The Physics of Digestion

Alan Mackiea.r.mackie@leeds.ac.uk

Introduction

• Digestion is a complex process but the ability to predict digestion kinetics would be helpful for both Food and Pharma applications

• Surprisingly transport phenomena in the gut are still not well-understood processes

• Can multi-scale approaches help us understand gastrointestinal digestion and thus risk factors for non-communicable disease?

Stomach volume changing with time (as highlighted)

Measuring Changes in Stomach Volume

Two isocaloric meals: The impact on Gastric retention

Liquid - - - - , Solid t½ = 70 min. and 100 min.

Gastric Processing

Ferrua et al. Trends in Food Science & Technology. (2011) 22(9) 480–491

• Preparing interfacial scenario for colipase/lipase adsorption– Further emulsification of lipids

– Desorption of surface active agents

• Solubilisation of hydrolysed lipids

Lipid digestion

+ 2H+

Triglyceride (TG)Monoglyceride (MG)

Free fatty acids (FFA)

Stomach

Undispersed oil

Gastric lipase

Oil droplets

Pancreas

Small intestine

Liver

Gall bladder

Lipid

droplet

(TG)

Pancreatic lipaseColipase

BS

Mixed micelle of BS and hydrolysed lipids

Lipid digestion

Products are mixed by intestinal contractions and then must diffuse through the intestinal mucus layer to be absorbed

What factors affect the kinetics of this mixing and diffusion?

Nutrient transport and mixing

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Nutrient transport and mixing

Displacement of a diffusive probe (D =3×10−10 m2 s−1), initially aligned in a horizontal plane, during a contraction and relaxation cycle with finger-like aligned villi with

(a) Newtonian (left)

(b) Shear-thinning (n = 0.4, ˙γc = 0.01) fluids. Colour keys are the concentration. (right)

Diffusion coefficients of sodium alginate in ex vivo mucus or buffer as measured by FRAP

Diffusion of sodium alginate through mucus

Does Alginate stick to intestinal mucus?

QCM measurements of mucin absorption followed by the introduction of alginate above the absorbed layer

Intestinal Mucus permeability

Measuring particle diffusion

Mean square displacement

(MSD) <Δr2(Δt)> = <Δx2 + Δy2>

Deff = MSD/4Δt= kT/6πr

viscosity

The effect of food components on gut mucus permeability

Using pigs as a model system

Increasing dietary oat fibre decreases the permeability of intestinal mucus

100 nm bile salt coated carboxylated polystyrene beads through small intestinal mucus ex vivo from pigs fed Control (open circles) or OM10 diets (closed circles).

Increasing dietary oat fibre decreases intestinal mucus permeability

The data from this study showed that increasing dietary fibre reduced mass transfer of lipid through the intestinal mucus layer. Journal of Functional Foods 26 (2016) 418–427

Fluorescently labelledlipid from the pig diet

Diffusion coefficient of 500nm latex beads plotted as a function of polymer concentration.

Water

DNA

mucin ex vivo mucus

The role of endogenous polymers

Conclusions

• Digestion is a complex process / series of processes.• Kinetics is important for health.• Gastric emptying is a significant controller of digestion kinetics but

what are the factors driving gastric emptying rate?• Mixing and rheological properties are key controllers of digestion

and absorption (fluid dynamics).• Finally, diffusion of nutrients and bioactives through the unstirred

mucus layer can also have a significant impact on absorption and bioavailability.

Acknowledgements

My Staff and Colleagues at Quadram and Leeds

Adam Macierzanka, Balazs Bajka, Neil Rigby, Louise Salt, Pete Wilde, Mike Ridout, and many others

Alan Mackie: a.r.mackie@leeds.ac.uk