Fermentation as a Means of Optimizing the Glycaemic Index - Food Mechanisms and Metabolic Merits with Emphasis on Lactic Acid in Cereal Products

In current recommendations from FAO/WHO, foods that elicit low glycaemic responses and thus have low glycaemic indices (GIs) are advocated. The rationale for this recommendation is that low-GI diets have been shown to reduce risk factors for e.g. type II diabetes and cardiovascular disease. A major problem in this context is that most important starch rich staple foods in our diet, such as potato and bread products, have high GIs. One important task is, therefore, to identify food factors that are able to lower the rate of starch digestion and/or prolong the uptake of glucose to the blood. The overall purpose of the present thesis was to examine the use of fermentation as a means of lowering the GI characteristics of carbohydrate foods, i.e. cereal and dairy products. Previous studies have shown that addition of lactic acid to bread reduce the glycaemic and insulinaemic properties in healthy subjects. A reduced rate of starch hydrolysis has been suggested as a cause, but the mechanism has not yet been fully revealed. An important topic of the thesis was to evaluate the food mechanism for a reduced availability of starch in lactic-acid-containing bread. Another purpose was to study the potential effect of lactic acid in other food systems such as gruel or milk products. Finally, the metabolic effects of lactic-acid-containing bread on glucose tolerance and related parameters were evaluated in a “second-meal study” in healthy subjects, and in a dietary intervention in hyperinsulinaemic rats. The major findings of the thesis are the following; 1) The presence of lactic acid in the dough reduces the rate of in vitro starch hydrolysis in wheat or barley bread products and bread-like wheat starch systems by 15-25% compared with controls with no acid. This is in line with previous observations and suggests that lactic acid may obstruct the digestive process in the case of certain cereal products, and probably explain the lowered GI and insulinaemic responses to lactic-acid-containing bread. 2) The effect of lactic acid on the rate of starch amylolysis and/or glucose uptake to the blood appears to differ between food systems, and no effect was seen in milk and barley based gruel products. 3) In order to reduce the rate of starch hydrolysis in cereal-based foods, lactic acid must be present during starch gelatinization. Presence of gluten was also a prerequisite for a lactic acid effect in bread-like wheat starch systems. 4) Lactic-acid-containing barley bread with a 25% reduction of acute glycaemia improved the insulin economy at a subsequent standardized lunch meal in healthy subjects. 5) A diet based on wheat bread to which lactic acid was added prior to baking improved the glucose tolerance after 14 days in hyperinsulinaemic rats, compared with wheat bread where the lactic acid was added after baking. 6) Fermentation and presence of lactic acid in milk products did not significantly affect either the glucose or insulin responses. For all milk products the glucose responses were surprisingly low and the insulin responses unexpectedly high. The inconsistency between the glucose and insulin responses indicates that some component, other than lactose, stimulates insulin secretion. With pure lactose the insulin response was in correspondence with the glycaemic response.