Background: Based on in vitro measurements, it is assumed that starch in wholemeal bread is rapidly digestible, which is considered to be less desirable for health.
Aim of the study: To evaluate the in vitro prediction, we characterized starch digestion of wholemeal wheat bread (WB) and postprandial glucose kinetics in healthy volunteers.
Methods: In a crossover study 4 healthy men ingested either intrinsically (13)C-enriched WB (133 g) or glucose (55 g) in water. Plasma glucose and insulin concentrations were monitored during 6 h postprandially. Using a primed continuous infusion of D-[6,6-(2)H(2)] glucose, the rate of systemic appearance of glucose was estimated (reflecting glucose influx) and the endogenous glucose production calculated.
Results: The glucose influx rate after WB was comparable with that after glucose in the early postprandial phase (0-2 h) (P = 0.396) and higher in the late postprandial phase (2-4 h) (P = 0.005). Despite the same initial glucose influx rate the 0-2 h incremental area under the curve (IAUC) of insulin after WB was 41% lower than after glucose (P = 0.037). Paradoxically endogenous glucose production after WB was significantly more suppressed than after glucose (0-2 h IAUC: P = 0.015, 2-4 h IAUC: P = 0.018).
Conclusions: Starch in WB seems to be partly rapidly and partly slowly digestible. Postprandial insulin response and endogenous glucose production after WB ingestion might not solely be determined by the digestive characteristics of starch; other components of WB seem to affect glucose homeostasis. In vitro measurements might not always predict in vivo starch digestion precisely.