Whole flaxseeds are known to lower total and LDL cholesterol
levels, reduce postprandial glucose absorption, decrease
some markers of inflammation, and raise serum levels of
omega-3 fatty acids. This is of obvious benefit to diabetics
who not only need help controlling their blood glucose
levels, but who are also at much greater risk of developing
cardiovascular disease than the general population.
The health benefits of flaxseeds are attributed to
three classes of nutrients.
* Flaxseeds are an excellent source of omega-3 fatty
acids. The essential fatty acid alpha-linolenic acid
(ALA) makes up more than 50% of this mix.
* Flaxseeds are by far the major source of a class of
phytoestrogens called lignans. The major component, a
lignan called secoisolariciresinol diglucoside (SDG),
is the main precursor of enterodiol and enterolactone,
the active phytoestrogens in humans.
* Flaxseeds are an excellent source of soluble fiber (mucilage,
The lipid-lowering effect of flaxseeds has to be due to
the lignans and/or fiber, since defatted (ALA-free) flaxseeds
have the same cholesterol-lowering effect as whole flaxseeds.
It wasn't clear though which of the two components is
responsible; there were no human studies of the effect
of lignans on lipid levels.
The fiber in flaxseed
may be responsible for the improvement in glycemic control
by delaying postprandial glucose
absorption. However, it was suspected that lignans also
play a role. First, lignans may act as antioxidants. Secondly,
it is known from cell culture studies that the main flaxseed
lignan, SDG, can suppress the gene expression of a key
enzyme in hepatic gluconeogenesis (glucose synthesis in
the liver). Again, there were no human trials to test
To test the effect of SDG, the main flaxseed lignan, on
glycemic control and lipid levels, a clinical trial was
conducted in Shanghai, China. 73 type 2 diabetics,
50 - 79 years of age, with mild hyperlipidemia participated;
68 patients finished the trial and provided the data for
the final analysis. The intervention lasted twelve weeks.
Since this was a cross-over study, i.e. all participants
entered both an active treatment and a placebo phase,
there were two intervention periods of twelve weeks each,
separated by an 8 week wash-out period and cross-over.
Active treatment consisted of a supplement standardized
to 360 mg SDG per day, corresponding to 27 - 60 g of whole
The results of the trial showed a small but statistically
significant improvement in glycemic control, as measured
by the reduction in the level of glycated hemoglobin HbA1c,
from 7.17% to 7.06%. There were no changes in fasting
glucose or insulin concentrations, nor was there any change
in lipid levels.
HbA1c is a measure of the degree of hemoglobin
glycation. The open-chain aldehyde form of glucose reacts
non-enzymatically with the free amino groups of N-terminal
and lysyl protein residues, damaging the protein in the
process. Diabetics and non-diabetics alike need glucose
and are therefore subject to protein glycation, but the
degree of glycation damage increases with glucose levels.
The glycation rate reflects the glucose level that hemoglobin
is exposed to during the red blood cell's life span, typically
around 120 days. Everyone has, and stays very close to,
their own HbA1c "set point". In non-diabetics
this set point varies from 4% to 6%. Clinical practice
recommendations in the U.S. call for levels < 7% to reduce
the risk of cardiovascular complications, with values
> 8% suggesting a need for intervention.
The average HbA1c level for this cohort was
close to the upper tolerable limit of 7% at the outset
of this study; further improvements were likely to be
marginal. Secondly, these patients consumed a diet rich
in soy products which are also known to have beneficial
effects on cardiovascular risk factors. Soy products are
the major source of isoflavones,
another class of phytoestrogens. Interestingly, a clinical
trial with a supplement containing soy isoflavones did
not find a reduction in glucose levels.
Is this improvement in glycemic control significant? The
authors concede that "...the magnitude of improvement
may not be clinically meaningful." and that "...further
studies are needed to confirm the current findings in
subjects with different metabolic profiles and/or different
Remember though that the study was designed to test the
role of one of the constituents of flaxseed, namely the
phytoestrogen SDG, on cardiovascular risk factors. The
health benefits of whole flaxseeds are not in question.
To quote the authors once more: "Given the overall beneficial
effects of whole flaxseed consumption for cardiovascular
disease and wide distribution of dietary lignans in plant-based
foods, especially flaxseed, there is considerable potential
to use these whole foods as adjuvant therapies in type
without apparent side effects."