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ORIGINAL ARTICLE
Year : 2020  |  Volume : 64  |  Issue : 3  |  Page : 295-299  

Are the tribal highlanders protected from hypertension? A meta-analysis on prevalence of hypertension among high altitude tribal population of India


1 Assistant Professor, Department of Community Medicine, ESIC Medical College and Hospital, Faridabad, Haryana, India
2 Professor and Head, Department of Community Medicine, Dr. Rajendra Prasad Government Medical College, Kangra, Himachal Pradesh, India
3 Senior Resident, Department of Community Medicine, Dr. Rajendra Prasad Government Medical College, Kangra, Himachal Pradesh, India

Date of Submission09-Oct-2019
Date of Decision14-Dec-2019
Date of Acceptance18-Jun-2020
Date of Web Publication22-Sep-2020

Correspondence Address:
Sunil Raina
Department of Community Medicine, Dr. Rajendra Prasad Government Medical College, Kangra, Tanda, Himachal Pradesh
India
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/ijph.IJPH_509_19

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   Abstract 


Background: The so-called protected tribal population are also facing burden of noncommunicable diseases. The high altitude tribes are thought to be genetically and environmentally protected from hypertension (HTN) like diseases. Objective: The objective is to estimate the prevalence of HTN among tribes residing at high altitudes (>6000 feet above sea level) of India. Methods: The meta-analysis was undertaken during March to August 2018. National Library of Medicine's PubMed database, and Google scholar were comprehensively searched including search terms such as “blood pressure, hypertension and prevalence” combined with “tribal, tribes, high altitude, India.” Articles on the prevalence of HTN were searched first and then were segregated on the basis of high altitude (>6000 ft). Results: Initially, 69 references and a total of 16 abstracts were screened. After applying the inclusion and exclusion criteria on 16, eight studies were included. Meta-analysis of the prevalence of selected studies resulted in a pooled estimate mean prevalence of HTN among tribal population to be 11.43% (95% confidence interval: 6.72%–17.21%). Conclusion: The role of epidemiological transition needs to be read in the context of social anthropology to identify factors preventing HTN among high altitude tribes.

Keywords: High altitude, hypertension, indigenous tribes, noncommunicable diseases


How to cite this article:
Singh M, Raina S, Goswami S, Raj D. Are the tribal highlanders protected from hypertension? A meta-analysis on prevalence of hypertension among high altitude tribal population of India. Indian J Public Health 2020;64:295-9

How to cite this URL:
Singh M, Raina S, Goswami S, Raj D. Are the tribal highlanders protected from hypertension? A meta-analysis on prevalence of hypertension among high altitude tribal population of India. Indian J Public Health [serial online] 2020 [cited 2020 Oct 30];64:295-9. Available from: https://www.ijph.in/text.asp?2020/64/3/295/295796




   Introduction Top


Hypertension (HTN) has been established as an important entity leading to cardiovascular events and death. The interventions planned focus on lifestyle of general population residing in plains.[1]

Indigenous tribes are described as people who follow traditional nonindustrial lifestyle in areas that they have occupied for generations.[2] In HTN research, tribal populations provide an interesting epidemiological perspective, since studies world over in 20th century have shown that they have a lower prevalence.[3],[4],[5],[6],[7],[8],[9],[10] In India, tribes constitute 8.6% of the total population with an overwhelmingly diverse range of types.[11]

Exposure to hypoxia at high altitude has been recognized as a risk factor for hypertension.[12],[13] World over studies conducted on high altitude tribes of Andes,[14],[15],[16] Pamirs, and Tienshan,[17] Amaras region of ethiopia,[18] Tibetans of India,[19] and rural high altitude natives in Greater Himalayas[20] have shown blood pressure (BP) values of natives to be lower than the residents of plains and low altitude. However, there are evidences from Tibetan high landers and high altitude natives of Saudi Arabia where higher prevalence of hypertension (HTN) as compared to low land population.[21],[22],[23],[24] Hence, there are two theories. One where the low prevalence is attributed to chronic hypoxia stress (hypobaric hypoxia) due to genetic adaptation and the other where higher prevalence introduces the effect of acculturation, changing lifestyle, and better socioeconomic status.[25]

These indigenous nomadic scattered populations have always intrigued the researchers whether it is their lifestyle or genetic makeup that makes them different from the general population. Answer to these questions may pave way in preserving the health of our indigenous population. The lifestyle especially diet related differences in these tribes; if are protective for emergence of lifestyle diseases can be adopted in planning interventions for general population.

This review and meta-analysis were conducted to observe the prevalence of HTN among tribes residing at high altitudes (>6000 feet above sea level) of India.


   Materials and Methods Top


Search strategy

We comprehensively searched National Library of Medicine's PubMed database, and Google scholar from March to August 2018. We used combinations of medical subject headings (MESH) and free text words that included search terms such as BP, hypertension, and prevalence. These were combined with search terms related to the tribal, tribes, high altitude, India. The filters included were: English for the language category and humans for the study category. We identified articles eligible for further review by performing an initial screen of identified titles or abstracts, followed by a full-text review [Annexures 1].



Selection criteria

Articles included in the review were cross-sectional, cohort, and any published study till date. Articles on the prevalence of hypertension were searched first and then were segregated on the basis of high altitude. Further “tribal population” was added in the MESH to search hypothesized outcome (HTN) in the particular group. Surveys conducted in areas on and above 6000 feet (above sea level) were included. As per the lake Louis criteria a height above 1500 m (5000 feet) is considered high altitude. However, we chose a higher cut off of 6000 feet based on the fact acute mountain sickness (a common negative health effect of high altitude) invariably starts affecting any visitor sleeping higher than 6000 feet and above.[26],[27]

Majority of the studies included used JNC VII criteria[28] to define HTN; systolic BP (SBP) more than or equal to 140 mm of Hg and or diastolic BP (DBP) more than or equal to 90 mmHg. However, two articles used the criteria of SBP ≥160 mm of Hg and or DBP ≥95 mm of Hg. Articles were excluded if they were letters, abstracts, conference proceedings, case series, case–control studies, reviews, and meta-analysis; not conducted on humans and not community-based studies.

Study selection

Two independent reviewers screened the titles and abstracts of the initially identified studies to determine whether they would satisfy the selection criteria. Any disagreements about selection were resolved through consensus or consultation with another reviewer. Full-text articles were retrieved for the selected titles. Reference lists of the retrieved articles were searched for additional publications. The retrieved studies were assessed again by one of the researcher to ensure that they satisfied the inclusion criteria.

Quality assessment and data extraction

A critical appraisal check list with few modifications was used for each article to assess quality by two authors [Annexures 2]. This was used by the reviewers to extract the relevant information from the selected studies. The data collection form included questions on year of publication, design, study setting, geographic origin, inclusion and exclusion criteria, response rate, information on the reported variables, i.e., percentage of population with hypertension [Annexures 3].[20],[25],[29],[30],[31],[32],[33],[34]



Statistical analysis

All statistical analysis was done using MedCalc statistical software version 16.4.3 (MedCalc Software bv, Ostend, Belgium).[35] Meta-analysis was conducted using random effects model to study the numerical estimate of the effect of interest that is the prevalence of hypertension. I2 statistic was also calculated to observe total variation across the studies and a forest plot was constructed with 95% confidence interval (CI) of the prevalence. Random effects meta regression analysis was applied to determine role of covariates in heterogeneity using Comprehensive Meta-Analysis software version 3 (Biostat inc. NJ, USA).[36]


   Results Top


The PubMed and Google scholar search with different Boolean combinations initially identified 69 references and a total of 16 abstracts were screened. Following the inclusion criteria only eight surveys were included in our review. The excluded articles included following: did not mention tribal population (1), BP was not categorized into hypertension (3), low and medium altitude areas were studied (3), and only abstract available (1) [Figure 1].
Figure 1: Flow chart showing selection of studies.

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[Table 1] summarizes the major characteristics of studies. All except one (Jammu and Kashmir [J&K]) study was conducted among high altitude tribes of Himachal Pradesh (HP). All of the studies used random sampling strategy except Dasgupta et al., where house-to-house survey was conducted in Pooh village of Kinnaur district of HP.[20] Five of these included adults equal to and above 20 years of age, two included more than 14 years of age group and the national survey for a district included adults between 15 and 49 years of age group. Except for the national family health survey-district factsheet of Lahaul and Spiti (NFHS-4)[31] which included maximum age of 49 years; all other studies included older population. All studies provided sex wise prevalence of hypertension except one (that is Negi et al.).[25] Age-wise prevalence was reported by all except Negi et al.,[25] Dasgupta et al.,[20] and NFHS-4 survey (L&S).[31]
Table 1: Characteristics of studies included

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The prevalence of HTN was lowest (1.9%) among tribes of Pooh, Kinnaur (HP) as Dasgupta et al., used a higher cut off to label hypertension.[20] The prevalence in high altitude tribes of Chamba (HP)[33] and L&S[31] ranged from 10.5% to 12.2%. A higher prevalence (22.5%) was reported from Spiti valley (HP)[25] and even higher (27.7%) among nomads of Ladakh (J&K).[31] Norboo et al.,[31] and Negi et al.,[25] have also presented altitude-wise prevalence of HTN. The altitude-wise prevalence from Ladakh region reported increase in the prevalence of HTN as the altitude increased. The prevalence of HTN was lowest among nomads of Ladakh at altitude of 4000–4499 m (22.4%) and this increased to 29% at altitude more than 4500 m. The prevalence was highest among farmers of the region.[32] The prevalence of hypertension in Spiti valley was significantly lower among residents of higher altitude as compared to lower ones (19.0% vs. 27.5%, P < 0.041).[25]

The pooled estimate for mean prevalence of hypertension among adult tribal populations of India was 11.43%, 95% CI: 6.72%–17.21% [Table 2] and [Figure 2]. There was significant heterogeneity between the studies (I2 = 98.35% and Cochran's Q = 424.94, df = 7, P < 0.0001). I2 denotes the percentage of observed total variation across studies due to real heterogeneity rather than chance. A higher value in this meta-analysis denotes higher heterogeneity between studies. Random effects meta regression analysis was applied to determine role of altitude and year of study in heterogeneity. Regression coefficient was 0.0001 and statically insignificant (P = 0.38) for altitude as covariate and a significant effect of year of study as covariate (coefficient: 0.0474, P = 0.0002) was observed. A substantial heterogeneity persisted with I2 = 94.97% (Q = 99.44, P < 0.001).
Table 2: Random effects mean percentage of hypertension prevalence

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Figure 2: Forest plot of prevalence of hypertension with 95% confidence interval.

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   Discussion Top


The tribes residing in high altitude live in relative cultural and geographical isolation. This isolation ensures a continuity of ancestral traditional custom, belief, and myth. The prevalence of hypertension among adults in India as per district level household survey-IV (2015–2016) was 25.3%.[37] In a meta-analysis of studies conducted among adult tribal population of India (1982–2011) the pooled prevalence of HTN was 16.1% (95% CI: 13.5%–19.2%).[1] The current analysis on high altitude tribal population gave a pooled estimate of the prevalence of 11.43%.

Therefore, as per the current analysis, the prevalence of hypertension among tribals at high altitude is lower not only compared to population residing in plains but also to nonhill tribals. The hypobaric hypoxia at high altitude may be a reason for lower prevalence. However, can the lower prevalence be entirely explained by this single phenomenon? Natural stressors at high altitude have been driving the homeostatic processes to evolve by natural selection to produce genetic adaptations and thereby acting as protective factors against development of hypertension. Greater autonomic or sympathetic activities at high altitude have also been hypothesized as cause of increase BP.[38]

The studies on Andean and Tibetan highlanders have also discussed the role of natural selection and culture in biological adaptation. Beall et al. hypothesized that there is difference in natives of high altitude in different parts of the world due to their different genetic bases.[39] Furthermore, the role of heritable biological adaptability is not applicable to all the tribes in high altitude.[40] Lewis et al., in their study on conduit artery structure in high and lowlanders concluded that natives to high altitude are at greater risk of advanced vascular aging, due to impairment in endothelial and smooth muscle function. Hence, lifelong adaption to high altitude does not appear to provide any functional cardio-protective benefits.[41]

Beyond that, the unique lifestyle led by indigenous people (specifically their diet) needs attention. The role of diet and lifestyles assumes significance in view of an increase in prevalence of hypertension observed in last few decades among tribal's at high altitude.[30] Studies conducted among tribes of South America, Malaysia, and Africa have suggested that population undergoing cultural changes due to acculturation has recorded prevalence in the range of 10%–35%.[1] With the advanced modernization and the benefits of same reaching the unreached, the tribal at high altitude are also recording change in diet and lifestyle, hitherto potential protective factors against development of hypertension. Importantly, the prevalence of obesity and diabetes is less in the higher altitude population suggesting likely differences in the level of physical activity. The higher physical activity index may have protective effect independent of body mass index (BMI) which needs to be explored before concluding the protective effects of hypobaric hypoxia induced vascular adapted responses as a mechanism for lower prevalence of hypertension in high altitude tribes. Studies from across the world and conducted in different populations have also shown association of urbanization, increase in age, socioeconomic status, waist circumference, and BMI with increase in the prevalence of hypertension.[30]

This review and meta-analysis might have some limitations too. The included studies were all cross sectional, and therefore not ideal for establishing causality. The higher heterogeneity between studies was observed could have existed due to difference in sampling strategy, difference in cut off value and other confounders.


   Conclusion Top


While extrapolating potential protective factors for the occurrence of hypertension among high altitude tribals, research needs to focus beyond genetics. There is an urgent need to understand the role of epidemiological transition among the so called protected tribal population. Expanding role of social anthropology to understand benefits if any of folklore associated health beliefs among tribal's at high altitude may open up a window of opportunity in understanding diseases like hypertension.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
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