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Banana starch breakdown in the human small intestine studied by electron microscopy
N Faisant1, D J Gallant, B Bouchet
1Laboratoire de Technologie Appliquée à la Nutrition I.N.R.A., BP 1627, Nantes, France.
Objective:
To determine the origin of the poor digestibility of banana starch granules in the human small intestine.
Design:
The subjects received the same experimental meal.
Setting:
Nutrition Research Unit, Laënnec Hospital, CHU, Nantes.
Subjects:
Six healthy young subjects.
Interventions:
The digestion of raw green banana flour in the upper part of the gut was studied by the intubation technique. After ingestion of 30 g banana flour mixed with a complex meal, ileal samples were continuously collected during 14 h. In order to determine the structural nature of this resistant starch, the dried ileal samples were observed with scanning and transmission electron microscopy. Transmission electron microscopy was performed after treatment with periodic acid-thiosemicarbazide-silver nitrate.
Results:
Banana starch proved very resistant to in vivo amylase hydrolysis since 84% of the starch ingested reached the terminal ileum. The microscopic observations showed that raw banana flour contained irregularly shaped dense starch granules with smooth surfaces. After their passage through the small intestine, starch granules appeared exocorroded, with porous surfaces, and some exhibited several irregular pits, crevices or holes by which the enzymes had penetrated and hydrolysed the inner part. Cell walls closely associated with starch granules could have hindered enzyme access to starch.
Conclusions:
Encapsulation could be partly responsible for the low digestibility of starch in banana flour, together with the intrinsic resistance of banana starch granules.