Safety evaluation of certain food additives - ipcs inchem

STEVIOL GLYCOSIDES (addendum) 187
compounds, and steviol was detected primarily in the faeces at low levels. The study
was approved by the local ethics committee and met the requirements of the
Declaration of Helsinki (Wheeler et al., 2008).
2.1.2 Biotransformation
The metabolism of stevioside (purity not stated) was investigated in human
saliva, gastric secretions and faecal bacteria, as well as intestinal brush border
membranes and intestinal microflora from rats, mice and hamsters. Stevioside was
unchanged following incubation with human saliva and gastric secretions or with
intestinal brush border membrane vesicles from rats, mice and hamsters. Microflora
from rats, mice, hamsters and humans was found to metabolize stevioside to steviol.
Steviol-16,17-epoxide was found to be produced by human faecal bacteria, but
this was converted back to steviol by further action of faecal bacteria (Hutapea et
al., 1997). A review of microbial hydrolysis of steviol glycosides noted that several
other similar studies have been carried out with no epoxide formation being detected
and concluded that this may be an incorrectly identified metabolite or may be due
to the aerobic nature of the medium allowing oxidation to occur (Renwick & Tarka,
2008).
In the studies described in section 2.1.1 above, in which [ 14 C]rebaudioside A,
[ 14 C]stevioside and [ 14 C]steviol were administered by gavage to intact and bile duct–
cannulated male and female Sprague-Dawley rats, the faecal metabolite profiles
were similar between the three test substances, with the predominant metabolite
being steviol in all cases, with a smaller amount of steviol glucuronide being found,
along with a very small percentage of unidentifiable metabolites. Steviol glucuronide
was the predominant radioactive component in the bile, indicating that
deconjugation occurs in the lower intestine (Roberts & Renwick, 2008).
Five male and five female healthy volunteers (aged 21–29 years) were
provided with capsules containing 250 mg stevioside (97% stevioside, 2.8%
steviolbioside, 0.2% rebaudioside A) to be taken 3 times per day for 3 days. Doses,
expressed as steviol, were 299 mg/day or 4.60 mg/kg bw per day for females and
4.04 mg/kg bw per day for males. Twenty-four-hour urine samples were taken at
enrolment and after dosing. Urine samples were analysed for bound steviol and
steviol glucuronide. Blood samples were also taken before and after dosing and
analysed for alkaline phosphatase, alanine aminotransferase (ALT), glutamic–
pyruvic transaminase (GPT), creatine kinase and lactate dehydrogenase. No
significant differences in electrolytes or markers of tissue damage were observed.
The only metabolite detected in urine was steviol glucuronide. The authors concluded
that because of its molecular size, the uptake of stevioside by the intestinal
tract is likely to be very low and that stevioside is not degraded by enzymes in the
gastrointestinal tract. However, bacteria found in the gut microflora are able to
metabolize stevioside into free steviol, which is easily absorbed. The authors
suggested that following degradation by the microflora, part of the steviol is
absorbed by the colon and transported to the liver by portal blood, where it is
conjugated with glucuronide, which is subsequently excreted in the urine. This study
was approved by the local ethics committee (Geuns et al., 2006).