Dietary Milled Flaxseed and Flaxseed Oil Improve N-3 Fatty Acid Status and Do Not Affect Glycemic Control in Individuals with Well-Controlled Type 2 Diabetes

To determine the effects of dietary consumption of milled flaxseed or flaxseed oil on glycemic control, n-3 fatty acid status, anthropometrics, and adipokines in individuals with type 2 diabetes. Thirty-four participants were randomized into a parallel, controlled trial. The participants were adults with type 2 diabetes (age 52.46 +/- 1.5 years, body mass index 32.46 +/- 1.0 kg/m2, n= 17 men and 17 women). Participants consumed a selection of bakery products containing no flax (control group [CTL], n=9), milled flaxseed (FXS, n=13; 32 g/d),or flaxseed oil (FXO, n=5 12;13 g/d)daily for12 weeks. The FXS and FXO groups received equivalent amounts of alpha-linolenic acid (ALA; 7.4 g/day). The primary outcome measures were fasting plasma hemoglobin A1c, glucose, insulin, and phospholipid fatty acid composition. The secondary outcome measures were fasting circulating leptin and adiponectin, as well as body weight, body mass index, and waist circumference. Dietary intake assessment and calculations for homeostasis model assessment for insulin resistance and quantified insulin sensitivity check were also completed. The FXS and FXO groups had increases in plasma phospholipid n-3 fatty acids (ALA, eicosapentaenoic acid [EPA], or decosapentaenoic acid [DPA], but not docosahexaenoic acid), and the FXO group had more EPA and DPA in plasma phospholipids compared to the FXS group. All groups had similar caloric intakes; however, the CTL group experienced a 4% weight gain compared to baseline (p<0.05), while both flax groups had constant body weights during the study period. All other parameters, including glycemic control, were unchanged by dietary treatment. Milled FXS and FXO intake does not affect glycemic control in adults with well-controlled type 2 diabetes. Possible prevention of weight gain by flax consumption warrants further investigation. (Authors abstract)

Research examining the effects of dietary ALA as the n-3 fatty acid source on glycemia or insulinemia is lacking. However, it has been demonstrated in an animal model of diabetes (ob/ob mice) that ALA may have a favorable effect on insulin sensitivity, while dietary EPA or DHA may not. Thus, it is important to differentiate the effects of ALA supplementation versus those of EPA and DHA. Lignans in flaxseed could also contribute to glycemic control, since SDG-fed rats have delayed development of hyperglycemia and SDG-supplemented hypercholesterolemic people have attenuated fasting plasma glucose levels. Thus, the primary objective of this study was to compare the effects of dietary consumption of milled flaxseed or flaxseed oil on glycemic control and plasma phospholipid fatty acid composition in adults with T2DM managed without  anti hyperglycemic agents or insulin. As a secondary objective, the effects of milled flaxseed or flaxseed oil on body weight and circulating adipokines were investigated. Dietary milled flaxseed and flaxseed oil consumption for 12 weeks did not positively or negatively affect HbA 1c or fasting glucose concentrations in this study of adults with well controlled T2DM. Fish oil supplementation (5.9 g n-3 fatty acids/d from fish oil including 1.8 g EPA+3.0gDHA) in people with diabetes had shown negative effects on insulin sensitivity.
The current study, using ALA as the n-3 fatty acid source, did not show this negative effect based on HOMA and QUICKI calculations. The soluble fiber or SDG content of the FXS diet did not appear to have any benefits on glycemic control, but these components may have benefits beyond the parameters assessed in this study, such as postprandial glucose response. Additionally, the dose of SDG provided may not have been sufficient to affect glycemic control, but there have been no studies investigating this in humans.  Both milled flaxseed and flaxseed oil consumption elevated plasma phospholipid ALA, EPA, and total n-3 fatty acids, but flaxseed oil was more effective for increasing EPA and DPA. Neither flax treatment elevated plasma phospholipid DHA levels. Adiponectin was positively correlated with both ALA and total n-3 and negatively correlated with the n6:n3 of plasma phospholipids. Adiponectin is an anti-inflammatory adipokine that is known to be lower in people with diabetes compared to those who are healthy and may be associated with glycemic control. The FXS group experienced a reduction in waist circumference, a measure of visceral fat that is recognized as an important risk factor for chronic disease and insulin sensitivity. This observation requires confirmation with a larger sample size and other measures of body composition since body weight and caloric intake of the FXS group was not significantly altered. It has been shown that dietary lignin intake is inversely correlated with the waist to hip ratio in women; thus, lignin may explain the reduction in waist circumference in the FXS group and not the FXO group. The anthropometric data also showed that the FXS and FXO groups maintained their body weight while the CTL group gained 4% body weight, despite similar energy intakes. Further investigation related to FXS and weight control appears warranted. (Editors comments)