Flaxseed dietary fibers lower cholesterol and increase fecal fat excretion, but magnitude of effect depend on food type

January 1, 2012 Human Health and Nutrition Data 0 Comments

Flaxseed dietary fibers lower cholesterol and increase fecal fat excretion, but magnitude of effect depend on food type

Year: 2012
Authors: Kristensen, M. Jensen, M.G. Aarestrup, J. Petersen, K.E.N. S�ndergaard, L. Mikkelsen, M.S. Astrup, A.
Publication Name: Nutrition & Metabolism
Publication Details: doi:10.1186/1743-7075-9-8


Background: Dietary fibers have been proposed to play a role in cardiovascular risk as well as body weight management. Flaxseeds are a good source of dietary fibers, and a large proportion of these are water-soluble viscous fibers. Method: Here, we examine the effect of flaxseed dietary fibers in different food matrices on blood lipids and fecal excretion of fat and energy in a double-blind randomized crossover study with 17 subjects. Three different 7-d diets were tested: a low-fiber control diet (Control), a diet with flaxseed fiber drink (3/day) (Flax drink), and a diet with flaxseed fiber bread (3/day) (Flax bread). Total fat and energy excretion was measured in feces, blood samples were collected before and after each period, and appetite sensation registered 3 times daily before main meals. Results: Compared to control, Flax drink lowered fasting total-cholesterol and LDL-cholesterol by 12 and 15%, respectively, (p < 0.01), whereas Flax bread only produced a reduction of 7 and 9%, respectively (p < 0.05). Fecal fat and energy excretion increased by 50 and 23% with Flax drink consumption compared to control (p < 0.05), but only fecal fat excretion was increased with Flax bread compared to control (p < 0.05). Conclusion: Both Flax drink and Flax bread resulted in decreased plasma total and LDL-cholesterol and increased fat excretion, but the food matrix and/or processing may be of importance. Viscous flaxseed dietary fibers may be a useful tool for lowering blood cholesterol and potentially play a role in energy balance. (Authors abstract)
The cholesterol lowering effect of soluble viscous dietary fiber has been known for decades. Extracted flaxseed fiber added to bread was found to lower cholesterol in diabetics. Flaxseeds contain 30% dietary fibers of which one third are water-soluble group of heterogeneous polysaccharides found as a mixture of three major families of polymers: arabinoxylans with a A/X ratio of 0.25, and various amount of galactose and fucose residues. Thus, the lower A/X ratio compared to wheat arabinoxylans, which are mainly insoluble, results in different physicochemical properties. Also, flaxseeds contain some pectins. Flaxseed fibers form highly viscous solutions upon hydration, which is similar to those observed for other gums. Dietary fibers may also play a role in body weight regulation, through both hunger suppression and diminished nutrient absorption. This research group showed that the addition of flaxseeds to rye breads (6 g/100 g) significantly reduced the digestibility of fat and energy in humans. Particularly viscous fibers appear effective in suppression of hunger. Doses of 5 and 10 g of flaxseed fibers increased satiety and gave a prolonged decrease in ghrelin, a hunger-signaling gut peptide. In the present study, the hypothesis that addition of flaxseed fibers to a controlled diet will increase excretion of fat and energy, lower blood cholesterol and suppress hunger was tested. A potential difference due to food types was explored as the flax dietary fiber was fed both as a viscous drink and baked into bread. The results of the present study demonstrate that consumption of 5 g of dietary fibers from flaxseeds daily for one week significantly increased fecal excretion of fat and reduced total and LDL cholesterol markedly. Despite similar doses of dietary fiber, the effect of the dietary fiber was less pronounced when incorporated into bread than when administered as a drink, which emphasizes the importance of food matrix. A lowering of both total cholesterol and LDL cholesterol by 12 and 15%, respectively, within seven days in young healthy adults with normal blood cholesterol concentrations was found. A recent meta analysis on the effects of flaxseed on blood lipids showed that flaxseed consumption lower both total and LDL cholesterol, whereas flaxseed oil does not, and the role of lignans is still controversial. Thus, the responsible component for the assumed cardioprotective effect of flaxseeds may well be the fiber component. The 5 g dose of fibers used was relatively low compared to other studies. The most plausible mechanism of action is through an interference with bile acid metabolism, where an increased intraluminal viscosity can 1) hinder micelle formation and thus diminish lipid uptake and 2) inhibit re-uptake of bile acids causing increased hepatic synthesis of bile acids which diverts cholesterol away from lipoprotein synthesis in the liver, thereby reducing serum cholesterol. Fecal energy excretion increased by 129 kJ/d with Flax drink consumption compared to Control. The amount of fat and energy which escapes digestion may appear small in numbers, but corresponds to ~47 MJ excreted per year and thus of relevance in the prevention of weight gain as a decrease in energy uptake of this magnitude equals a difference in body weight of ~1.6 kg.  Appetite sensation was assessed three times daily throughout the present study, and there was an overall trend towards increased fullness during the Control period compared to both Flax drink and bread. This was unexpected as the researchers have previously observed that flax fibers significantly increase both satiety and fullness and no studies on viscous dietary fibers indicate that they should induce hunger. However, the Control drink based on MCS may have caused bloating, which may have been mistaken for fullness. Appetite sensation was assessed 30 minutes prior to main meals; however, the subjects were not instructed to have their lunch and dinner at fixed time points, which may have influenced the results. The effect of Flax bread was less pronounced than with a similar dose of dietary fiber provided as a drink, both with regard to cholesterol-lowering properties and reduction of apparent fat digestibility, although only the latter was significantly different between Flax drink and Flax bread. The different effects observed between drinks and baked products may be due to differences in their ability to induce viscosity resulting from either reduced hydration of the dietary fiber or reduced molecular weight in the baked products due to processing and/or storage. Flax fiber�s ability to directly adsorb fat and bile acids may have been compromised by processing or the inclusion of a viscous drink with MCS in the Flax bread period may have interfered with the water-interaction of the flaxseed fibers. (Editors comments)

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