The body
requires major and minor minerals to maintain good health. Fresh fruits and vegetables are good sources
of vitamins, but contain relatively low levels of essential minerals. Even
vegetables believed to be high in mineral content usually do not provide
adequate dietary concentrations when consumed in normal sized portions.
Screening
studies have recently identified cultivars of Indian mustard (Brassica juncea),
an edible plant, that accumulate large amounts of various elements in their
above ground biomass under certain cultivation conditions. In some cases the trace element concentrations
exceed 1% of the plant dry weight, and generally are over two orders of
magnitude higher than typical mineral concentrations in conventional edible
plants. These discoveries have raised for the first time the possibility that
plants can be used as a concentrated form of essential mineral nutritional
supplements. The form of the mineral in the plants can also offer a potential
benefit, since the bioavailability, or amount of mineral taken up and utilized
by the body, can be modified depending on the mineral source. Although plant
components such as phytates can interfere with mineral uptake by the body,
plants are generally considered to be superior sources of supplements.
The
bioavailability of select minerals in the mineral enriched plant tissues was
measured in vitro through two independent procedures. One involved a series of
sequential extractions [1] that is used to assess bioavailability of trace
metals in solid media [2]. The procedure uses increasingly stringent
extractants to characterize the solubility (water soluble, acid soluble,
reducible, oxidizable, residual) of trace elements in a given matrix. The
second procedure utilizes simulated gastric fluid to predict the solubility of
the metal in the stomach [3]. These analyses measure the solubilization of the
minerals from the plant tissue but not absorption as defined for
bioavailability [4]; however, minerals must be in a soluble form before taken
up or used by the body.
The
nutritional minerals Cr, Fe, Mn, Se, Zn accumulated by the Brassica plants were
found to be easily solubilized by the sequential extraction procedure (Table
1). More than 80%, and in most cases 100% of the total mineral content in the
enriched plants was solubilized by the sequential extraction method. Each of
these five plant bound minerals was found to be much more soluble than those in
an over-the-counter multivitamin tested in the same procedure, suggesting that
a plant based source of these minerals produces a more available mineral form
compared to the form of minerals found in popular commercial supplements.
Table 1.
Solubility of Fe, Zn, Cr, Mn, and Se based on sequential extraction of trace
mineral enriched B. juncea plants compared with a common trace
element/multivitamin supplement.
|
Supplement |
Metal |
Water-Soluble |
Acid-Soluble |
Reducible |
Oxidizable |
Residual |
|
---------------------%
of total metal concentration--------------------- |
||||||
|
B. juncea |
Cr |
76.95a |
13.30a |
3.06a |
3.94a |
2.74a |
|
Multivitamin
I |
|
0.00b |
0.00b |
0.00b |
0.00b |
100.00b |
|
B. juncea |
Fe |
47.85a |
19.10a |
16.91a |
3.41a |
12.71a |
|
Multivitamin
I |
|
1.82b |
0.10b |
1.36b |
0.21b |
96.51b |
|
B. juncea |
Mn |
87.14a |
12.38a |
0.28a |
0.16a |
0.04a |
|
Multivitamin
I |
|
67.81a |
15.31b |
15.55b |
0.73b |
0.59b |
|
B. juncea |
Se |
93.91a |
3.63a |
0.35a |
1.84a |
0.27a |
|
Multivitamin
I |
|
0.00b |
0.00b |
100.00b |
0.00b |
0.00b |
|
B. juncea |
Zn |
92.10a |
7.41a |
0.37a |
0.06a |
0.05a |
|
Multivitamin
I |
|
3.73b |
19.06b |
64.96b |
5.57b |
6.68b |
Results of the
simulated gastric fluid digestion confirm the data obtained from the sequential
extraction study. Simulated gastric fluid extracted 31% of the total Fe in the
plant sample and 81%, 69%, 100%, and 100% of the total Cr, Mn, Se, and Zn,
respectively. The high solubility of these metals in the enriched plant
material produced RDI values significantly greater than those found in
commercially available supplements.
.
Simulated
intestinal fluid digestion studies showed that the solubility of the commercial
supplements decreased in comparison to their behavior in the gastric fluid
digestion whereas the metals within the enriched plant matter remained soluble.
It is believed that the high pH of the intestinal fluid precipitates cationic
metals that were soluble in the acidic simulated gastric fluid as
oxyhydroxides, whereas cationic metals associated with the plant matter are
apparently chelated with organic complexes which prevents their precipitation.
Solubility of these metals in the enriched plant material produced
soluble-based %RDI values significantly greater than those of commercially
available supplements. The high total mineral concentrations in the plant
biomass coupled with a high degree of solubility in both the simulated gastric
and intestinal fluid supports the use of these plants as mineral supplements.
The low
solubility displayed in this study of Cr, Fe, Mn, Se, and Zn in several leading
nutritional supplements clearly shows a need for a supplement that supplies
greater available levels of these minerals for absorption to attain the
recommended daily intake (RDI), not only in total metal form, but in a soluble
form. For example, the multivitamins tested in this study contain 100% of the
RDI for Fe and Zn; however, only a maximum of 9% and 8% of the RDI for Fe and
Zn, respectively, were solubilized by simulated gastric fluid. Because eventual
absorption of the metals by the body requires the metals to be soluble,
achieving the 100% RDI criterion by total metal content alone is insufficient
to meet daily nutritional requirements.
Bioavailability
requires solubility, absorption, and eventual metabolism by the body. Even
though solubility in stomach acid is important for digestion of food intake,
metals must remain soluble in the intestinal fluid before absorption can occur.
Soluble ions, not solid precipitates, are required for absorption. The poor
performance of the mineral-based supplements, particularly the multivitamins,
in providing the metals in soluble form in the intestinal fluid simulant
clearly shows that these supplements do not provide their metals in
bioavailable form. The enriched plant matter can provide intestinal fluid
soluble Cr, Mn, Se, and Zn at levels that exceed at least 50% of each metal's
respective RDI in as little as 0.25 g of plant material, thereby providing a
bioavailable source of these metals.
1. Ramos, L.,
Hernandez, L.M., and Gonzalez, M.J. Sequential extraction of copper, lead,
cadmium, and zinc in soils from or near Donana National Park. Journal of
Environmental Quality 1994. 23:50-57.
2. Berti, W.,
Cunningham, S.D., and Jacobs, L.W. Sequential chemical extraction of trace
elements: Development and use in remediating contaminated soils. Proc. 3rd
International Conference on Biogeochemistry of Trace Elements. 1995.
3. Glahn,
R.P., Lai, C., Hsu, J., Thompson, J.F., Guo, M., and Van Campen, D.R. Decreased
citrate improves iron availability from infant formula: application of an
in-vitro digestion/Caca-2 cell culture model. Journal of Nutrition. 1998.
128:257-264.
4. Newman,
M.C. and Jagoe, C.H. Ligands and the bioavailability of metals in aquatic
environments. in Bioavailability: Physical, Chemical, and Biological
Interactions, Hamelick, J.L., Landrum, P.F., Bergman, H.L., and Benson, W.H.,
eds. CRC Press, Lewis Publishers, Boca Raton, FL. 1994.Pages 39-61