Vol.3, No.5, 697-701 (2012) Agricultural Sciences
Macro-nutrients in edible parts of food crops in the
region of Moanda, Gabon
Jean Aubin Ondo*, Richard Menye Biyogo, Magloire Ollui-Mboulou, François Eba,
Joseph Omva-Zue
Laboratoire Pluridisciplinaire des Sciences de l’Ecole Normale Supérieure, Libreville, Gabon;
*Corresponding Author: laplus_ens@yahoo.fr
Received 11 June 2012; revised 18 July 2012; accepted 28 July 2012
The assessment of nutritive foodstuffs quality is
an important step in the estimation of dietary
intake of metals. This study aimed to assess
accumulation and the daily intake of macro-
elements magnesium, calcium, potassium and
sodium in edible parts of plants grown in Moan-
da area. The ranges of conc entrations were 246 -
5645 mg/kg, 1167 - 35,105 mg/kg, 158 - 12,563
mg/kg and 88 - 795 mg/kg for Mg, Ca, K and Na,
respectively. Mg and K levels were significantly
higher in fruits than in tubers. The trend of ac-
cumulation in the edible parts of plants was
generally: Ca > K > Mg > Na. The daily intakes of
Mg and Ca were the highest and indicated food
crops in Moanda had a good nutritive quality.
Keywords: F ood Crops; Macro-Nutrients; Daily
Intake; Moanda
Food crops hold an important place in well-balanced
diets, and increasing consumption of vegetable and fruits
is advisable [1]. For example, leafy vegetables represent
an important source of proteins, vitamins and minerals for
humans, and they act as buffering agents for acidic
products formed during the digestion process [2]. Eleven
elements are named essential nutrients. Six of them are
used in large quantities (magnesium, phosphorus, potas-
sium, calcium, nitrogen and sulphur) and are named
macro-elements, and five elements are used in small
quantities (boron, copper, iron, manganese and zinc) and
named trace-elements. They are transferred from soil to
plant and are receiving major attention [3]. The level of
these elements in soils is very important for crop growth
successful. If they are deficient or improperly balanced,
normal growth does not occur. Plant growth maybe re-
tarded because these elements are actually lacking in the
soil, they become available too slowly. Because that, they
are not adequately balanced by other nutrients. Likewise,
insufficient levels of essential elements in crops can to
cause serious diseases to human and animal [4-6].
WHO has recommended selective studies of individual
foodstuffs as an important step in the estimation of dietary
intake of metals [7]. The present work aimed to assess the
accumulation of major essential nutrients (Ca, Mg, K and
Na) in food plants, and to estimate their daily intake by
Moanda inhabitants who consume these plants.
2.1. Sampling, Preparation and Analyses
Ten kinds of food crops samples were randomly col-
lected in plantations located 15 km south of Moanda City,
South-East Gabon (13˚10'E - 13˚15'E, 1˚25'S - 1˚35'S) in
December 2009. All collected plant samples reached the
maturation. Samples were washed with distilled water and
with deionized water to eliminate the air-borne pollutants
and soil particles. Then they were blotted dry with tissue
paper. The edible part of each vegetable was removed
with a knife; they were dried at 70˚C until constant weight.
Samples were finely ground (0.2 mm) and kept in poly-
ethylene bags. The selected plants are listed in Table 1.
Dried plant materials were put into a platinum dish. The
capsule was introduced in an oven whose temperature was
gradually increased up to 500˚C and maintained for 2
hours. The ash obtained was mineralized by hydrochloric
acid 6 N. Calcium, magnesium, potassium and sodium in
the sample were determined using atomic absorption
spectrometry. Instrument optimization was set according
to the manufacturer’s instruction manual. Prior to analysis,
the nebulizer was cleaned by aspirating 20 ml of deion-
ized water. The blank and standards were aspirated first,
followed by the unknown samples. Deionized water was
aspirated between each sample after duplicate readings
were taken. The unknown element concentrations were
determined from the calibration graph.
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J. A. Ondo et al. / Agricultural Sciences 3 (2012) 697-701
Table 1. Studied plants and edible parts.
Usual plant name Edible part Scientific name of plant
Pepper Fruits Capsicum frutescens
Nkoumou Leaves Gnetum africanum
Okra Fruits Abelmoschus calei
Eggplant Fruits Solanum melongena
Lemon grass Leaves Cymbopogon citratus
Yam Tubers Dioscorea spp
Manihot esculenta Crantz
Amaranth Leaves Amaranthus cruentus
Roselle with small red leaves Leaves Hibiscus sabdariffa
Roselle with large green leaves Leaves Hibiscus sabdariffa
Appropriate quality assurance procedures and precau-
tions were carried out to ensure reliability of the results.
Double distilled deionized water was used throughout the
study. Glassware was properly cleaned, and the reagents
were of analytical grade. Standard plant reference mate-
rials (DC 73349) from China National Analysis Center for
Iron were analyzed as a part of the quality assurance-
quality control protocol (accuracies within 100% ± 10%).
Blank and drift standards were run after ten determina-
tions to maintain instrument calibration. The coefficient
of variation of replicate analyses was determined for the
measurements to calculate analytical precision.
2.2. Daily Intake of Elements
The estimated daily intake (EDI) of elements depended
on both the element concentration in crops and the amount
of consumption of the respective food crop. In this study,
the daily intake was considered for each edible part of
plants. The estimated daily intake (EDI) of Mg, Ca, K and
Na through edible parts of food crops was calculated
according to the following Eq.1 [8]:
DIMMK I (1)
where [M]: macro-element concentration in plants (mg·kg1),
K: conversion factor used to convert fresh part consumed
of plant weight to dry weight, estimated to 0.085, I: daily
intake of consumed plants in kg. The average daily food
crops intakes for adults and children were considered to
be 0.345 and 0.232 kg/person/day, respectively [9].
2.3. Statistical Analysis
Pearson’s correlation coefficients were calculated and
one-way ANOVA method to test the significant differ-
ences of macro-element content in edible parts of crop
foods regions was employed, the level of significance was
set at p < 0.05. Statistical analyses were performed with
the software XLSTAT (version 2010).
3.1. Metal Concentrations in Edible Parts of
Food Crops
Elemental composition of samples, reported on dry
weight basis, is given in Ta b l e 2 . On comparative basis,
the tubers had less Mg and K levels than the fruits (p <
0.042 and p < 0.049, respectively). Ca and Na presented
no significant differences for the three plant parts. Fur-
thermore, certain edible parts showed interesting element
uptake. Thus, Abelmoschus calei (fruits), Amaranthus
cruentus and green Hibiscus sabdariffa (leaves) had the
highest levels of Mg (5154, 5374 and 4247 mg/kg re-
spectively); Solanum melongena (fruits), Amaranthus
cruentus and green Hibiscus sabdariffa (leaves) had the
highest levels of Ca (15,049, 21,384 and 32,721 mg/kg
respectively); Capsicum frutescens had the highest levels
of K (11,902 mg/kg) green Hibiscus sabdariffa (leaves)
had the highest levels of Na (745 mg/kg). When Gnetum
africanum was the most consumed vegetable in the East
region of Gabon, it was the poorest of food crops studied
because it accumulated the lowest macro-element con-
centrations. The trend of accumulation in the edible parts
of plants was generally: Ca > K > Mg > Na.
Macro-elements are essential minerals required for the
diverse physiological and biochemical functions in the
human body. They are absorbed by human and animals
via foods as plants. The levels of macro-elements and
others minerals in plants are influenced by their interac-
tions and the soil nature and chemical composition [10].
Milk and other dairy products are excellent sources of
calcium, both qualitatively and quantitatively. However,
in some countries, milk consumption is limited due to the
undesirable effects of lactose intolerance [11]. Green lea-
fy vegetables, seeds and legumes are good alternative
sources of calcium, in addition to cow milk and fish with
bones. Thus, Amaranthus cruentus, green Hibiscus sab-
dariffa and Solanum melongena could be considered good
or excellent source of calcium because its concentrations
were higher than 1000 mg/kg [12].
The magnesium chief function in the body includes
bone mineralization, building of proteins, and a magne-
sium deficiency results in osteoporosis [13]. Therefore,
magnesium role with regard to bone quality and nutrition
is really indispensable. In this study, fruits and leaves had
higher Mg concentrations than tubers. Mg, as a constitu-
ent of chlorophyll, is contained in large quantities of green
leafy vegetables. Mainly centered on plant foods, the
African traditional diet appears to have sufficient Mg
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J. A. Ondo et al. / Agricultural Sciences 3 (2012) 697-701
Copyright © 2012 SciRes. OPEN ACCESS
Table 2. Metal concentrations in edible parts of dry food crops (mg/kg).
Mg Ca K Na
Gnetum africanum 257 ± 12 f 1382 ± 310 e 168 ± 10 g 94 ± 6 d
Cymbopogon citratus 562 ± 27 ef 5447 ± 380 d 6652 ± 387 de 479 ± 108 b
Amaranthus cruentus 4248 ± 436 b 21,384 ± 1492 b 10,269 ± 598 ab 471 ± 31 b
Hibiscus sabdariffa (red) 2245 ± 108 c 3248 ± 549 de 7935 ± 462 cd 455 ± 29 b
Hibiscus sabdariffa (green) 5375 ± 258 a 32,722 ± 2283 a 8378 ± 1243 bcd 745 ± 48 a
Manihot esculenta 2235 ± 107 c 2473 ± 430 e 7974 ± 464 cd 436 ± 28 b
Capsicum frutescens 2002 ± 630 cd 3032 ± 212 de 11,903 ± 1649 a 531 ± 63 b
Abelmoschus calei 5155 ± 247 a 2344 ± 164 e 9932 ± 473 f 457 ± 30 b Fruits
Solanum melongena 1270 ± 61 de 15,049 ± 1050 c 4357 ± 254 abc 282 ± 16 c
Dioscorea spp 623 ± 30 ef 4010 ± 280 de 3930 ± 652 f 397 ± 26 bc
Tubers Manihot esculenta 412 ± 62 f 3091 ± 216 de 5407 ± 315 ef 480 ± 31 b
content. However, due the increasing consumption of
fatty and animal foods, plant diet decreases.
The potassium level was > 3000 mg/kg in all samples,
exception of Gnetum africanum. Na level was < 1000
mg/kg in all samples. K is an abundant ion in the plant and
human cells and is required for a wide array of functions,
ranging from the maintenance of electrical potential gra-
dients across cell membranes, to the generation of turgor,
to the activation of numerous enzymes [14], while Na is
the principal ion in the fluid outside of cells. Na and K
together with Cl are electrolytes that maintain normal
fluid balance inside and outside cells and a proper balance
of acid and bases in the body. A large portion of energy in
the body is dedicated to maintaining sodium/potassium
concentration gradients, underscoring the importance of
the balance between sodium and potassium in sustaining
life. Tight control of cell membrane potential is critical for
heart function, as well as nerve impulse transmission and
muscle contraction [15].
Nutrient correlation analysis showed significant rela-
tions between the four elements, exception between Ca
and K (Table 3). Interactions between chemical elements
may be both antagonistic and synergistic, and their im-
balanced reactions may cause a real chemical stress in
plants. These interactions may also refer to the ability of
one element to inhibit or stimulate the absorption of other
elements in plants. Interaction processes are controlled by
several factors and these mechanisms are still poorly
understood [16]. Thus, an antagonistic effect of K on the
Ca content was exerted in the studied edible parts of
plants. This antagonism between Ca and K is quoted in the
literature [17].
3.2. Average Daily Mineral Intake
The levels of the minerals indicated that the local food
crops could be valuable and important contributors in the
Ta b l e 3 . Correlation matrix of accumulated macro-nutrients in
food crops.
Mg Ca K Na
Mg 1
Ca 0.585*** 1
K 0.645*** 0.220 1
Na 0.594*** 0.491** 0.667*** 1
diets of people of Gabon. The mean food crops contribu-
tion of daily intakes and the recommended Intakes of Mg,
Ca, K and Na for both adults and children were presented
in Table 4 [18,19]. The average daily intake was 5.1 -
157.6 mg/day for Mg, 27.2 - 959.6 mg/day for Ca, 3.3 -
349.0 mg/day for K and 1.9 - 21.9 mg/day for Na. In
general, local food crops offered greater nutritional con-
tributions to the recommended intakes of Mg and Ca (2%
- 82% and 3% - 81% of recommended daily intakes, re-
spectively) than K and Na (2% - 8% and 0% - 2% of
recommended daily intakes, respectively). Furthermore,
because of the high recommended daily intakes of Mg
between children and adults, contributions to this element
were more two times for children than for adults.
The mineral composition of food crops in this study
showed that they could be rich sources of macro-nutrients,
and contribute significantly to daily intake of calcium and
magnesium, particularly. International and national health
agencies are working in the world to knowledge the food
composition and developing new standards, but in de-
veloping countries like Gabon these studies are still scarce
or lacking. Thus, more studies could be led in the order to
assess the complete nutritional contribution of principal
eals. m
J. A. Ondo et al. / Agricultural Sciences 3 (2012) 697-701
Table 4. Estimated daily intake of metals by edibles parts of plants of Moanda.
Mg Ca K Na
Children Adults Children Adults Children Adults Children Adults
Gnetum africanum 5.1 7.5 27.2 40.5 3.3 4.9 1.9 2.8
Cymbopogon citratus 11.1 16.5 107.4 159.7 131.2 195.1 9.4 14.0
Hibiscus sabdariffa (red) 44.3 65.8 64.0 95.2 156.5 232.7 9.0 13.3
Hibiscus sabdariffa (green) 106.0 157.6 645.3 959.6 165.2 245.7 14.7 21.9
Amaranthus cruentus 83.8 124.6 421.7 627.1 202.5 301.1 9.3 13.8
Manihot esculenta 44.1 65.6 48.8 72.5 157.2 233.8 8.6 12.8
Capsicum frutescens 39.5 58.7 59.8 88.9 234.7 349.0 10.5 15.6
Abelmoschus calei 101.6 151.2 46.2 68.7 195.9 291.2 9.0 13.4 Fruits
Solanum melongena 25.0 37.2 296.8 441.3 85.9 127.8 5.6 8.3
Dioscorea spp 12.3 18.3 79.1 117.6 77.5 115.2 7.8 11.6
Tubers Manihot esculenta 8.1 12.1 60.9 90.6 106.6 158.6 9.5 14.1
Recommended Intakes for Individual Elements1130.0 420.0 800.0 1300.03800.0 4700.0 1200.01500.0
1: [18,19].
The authors acknowledge Edna Michelle BISSO BI MBA and Agnès
Yolaine MIGUIBA MAROGA for their corrections and critics, and
Jean Félix NDZIME for his technical assistance in laboratory analysis.
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