Past research has shown that long-term dietary consumption of soy
products, containing isoflavones, has beneficial effects on
cardiovascular health, bone mineral density, and risks for certain
cancers. In addition, there is evidence that long-term dietary soy
consumption improves cognitive performance. However, no such studies
have examined the impact of acute doses of soy isoflavones on cognition.
Therefore, this experiment was designed to evaluate whether acute doses
of soy isoflavones would enhance memory in young adults who were not
regular consumers of soy products. The results indicate that individuals
consuming soy isoflavones, compared to control participants, did not
show enhanced memory on visual-spatial or verbal tasks. These results
suggest that the cognitive benefits derived from isoflavone consumption
may be primarily due to long-term dietary soy intake and not acute
Health benefits associated with consumption of soy-based food
products have previously been identified, including reducing the risk of
coronary heart disease (Clarkson, Anthony, & Hughes, 1995) and some
forms of cancer (Aldercreutz, Honjo, Higashi, Fotsis, Hamalainen,
Hasegawa, & Okada, 1991), positively impacting bone density (Ho,
Chan, Yi, Wong, & Leung, 2001), and improving cognitive function
(File, Jarrett, Fluck, Duffy, Casey, & Wiseman, 2001;
Kritz-Silverstein, Von Muhlen, Barrett-Connor, & Bressel, 2003).
While some epidemiological and clinical data have suggested a
relationship between dietary soy consumption and health, the evidence
related to improvements on cognitive function are less conclusive
(White, Petrovitch, Ross, Masaki, Hardman, Nelson, Davis, &
Markesbery, 2000). The soy and postmenopausal health in aging study
(SOPHIA) evaluated the impact of 110 mg of soy isoflavones on cognition
in a 6-month trial and showed significant improvements in cognitive
function assessments among their participants and concluded that soy
supplements may help prevent or delay the decline in verbal memory
observed with aging (Kritz-Silverstein et al., 2003). Research conducted
by File et al. (2001) assessed the impact of a high-soy diet (100 mg
isoflavones) administered for a 10-week period and observed improvements
in both short and long-term memories among healthy young adults.
Improvements in cognitive functions were also observed among a sample of
postmenopausal women receiving 60 mg of soy isoflavones for a 12-week
trial (Duffy, Wiseman, & File, 2003). However, additional
investigations involving soy isoflavones (99 mg) and postmenopausal
women indicated no improvements in cognitive functions among their
samples after a one-year period (Kreijkamp-Kaspers, Kok, Grobbee, de
Haan, Aleman, Lampe, & van der Schouw, 2004).
Many of the proposed health benefits obtained from dietary intake
of soy have been attributed to its isoflavone content, primarily the
aglycones genistein and diadzein (Wagner, Anthony, & Cline, 2001),
and the neurological impact of isoflavones may be due to their
phytoestrogenic effects on brain function (Lepart, West, Weber, Rhees,
Setchell, Adlercreutz, & Lund, 2002). Although the exact biological
mechanisms of soy's impact on memory have not been completely
identified, soy isoflavones have been shown to affect the synthesis of
acetylcholine, and neurotrophic factors including nerve growth factor
(NGF) and brain-derived neurotrophic factor (BDNF; Pan, Anthony, &
Clarkson, 1999). In addition, soy isoflavones impact tyrosine kinase in
the hippocampus, a brain region which is vital to memory and learning
(Akiyama, Ishida, Nakagawa, Ogawara, Watanabe, Itoh et al., 1987;
O'Dell, Kandel, & Grant, 1991). Investigations examining
soy's effect on cognition have mostly focused on longitudinal
consumption (>8 weeks) among adults, specifically, pre- or
post-menopausal women (Kritz-Silverstein et al., 2003). No studies have
assessed the effect of an acute dose of soy isoflavones on cognitive
function. There is precedent that acute administration of certain
substances can impact memory performance in both animals and humans.
Studies have demonstrated that acute administration of compounds such as
glucose, epinephrine, caffeine, and ACTH has a positive impact on memory
(Gold, 1986; Haskell, Kennedy, Wesnes, & Scholey, 2005; Metzger,
2000; Richardson, Riccio, & Ress, 1988; Stone, Rudd, & Gold,
1990). Therefore, the purpose of this study was to assess the effects of
an acute administration of soy isoflavones on memory among a sample of
healthy young adults.
Fifty college students (37 females and 13 males with a mean age of
20.1 [+ or -] 2.89 years) served as participants. Their mean weight,
height, and body mass index (BMI) were 75.2 [+ or -] 24.2kg, 160.1 [+ or
-] 40.3cm, and 24.7 [+ or -] 4.82, respectively. Participants completed
a family medical history questionnaire prior to the experiment, and
indicated no soy allergies or other health-related problems that would
prevent them from participating. In addition, participants were not
regular consumers of soy products (e.g., soy milk, tofu, soy cheese or
soy nuts). This study was approved by the Ashland University Human
Subject Review Board; all participants gave written informed consent.
The experimental group consumed fifty grams of soy protein isolate
powder, containing 54 mg of total isoflavones, which was diluted in 400
ml of tap water. The control group consumed fifty grams of whey protein,
containing no isoflavones, which was also diluted in 400 ml of tap
water. The soy protein isolate was obtained from Cargill Health and Food
Technologies (Wayzata, Minnesota) and the whey protein was obtained from
Clofine Dairy Products (Linwood, New Jersey). To increase the
palatability and visual appeal of the protein drinks, three packets of
Equal[R] (Merisant, Chicago, Illinois) and three drops of red food
coloring were added to each shake.
The stimulus lists for the word recall and recognition tests were
constructed using normed words that were similar in concreteness,
imagery, and meaningfulness (Pavio, Yuille, & Madigan, 1968).
Photographs of common objects (hammer, coffee mug, basket, etc.) for the
visual-spatial memory (VSM) task were likewise standardized, and were
taken from the International Affective Picture System (IAPS) (Lang,
Ohman, & Vaitl, 1988). These stimuli were incorporated into a
PowerPoint[R] slideshow, and projected onto a large screen with an LCD
projector. Participants penciled their responses onto prepared answer
Participants were randomly assigned to either the SOY group (n =
25) or the WHEY group (n = 25). Each participant first filled out
appropriate consent documentation, completed the family medical history
questionnaire, and indicated that they had fasted for at least 8 hours
prior to the experiment. During baseline testing, participants first
completed the VSM task. Sixteen photographs were projected onto the
screen in a 4 x 4 grid for ten seconds, after which the objects were
replaced by a blank screen. Each subject was given a blank 4 x 4 grid
and reproductions of each photograph at their desks, and they were
required to place the objects in the correct locations on their grid.
Once all participants had completed the first trial, their photograph
reproductions were removed and two more identical trials of the VSM were
completed (using the same objects in the same location). After
completion of the VSM, a verbal task ensued. Twenty words were projected
sequentially onto the screen for 2-seconds each (with a 2-second
inter-stimulus interval). Immediately after the word list was presented,
a recall test was given where participants had two minutes to write down
as many words that they could recall from the projected list. A
recognition test immediately followed the recall task, during which the
participants identified the words from a list of 40 items (20 original
and 20 new words). After the baseline testing, participants within the
experimental and control groups were given up to five minutes to consume
either the soy or whey protein shake, respectively. All participants
were blind regarding the type of protein they consumed. The subsequent
testing session occurred 1.75 hours after protein consumption to allow
for isoflavone absorption (Izumi et al. 2000; Zubik & Meydani,
2003). The second testing procedure was identical to that of the
baseline testing, other than the stimuli for both the VSM and verbal
tasks were different from the first testing session.
The SOY and WHEY groups performed similarly on baseline cognitive
tests prior to protein consumption (see Table 1). As expected, the means
for both groups improved across repeated trials of the VSM task
([F.sub.2,147] = 57.37, p < .01). As three identical trials of the
visual-spatial task were administered, it was predicted that
participants' scores would improve across the trials. However, the
only difference between the groups occurred on the first trial of the
VSM, as the WHEY group scored higher than the SOY group ([F.sub.1,48] =
4.11, p < .05). For the word recall and recognition tasks, regardless
of which task was analyzed, performance between participants in the SOY
and WHEY conditions was similar (p>.05 for all comparisons).
Cumulatively, these results indicate that participants in the SOY and
WHEY groups performed similarly on the baseline assessment.
The performance of the SOY and WHEY groups on the subsequent
cognitive testing was similar to that of baseline testing (see Table 2).
The groups performed similarly on the VSM and verbal tasks, and
statistical analyses confirmed these impressions. As with the baseline
measures of cognition, performance did improve over the three trials of
the VSM ([F.sub.2,147] = 37.76, p < .01). There were, however, no
differences between the groups on any of the VSM trials, nor on the
recall or recognition tasks (for all comparisons, p > .05).
After assessing baseline cognitive performance on visual-spatial
and verbal tasks, participants consumed either a soy or whey protein
shake and were reevaluated using similar methods. The results indicate
that an acute administration of soy isoflavones did not affect memory in
the young adults tested. This conclusion is based on the observation
that participants in the soy group did not demonstrate enhanced
cognitive performance relative to the whey (control) group.
It is clear that isoflavone administration, in this dose, was not
effective in enhancing memory performance. The soy isoflavone levels
chosen for this experiment were consistent with those amounts
administered in longitudinal studies (Kritz-Silverstein et al., 2003;
Duffy et al., 2003). While it has been observed that longitudinally
administered isoflavones enhance memory at this dosage level, this same
amount was not effective when administered acutely. It is possible that
the effects observed in longitudinal studies are due to sustained
isoflavone levels in blood plasma that modulate the mechanisms
associated with memory.
Our results do not permit us to determine whether soy isoflavones,
administered acutely to older adults and/or pre/post menopausal women,
would enhance memory. One of the proposed benefits of long-term
consumption of soy isoflavones among older populations is the prevention
of cognitive decline associated with aging (Lee, Lee, & Sohn, 2005).
As the mean age of our study sample was 20.1 years, age-related
cognitive decline was not a factor. Since soy isoflavones administered
longitudinally to young adults have been shown to improve cognition
(File et al., 2001), this implies that there may be several mechanisms
associated with improved cognition after long-term consumption of soy
As the results of this acute experiment differ from that of
longitudinal studies, additional research on the impact of soy
isoflavones on cognitive function is needed. Future investigations
should focus on assessing the dose response curve associated with acute
administration of soy isoflavones. Only after a dose response curve has
been determined, will the benefits, if any, of acute consumption be
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Author info: Correspondence should be sent to: Dr. David Vanata,
Ashland University, 401 College Ave., Ashland, OH 44805. E-mail
David F. Vanata and Mitchell M. Metzger
TABLE 1 Baseline Group Means for Three Trials of Visual Spatial Memory
Task (VSM), Word Recall, & Word Recognition Tests
Group VSM1 VSM2 VSM3
SOY 6.32 (2.6) 10.24 (3.4) 13.00 (3.5)
WHEY 7.80 (2.6) 11.88 (3.0) 13.60 (2.1)
Group RECALL RECOG
SOY 10.76 (3.8) 16.60 (2.8)
WHEY 11.56 (3.0) 16.68 (3.1)
Note: Standard deviations are presented in parentheses
TABLE 2 Group Means for Three Trials of Visual Spatial Memory Task
(VSM), Word Recall, & Word Recognition Tests after Protein Consumption
Group VSM1 VSM2 VSM3
SOY 4.96 (2.5) 8.76 (3.6) 10.16 (3.8)
WHEY 5.28 (2.4) 8.44 (3.5) 11.20 (3.5)
Group RECALL RECOG
SOY 11.04 (3.3) 15.32 (2.8)
WHEY 11.84 (3.4) 16.36 (2.9)
Note: Standard deviations are presented in parentheses