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Clinical Trial
. 2015 Jan;101(1):25-33.
doi: 10.3945/ajcn.114.088328. Epub 2014 Nov 12.

Effect of Mastication on Lipid Bioaccessibility of Almonds in a Randomized Human Study and Its Implications for Digestion Kinetics, Metabolizable Energy, and Postprandial Lipemia

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Free PMC article
Clinical Trial

Effect of Mastication on Lipid Bioaccessibility of Almonds in a Randomized Human Study and Its Implications for Digestion Kinetics, Metabolizable Energy, and Postprandial Lipemia

Myriam M L Grundy et al. Am J Clin Nutr. .
Free PMC article

Abstract

Background: 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 μm) 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.

Keywords: almonds; lipid bioaccessibility; mastication; mathematical model; microstructure.

Figures

FIGURE 1
FIGURE 1
Particle size distributions of masticated almonds were measured by mechanical sieving (A) and laser diffraction (B); NA (dark-gray line) and RA (light-gray line) boluses. Size data are presented on a log scale plotted against percentage weight recovered (A) or percentage volume (B). Some sieve fractions with similar sieve apertures were combined (850 with 1000 μm and 1700 with 2000 μm), so that the total number of experimental points shown in the figure is 9, which also includes the 20-μm nylon mesh “sieve.” Student's paired t test indicated significant differences (P < 0.001) in particle size at all size fractions found between raw and roasted almonds, except for sizes 850 and 1000 μm and sizes 141, 159, 178, and 200 μm for sieving and laser methods, respectively. Individual experimental points on the size distribution profiles are means ± SEMs (n = 15). NA, natural almond; RA, roasted almond.
FIGURE 2
FIGURE 2
Light microscopy images of masticated NAs: whole particles of decreasing size (A, B, and C), parenchyma cells located in the center of the particles (D and E), and cells situated at the edge of the particles (F). Note the presence of coalesced lipid droplets (C, E, and F). Scale bars: A, 100 μm; B, 50 μm; C–E, 20 μm; F, 10 μm. Approximate sizes of NA particles: A, 1200 μm; B, 500 μm; C, 250 μm. NA, natural almond.
FIGURE 3
FIGURE 3
Scanning electron microscopy images of particles from masticated NAs. Scale bars: A and B, 200 μm; C, 100 μm. Approximate sizes of NA particles: B, 2000 μm; C, 550 μm. NA, natural almond.
FIGURE 4
FIGURE 4
TEM images (A and B) of masticated NAs show intact cells and their content. The TEM image in panel C shows ruptured cells at the surface of the masticated NA particle; note the coalesced lipid bodies. Scale bars: A, 6 μm; B and C, 5 μm. NA, natural almond; TEM, transmission electron microscopy.
FIGURE 5
FIGURE 5
Light microscopy images of masticated natural almonds stained with Nile red indicating the presence of lipid. Scale bars: A–D, 20 μm.

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