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Effect of mastication on lipid bioaccessibility of almonds in a randomized human study and its implications for digestion kinetics, metabolizable energy, and postprandial lipemia

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.

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Alex’s Notes: How many of you reading this have thought about chewing? I mean, we chew our food multiple times every day and yet it seems to be an overlooked part of digestion. I’m sure even that word brings images of the stomach and intestines to mind, but not the mouth or teeth. Well chewing is pretty damn important to break down food and not choke. Although our stomach acid and numerous enzymes are great for doing that too, they are limited by the size of the food entering. Consider the almond. Previous work has shown that a majority of the lipid content resides within the cells of the almond tissue that make it less accessible for digestion and absorption. This concept is reinforced by studies demonstrating that almonds contain about 32% less metabolizable energy than what is determined with the bomb calorimeters.

So how does chewing come into play? To find out, the researchers of the current study recruited 15 young (average age 25.4 years) and healthy (average BMI 21.6kg/m2) men and women to undergo a single-blind crossover study of 4 chewing sessions (2 with raw almonds and 2 with roasted almonds). During each session the participants chewed 4-5 grams of almonds ten times with two minutes of rest between each trial. The first two trials had the participants chew and swallow normally to allow recording of the number of chews and length of chew time. The subsequent eight trials had the participants chew a specified number of times and spit the content into a collection dish. They then rinsed their mouth with water and spat that out as well to maximize recovery of almond pieces before beginning the next trial.

With chewed almonds in hand, the researchers used mechanical sieving and laser diffraction to determine the almond piece size and lipid content. They then plugged this information into their fancy equation for determining lipid bioaccessibility and compared it to a solvent extraction method that physically measured the amount of lipid released during chewing. Finally, they looked under some microscopes to analyze the almond’s microstructure.

Nom, nom, nom, nom

Amusingly (at least to me), there was no significant difference in the number of chews between raw and roasted almonds, but the duration of the chewing time was significantly lower for the roasted variety, suggesting that the subject ate the roasted almond more quickly. I suppose they may have tasted better.

This did lead to slight differences in average almond particle size between the varieties, with raw almonds having more large particles and less small particles than roasted almonds. Nonetheless, both the predicted and experimentally determined lipid bioaccessibility values were only 8-11%. The smaller almond particles demonstrated far greater cell wall disruption than the larger particles when viewed through a scanning electron microscope.

So if metabolizable energy is only about 68% and chewing gets us 8-11% of the way there, then you really have to give props to the stomach and intestines for breaking down the almonds. Even so, it appears that can’t fully reap the lipid of the almond. Something that is often overlooked when assessing nutrition is the complex behavior of food within our digestive tract. Not just the food itself, but also how foods interact with one another. I like the saying “it is not about calories, it is about what your body does with those calories.”


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