What are stevia's therapeutic uses?

Overview

Stevia, a member of the Aster family, has a long history of native use in Paraguay
as a sweetener for teas and foods. It contains a substance known as stevioside
that is one hundred to three hundred times sweeter than sugar but provides no
calories.

In the early 1970s, a consortium of Japanese food manufacturers developed stevia
extracts for use as a zero-calorie sugar substitute. Subsequently, stevia extracts
became common ingredients in Asian soft drinks, desserts, chewing gum, and many
other food products. Extensive Japanese research has found stevia to be extremely
safe. However, there have not been enough studies in the United States for the
Food and Drug
Administration to approve stevia as a sugar substitute.
Without its being identified as such, stevia is nonetheless widely used by savvy
manufacturers to sweeten commercial beverage teas and other products.

Although stevia is best known as a sweetener, stevia extracts also can be taken in
very high doses to possibly reduce blood pressure, according to two large Chinese
studies.

Therapeutic Dosages

Stevia is sold as a powder to be added to foods as needed for appropriate sweetening effects. It tastes slightly bitter if placed directly in the mouth. In liquids, however, bitterness is generally not noticeable, and most people find the taste delightfully unique.

In the studies of stevia that have shown an effect on blood pressure, stevia was given as a standardized extract supplying 250 to 500 milligrams (mg) of stevioside three times daily (a dose considerably higher than any reasonable use of stevia as a sweetener).

Therapeutic Uses

Stevia is primarily useful as a sweetening agent. In addition, two double-blind
studies suggest that it may also offer potential benefits for hypertension.
Weak evidence hints at potential benefits for diabetes.

Scientific Evidence

A one-year double-blind, placebo-controlled study of 106 persons with high blood pressure evaluated the potential benefits of stevia for reducing blood pressure. In the treated group, the average blood pressure at the beginning of the study was about 166/102. Participants were given either placebo or stevioside (stevia extract) at a dose of 250 mg three times daily. By the end of the study, the average blood pressure had fallen to 153/90, a substantial if not quite adequate improvement. Note that this is a high dose of steviosides, the sweetness equivalent of more than one-third of a pound of sugar daily. However, this study is notable for finding no benefits at all in the placebo group. This is unusual and tends to cast doubt on the results.

Benefits were also seen in a two-year double-blind, placebo-controlled study of
174 people with mild hypertension (average initial blood pressure of approximately
150/95). This study, performed by some of the same researchers who worked on the
study just described, used twice the dose of the previous study: 500 mg three
times daily. A reduction in blood pressure of approximately 6 to 7 percent was
seen in the treatment group compared with the placebo group, beginning within one
week and enduring throughout the entire two years. At the end of the study, 34
percent of those in the placebo group showed heart damage from high blood pressure
(left ventricular hypertrophy), while only 11.5 percent of the stevioside
group did, a difference that was statistically significant. No significant adverse
effects were seen.

However, once again, no benefits at all were seen in the placebo group. This result means the that study design had problems. Both studies were performed in China, a country that has a documented history of questionable medical study results.

Furthermore, a study by an independent set of researchers failed to replicate these findings. In this study, stevioside was given according to body weight, at a dose of 3.75 milligrams per kilogram (mg/kg) per day, 7.5 mg/kg per day, or 15 mg/kg per day. Compared with placebo, none of these doses affected the blood pressure of the study participants, all of whom had mild high blood pressure. These finding do not entirely refute those described above, however, as the dosage of stevia used was somewhat on the low side. For example, for a man weighing 60 kg (132 pounds), the highest dose would be 300 mg three times a day.

Another study involving diabetics as well as healthy subjects found that stevia, at a dose of 250 mg three times daily, had no significant effect on blood pressure after three months of treatment.

Safety Issues

Animal tests and the extensive Japanese experience with stevia suggest that this is a safe herb. Based primarily on the apparently incorrect belief that stevia has been used traditionally to prevent pregnancy, some researchers have expressed concern that stevia might have an antifertility effect in men or women. However, evidence from most (though not all) animal studies suggests that this is not a concern at normal doses.

The two studies described above in which use of very high dosages of a stevia extract led to reductions in blood pressure raise at least theoretical concerns about stevia’s safety. In theory, the herb could excessively reduce blood pressure in some people. Furthermore, if stevia can reduce blood pressure, that means that it is, in some fashion, acting on the cardiovascular system.

Because sugar substitutes are meant to be consumed in essentially unlimited quantities by a very wide variety of people, the highest levels of safety standards are appropriate, and unknown effects on the heart and blood circulation are potentially worrisome. This concern is somewhat mitigated by the fact that the daily dose of stevioside used in those studies was considerably higher than is likely to be consumed if whole stevia is used for sweetening purposes. Reassurance also comes from the study that found no effect with a dose of 15 mg/kg per day.

Safety of stevia use in young children, pregnant or nursing women, and those with severe liver or kidney disease has not been conclusively established. Because of the concerns noted above, individuals with cardiovascular disease should use high doses of stevia extracts only under physician supervision.

Bibliography

Barriocanal, L. A., et al. “Apparent Lack of Pharmacological Effect of Steviol Glycosides Used as Sweeteners in Humans: A Pilot Study of Repeated Exposures in Some Normotensive and Hypotensive Individuals and in Type 1 and Type 2 Diabetics.” Regulatory Toxicology and Pharmacology 51 (2008): 37-41.

Chan, P., et al. “A Double-Blind Placebo-Controlled Study of the Effectiveness and Tolerability of Oral Stevioside in Human Hypertension.” British Journal of Clinical Pharmacology 50 (2000): 215-220.