Plasma uptake

In a study of five adults, our laboratory (Freeland-Graves et al., Trace Element Metabolism in Man and Animals. In press.) found that an oral dose of 800 mg of calcium significantly depressed the plasma uptake of 40 mg inorganic manganese. Whether or not this antagonistic relationship exists in normal food or diets is unclear. 

The influence of three dietary factors - pectin, cellulose and phytates - on plasma uptake of manganese (Mn) was studied in adults administered Mn tolerance tests. Plasma samples were collected at hourly intervals following administration of various doses of several Mn salts. It was found that a 40 to 50 mg dose of elemental Mn was necessary to produce consistent plasma responses and that manganese chloride (MnCl ) was better absorbed than the sulfate or acetate form. In fasting subjects, plasma Mn was 0.64, 1.29, 1.12, 0.95, and 0.75 ug/L at hours 0, 1,... 

A series of experiments were initiated to examine the effects of dietary fibers on the plasma uptake of an oral dose of manganese in humans. Stable isotope techniques are the ideal methods for measuring the intestinal absorption of minerals however this technique is impossible for manganese because of its monoisotopic nature (14). Also, ethical considerations prohibit the use of radioactive forms of manganese. Thus, a technique which was both relatively easy to conduct and safe to administer repeatedly to the same subject was developed. 

The manganese tolerance test measures the plasma uptake of pharmacological doses of manganese. This test is analogous to the plasma tolerance test for zinc, which was described in 1973 (15) and has subsequently been used extensively in this (16) and other laboratories as a qualitative indicator of absorption (17,18). 

Influence of Dietary Factors on Plasma Uptake of Manganese... 

Figure 3 Mean plasma uptake of 40 mg manganese as MnCl is...

Experiments In this laboratory examined the effects of small (100 and 300 mg) doses of sodium phytate administered In capsule form on plasma uptake of 50 mg of MnCl. As shown In Figure 5, the 300 mg dose of phytate produced a moderate reduction In manganese response, which was manifested chiefly at hours three and four of the test. The relatively modest effect of phytate on manganese uptake Is somewhat unexpected, since It has been shown that phytic acid may be a more potent Inhibitor of trace element absorption than dietary fibers (35). However, the doses of sodium phytate administered to our subjects were much smaller than the typical daily Intake of 600 to 800 mg (54). Thus we are now Investigating the effects of larger doses of phytate on manganese uptake. 

In a series of manganese tolerance tests, three dietary components —cellulose, pectin, and phytate—were found to reduce plasma uptake of manganese. Although the amount of manganese administered (40-50 mg) in these tolerance tests was much larger than that typically consumed (0.9 to 7.0 mg per day) (55,56), the results were similar to those obtained in bioavailability studies with other trace elements. Thus it appears that diets high In fiber and phytates also reduce the bloavailabllity of manganese. 

Figure 5 Effect of 100 and 300 mg of sodium phytate on plasma uptake of 50 mg of manganese (n=3).

In volunteers who inhaled a tracer dose of metallic mercury vapor for 20 minutes, approximately 2% of the absorbed dose was deposited per liter of whole blood after the initial distribution was complete (Cherian et al. 1978). Uptake into the red blood cells was complete after 2 hours, but plasma uptake was not complete until after 24 hours. Mercury concentration in red blood cells was twice that measured in the plasma. This ratio persisted for at least 6 days after exposure. However, the ratios of 1-2 have been reported for metallic mercury vapor (Miettinen 1973). 

Freeland-Graves JH, Lin PH. 1991. Plasma uptake of manganese as affected by oral loads of manganese, calcium, milk, phosphorus, copper, and zinc. J Am Coll Nutr 10(1) 38-43. 

Another factor to be taken into consideration is the dosage of the intravenous isotopic label. Dosages need to be kept as low as possible to ensure that they do not trigger a homeostatic response of the body, as this may result in an increased discharge of both labels into other body compartments and urine. Because it is almost impossible to mimic plasma uptake of the oral isotopic label exactly by label infusion, this is a potential source of bias. This makes the isotope with the lowest natural abundance the preferred label for intravenous administration. The lower the natural abundance, the lower is the amount of label required to induce a measurable shift in the isotope ratios (see below). 

---