Protein digestion and the absorption of mineral elements

In vitro availability of iron and zinc: effects of the type, concentration and fractions of digestion products of the protein

An in vitro dialysis method was employed to determine the effect on the Fe and Zn absorption of the type (beef, pork and soyabean) and the amount (10 and 30 g/kg) of protein present. In addition, the effects of low- and high-molecular-weight (LMW and HMW respectively) digestion products were investigated. After in vitro digestion and dialysis a lower percentage of N, Fe and Zn was found in the LMW fractions from beef, pork and soyabean proteins when the protein level was increased from 10 to 30 g/kg; the higher level of protein being associated with a lower percentage of hydrolysed protein. The highest percentage levels of intrinsic Fe were always found in the HMW fractions, independent of the type and the level of proteins studied, while in the case of Zn, both HMW and LMW fractions gave similar values. An interaction was found between inorganic Zn and non-haem-Fe. The addition of inorganic Zn (10 micrograms/ml) caused a significant decrease in the in vitro availability of Fe from soyabean protein, while it did not affect the dialysability of intrinsic Fe from beef and pork proteins. Our results showed that the type and the level of the protein had a positive effect on the dialysability of extrinsic Fe. We postulate that the effect of a protein on the absorption of extrinsic Fe could be accounted for by free amino acids and/or small peptides released during the digestion process and also by the undigested or partially-digested HMW fractions of hydrolysed proteins which could play a fundamental role in the availability of this essential element.

Phytate and zinc bioavailability

This review discusses evidence from human studies on the effects of dietary phytate on zinc bioavailability. In vitro and animal experiments have implicated calcium as a potentiating factor because it reacts with phytate, and zinc binds to the precipitate. Magnesium also reacts similarly to calcium, but most studies have not considered this factor. Protein provides amino acids, some of which are able to desorb zinc from the precipitate and improve bioavailability. Some predictive ratios, derived from animal studies, have been directly applied to human studies. The studies reviewed included those on: zinc status of groups, apparent absorption of zinc in normal subjects and ileostomists, true absorption using a stable isotope, plasma tolerance, and the accumulation in the body of a radioisotope. It was concluded that detrimental effects of phytate could be demonstrated on zinc bioavailability, but that the studies had not been designed specifically to demonstrate whether the interactions found in animal studies also apply to humans. It is suggested that more targeted research is required before predictive ratios are used for humans.

Bioavailability of magnesium diglycinate vs magnesium oxide in patients with ileal resection

BACKGROUND

Patients who have undergone ileal resection are at risk for developing magnesium depletion/deficiency because of poor absorption and decreased intake as well as increased endogenous losses. Magnesium repletion is difficult to accomplish because of the cathartic action of most oral magnesium supplements at therapeutic doses. The results of in vitro and in situ studies show that magnesium diglycinate (chelate) represents a highly available form of magnesium that is absorbed in part as an intact dipeptide in the proximal small intestine.

METHODS

We conducted a double-blind, randomized crossover trial with 12 patients who had ileal resections in order to compare the bioavailability of a 100-mg dose of 26Mg-labeled chelate with MgO in this patient population.

RESULTS

For the patient group as a whole, 26Mg absorption was low but was not different for the two supplements (23.5% vs 22.8% for magnesium chelate and MgO, respectively). However, 26Mg absorption was substantially greater from the chelate (23.5% vs 11.8%; p < .05) in the four patients who showed the greatest impairment of magnesium absorption with MgO and was better tolerated by all patients. Peak isotope enrichment also occurred significantly earlier after 26Mg chelate than after 26MgO ingestion (mean difference 3.2 +/- 1.3 hours; p < .05), and the area under the enrichment vs time curve was greater after chelate ingestion (p < .05).

CONCLUSIONS

Data from this study support the suggestion that some portion of magnesium diglycinate is absorbed intact, probably via a dipeptide transport pathway. Magnesium diglycinate may be a good alternative to commonly used magnesium supplements in patients with intestinal resection.