Population variation and differences in serum leptin independent of adiposity: a comparison of Ache Amerindian men of Paraguay and lean American male distance runners.

Abstract

Serum leptin variation is commonly associated with fat percentage (%), body mass index (BMI), and activity. In this investigation, we report population differences in mean leptin levels in healthy men as well as associations with fat % and BMI that are independent of these factors and reflect likely variation resulting from chronic environmental conditions.
Serum leptin levels, fat %, and BMI were compared between lean American distance runners and healthy Ache Native Americans of Paraguay. Mean levels were compared as were the regressions between fat %, BMI, and leptin. Comparisons were performed between male American distance runners (n = 13, mean age 32.2 +/- 9.2 SD) and highly active male New World indigenous population (Ache of Paraguay, n = 20, mean age 32.8 +/- 9.2) in order to determine whether significant population variation in leptin is evident in physically active populations living under different ecological circumstances independent of adiposity and BMI.
While the Ache were hypothesized to exhibit higher leptin due to significantly greater adiposity (fat %, Ache 17.9 +/- 1.8 SD; runners 9.7 +/- 3.2, p < 0.0001), leptin levels were nonetheless significantly higher in American runners (Ache 1.13 ng/ml +/- 0.38 SD; runners 2.19 +/- 1.15; p < 0.007). Significant differences in the association between leptin and fat % was also evident between Ache and runner men. Although fat % was significantly related with leptin in runners (r = 0.90, p < 0.0001) fat % was negatively related in Ache men (r = -0.50, p < 0.03).
These results illustrate that chronic ecological conditions in addition to activity are likely factors that contribute to population variation in leptin levels and physiology. Population variation independent of adiposity should be considered to be an important source of variation, especially in light of ethnic and population differences in the incidence and etiology of obesity, diabetes, and other metabolic conditions.

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BioMed Central
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Nutrition & Metabolism
Open Access
Research
Population variation and differences in serum leptin independent of
adiposity: a comparison of Ache Amerindian men of Paraguay and
lean American male distance runners
Richard G Bribiescas*1 and Matthew S Hickey2
Address: 1Reproductive Ecology Laboratory, Department of Anthropology, Yale University, New Haven, CT 06520–8277, USA and 2Department
of Health and Exercise Science, 220 Moby-B Complex, Colorado State University, Fort Collins, CO 80523–1582, USA
Email: Richard G Bribiescas* - richard.bribiescas@yale.edu; Matthew S Hickey - Matthew.Hickey@ColoState.edu
* Corresponding author
Abstract
Background: Serum leptin variation is commonly associated with fat percentage (%), body mass
index (BMI), and activity. In this investigation, we report population differences in mean leptin levels
in healthy men as well as associations with fat % and BMI that are independent of these factors and
reflect likely variation resulting from chronic environmental conditions.
Methods: Serum leptin levels, fat %, and BMI were compared between lean American distance
runners and healthy Ache Native Americans of Paraguay. Mean levels were compared as were the
regressions between fat %, BMI, and leptin. Comparisons were performed between male American
distance runners (n = 13, mean age 32.2 ± 9.2 SD) and highly active male New World indigenous
population (Ache of Paraguay, n = 20, mean age 32.8 ± 9.2) in order to determine whether
significant population variation in leptin is evident in physically active populations living under
different ecological circumstances independent of adiposity and BMI.
Results: While the Ache were hypothesized to exhibit higher leptin due to significantly greater
adiposity (fat %, Ache 17.9 ± 1.8 SD; runners 9.7 ± 3.2, p < 0.0001), leptin levels were nonetheless
significantly higher in American runners (Ache 1.13 ng/ml ± 0.38 SD; runners 2.19 ± 1.15; p <
0.007). Significant differences in the association between leptin and fat % was also evident between
Ache and runner men. Although fat % was significantly related with leptin in runners (r = 0.90, p <
0.0001) fat % was negatively related in Ache men (r = -0.50, p < 0.03).
Conclusion: These results illustrate that chronic ecological conditions in addition to activity are
likely factors that contribute to population variation in leptin levels and physiology. Population
variation independent of adiposity should be considered to be an important source of variation,
especially in light of ethnic and population differences in the incidence and etiology of obesity,
diabetes, and other metabolic conditions.
Background
Leptin is a polypeptide hormone that is secreted primarily
by adipose tissue and acts as a lipostat to the hypothala-
mus affecting numerous aspects of physiology including
metabolism, immune function, and reproduction [1].
While leptin is often highly correlated with fat percentage,
Published: 30 August 2006
Nutrition & Metabolism 2006, 3:34doi:10.1186/1743-7075-3-34
Received: 04 May 2006
Accepted: 30 August 2006
This article is available from: http://www.nutritionandmetabolism.com/content/3/1/34
© 2006 Bribiescas and Hickey; licensee BioMed Central Ltd.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0),
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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other sources of variation are important. Diet [2], activity
[3], genetics [4], and infectious status [5], all contribute to
leptin variation to some extent independent of adiposity.
Other tissues besides fat also contribute to leptin levels
although their relative contribution is minor [6].
Although adiposity is a major source of variation in leptin
levels, significant population variation is evident with lep-
tin being significantly lower in non-industrialized popu-
lations compared to more developed communities [7-9].
The source of this variation remains to be fully described
but chronic environmental and nutritional conditions
have been suggested [10]. Among Ache Amerindian
women of Paraguay, such marked variation has been
reported. Curiously, Ache women exhibit leptin levels
that are not significantly different from American anorec-
tic women despite significantly greater adiposity (Figure
1) [11]. Comparisons of leptin among Ache men are less
well described.
To determine whether differences in the association
between leptin and adiposity extend also to Ache men,
this investigation compares Ache male leptin with a group
of comparable adiposity in a developed society, American
runners. The significance of this comparison is gain
insights into ethnic variation in leptin physiology and to
provide a greater understanding of the etiology of meta-
bolic syndrome among Native American groups [12,13].
Both groups are lean and exert significant physical activ-
ity. While the Ache do not engage in habitual exercise,
their activity levels and aerobic profiles reflect significant
daily physical exertion resulting from their daily regimen
of manual farming and foraging for food products in the
surrounding forest [14]. This comparison allows the
assessment of interpopulation variation in leptin associa-
tions with adiposity within a non-pathological context. It
was therefore hypothesized that according to our current
understanding of leptin physiology, 1) leptin should be
higher in the group exhibiting greater adiposity; 2) leptin
should be positively associated with adiposity in each
group.
Methods
Leptin and anthropometric data was collected from the
Ache by the author and compared with pre-exercise con-
trol measures from runners reported in Hickey et al. [15].
Specific anthropometric and hormone assessment meth-
ods are available from their respective references [9].
Although leptin assay methods and kits are highly corre-
lated [16], these data sources were chosen since each uti-
lized similar leptin iodine 125 based radioimmunoassays,
thereby reducing interlaboratory variability. Unpaired 2
tailed t-tests of fat%, BMI, age, and leptin were conducted.
Welch's correction was employed when variances were
significantly different. Standard linear regression was used
to determine associations between anthropometric meas-
ures and leptin. Comparison of regression slopes was con-
ducted using the methods of Zar, 1999 [17]. Results were
calculated using Prism 4.0 for Macintosh (GraphPad Soft-
ware, San Diego, CA). Alpha was set at 0.05. These proto-
cols were approved by the human subjects protection
committees of Yale and Colorado State Universities.
Results
Despite significantly higher body fat percentage (Ache
17.9 ± 1.78; Runners 9.7 ± 3.2; p < 0.0001), Ache leptin
was significantly lower than runners (Ache 1.13 ± 0.38;
runners 2.2 ± 1.15; p < 0.007). There was no significant
difference in BMI (Ache 23.8 ± 1.4; Runners 22.9 ± 2.0; p
< 0.16) or age (Ache 32.8 ± 16.1; runners 32.2 ± 9.2; p <
0.89) (Figures 2a–d). In addition, the associations
between leptin and fat percentage were quite different.
While runners exhibited the expected positive relation-
ship between leptin and fat % (r = 0.90, p < 0.0001), the
association in Ache men was negative (r = -0.50, p < 0.03),
although the association is not significant with the elimi-
nation of one outlier (Figure 3a). Not surprisingly, the
slopes were significantly different (p < 0.0001, F = 40.0).
Similarly, leptin associations with BMI were highly signif-
icant among the runners (r = 0.75, p < 0.003) but not Ache
(r = -0.17, p < 0.50). Again, the slopes are significantly dif-
ferent (p < 0.001, F = 13.5) (Figure 3b).
Mean leptin comparisons between Ache women and Ameri- can anorectics and controls
Figure 1
Mean leptin comparisons between Ache women and
American anorectics and controls. Mean comparisons
of leptin and fat percentage between Ache women (leptin 5.6
± 3.2 ng/ml; fat % 33.3 ± 4.4 SD), American Anorectics (5.6 ±
3.7; 7.0 ± 2.0), and American controls (19.1 ± 8.1; 28.0 ± 5.0)
(Ache leptin vs. Anorectic, p < 0.98; Control, p < 0.0001;
Ache fat % vs. Anorectic, p < 0.0001; Control, p < 0.004)
[11].
0 1020 3040
0
5
10
15
20
25
Anorectic
Ache
Control
Fat %
Leptin (ng/ml)

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Discussion
The hypothesis that leptin should be higher in the group
exhibiting greater adiposity was not supported in this
investigation. In addition, the hypothesis that leptin
should be positively associated with adiposity is only evi-
dent among American runners. Therefore other sources of
leptin variation, particularly among the Ache, need to be
considered. The exact mechanisms that underlie popula-
tion variation independent of adiposity remain unclear,
however genetic differences between populations, chronic
environmental influences such as maternal/fetal or child-
hood nutritional status, and acute lifestyle differences
Mean leptin comparisons between Ache men and American male runners
Figure 2
Mean leptin comparisons between Ache men and American male runners. Mean comparisons between Ache men
and runners (a) Leptin (p < 0.007), (b) Fat % (p < 0.0001), (c) BMI (p < 0.16), and (d) age (p < 0.89).

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such as diet and physical activity may be contributory.
Since American runners were chosen due to their leanness
and physical activity compared to the Ache, it is unlikely
that acute differences in daily lifestyle patterns related to
adiposity and physical activity were a factor in this study.
Genetic differences in the Ob gene that codes for leptin as
well as its receptor are also possible sources of variation.
For example among Taiwanese aboriginal populations,
greater incidences of obesity were associated with the G-
2548A polymorphism in the promoter region of the lep-
tin gene and the Gln223Arg (Q223R) polymorphism of
the leptin receptor gene [18]. Similar findings are evident
among Brazilian populations [19]. Variation in the proo-
piomelanocortin gene has also been associated with vari-
ation in leptin levels [20]. While various genetic analyses
have been conducted among the Ache, descriptions of
potential genetic leptin polymorphisms remain to be con-
ducted. The Ache are a genetically homogeneous popula-
tion and unique leptin polymorphisms may be possible
[21].
McMillen and colleagues have emphasized the potential
role of early developmental processes that may affect lep-
tin and adipose physiology later in adulthood. They sug-
gest high maternal BMI may lead to greater circulating
glucose in fetuses and altered adipocyte metabolism dur-
ing childhood, ultimately leading to greater leptin levels
in adulthood and a propensity towards obesity [10].
However the inverse may be at play with the Ache. Among
the Ache, maternal undernutrition may lead to lower
adult leptin levels. One Ache woman, nine months preg-
nant, exhibited very low leptin levels (bottom 5th percen-
tile compared to American
(unpublished data). However clearly further data on preg-
nancy and leptin are needed among the Ache.
pregnant women)
In addition, a comparison of leptin between well-fed Ital-
ian and undernourished Gambian boys and girls revealed
that Gambian children exhibited consistently lower leptin
even after controlling for BMI [22]. Animal models have
also shown that early exposure to leptin increases receptor
formation in the hypothalamus [23]. Leptin clearance
rates are also positively associated with adiposity,
although relationships with population variation are
unknown [24]. Conversely, a comparison of leptin among
Pima Indian populations living under differing lifestyle
and environmental conditions revealed that under condi-
tions of high caloric intake/low energy expenditure
(American) compared to low caloric intake/high energy
expenditure (Mexican), leptin was significantly higher
among the Mexican Pima independent of sex, adiposity,
and insulin resistance state [25].
In adults, leptin levels and adipose tissue production rates
exhibited a more prominent decline in fasted lean women
compared to fasted obese women [26], while five days of
intense active military training also decreased leptin levels
in healthy men, most likely due to hypoinsulinemia and
increases in catecholamines [3]. However both Ache and
American runner men are very physically active. Therefore
daily physical exertion may contribute to their low leptin
levels compared to more sedentary men, but it does not
account for the lack of association between leptin and adi-
posity among the Ache. Interestingly, elite male and
female gymnasts exhibit hypoleptinemia and associated
delays in puberty and other developmental challenges,
suggesting continued effects on lower leptin production
in adulthood [27]. However leptin responses to heavy
endurance training exhibited only a modest increase in
elite runners [28]. The Ache also undergo later puberty
compared to American counterparts [29], intimating the
potential role of energetic status during pubertal develop-
ment and later adipose leptin production rates. Finally,
Associations between leptin and anthropometric measure-ments
Figure 3
Associations between leptin and anthropometric
measurements. Linear regression of leptin and fat % (a) in
Ache men (r = -0.50, p < 0.03) and runners (r = 0.90, p <
0.0001). Slopes are significantly different (p < 0.0001, F =
40.0); (b) in Ache men (r = -0.17, p < 0.50) and runners (r =
0.75, p < 0.003). Slopes are significantly different (p < 0.001, F
= 13.5).

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zinc deficiency induces acute hypoleptinemia [30-33],
however oral zinc supplementation did not affect Ache
male leptin levels [34].
Immunological stress, while not quantified in this study,
likely differed between Ache and American runner men.
Chronic and acute infections affect and are influenced by
leptin levels [5,35]. Specific aspects of Ache disease epide-
miology are not well described. However, tuberculosis is
becoming more common within the Ache community
and may be a factor to consider [36]. Curiously, Yüksel
and colleagues [37] reported that serum leptin levels were
higher among active tuberculosis patients compared to
controls. Treatment elevated leptin levels even higher.
Schwenk et al. [38] also reported that wasting associated
with tuberculosis was unrelated to leptin. It is therefore
unlikely that possible tuberculosis infection made any sig-
nificant contribution to Ache leptin profiles although the
role of other infectious agents remain to be investigated.
While leptin likely serves as a fat cell mass/energy signal to
the central nervous system, acute underfeeding can dra-
matically lower systemic leptin independent of changes in
adipose tissue mass [2]. This is in accordance with the
hypothesis that leptin may serve as an anti-starvation as
opposed to an anti-obesity signal. The unusually low lep-
tin in Ache males and females may be part of a coordi-
nated metabolic milieu that is geared toward restrained
energy intake and storage, as the environment is much
more tenuous than a Western environment with abun-
dant energy dense foods and little physical activity. Such
a state is consistent with the thrifty genotype hypothesis
and may contribute to contemporary high rates of obesity
among Native American populations [39,40].
Foraging populations are excellent models for assessing
the evolutionary implications of exercise physiology.
Assessments of physical performance of the Ache have
provided evolutionary physiologists with important
insights into human biology and their responses to phys-
ical activity, both recreational and non-recreational [14].
Indeed, long distance running is a popular exercise activ-
ity may be rooted in the evolution of human physiology.
Not only is skeletomuscular adaptation required, an effi-
cient suite of metabolic mechanisms are also necessary to
manage energy usage during this period of acute activity.
Indeed, running has been suggested to have been a central
factor in the evolution of habitual bipedalism [41].
Although investigations of our hominid ancestors are lim-
ited to examinations of the fossil record, research among
contemporary hunter/gatherers and other non-industrial
populations are useful models of the ecological, energetic,
and lifestyle challenges that were probably not unlike
those that confronted human ancestors. The effects of
acute activity among forager populations suggests that
chronic differences in lifestyle and ecological circum-
stance may influence physiological function [14,42].
The significant non-pathological range of variability in
leptin and other metabolic hormones merits greater
awareness and appreciation on the part of human biolo-
gists. Moreover, the growing incidence of obesity, meta-
bolic syndrome, and diabetes, along with increasing
population heterogeneity due to immigration, strongly
advocates that diagnostic and research clinicians would
surely benefit from recognizing the importance of leptin
variation independent of adiposity.
Conclusion
While adiposity has been repeatedly shown to be central
to leptin production and circulating levels, we have pro-
vided further evidence that leptin levels exhibit a signifi-
cant range of variation independent of somatic condition,
most importantly adiposity. Leptin production rate varies
in association with environmental conditions and con-
tributes to population variation. Further research is neces-
sary to ascertain the potential implications for differences
in the incidence and etiology of obesity, diabetes, and
other metabolic disorders among ethnicities and popula-
tions.
Competing interests
The authors(s) declare that they have no competing inter-
ests.
Authors' contributions
Dr. Richard G. Bribiescas initiated the research question
addressed in this manuscript, performed the field collec-
tion and conducted the laboratory analysis of the Ache
hormonal and anthropmetric data, performed the statisti-
cal analysis, as well as contributed to the preparation and
editing of the manuscript. He also initiated and procured
the grants necessary to collect the Ache data.
Dr. Matthew S. Hickey contributed to the collection and
laboratory analysis of the American runner hormonal and
anthropometric data. He also assisted in the preparation
and editing of the manuscript. Both authors were
involved with the preparation and editing of the final
manuscript.
Acknowledgements
The author thanks the Ache community of Puerto Barra and members of
the Yale University Reproductive Ecology Laboratory for their cooperation
and support. This research partially funded by a Yale Faculty Social Science
Research Grant to RGB.
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