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.
We observed a lowering of both total-cholesterol and LDL-cholesterol by 12 and 15%, respectively, within just seven days in young healthy adults with normal blood cholesterol concentrations. It can be speculated that the effect may be overestimated as the short intervention period does not allow for equilibrium to occur, but the short duration was chosen due to the high level of controlled diet and the fact that fecal parameters were the main end point. However, in support of our findings, effects of similar magnitude are reported in a recently published study, in which 5 g of flaxseed gum per day for three months reduced total and LDL cholesterol by 10 and 16%, respectively in type 2 diabetics [8
], although one could expect a more pronounced effect among diabetics with dyslipidemia. 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 [23
]. 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. Flaxseeds contain ~30% dietary fibers, with one third being viscous; thus the dose used would correspond to ~50 g of whole flaxseeds or slightly less, as non-viscous fibers may also contribute. 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 [7
]. Short chain fatty acid production has also been proposed to play a role, and although flaxseed dietary fibers have been shown to be highly fermentable in rats [24
], this mechanism has not been confirmed in humans [7
]. Flax drink at the moment of ingestion (10°C) and at physiological temperature (37°C) showed significant higher viscosity (300-500 fold) compared to control MCS drink. Although it is possible that the flax dietary fibers may be subject to depolymerization and hence loss of viscosity with ingestion, we assume that the overall luminal viscosity is higher for flax dietary fibers compared to control (MCS) drink.
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. The increase in fecal energy and fat excretion is in accordance with our previous studies on flaxseed fibers showing a reduction in energy digestibility and weight gain in growing Wistar rats fed a diet with 10% of flaxseed dietary fibers (Kristensen M, Knudsen KEB, Jørgensen H, Oomah D, Bügel S, Toubro S, Tetens I, Astrup A. Flaxseed dietary fibers reduce apparent energy and fat digestibility and weight gain in growing rats, submitted). Studies using other viscous dietary fibers have shown similar results [25
], and Eastwood and colleagues (1986) compared different dietary fiber sources and found that gum tragacanth, gum arabic and raw carrot increased faecal fat excretion, whereas non-viscous potato dietary fibers did not [28
]. In a study on broiler chickens fed either flaxseeds or demucilaged flaxseeds found that removing the mucilage layer reduced energy utilization and viscosity of the jejunal digesta [15
], strongly suggesting that the physiological effects are closely linked to their ability to form viscous gels upon hydration. In contrast, a recent study found that insoluble (cereal) rather than soluble fiber (guar gum) increased fecal energy excretion in growing mice [29
]. Interestingly, the soluble fiber also resulted in a larger weight gain than insoluble fibers, which the authors linked to fermentation of soluble fibers leading to increased energy extraction. This cannot be extrapolated to humans due to differences between species, but does warrant further research into the importance of large intestinal contributions to energy balance.
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 we have previously observed that flax fibers significantly increases both satiety and fullness [19
] and no studies on viscous dietary fibers indicate that they should induce hunger. However, we believe that the Control drink based on MCS may have caused bloating, which may have been mistaken for fullness. This is supported by the subset analysis on subjects not reporting increased bloating during any dietary intervention period, which eliminated the differences in fullness ratings. 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.
One of the objectives of the present study was to explore the effect of food matrix on physiological responses. We found that 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. An effect of food matrix was also seen in a study, in which β-glucan enriched juices produced a more pronounced effect on total and LDL-cholesterol compared to β-glucan enriched cookies [30
], but in general evidence on the importance of food matrix are lacking. 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, as reported for β-glucans [31
]. Further, 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.
In conclusion, we studied the effect of flax fiber-enriched drinks and breads on fecal energy and fat excretion, blood lipids as well as subjective appetite sensation. We found that addition of a flax dietary fiber extract rich in viscous dietary fibers significantly increased fat excretion and lowered total and LDL-cholesterol although no effect on appetite was observed. Viscous flaxseed fibers appear useful for lowering blood cholesterol and may play a role in energy balance; however, food type and/or processing may be of importance.