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, gum).

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 this hypothesis.

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 flaxseeds.

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 HbA_1c, from 7.17% to 7.06%. There were no changes in fasting glucose or insulin concentrations, nor was there any change in lipid levels.

HbA_1c 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 HbA_1c "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 HbA_1c 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 ethnic backgrounds."

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 2 diabetes without apparent side effects."