Background: Milk fat, which has different structures in the various dairy products, is a major and controversial lipid source in the Western diet. However, information about the digestion fate of milk fat depending on its supramolecular structure for a given composition is scarce.
Aim of the study: In this study, 13CO2 breath tests were performed with fasted rats force-fed different dairy preparations of similar composition but differing in fat suprastructure in order to highlight differences of general lipid metabolism.
Methods: Each preparation consisted of a NaCl solution, anhydrous milk fat labelled with a 13C mixed triacylglycerol, casein (as native phosphocaseinate powder with some lactose), and dipalmitoylphosphatidylcholine. Milk fat was either fed (i) unemulsified consecutively to the aqueous phase, or emulsified as (ii) coarse droplets of approximately 10 microm covered mainly with the phospholipid, or (iii-iv) fine droplets of approximately 1 microm covered mainly with casein, force-fed either in the liquid state or in a semi-crystallized state. 13C abundance in expired air samples was measured by isotope ratio mass spectrometry; results were expressed as 13C enrichment and were submitted to an ANOVA analysis.
Results: The 13CO2 excretion curves of the unemulsified preparation and the coarse emulsion were similar and presented a sharp peak, both significantly different from the fine emulsion curves characterized by a nearly linear cumulative recovery. The crystalline state of the fine emulsion droplets and the viscosity of these emulsions did not affect significantly their excretion curves. The lipid metabolization (indicated by the 13C recovery) was significantly slower for the fine droplets coated with casein than for the large droplets coated with the phospholipid and the unemulsified fat. For the latter, a single 13C peak rapidly appeared, while for small droplets coated with caseins, 13C excretion was continuous up to 6 h.
Conclusions: Global lipid metabolism based on oxidation to CO2 was decreased with smaller compared to larger emulsified milk fat particles with different coatings. These data support the concept that dairy products with different fat suprastructures are digested and metabolized differently.