Background
Chronic kidney disease of unknown aetiology (CKDu) has been reported in developing countries including Nicaragua, countries of Balkan region, Tunisia and Sri Lanka [
1‐
3]. CKDu has been reported from certain parts of Sri Lanka, including the North Central, North Western and Uva provinces and the prevalence is highest in the North Central Province. The affected areas belong to the dry and intermediate climatic zones of Sri Lanka. The majority of the CKDu affected inhabitants in these areas, belong to the low –socioeconomic farming community. The prior studies indicated the affected populations to predominantly be young males, whilst the more recent studies have shown high prevalence among females and more severe stages of CKDu among males [
4,
5].
Numerous studies of diverse disciplines have been carried out to reveal the credible contributory factors for the disease. The prevalence of CKDu, epidemiological studies and environmental risk factors were meticulously scrutinized in these studies. The scientific correlation between the incidence of CKDu and the suggested risk factors still remain detached.
Urinary β
2 microglobulin (β
2m) has been used as a marker of heavy metal poisoning that includes cadmium, arsenic and lead [
6]. Current research conducted on CKDu in Sri Lanka suggests heavy metals as the major aetiological factor and food chains as the possible route of heavy metal entry into the bodies of those afflicted by the disease [
5,
7]. When the endemic areas are concerned, non-affected individuals reside in the same vicinity and household as affected for long periods of time without being affected by CKDu and studying different aspects of lifestyle, occupations, dietary patterns etc. of these two groups would be much intriguing. Main objectives of the current study were to compare the variation in urinary β
2m excretion of CKDu patients and non-CKDu subjects, inhabiting in a CKDu endemic area, and to compare the dietary patterns of CKDu patients and non-CKDu subjects from the same area.
Discussion
Mean urinary β2m level of the CKDu patients of Medawachchiya was approximately 7 times higher than that of the non-CKDu subjects. The CKDu patients were medically confirmed to be otherwise normal. Almost all β
2m filtered by the glomerular capillary membrane is reabsorbed and catabolized by the cells of the proximal tubules [
10]. Therefore the elevated urinary β
2m levels are confirmative of the tubular damage reported in the biopsy reports of CKDu patients indicating proximal tubular damage. Studies conducted in the Balkan region, where a similar clinical scenario to that of CKDu exists, have suggested urinary β
2m as an early maker to differentiate Balkan endemic nephropathy from other forms of nephropathies, as healthy individuals from the same situ have demonstrated elevated urinary β
2m levels [
11,
12]. In the current study, the urinary mean β
2m levels of the non-CKDu subjects was within the normal reference limits (0–0.3 μg/mL) for spot urine samples (Table
1), indicating normal tubular functions despite the normal subjects living under the equivalent environmental conditions as CKDu patients. The mean urinary β
2m levels of CKDu patients when compared against the stage of their kidney disease gradually increased with the severity of kidney damage (Table
2), probably owing to amplified tubular damage.
Chronic exposure to heavy metals is indicated as the possible cause of CKDu in Sri Lanka by most recent studies [
5,
7]. Urinary level of β
2m is increased in heavy metal toxicities causing tubular damages [
13]. The elevated urinary β
2m values obtained in the current study is supportive of the aforementioned findings. Prevailing high cost of the β
2m test has limited it being used as a biomarker in the diagnosis or monitoring of nephropathies in the local setup.
Food consumption study revealed that the majority of the subjects, among both CKDu patients and non-CKDu subjects consumed rice, for all three meals. The variety of rice, the majority consumed, was mostly white raw rice. Red raw rice and parboiled rice, were also eaten, but to a lesser extent by both groups. Out of the tested meals, approximately 91.0% meals of CKDu patients, and 95.8% meals of the non-CKDu subjects, included rice as the staple food. Further, more than 90% of CKDu patients and non-CKDu subjects indicated that rice consumed by them is grown in their own paddy fields. Their preference for white raw rice was linked to its trouble-free availability. Rice grown in their own paddy fields is harvested and milled in local rice mills. The convenience in processing, avoiding parboiling efforts had made them consume, white raw rice over many years, perhaps throughout their lifetime. When the percentages of main meals, including different rice varieties were compared, between the two groups, the pattern of variation was almost the same. There were slight differences between the percentages of other food categories eaten as staple foods, including bread, wheat and rice flour products. Consumption of rice was significantly high (p < 0.05) when compared with the reference food bread and wheat flour products, but the percentage of meals of non-CKDu subjects (96.3%) recorded with rice and rice flour products were higher than that of CKDu patients (91.6%) (Table
4). Therefore, it could be interpreted that rice is less likely to contribute towards the development of CKDu. The low consumption of rice and rice flour products by the CKDu patients compared to non-CKDu subjects may be attributable to loss of appetite in these patients as a result of accumulating uremic substances in blood in the latter stages of CKDu. The percentages of reported main meals with millet and pulses and seeds with CKDu patients and normal subjects were significantly less than that of the reference food. Therefore, their consumption could not be tied up with the development of CKDu.
There’s a hypothesis that heavy metals in rice, although not present at toxic levels, may also contribute towards the etiology of CKDu, when consumed over a long period of time, due to its accumulation in the body [
5,
7]. However, the frequency of rice consumption between CKDu patients and normal subjects in this study were approximately equal. Hence, evidence to support the above hypothesis is limited from the current study. However, a detailed quantitative analysis of food intake may be necessary, before coming to a definite conclusion on this matter. There might be individual variations in the amount of rice consumed. Generally males consume more food than females, especially those males engaged in manual labour, like farming. High incidence of severe stages of CKDu reported among males [
5] could be assumed to be due to their higher consumption. But the unaffected normal subjects consuming rice grown in the same area remains questionable. Though rice alone might not be a contributory factor towards the development of CKDu, long term consumption of contaminated rice with other risk factors including life style, exposure to environmental toxins, genetic susceptibility and selenium deficiency could be augmenting the incidence in certain individuals while sparing others. Analysis of rice consumed by CKDu patients and non-CKDu subjects, for heavy metals, combined with the quantitative analysis of rice intake, would provide a solid background for assessing the effect of heavy metals ingested through rice, on CKDu. The newly introduced hybrid varieties of rice: like the common white raw rice eaten is the 3.5 months variety and red rice is the 3.0 months variety; are harvested within a short period of time. If rice is a contributor towards the etiology of CKDu, then a longer time taken for harvesting may increase the risk of promoting such a condition.
With the objective of finding any significant variation among the consumption of locally grown food items, by CKDu patients and normal subjects, they were questioned about consumption of these items. Green gram, cowpea like pulses and a wide variety of other vegetables were found to be grown by some on a small scale and by a few on a larger scale. Certain food accompaniments like jack fruit, siyambala (Tamarindus indica) and elabatu (Solanum melongena) were grown in and around their compounds.
In assessing the effect of accompaniments of the main meals on CKDu, other vegetable accompaniments which consisted of roots and tubers which were not grown locally were considered as the reference accompaniment. The most commonly used main meal accompaniment (Table
5), was animal sources in both groups. It included varieties of fish, dry fish, meat and eggs and the recorded number of meals with animal sources was higher with non-CKDu subjects than CKDu patients. This indicates that the effects of accompaniments are more favorable to healthy living than being a risk factor. Though the CKDu patients have been advised to include lesser amount of protein via animal sources in their meals the highest recorded accompaniment in the tested meals were animal sources. However, the consumption was significantly lower than that of the normal subjects and this could be due to facts including CKDu patients following the medical advice and lowering their consumption of animal sources and their appetite being affected by the disease.
When consumption of fruit vegetables was considered, the number of meals reported with CKDu patients was higher than that of non-CKDu subjects and the difference was statistically significant (p < 0.05). Most of the fruit vegetables were grown locally and therefore there is a possibility of them acting as sources containing nephrotoxic agent/s. Analysis of these food items consumed by CKDu patients for possible nephrotoxic agents would enlighten this qualm.
The percentages of meals reported with pulses and products as an accompaniment were almost equal in the two groups. Therefore, though a statistical significance is shown when compared with the reference, a possible risk association could not be justified. Further, out of the pulses and products, lentil was the major one consumed and this was not grown locally.
The meals reported with green leaves were significantly low (p < 0.05) when compared with the reference food, which included other vegetable accompaniments. Therefore, though almost all green leaves were locally grown, a definite relationship with CKD cannot be established. The food items like storage rhizomes of of
N. nucifera (Nelumbonaceae), is highly available in this region and reported to contain high concentrations of Cd [
14]. This study hypothesized that including these rhizomes regularly in the diet, may be a predisposing factor for CKDu. However, none of the tested meals, of both CKDu patients and non-CKDu subjects, included rhizomes of
N. nucifera as an accompaniment. Though food items like these are much available in the region inhabitants may not be consuming them regularly but exporting to the other parts of the country.
There were limitations encountered in the dietary study. Generally almost all of the subjects consumed 3 main meals per day. However, some were found to have skipped a meal, or taken an extra meal, on the day the dietary survey was conducted. Further, few of the subjects in the normal group, commented that they could not recall what was included in their meals on the previous day. Sometimes second and third visits had to be made, to collect dietary recall data, as on some visits, the study participants were not available. Sri Lankan staple diet is a rice based mixed diet which generally includes rice as the major component and vegetables, green leaves, animal sources etc. in smaller amounts as accompaniments. The choice for accompaniments and the amount of accompaniments consumed by an individual may vary by factors including socioeconomical background and local availability of food. The current study analyzed only the qualitative aspect of the diet of the tested groups, by means of assessing the food types consumed by these populations and number time each food type was reported in the tested meals. A quantitative analysis of the food items consumed by CKDu affected and normal people of the same area would have provided more detail as to quantitative variations among the two groups. The practical hitches as unavailability of food composition tables for cooked local food items, the subjects not been inward patients and their poor compliance in measuring their own food intake at home and training a literate family member for accurate measurement of foods consumed limit the implementing of a quantitative dietary study in the selected populations. The appetite for food could get lessen by the disease among CKDu patients and the current study lacks tracking such defects among the subjects.
There were strengths associated with this type of the dietary study, when repeated 24-hour recalls were made on 3 to 7 occasions [
15]. The present dietary study was repeated at 6 month intervals on 3 occasions. The entire dietary study was extended over a period of one and half years, to include all CKDu patients and non-CKDu subjects, and also allowing the capturing of any seasonal or individual variations, that could affect the dietary patterns of the study population.
In conclusion, the types of staple food and their consumption patterns among CKDu patients and non-CKDu subjects were found to be similar. However, dietary studies have not been reported up to date, on CKDu affected populations, in Sri Lanka. Hence, in-depth dietary studies which include the quantitative analysis of dietary intake, total energy and protein intake of the CKDu affected and non-CKDu affected subjects and testing of associated metals in the foods they consume are required. Multifactorial studies assessing all the risk factors associated with CKDu comprehensively and simultaneously would shed more light in revealing in exact mechanism of causation of the disease.
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
EARIES, PAJP, DBN and RS designed the study, TA performed clinical investigations and selected cases and controls, EARIES performed laboratory investigations, EARIES acquired data, EARIES and KGADW performed the statistical analysis and interpreted the data, EARIES drafted the manuscript, PAJP, DBN, RS revised the manuscript for important intellectual content. PAJP, DBN, RS, TA, EARIES, KGADW read and gave final approval of the version to be published.