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Dietary Raw Versus Retrograded Resistant Starch Enhances Apparent but not True Magnesium Absorption in Rats^1,2,

Marie-Louise A. Heijnen³, Gerrit J. van den Berg^* and Anton C. Beynen

Department of Human Nutrition, Wageningen Agricultural University, Wageningen ^* Interfaculty Reactor Institute, Delft University of Technology, Delft Department of Laboratory Animal Science, Utrecht University, Utrecht, The Netherlands

Dietary raw (RS₂) vs. retrograded resistant starch (RS₃) raises apparent magnesium absorption in rats. The mechanism proposed is that RS₂ enhances magnesium availability for absorption; it does this by increasing ileal solubility of magnesium due to a reduction in pH as a consequence of RS₂ fermentation in the gut. The mechanism implies that dietary RS₂ vs. RS₃ would raise true magnesium absorption and stimulate reabsorption of endogenous magnesium, leading to a lower fecal excretion of endogenous magnesium. Dietary lactulose vs. glucose raises apparent magnesium absorption, and the mechanism proposed is similar to that for the stimulatory effect of RS₂ vs. RS₃. Thus, we measured in rats fed RS₃, RS₂, glucose or lactulose true magnesium absorption on the basis of the retention of the orally and intraperitoneally administered radiotracer ²⁸Mg. Feeding rats RS₂ instead of RS₃ significantly enhanced apparent but not true magnesium absorption, because RS₂ lowered fecal excretion of endogenous magnesium. When compared with dietary glucose, lactulose significantly raised both apparent and true magnesium absorption, but did not affect fecal excretion of endogenous magnesium. It is suggested that the proposed mechanism by which RS₂ and lactulose would enhance magnesium absorption is disproved by the present data.

KEY WORDS: • resistant starch • magnesium absorption • lactulose • rats

¹ This study was conducted within the framework of the European Resistant Starch (EURESTA) research group working within the European Flair Concerted Action No. 11 and funded by Unilever Research Laboratorium, Vlaardingen, The Netherlands.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence should be addressed.

Manuscript received 5 March 1996. Revision accepted 3 June 1996.