Linseed Dietary Fibers Reduce Apparent Digestibility of Energy and Fat and Weight Gain in Growing Rats

Dietary fibers (DF) may affect energy balance, an effect often ascribed to the viscous nature of some water soluble DF, which affect luminal viscosity and thus multiple physiological processes. We have tested the hypothesis that viscous linseed DF reduce apparent nutrient digestibility, and limit weight gain, in a randomized feeding trial where 60 male, growing, Wistar rats, with an initial weight of about 200 g, were fed different diets (n equals 10 per group) to low DF control (C), 5 percent DF from cellulose (5 CEL), CEL plus 5 percent DF from whole (5 WL) or ground linseed (5 GL), CEL plus 5 percent DF from linseed DF extract (5 LDF), and CEL plus 10 percent DF from linseed DF extract (10 LDF). Diets were provided ad libitum for 21 days. Feed intake and faecal output were measured during days 17 to 21. Faecal fat excretion increased with increasing DF content and was highest in the 10 minus LDF group. Apparent fat digestibility was highest with the C diet (94.9 percent 0.8 percent) and lowest (74.3 percent 0.6 percent) with the 10 LDF diet, and decreased in a non linear manner with increasing DF. Apparent fat digestibility also decreased with increased accessibility of DF (5 WL vs. 5 GL) and when the proportion of viscous DF increased (5 GL vs. 5 LDF). The 10 LDF resulted in a lower final body weight (258 plus minus 6.2 g) compared to C (282 plus minus 5.9 g), 5 CEL (281 plus minus 5.9 g), and 5 WL (285 plus minus 5.9 g). The 10 LDF diet reduced body fat compared to 5 CEL. In conclusion, DF extracted from linseed reduced apparent energy and fat digestibility and resulted in restriction of body weight gain in growing rats. (Authors abstract)

Dietary fibers (DF) have received increasing attention for their potential role in weight regulation because high intakes of DF have been associated with a smaller weight gain in prospective observational studies. Viscous soluble DFs in particular seem to play a role in short term appetite regulation. Linseeds (flax) contain approximately 30 percent DF of which one third is water soluble. The majority of the water soluble DF belongs to a group of heterogenic polysaccharides consisting of neutral arabinoxylans and highly acidic rhamnose containing polysaccharides present on the outside of the seed coat (the mucilage), which form highly viscous solutions when mixed with water. These polysaccharides are, thus, easily extractable using only water. A diet with linseed DF derived from the mucilage fraction was tested to decrease apparent energy and fat digestibility, thereby affecting body weight gain in growing rats.

Rats fed the 10 LDF diet had a lower daily weight gain compared to the rats fed the C, 5 CEL and 5 WL diets, and body weight was lower compared to all other rats. Apparent fat and energy digestibility were decreased by 10 percent to 20 percent with the addition of a high dose of linseed DF to the feed, which could be modeled as a second degree polynomial relationship. As
weight gain was reduced and faecal volume and energy excretion significantly increased with the 10 LDF diet, it appears that viscosity played a larger role than fermentation of the linseed DF in the present study. Nonetheless the significantly larger contents of caecum of the 10 LDF fed rats suggest an increased bacterial fermentation.

It was hypothesized that similar doses of DF provided as whole or ground linseed, or as extract, would affect nutrient digestibility differently due to differences in botanical integrity, particle size, and DF type. In the 5 WL and 5 GL diets the DF were a mix of soluble and insoluble DF, whereas the 5 LDF diet contained only water-extracted soluble DF. These differences in DF type did not result in any significant differences in body weight gain, whereas apparent fat digestibility, faecal volume and weight of intestinal contents was larger when rats were fed extracted viscous linseed DF compared to a mix of DF types.

In conclusion, a high dose of DF extracted from linseed increased faecal fat and fat excretion, and resulted in decreased body weight gain in growing rats, whereas diets with similar DF content for the whole or ground linseed or DF extracted from linseed did not affect body weight differently, despite differences in apparent fat and energy digestibility. The present findings should be regarded as a proof of concept for this new source of extracted DF. The authors noted that linseed DF may be a useful food ingredient for its effect on energy balance, also in humans, although this needs to be confirmed in long-term human intervention trials. (Editors comments)