Insulinogenic Effects of Milk- and Other Dietary Proteins, Mechanisms and metabolic implications

Sammanfattning: The metabolic syndrome (type 2 diabetes, obesity, hypertension, atherosclerotic cardiovascular disease, dyslipidaemia, and hyperinsulinaemia), is increasing in prevalence world-wide. The progression of this syndrome proceeds through a step-wise deterioration of metabolic events where deterioration of insulin sensitivity appears to have a key role in a ?vicious circle? of hyperinsulinaemia/hyperglycaemia and insulin resistance. Food factors inducing low postprandial glycaemic and insulin responses, and improving insulin sensitivity might thus be advantageous. Recent data suggest that certain proteins and protein-containing foods e.g. milk, may exert insulinotrophic effects in healthy subjects, without a concomitant postprandial hyperglycaemia. The longer?term metabolic effects of non-glucose mediated insulin secretion remain to be elucidated, and reports concerning the impact of milk proteins, or type and/or amount of other dietary protein sources on metabolic risk factors are, however, contradictory. The present thesis investigates the role of certain food proteins on insulin secretion and blood glucose regulation, focusing on milk proteins. In particular, the key determinants at food- and physiological level, respectively, have been examined. Dairy proteins, in particular the whey protein fraction, were found to be potent insulin secretagogues in healthy subjects, whereas cod and gluten did not stimulate postprandial insulin release. A positive correlation was seen between postprandial insulinaemia and responses of leucine, isoleucine, valine, threonine and lysine. Whey also induced a high GIP response compared with milk, cheese, cod and, gluten meals. Breakfast and lunch meals supplemented with whey, as opposed to a protein equivalent amount of ham, increased insulin response by 31 % and 57 % following breakfast and lunch, respectively, in subjects with diabetes type 2. The whey supplementation reduced glycaemia at lunch by 21 % (P<0.05), and reduced glycaemic excursions from fasting value over the course of the day (0-7 h) by 12 % (P<0.05). Insulinogenic properties of different amino acids, or of postprandial serum obtained following ingestion of whey or white wheat bread (WWB), were investigated in vitro using isolated Langerhans islets. Serum, withdrawn at 15 and 30 min, respectively, after a whey meal, increased insulin release in vitro (+87 % at 15 min, +139 % at 30 min) compared with corresponding serum after WWB. Further, leucine (+105 %), threonine (+97 %) and to a lesser extent isoleucine (+45 %) potentiated insulin secretion in the presence of glucose (8.3 mM), compared with glucose alone, whereas lysine and valine did not. Exposing Langerhans islets to a combination of the above amino acids increased insulin secretion further (+270 %), particularly when also including GIP (+558 %). In healthy subjects, a test drink with leucine, isoleucine, valine, and glucose resulted in higher insulin responses than pure glucose (+40 %), whereas the combination of lysine, threonine, and glucose had no effect. A drink with all five amino acids and glucose mimicked the glycaemic and insulinaemic responses seen after whey ingestion. The drink with the five amino acids also induced similar postprandial plasma amino acid responses, except for leucine which caused a higher increment (P<0.05). The whey meal was accompanied by a higher GIP response (+80 %, P<0.05), whereas the drinks containing free amino acids did not affect GIP. The effect of milk induced hyperinsulinaemia on semi-acute metabolic responses at a second standardised meal was studied in healthy subjects. Breakfast meals differing in GI/II characteristics were included to study the impact of non-glucose mediated hyperinsulinaemia (whey GI=53/II=140) versus the impact of differences in insulinaemia caused by differences in the rate of glucose delivery to the blood (pasta GI=56/II=32 vs white bread GI=100/GI=100). In addition, a rye bread product with a high GI (81), and a low II (56), mediated by unidentified food factors, was included. The postprandial glycaemia after the standardised lunch post pasta breakfast was lower (-48 %, P<0.05) compared with the WWB breakfast. Although the whey breakfast had low GI properties similar to the pasta, no effect on postprandial glycaemia was seen after the standardised lunch. Nor did whey-induced hyperinsulinaemia (II=140) at breakfast deteriorate glucose tolerance at the proceeding standardised lunch compared with a WWB breakfast (II=100). It is concluded that milk induced hyperinsulinaemia primarily relates to the whey fraction. The insulinotrophic features of whey could be simulated in healthy subjects with a drink containing a mixture of leucine, isoleucine, valine, threonine and lysine. In accordance with whey, these amino acids increased in postprandial plasma, but with no concomitant increase in GIP, indicating that the insulinotrophic effect of whey is mediated by a rapid postprandial response of these amino acids, rather than by stimulation of the incretin hormones. No detrimental effects of whey-induced hyperinsulinaemia were seen on glucose tolerance in healthy subjects in the perspective from a test breakfast to a standardised lunch. Enclosure of whey at breakfast and lunch facilitated blood glucose regulation in type 2 diabetics, indicating a therapeutic role of certain proteins in individuals with diminished insulin secretory capacity.