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Flaxseed Not Flaxseed Oil Can Reduce Blood Lipids
Posted: Wednesday, August 05, 2009
Whole flaxseed and flaxseed lignans significantly reduce circulating total cholesterol and low-density lipoprotein (LDL) cholesterol levels, showing their greatest effect in postmenopausal women and individuals with high initial cholesterol concentrations, according to the results of a published study.
An Pan, MSc, from the Key Laboratory of Nutrition and Metabolism, Institute for Nutritional Sciences, Chinese Academy of Sciences, Shanghai, China, and colleagues write, "Owing to the promising results in preclinical models, many clinical trials have been performed to determine the outcomes of flaxseed intervention (whole flaxseed, flaxseed oil, or lignans) on various cardiometabolic risk factors, particularly blood lipids.… However, the findings from most of the previous clinical trials were inconsistent, and the discrepancies could be attributed to small sample size, insufficient study duration, variation in study designs, and diversity of the test product."
The aim of this study was to evaluate whether administration of flaxseed or its derivatives could improve total, LDL, and high-density lipoprotein (HDL) cholesterol levels and triglyceride levels by making use of the increased statistical power afforded by a meta-analysis.
The researchers searched all randomized controlled trials of flaxseed or its derivatives on lipid profiles in adults, which were published in English from January 1990 to October 2008. They also attempted to contact authors of unpublished data.
The 28 trials used in the meta-analysis comprised a total of 1,381 study subjects. Five were conducted exclusively in postmenopausal women, 1 was done in premenopausal women, 10 trials were conducted in men, 10 trials were conducted in both sexes, 1 trial did not indicate the sex composition of the study sample, and 1 trial reported results separately by sex.
The median duration of the trials was 8.5 weeks, (range, 2 - 52 weeks). Flaxseed in whole, ground, or defatted form was tested in 10 of the 28 trials in doses that ranged from 20 to 50 g (2 - 5 tablespoons), and flaxseed oil was tested in 13 of the trials.
A significant reduction in total cholesterol levels was found in studies using whole flaxseed, with a net change of –0.19 mmol/L (95% confidence interval [CI], –0.29 to –0.09 mmol/L) and also in studies using lignan supplements, where the net change was –0.28 mmol/L (95% CI, –0.55 to –0.01 mmol/L). Similarly, a significant reduction in LDL cholesterol levels was found with whole flaxseed (net change, –0.16 mmol/L; 95% CI, –0.25 to –0.06 mmol/L) and lignan supplements (–0.16 mmol/L; 95% CI, –0.31 to –0.01 mmol/L). However, there were no significant changes in total and LDL cholesterol levels with the intervention of flaxseed oil, the study authors report.
Women versus men had greater reductions in total cholesterol levels with flaxseed. The mean reduction for women was –0.24 mmol/L (95% CI, –0.36 to 0.12 mmol/L), and for men, the mean reduction was –0.09 mmol/L (95% CI, –0.05 to 0.23 mmol/L). The same was found with reductions in LDL cholesterol levels: for women, the mean reduction was –0.17 mmol/L (95% CI, –0.28 to –0.06 mmol/L), and for men, the mean reduction was –0.07 mmol/L (95% CI, –0.04 to 0.18 mmol/L).
When the studies were stratified according to initial lipid status, significant reductions in total cholesterol levels were found in the studies including subjects with high initial concentrations, defined as 5.7 mmol/L for total cholesterol and 3.4 mmol/L for LDL cholesterol. For these individuals, the mean change in total cholesterol level was –0.17 mmol/L (95% CI, –0.32 to –0.03 mmol/L), and the mean change in LDL cholesterol level was –0.13 mmol/L (95% CI, – 0.23 to –0.02 mmol/L). In comparison, the mean change in total cholesterol levels for subjects with lower initial concentrations was 0.03 mmol/L (95% CI, –0.11 to 0.17 mmol/L), and the mean change in LDL cholesterol levels was 0.00 mmol/L (95% CI, –0.12 to 0.12 mmol/L).
There were no significant changes in HDL cholesterol or triglyceride levels.
The authors point out that the limitations of their study include the heterogeneity of the studies in the meta-analysis, and a lack of information on the quality, quantity, and bioavailability of the flaxseed that was used. Therefore, they write, their findings must be interpreted with caution.
The results of this meta-analysis indicate that flaxseed consumption may be a useful dietary approach for the prevention of hypercholesterolemia, especially in some patient subgroups, the authors conclude. They add that further studies should be done to investigate the effectiveness of flaxseed supplementation on cardiometabolic risk factors other than blood lipids and, ultimately, on cardiovascular disease–related morbidity and mortality.
Source: Diabetes In Control: Am J Clin Nutr. August 2009;90:288-297.Abstract
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