Experts,
researchers clarify results of most recent meta-analysis
Urinary tract infections (UTI) are among the most
common type of infection in adults and result in more than 8 million visits to
healthcare providers annually. UTIs — which are 50 times more likely to occur
in women — can arise in any part of the urinary tract, including the kidneys,
bladder, or urethra, and are most frequently caused by bacteria such as E. coli. Currently, antibiotics are
considered the most effective treatment for the infection, and some women who
experience frequent UTIs are prescribed low-dose antibiotics as a preventative
measure.1 With antibiotic resistance a growing concern in recent
years, however, researchers are studying plant-based UTI prevention strategies,
including formulations of American cranberry (Vaccinium macrocarpon).
Cranberry has been a popular prophylactic for
urinary tract infections for decades and has been used by indigenous North
American peoples for centuries to treat certain urinary conditions.2
Since 1998, cranberry has been the focus of four major reviews from the
Cochrane Collaboration, an independent research organization that advocates
evidence-based decision-making in healthcare.3,4 Its popularity as a
urinary tract health supplement, in part, helped make cranberry the
best-selling single herb supplement in the US Food, Drug, and Mass Market
retail channel in 2011, commanding sales of more than $40 million.5
A 2008 Cochrane Review, which analyzed 10 studies
of cranberry for the prevention of UTIs, concluded that cranberry products —
such as juices or capsules — significantly reduced the occurrence of UTIs at 12
months, particularly in women with recurrent infections.6 An update
of the review, published in October 2012, however, found that “there was a
small [but insignificant] trend towards fewer UTIs in people taking cranberry
products compared to placebo or no treatment.” Although the authors noted a
number of potential weaknesses in the reviewed studies, they concluded that
“until there are more studies of products containing enough of the active ingredient [emphasis added],
measured in a standardised way, cranberry products cannot be recommended for
preventing UTIs.”4
Unusually high dropout rates and methodological
issues with many of the studies included in the 2012 review — such as failure
to quantify the main active ingredient for
UTI prevention in cranberry and small sample sizes — have led some to question
the validity of the Cochrane group’s findings.
“It is essential that cranberry continue to be regarded and researched
as an important area of study to help address the public health challenge that urinary tract infections and
their treatment presents to antibiotic resistance,” said Amy Howell, PhD, an associate research scientist at Rutgers University’s Marucci Center for Blueberry and Cranberry
Research (email, November 27, 2012). “UTIs are a significant public health challenge with more than 15
million cases in the US each year, and their
treatment accounting for 15 percent of all community-prescribed antibiotics. I
think that cranberry has great potential to help slow the pace of resistance
development in an era when consumers are concerned with having to rely on
pharmaceuticals to prevent and treat disease. ”
Cranberries as a Preventative Measure
Fresh cranberries are composed of roughly
90 percent water and are
known for their elevated concentration in total polyphenols, such as
anthocyanins, tannins, flavonoids (flavonols and flavan-3-ols), and most
notably proanthocyanidins (PACs). The amount of polyphenols in cranberry,
however, comprises just a small percentage of the fruit’s total organic
constituents. Previously, scientists believed that
organic acid in cranberry acidified the urine, which acted as an antimicrobial
agent. However, current hypotheses revolve around a specific type of PACs — those with type-A linkages — that are thought to be responsible for
the fruit’s ability to inhibit bacteria from sticking to the urinary tract
lining, thus preventing infection. According to authors of an unrelated 2012
review of cranberries for lower UTI prevention, PACs “function as a natural
plant defense system against microbes.”7 However, this well-studied in vitro effect did not translate to a
measurable effect in the populations analyzed by the Cochrane group.
Dr. Howell, who has been
researching cranberries for two decades, did not agree with the Cochrane
group’s essential dismissal of cranberries — particularly cranberry
juice — for the prevention of UTIs. “I was disappointed by the authors'
conclusions given that, as a cranberry researcher, my lab has consistently
found that cranberries effectively help prevent bacterial adhesion to bladder
cells, the first step in the initiation of a UTI,” she said. “If the bacteria
are prevented from adhering, they will not be able to grow and cause an
infection. They are washed out of the body in the urine stream.”
Methodological Issues
Ruth Jepson, PhD, of the Department of
Nursing and Midwifery at the University of Sterling, in Scotland, and lead
author of the 2012, 2008, and other previous Cochrane Reviews of cranberry, mentioned
in an email a number of possible weaknesses in the chosen studies. In total, the latest analysis included 24 studies — 14 more than the 2008 review — totaling 4,473 participants. The 14
new studies were published after the group’s original literature search in
January 2007. To meet inclusion criteria, all studies had to be randomized
controlled trials (RCTs) or quasi-RCTs of cranberry products for UTI
prevention. Even with this criteria, studies varied greatly in terms of
cranberry product used, type of placebo or control, and study design.
Funding for the 2012 Cochrane Review
came from the UK National Health Service’s National Institute for Health
Research, a government initiative. Importantly, Dr. Jepson noted that the
authors of the review did not receive any funding from cranberry product
manufacturers or drug companies.
As noted in the review, of the 24
studies included, 11 used a cranberry juice product, nine evaluated cranberry
tablets or capsules, two used a liquid cranberry concentrate or syrup, one
compared juice and tablets, and one compared capsules and tablets. Of the studies
that examined the effectiveness of cranberry tablets or capsules, only one
reported the amount of PACs in the product. Without prior analysis, the precise
amount of PACs in cranberry juice products — which are often not marketed as dietary supplements — is impossible to determine.
Dr. Jepson said that the lack of
active ingredient quantities for products in many of the studies might have had
an impact on the review’s outcome. “I think there are two reasons for why we
did not see [the in vitro effects]
translate to a living population, both related to which cranberry product is
being consumed,” she said (email, November 9 and December 6, 2012). “Firstly,
the effects of the PACs only last for about 10 hours. So to get maximum
benefit, someone would have to drink two glasses of juice a day every day.…
Indeed many people dropped out of the studies, possibly because it was
difficult to drink these amounts.” However, Dr. Howell pointed out that the
10-hour effect is from in vitro data
and that previous clinical work has shown that cranberry juice can be effective
if a single serving is consumed only once per day.
Dropout or withdrawal rates ranged
from 3 to 55 percent in studies where the data were available. The Cochrane
group noted that adherence was varied, with several studies reporting
“participants withdrawing because of the unpalatable or intolerable nature of
the cranberry product.”4 The resulting large number of dropouts is
one reason why there has been an increased interest in cranberry tablets and
capsules, which may be more suitable to consume on a daily basis as a
preventative measure.
Dr. Jepson noted that future studies
should focus on cranberry tablets or capsules, where amounts of PACs can be
more accurately determined. “We know now that a specific process is needed to
make sure that the PACs remain active in the dried preparations,” she said. “However,
many of the studies did not specify whether this process was undertaken, or
indeed how much of the active ingredient was in the preparations (if any).”
Room for Improvement
Similarly, Dr. Howell sees room for improvement in many of the studies’
designs. “Cranberry
researchers use different dosages and product types which in many cases were
not standardized to the active anti-adhesion compounds (proanthocyanidins) and
may not have had sufficient amounts to achieve efficacy. I agree that this has
been a problem in past studies and has most likely led to the results showing
little effect, but I strongly believe that this is a very good reason to
continue with clinical research and do it the right way, using
well-characterized products and protocols,” she said.
Gunter
Haesaerts, founder and CEO of Pharmatoka — which manufactures Ellura®,
a cranberry capsule with a significantly high standardized amount of PACs8
— also takes umbrage
with the Cochrane Review’s apparent dismissal of cranberry juice products.
“Cochrane’s jumping
to the conclusion that it would be a waste of time to conduct more juice
studies is a little bit unfair [to] the juice industry (led by Ocean
Spray). But that same juice industry should realize once and for all that
either they conduct and finance serious trials or they abstain from
further inadequate clinical trials that can only irritate scientists and
regulators,” said Haesaerts (email, November 30, 2012). “On the other hand, the
‘capsules and tablets’ industry, to which Pharmatoka belongs,
is explicitly encouraged by Cochrane to conduct more clinical
studies on [the] condition that they use efficacious products.”
Specifically, Haesaerts mentioned three
prerequisites for the inclusion of cranberry capsules or tablets to achieve measurable
results, two of which were mentioned in the Cochrane Review. “To ensure potency
in cranberry powders, levels of PACs must be quantified properly; and the
4-dimethylaminocinnamaldehyde [DMAC] method is currently the most validated
standard method,” Jepson wrote. One peer reviewer of this article, however,
noted that the DMAC method might not adequately account for larger molecules in
cranberry, including some PACs. Further, the Cochrane Review noted that a
recent study “found that to achieve a bacterial anti-adhesion effect in urine,
36 mg of cranberry PAC equivalents per day is effective, but 72 mg may offer
better protection in some cases.”
In
order to maximize bioactivity, Haesaerts also suggested that bioactive PACs
should be extracted from the juice, not from the cranberry presscake — the remaining material after juice
extraction that includes seeds and skin — that
also contains PACs but shows little bioactivity.9
“The mechanism of anti-adhesion of cranberry
PAC is quite well known, even though certain aspects of the mechanism are still
under study,” he added. “Therefore, you cannot say that cranberry does not
work. It does work. But to prove it’s working clinically is another issue, and an immense challenge for the
juice producers.”
Looking Beyond the
Cochrane Analysis
Dr. Howell urges consumers
to be skeptical when dealing with meta-analyses that attempt to find answers to
complicated questions. Whenever large amounts of data are involved, there is
usually room for varying interpretations.
“The recent review on
cranberry needs to be put into perspective and weighed against the other
positive clinical trials over the past couple of decades in which cranberry was
effective in maintaining urinary tract health.… There is a wealth of evidence
from independent research institutions globally that has demonstrated that
regular consumption of cranberry products helps to promote urinary tract health,”
she said. “Much
of the recent prevention research is actually quite positive. Those studies
that came out negatively all had issues with the products used, or design flaws
in the methodologies that resulted in poor outcomes. Consumers need to be aware
of these issues, especially when it comes to clinical trial results on
functional foods. Just because a study does not yield significant results,
it does not necessarily mean that the food is ineffective.”
In
particular, Dr. Howell noted the outcome of a recent analysis by Wang et al. published in The Archives of Internal Medicine, which included 13 clinical
trials with a total of 1,616 participants. According to Risa Schulman, PhD,
author of a research review of Wang’s paper that appeared in HerbalGram 96, “Cranberry juice was
shown to be more effective than capsules or tablets, which may be because it
provides better hydration or because there are other substances in the juice
that contribute to efficacy,” she wrote. “On the other hand, juice has the
potential drawbacks that it is high in sugar and may cause gastrointestinal or
other adverse side effects.”10 Importantly, Wang noted that the
results of his analysis should be treated with caution due to heterogeneity
among the studies.11
Additionally,
a 2012 paper in the Open Access Journal
of Clinical Trials by Uberos et al.
compared the efficacy of cranberry syrup against trimethoprim, a common
antibiotic, for the prevention of UTIs in children aged one month to 13 years.
“Our study confirms that cranberry syrup is a safe treatment for the pediatric
population,” Uberos wrote. The author also noted that, due to limits imposed on
studies of children by the Declaration of Helsinki, no placebos were given,
resulting in a simple “test of equivalence or non-inferiority.” Therefore, the
author concluded that “cranberry prophylaxis is not equivalent to trimethoprim,
but it is shown to be non-inferior versus trimethoprim in recurrent UTI.”12
Future Trials
Despite the negative
results of the 2012 Cochrane Review of cranberry for UTI prevention, many
experts agree that more and better research is needed. Dr. Jepson noted that
certain aspects of the included studies — such as the lack of quantified, standardized
PACs —
were cause for some concern. “[That] was
why we recommended further studies using [a] standardised amount. It was
difficult to say whether we were looking at a true estimate of effectiveness or
not. If the underlying hypothesis is correct I would expect that new studies
would show it,” she said. “It is very possible, that if studies were undertaken
using a standardised product which we were sure of the active ingredient, an
effect would be seen.”
Dr. Howell believes similarly and urges
consumers not to change their habits based solely on the recent meta-analysis.
“I have been doing research at Rutgers University for the past 20 years and
have found that cranberry consumption prevents bacterial adhesion to cells from
the urinary tract —
the initial step in the urinary tract infection process,” she said. “If women are currently
consuming cranberry products, the results of this one review do not provide a
reason for them to change their current practices.”
Haesaerts
of Pharmatoka believes cranberry’s fate now lies in the hands of researchers
conducting clinical trials. With better study designs, he hopes future results
will more accurately reflect cranberry’s potential to provide urinary tract health
benefits. “We are walking a very dangerous path. No new families of
[conventional pharmaceutical] antibiotics are in sight. Cranberry as a
prophylactic treatment of recurrent UTI is going to be dearly needed in the
future because of the fast-growing resistance of uropathogens against existing
antibiotics,” he said. “Let highly respected experts like Cochrane continue their
critical work, but give them study results that match their criteria and
expectations. Cranberries deserve no less.”
—Tyler Smith
References
1. Urinary
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2. Urinary
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3. About
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RG, Williams G, Craig JC. Cranberries for preventing urinary tract infections
(review). The Cochrane Library;
2012:10. Available here. Accessed November 12, 2012.
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27, 2012.
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Available here. Accessed November
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12. Uberos J, Nogueras-Ocana M,
Fernandez-Puentes V, Rodriguez-Belmonte R, et al. Cranberry syrup vs
trimethoprim in the prophylaxis of recurrent urinary tract infections among
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