Introduction

Several herbal medicines have been produced in the world for the treatment of different diseases caused in human [1,2,3]. The general belief that these herbal products are without any adverse side effects has fostered the use of herbal medicine for the treatment of various ailments [2, 3]. Herbal plants have also been found to nourish the body and provide vitamins, minerals, and many trace elements that are easy to absorb [3]. Herbal plants are known to contain a greater amount of trace elements than other plants. These elements are required by living beings for numerous biological and physiological processes that are necessary for the maintenance of good health for plants and animals [4,5,6,7,8,9,10].

According to several literatures, in order to optimize healthy digestions, there are a number of conventional drugs available for the treatment of gastrointestinal diseases. However, the clinical evaluation of several conventional drugs has shown considerable side effects [5, 6]. Therefore, relevant research investigations are needed to get drugs which are effective with less side effects.

Recent preclinical studies show that commonly used medicinal plants are the most promising in preventing various gastrointestinal ailments, but detailed investigations are required in order to bridge the gaps in the existing knowledge of these plants [7, 11]. WHO reported that more than 50% of world population still use medicinal plant preparation for gastrointestinal disorder and gets a symptomatic relief and improvement in the physiologic function of the gastrointestinal diseases [8, 12]. According to WHO report, the use of herbal medicinal products is expected to contribute insignificant exposure of heavy metal contaminants to the population due to rarely and limited intake of traditional medicine [12].

There is some information about Ethiopian plant species focussing on their corresponding medicinal applications [13, 14]. However, there is no study of multi elemental analysis of medicinal plants used for the treatment of gastrointestinal diseases. Therefore, the aim of this study is on the elemental composition of medicinal plants used for the treatment of gastrointestinal diseases and their impact on human health. Some of the medicinal plants considered as effective for the treatment of frequently observed diseases in the Ethiopia have been discussed [13,14,15]. The herbal plants, shown in Table 1, are among these herbs utilized for some GI disease treatment by the traditional healers [15,16,17].

Table 1 Details of traditional medicinal plants and gastrointestinal diseases [13,14,15,16,17]
Full size table

Materials and Methods

Materials

Sample preparation and irradiation of samples were done at Centre for Energy Research and Training (CERT), Ahmadu Bello University, using Nigeria Research Reactor (NIRR-1). It is a pool type Miniature Neutron Source Reactor (MNSR) with a nominal thermal power of 31 kW, a maximum thermal neutron flux of 1 × 1012 n/m2 s with pneumatic rabbit system is installed for safe sample transportation for irradiation processes. The gamma-spectrometry technique was applied using a high-purity germanium (HPGe) detector which is coupled to the integrated digital gamma-ray spectrometer hardware DSPEC jr 2.0, and dedicated software (Maestro-32 and Winspan-2004).

Sampling and Sample Preparation

Six healthy medicinal herbal plant samples were identified and collected from five Kebeles of Sidama and Gedeo zones using systematic random sampling technique. The samples were packed in polyethylene bags and taken to sample preparation laboratory. The raw sample was washed thoroughly by tap water and distilled water subsequently. The washed samples were dried using an air-oven at 45 °C for 24 h. Each sample collected were turned into different pre-washed metal sheet and mixed manually. The samples, therefore, homogenized using quartering technique manually until a total of 16 homogenized sub-samples were obtained. Finally, four sub-samples, one for each medicinal plant, were randomly selected among homogenized sub-samples then put in polyethylene bag and sealed.

All sub-samples were brought to the NAA sample preparation laboratory of Centre for Energy Research and Training (CERT), Ahmadu Bello University, Zaria Nigeria. These sub-samples were pre-weighed using four-digit electronic balance and put into a vacuum-oven at 65 °C about 1 h then weighed. The processes continued until constant mass was obtained. A certified reference material, NIST SRM-1515(Apple leaves), was prepared in a similar process. Materials needed for the preparation of samples, polyethylene bags and plastic vials, were pre-cleaned by socking them into HNO3 dilute acid.

The dried sub-samples were crushed and powdered using a pre-cleaned standard agate mortar and pestle until their matrix resembles the standard reference material. In order to avoid any cross contamination, in between two consecutive samples, the mortar and pestle were washed by clean tap water, detergent, distilled water, and acetone reagent. For short and long irradiation purposes, the powdered medicinal samples and SRM sample weighing about 250 mg were put into pre-cleaned polyethylene bag and sealed by standard heat sealer. They were coded based on their type and customer reference number given at CERT to avoid errors. Finally, samples put into a pre-cleaned plastic vials then covered by clean Wool (to avoid any miss-locations in the irradiation process) and tightly covered by a sticky cello-tape.

Standardization and Measurement

The qualitative and quantitative elemental analysis of these herbal plants was done by using comparative INAA technique. In this method, the samples do not undergo any chemical treatment, neither prior nor after the activation; it provides easy sample preparation, and it is the fastest method for the determination of multielement composition and concentrations [18]. The medicinal plants and SRM samples were subjected to same irradiation and gamma ray spectrometry protocols.

The concentration of an element can be obtained from the relation:

$$ {C}_x=\frac{C_{st}\left({m}_{st}{A}_{\left(c0,x\right)}\right)}{\left({m}_x{A}_{\left(c0, st\right)}\right)}.K $$

where mx and mst is unknown and standard material samples mass, respectively; C is the concentration of an element in ppm; K is the ratio factor of various parameters of sample and standard analyte; and Ac0 is the decay-corrected counting rate for indicator gamma-ray of the samples, which is given by:

$$ {A}_{c0}=\frac{N_c.\lambda {e}^{-\lambda {t}_d}}{\left(1-{e}^{-\lambda {t}_c}\right)} $$

where Nc is the number of counts in the indicator gamma-ray peak.

The experimental parameters are mass of samples, gamma intensity of each radionuclide, irradiation time, cooling time, and counting time. Energy and full-energy peak efficiency calibration of the detector was done for a qualitative and quantitative INAA analysis purposes. The reactor neutron flux parameters, standardization of irradiation process, data accusation, INAA calibration, and analysis process were taken from the previous work of Jonah et al. [19, 20]. The irradiation and counting process in NIRR-1 facility is shown in Table 2.

Table 2 The adopted irradiation and analysis schemes for multielement determination in NIRR-1
Full size table

All data files were subjected to calculations with necessary corrections with background subtraction, and error propagation rules were applied in final results at each stage of the calculations to determine overall combined uncertainty for measurements considering uncertainties in peak area, background, weighing, balance calibration, and uncertainties in certified values of reference materials (RMs) used for calibration.

For the purpose of validation of the analytical method, comparative INAA, the standard reference material, NIST SRM-1515(Apple leaves), were measured with the samples. The measured value of each element was compared with the certified elemental concentration, and the accuracy can be expressed by the standardized deviation, Z, between certified and measured values using the relation:

$$ Z=\frac{C_m-{C}_{cv}}{\sqrt{\delta_m^2+{\delta}_{cv}^2}} $$

where cm and ccv implies the measured and certified values, respectively.

Therefore, a total of 17 elements were measured with good accuracy as shown in Table 3.

Table 3 Elemental concentration comparison of NIST SRM-1515 (mean ± SD; n = 2) in ppm
Full size table

Results

The results of the same 17 elements found in the standard reference material were analyzed in all medicinal plants. Among analyzed elements, Ca, Mg, Cl, and K were measured as major minerals whereas Al, Na, and Fe were found to be minor minerals, and the rest minerals were found in trace levels as shown in Table 4. Out of the 17 elements determined in the SRM, V, Ba, and Eu were below detection limit (BDL). The detection limits for the experimental facilities and protocols used in this work have been determined in [20].

Table 4 Elemental concentration of six traditional medicinal plants in ppm
Full size table

The quantity of element ingested due to the use of these herbal medicines depends on the method of preparation by the traditional healer. Therefore, in order to assess the adverse effects related to the toxicity of the use of the herbal medicines, estimated daily intake (EDI) of elements in mg/day were calculated and compared with the tolerable limit set by international organization such as WHO/FAO. Considering that a 10 g intake of the medicinal plant is recommended, then we can calculate the estimated daily intake per person of the constituent elements [21], as shown in Table 5

Table 5 Comparisons of estimated daily intake (EDI) value with UL [22,23,24]
Full size table

Discussions

The metabolism of heavy metals is regulated by numerous factors such as the availability of nutrients with regard to the integrity of the gastrointestinal tract and on liver function. The potential of an element to be absorbed within the gastrointestinal tract depends on its chemical speciation at the absorption site. Therefore, based on their metabolic and natural properties of some metals, they can be utilized for the preparation of several metal-based drugs in modern laboratories [7, 25, 26].

Some heavy metals such as Al, Ca, Mg, and Na compounds are used for the treatment of some gastrointestinal diseases in conventional drugs [6, 11, 27]. Aluminum is one of those metals present in pharmaceuticals used for the treatment of a number of diseases. Some study shows that Helicobacter pylori-associated gastritis treatments are effective using the combination of metal salts containing aluminum [26,27,28,29].

The concentration of Al metal in medicinal plants measured from 217 to 570.1 ppm. It has been considered as an essential for some Al-resistant plants but has no function for human body; instead, it results in adverse effect when ingested beyond upper tolerable limit.

Calcium, magnesium, and sodium are essential minerals obtained from various diets, and they have several essential human body functions within a recommended dietary intake. However, there are adverse effects in gastrointestinal tissues associated with their deficiency and interaction with other minerals [5, 6, 11]. Among these minerals, magnesium was not measured in lemon grass (Cymbopogon citrates) and Ruta chaiepensis. On the contrary, higher concentration was measured in Leonotis ocymifolia (4080 ppm). High concentration of Ca and Na were measured in Taverniera abyssinica (an endemic herb of Ethiopia) widely known as effective herb for the treatment of fever and stomach complaints.

Manganese mineral is beneficial in several catalytic enzymes activities of human body systems. In addition, the therapeutic role of manganese compounds is a good reliever from inflammatory pain, gastrointestinal dysfunction associated with alcohol, and Helicobacter pylori diseases [30, 31]. Among the medicinal plants, highest Mn concentration was measured in Leonotis ocymifolia (198 ± 3 ppm), the but low concentration was found in Taverniera abyssinica (18 ± 1 ppm).

All the medicinal plant samples have trace levels of Zn concentration except Dill (Anethum graveolens). The remaining herbal plants were found to have appreciable zinc concentrations. The seed of Dill is traditionally utilized for most of gastrointestinal diseases as mentioned in the Table 1. There are some literature evidences on the zinc metabolism and gastrointestinal diseases [30]. The reduced Zn levels in the blood correlated with inflammatory bowel disease (IBD) and/or active ulcerative colitis [31]. Therefore, adequate Zn supplement increases the anti-gastrointestinal diseases activities such as stimulation of mucosal regeneration, enhances mucosal resistance to acids, stabilizes plasma membranes, and has antioxidant properties [8, 9].

According to Wu et al., increased concentrations of various elements such as Fe and K were independently associated with gastric cancer [31,32,33]. For instance, the metabolism of Fe results in high deposition of iron in gastrointestinal tissues like stomach and liver [6, 11]. The elevated potassium levels were related to the presence of lymphatic duct metastasis [32, 33]. Among the medicinal plants, Leonotis ocymifolia has the highest Fe (701 ± 46 ppm) concentrations followed by lemon grass. The concentration of potassium in all medicinal plant samples are the highest ranging from 7097 ppm in Taverniera abyssinica to 50,350 ppm in Leonotis ocymifolia.

The remaining elements determined in this work, such as Cl, Br, La, Sc, Sm, and Rb, were either found in trace level or below the detection limit. Among these elements, only Cl is useful for human body functions. The lower concentrations of these elements measured in the traditional medicinal plants have no significant adverse effect to human health [24].

Since there is no information about the multielement composition of indigenous medicinal plants used for gastrointestinal diseases, the present work can provide scientific evidence about the concentration of elements and associated impacts in human health. Moreover, the work can be used as a database of essential and nonessential elements in these herbal plants in particular.

Conclusion

A qualitative and quantitative investigation of major, minor, and trace elements of six medicinal plants, traditionally used for the treatment of gastrointestinal disease, was analyzed using INAA technique. The result of the analysis shows that major elements like Mg, Ca, and K concentration were found to be present in Withania somnifera. Furthermore, the concentrations of essential elements like Mn, Fe, and Zn were found to be highest in Leonotis ocymifolia followed by Anethum graveolens and Cymbopogon citrates, respectively. As can be seen, the average estimated daily intake of each element in the medicinal plants was found to be below the upper tolerable limit. The results obtained are informative and can support therapeutic usage, and also can let us to understand the mechanisms and interactions of some chemical compounds related to pharmacology and traditional medicine.