La scelta di consumare frutta secca con guscio, della quale sono noti i vantaggi correlati al contenuto di fibre, minerali e acidi grassi insaturi, è spesso condizionata dalla loro altrettanto nota densità energetica, dovuta in particolare all’elevato tenore lipidico (nelle mandorle, ad esempio i grassi contribuiscono al 50% circa delle calorie). Questo studio suggerisce che il valore energetico delle mandorle sia in generale sovrastimato. Infatti l’analisi degli effetti della masticazione, da parte di volontari sani, di mandorle fresche o tostate ha permesso di osservare che, al termine del processo, le particelle di alimento sono piuttosto voluminose e mantengano una certa integrità strutturale. In questo modo la bioaccessibilità del contenuto lipidico è ridotta nelle prime fasi della digestione. Vale a dire che fino al 92% dei lipidi contenuti nelle mandorle resta nella matrice intracellulare delle particelle masticate, facendo sì che la lipidemia post-prandiale non venga influenzata in modo significativo e che anche l’apporto energetico metabolizzabile risulti contenuto. Questo meccanismo può giustificare, almeno in parte, il calo ponderale osservato con diete mirate al controllo del peso che comprendono il consumo moderato di mandorle.
Glossario
Grassi insaturi
Grassi liquidi a temperatura ambiente. Gli oli vegetali sono grassi insaturi. Possono essere monoinsaturi o polinsaturi. Si trovano nelle olive, nel mais, negli oli di semi di girasole e nelle noci. Consumare cibi ad elevato contenuto di grassi monoinsaturi può aiutare ad abbassare il colesterolo. Tuttavia l’apporto calorico resta lo stesso, pertanto un consumo eccessivo può contribuire all’aumento di peso.
Effect of mastication on lipid bioaccessibility of almonds in a randomized human study and its implications for digestion kinetics, metabolizable energy, and postprandial lipemia.
Grundy MML, Grassby T, Mandalari G, Waldron KW, Butterworth PJ, Berry SEE, Ellis PR.
Am J Clin Nutr - Epub ahead of print - doi: 10.3945/ajcn.114.088328Background: The particle size and structure of masticated almonds have a significant impact on nutrient release (bioaccessibility) and digestion kinetics.
Objectives: The goals of this study were to quantify the effects of mastication on the bioaccessibility of intracellular lipid of almond tissue and examine microstructural characteristics of masticated almonds.
Design: In a randomized, subject-blind, crossover trial, 17 healthy subjects chewed natural almonds (NAs) or roasted almonds (RAs) in 4 separate mastication sessions. Particle size distributions (PSDs) of the expectorated boluses were measured by using mechanical sieving and laser diffraction (primary outcome). The microstructure of masticated almonds, including the structural integrity of the cell walls (i.e. dietary fiber), was examined with microscopy. Lipid bioaccessibility was predicted by using a theoretical model, based on almond particle size and cell dimensions, and then compared with empirically derived release data.
Results: Intersubject variations (n = 15; 2 subjects withdrew) in PSDs of both NA and RA samples were small (e.g., laser diffraction; CV: 12% and 9%, respectively). Significant differences in PSDs were found between these 2 almond forms (P<0.05). A small proportion of lipid was released from ruptured cells on fractured surfaces of masticated particles, as predicted by using the mathematical model (8.5% and 11.3% for NAs and RAs, respectively). This low percentage of lipid bioaccessibility is attributable to the high proportion (35–40%) of large particles (>500 micron) in masticated almonds. Microstructural examination of the almonds indicated that most intracellular lipid remained undisturbed in intact cells after mastication. No adverse events were recorded.
Conclusions: Following mastication, most of the almond cells remained intact with lipid encapsulated by cell walls. Thus, most of the lipid in masticated almonds is not immediately bioaccessible and remains unavailable for early stages of digestion. The lipid encapsulation mechanism provides a convincing explanation for why almonds have a low metabolizable energy content and an attenuated impact on postprandial lipemia.