REDUCED INSULIN LEVELS AND PREVENTION OF DIABETES

A high consumption of cereal fibre and whole grain is associated with a decreased risk of diabetes. Some factors in the dietary fibre complex in whole grain rye products seem to have favourable effects on insulin and glucose metabolism e.g. by increasing insulin sensitivity.

The majority of nutrients are digested and absorbed in the small intestine. Soluble fibre, such as b-glucan and soluble arabinoxylan, may slow down the evacuation of stomach contents into the small intestine by increasing the viscosity of the food mass. This leads to the delayed hydrolysis of starch and the absorption of nutrients, which helps to slow down the changes in blood glucose levels (Behall and Hallfrisch 2002; Brennan  and Cleary 2005). Whole grain rye has a high content of soluble fibre, especially soluble arabinoxylans. Number of clinical studies support the hypotheses that whole grain consumption is assosiated with a reduced risk of type 2 diabetes (Murtaugh et al. 2003; Venn and Mann 2004).

In healthy subjects it was found that postprandial plasma insulin responses were significantly lower after the intake of a whole grain rye bread as compared to a white wheat bread. No significant differences in plasma glucose levels were found (Leinonen et al. 1999; Juntunen et al. 2001, 2003). The lower insulin response after rye bread was not explained by the amount of fibre because low-fibre rye bread reduced the insulin responses similar to that of high-fibre rye bread (Juntunen et al. 2003). As compared to white wheat bread, both rye breads decreased postprandial insulin responses. This effect might be explained by differences in the structural properties of rye and wheat breads.

Cross-sectional results from the Botnia Dietary Study show that the energy-adjusted intake of rye was directly related to insulin sensitivity in both sexes (Ylönen et al. 2001). The consumption of other cereals, such as wheat, was not related to insulin sensitivity in this group of 555 high-risk individuals for type 2 diabetes. In an eight week cross-over study using an intravenous glucose tolerance test, insulin secretion (but not insulin sensitivity) was improved in 20 postmenopausal women when they were given a diet containing high-fibre rye bread compared with a diet containing white wheat bread (Juntunen et al. 2001). In another study, wheat bran added to test meals had very little effect on short-term glucose, insulin and lipid metabolism (Jenkins 2001).

In experiment on ileostomy subjects were given a wheat, low-fibre diet for 2 weeks, followed by a washout week, and a period of 2 weeks on a whole grain and rye bran supplemented high-fibre period (Lundin et al. 2001a). A cross-over design was used for meal frequency (three or seven times per day). During the seven meals per day period, the glucose and insulin peaks had clearly reduced at the end of the day during the whole grain rye period compared with the endosperm wheat control diet period. The daytime excretion of C-peptide was also decreased, which supports the results of decreased insulin secretion at the end of the day. C-peptide is a peptide, that is split off and secreted together with insulin. This delayed effect on glucose and insulin metabolism corresponds to similar effects on a morning meal as a result of a high fibre meal served during the previous evening ("over-night second meal effect") observed in other studies using high fibre diets (Björck et al. 2001; Björck and Elmståhl 2003; Lundin et al. 2004). It also corresponds to an increased insulin sensitivity, which has recently been observed associated with whole grain consumption (Pereira et al. 2002).

Rye bran and rye fibre have positive effects on the prevention of diabetes in experimental studies performed on both humans and animals. The effects of bread with a high content of rye bran were compared with those of either low-bran bread or a patients' usual bread in insulin-dependent diabetic patients (Nygren et al. 1984). When the bread with high rye bran content was included in the diet the glucose profile during the day improved and the insulin doses could be reduced.
In a study of Montonen et al (2003) an inverse association between whole-grain intake and the risk of type 2 diabetes was found. They assumed that the reduction may also be due to the bioactive compounds, such as lignans, tocotrienols, phytic acids and other antinutrients, found in wholegrain products.

The protective effects of rye bran on the diabetic syndrome were also found in rats and mice (Nygren et al. 1981; Lundin et al. 2001b). The diabetic rats fed the high-fibre bread lost less body weight, and exhibited lower blood glucose levels and lower urinary glucose excretion than the animals fed the low-fibre bread. Rye bran lowered blood glucose levels slightly and led to slower weight gain in normal rats and mice, and prolonged the survival of diabetic mice (Berglund et al. 1982). Obvious effects on the small intestinal morphology were also detected with an increased goblet cell volume and density in the ileum of hamsters (Lundin et al. 1993), broader ileal villi, a higher density of ileal microvilli, a thicker glycocalyx layer, and an abundance of filamentous microorganisms in the ileum of diabetic and control mice (Lundin et al. 2001b). The morphological changes in the intestine of the diabetic animals were associated with a favourable weight development and a disappearance of urinary glucose excretion.

A clear association between a high intake of dietary fibre from cereal grain and a reduced risk of diabetes has been observed in two large-scale prospective studies in females. In one of the studies a low glycemic index was concomitantly associated with a decreased risk (Salmeron et al. 1997) in comparatively younger women but not in the second more recent study (Meyer et al. 2000) on older women. Similar results were also obtained
with man in the study of Qi et al. (2005). The glycemic index is dependent on the food form and the way in which the food has been processed. Rye bread containing whole grains had a remarkably lower glycemic index than whole grain rye bread made of flour (Jenkins et al. 1986). The hypothesis that there may be differences in the effects of whole grain diets and diets based on refined grain, but with a similar total fibre concentration, has recently been tested in postmenopausal women (Jacobs 2000). A 17% lower mortality rate was seen in women consuming a predominantly whole grain diet. It was considered that the effect was perhaps caused by phytochemicals in the whole grain complex. Residual confounding (other explanations are not accurately adjusted for) is an alternative explanation. More studies are obviously needed in order to determine the effect of diets on survival.


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