Mechanisms responsible for the second meal response to maize porridge

Abstract

The short-term effect of a food on the carbohydrate metabolism is reflected in post-prandial variations of blood glucose levels, expressed as the glycaemic index (GI) of the food, as well as the effect of the food on the glucose response of a subsequent meal. The latter is referred to as the second meal response (SMR). In South Africa, maize porridge is the staple in the diet of many people from different ethnic groups. There is however, little information available on the GI of maize porridge and none on the SMR to maize porridge. The underlying mechanisms responsible for the SMR are also unknown. The main objective of this study was to examine the hypothesis that a suppression of circulating free fatty acids, glucagon and growth hormone secretion by a first meal containing carbohydrates which are slowly digested and absorbed, are responsible for the SMR. The GI of maize porridge, with and without added barley fibre, and the effect of this addition on the SMR were also examined. Six healthy black women voluntarily participated in the study. A Latin square design was used to randomly test three meals as a first meal (glucose, maize porridge and maize porridge with added barley fibre). After four hours, a second meal of glucose was taken by the subjects. All meals contained 50g carbohydrate. Serum levels of glucose, insulin, growth hormone and glucagon,

as well as plasma free fatty acids were measured at specific intervals. The results showed that the GI of maize porridge when eaten in a precooked, frozen and reheated form, was 57.7 ± 25.6%. The addition of barley fibre to the porridge did not change the GI: 57.5 ± 16.3%. The GI of glucose as second meal was 145.4 ± 33.3% when glucose was the first meal, 74.3 ± 38.1% with maize porridge as first meal, and 77 . 6 + 26.4% with maize porridge plus barley fibre as first meal. Therefore, a clear SMR with the two porridges was observed. The results further indicated that the levels in the blood of glucose, insulin and free fatty acids, four hours after the first meal just before the second meal was taken, had highly significant correlations with the area under the glucose response curve during the second meal (glucose: r = -o. 53, p = 0. 05; insulin: r = -0.70, p = 0.005; free fatty acids: r = + 0.80, p = 0.0005). Glucagon and growth hormone did not show these correlations. The results therefore support the hypothesis that a first meal with a low GI, which after four hours will be associated with "relative" high levels of glucose and insulin, but low levels of circulating free fatty acids, will elicit a SMR. Two factors are possibly responsible for the lack of effect of the added barley fibre on the GI. Firstly, the amount of soluble components in this fibre was probably too low to affect digestion and absorption rates of carbohydrate. Secondly, the effect of

retrogradation and consequent development of resistant starch during the cooling and freezing of maize porridge, probably overrided the effect of fibre on the GI. The results of this study have special applications in the planning of meal frequency - especially in the diet of persons with an abnormal glucose tolerance or with diabetes mellitus.