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| Date: 06-30-2016 | HC# 061621-547 |
Dion
C, Chappuis E, Ripoll C. Does larch
arabinogalactan enhance immune function? A review of mechanistic and clinical
trials. Nutr Metab (Lond). April 12, 2016;13:28. doi:
10.1186/s12986-016-0086-x.
Infections
caused by common cold viruses are generally self-limiting but result in the
loss of days at work and in school. In Western countries, adults and children
often have between two and four illnesses per year associated with common cold
viruses. Various treatments for the common cold are used to limit or shorten
the duration of symptoms. Some herbal treatments can alter the immune response
and may result in a decrease in duration of cold symptoms. Members of all
higher plant families contain the hemicellulose compound arabinogalactan, which
is a polymer of arabinose and galactan. Arabinogalactan is most often isolated
from larch (Larix spp., Pinaceae) for
commercial purposes and is used as an herbal medication. The goals of this
review were to present data on the structure of arabinogalactan and mechanisms by
which it modulates immune function.
Arabinogalactan
varies in the ratio of galactan to arabinose, with common ratios between 6:1 and
7.5:1. The variation is affected by the species and by growing and processing
conditions. Western larch (L. occidentalis)
is the most common commercial source of arabinogalactan. In western larch,
arabinogalactan is found in a triple-helical structure with a weight range
between 16,000 and 100,000 daltons. Arabinogalactan is stable over a wide range
of concentrations, pHs, and temperatures. In addition, protein is often
associated with arabinogalactan, making it resistant to degradation. This
resistance allows arabinogalactan to pass through the small intestine to the
colon without being degraded. Within the colon, arabinogalactan can improve the
gut flora composition, protect the mucosal layer, improve gastrointestinal
function, and enhance immune function.
The
effects of arabinogalactan on immune function have been studied in vitro, in
animal models, and in clinical trials. Several studies addressed the effect of
arabinogalactan on immune function. All clinical studies were conducted in
healthy human subjects. In a subset of these studies, the effect of
arabinogalactan supplementation on immune cells' response was measured. One
study found that arabinogalactan increased the production of tumor necrosis
factor-α (TNF-α), whereas another study found no change in this parameter. A
third study found an alteration in the number of immune cells produced, with an
increase in lymphocytes, specifically monocytes, and a reduction in CD8+
T-suppressor cells. Another study found that arabinogalactan consumption (4.5
g/day) significantly reduced the incidence of the common cold in adults
compared to placebo. In a last set of studies, the effect of arabinogalactan
was measured on the immune response after vaccination. Subjects taking arabinogalactan
supplements were found to have a greater immune response to vaccination with Streptococcus pneumoniae or tetanus (Clostridium tetani) toxins. This
response was seen in an increase in immunoglobulin G production.
In
vitro studies with human peripheral blood mononuclear cells have shown that the
addition of arabinogalactan can increase the toxicity of natural killer cells
and also increase the pro-inflammatory response by increasing production of the
cytokines TNF- α, interleukin-1β (IL-1β), and interleukin-6 (IL-6).
Arabinogalactan has also been found to increase the production of nitric oxide,
TNF-α, and IL-6 in macrophages. Animal studies also suggest that
arabinogalactan exposure can increase the number of immune cells, although some
animal studies contradict this finding.
Pharmacokinetic
studies have found that arabinogalactan reaches the colon intact. It can then
be transported across the gut wall intact or be broken down into short-chain
fatty acids, which include butyrate, acetate, and propionate, which are then
transported across the gut wall.
Arabinogalactan
may then act on immune function itself or through its metabolites. Short-chain
fatty acids are known to help regulate metabolism, alter the proliferation and
differentiation of colonic cells, and alter the immune response of the intestinal
tract. There is some evidence that butyrate may decrease leukocyte migration.
The intact arabinogalactan may affect the immunoreactive cells of the colon.
Studies
in human subjects, animal models, and in vitro suggest that arabinogalactan
consumption may alter immune function. Effects on the immune system include an
increase in the production of some types of white blood cells and of
pro-inflammatory compounds associated with the immune response. More high-quality,
randomized, controlled trials are needed in immune-challenged patients to
understand the effect of arabinogalactan on immune response and interactions
with the gut microbiome. Effective dosage also should be investigated. The
writing of this review was financially supported by Lonza Ltd. (Basel,
Switzerland).
—Cheryl McCutchan,
PhD
