I learned this about 5 years ago. Here's a recent review article by Janine Higgins:
Whole Grains, Legumes, and the Subsequent Meal Effect: Implications for Blood Glucose Control and the Role of Fermentation, Journal of Nutrition and Metabolism, January 2012
From the abstract:
"Whole grains and legumes are known to reduce postprandial glycemia and, in some instances, insulinemia. However, the subsequent meal effect of ingesting whole grains and legumes is less well known. That is, inclusion of whole grains or legumes at breakfast decreases postprandial glycemia at lunch and/or dinner on the same day whereas consumption of a whole grain or lentil dinner reduces glycemia at breakfast the following morning.I encountered the second meal effect while writing about resistant starch (e.g. Bacteria and Blood Sugar, Types of Resistant Starch). The mechanism back then, and described in this paper 5 years later continues to be fermentation of carbohydrates in the colon.
This effect is lost upon milling, processing, and cooking at high temperatures.
The subsequent meal effect has important implications for the control of day-long blood glucose, and may be partly responsible for the reduction in diabetes incidence associated with increased whole grain and legume intake.
"Fermentation of indigestible carbohydrates produces SCFA [Short Chain Fatty Acids] which have been associated with improved insulin sensitivity and glucose tolerance due to decreased hepatic glucose output and free fatty acid concentrations."Two types of carbohydrates that reach the colon undigested, and so, are available for fermentation by resident bacteria are:
- Oligosacharrides - Sources: legumes (beans, peas) and any member of the onion/garlic family.
- Resistant Starch - Sources: legumes, raw potato (but who could eat one!), green banana, cooled cooked potatoes, cooled cooked grains, cooled cooked pasta (cooling allows the heated starch to rearrange itself making it difficult for our bodies to digest), and high-amylose starch of the type found in some corn and rice.
"When whole grain barley was milled into flour and served as porridge, there was a detrimental effect on subsequent meal glycemia despite the presence of the same amount of insoluble and total fiber as in the intact barley kernel which significantly lowered subsequent meal glycemia. Thus, it is unlikely that insoluble fiber per se plays a significant role in the mechanism responsible for the subsequent meal effect."So it has to be the whole grain, not a flour made from the grain ... unless it's cooked, and cooled, like pasta.
For people with diabetes: If this is a bona fide quality of a food, the depression of blood glucose at subsequent meals, then meal-time insulin (bolus insulin) may have to be lowered to compensate (given individual variability). Indeed, long-term insulin needs may be reduced in the presence of long-term whole grain and legume consumption.