the studies I’ve now read, I have started using stevia again, but will make sure that I keep my intake below that acceptable daily intake recommended by the FDA.
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