Posted on http://scienceitches.com on 27 November 2016
“The Maya people descend from the indigenous inhabitants of southeastern Mexico, Guatemala, Belize, San Salvador, and Honduras. They are the largest Native American group (6-7 million people) and show the shortest average height of any non-pygmy human population” (1: 679).
The Maya carry a long legacy of political, educational, and socioeconomic deprivation that feeds racism and perpetuates a livelihood of poverty. These socio-political aspects negatively impact on the health of the Maya and bring about further cycles of adverse outcomes. I use a biocultural approach (2) on my research and include inputs from the Maya communities when defining research designs. This post summarises most of the research conducted by my research team. It is not a systematic review of the literature on the health of the Maya, but I hope it becomes a useful resource as a starting point for further research. This is a live post, I welcome comments and suggestions for improvement, and I will be adding information when relevant.
The measurement of physical growth and the accurate measurement of stature and other long bones – such as limb length – are powerful reflexions of the social, political, and moral conditions of a society. James Tanner (b1920-d2010) coined the expression “growth as a mirror of the human condition” and stated that:
“ the growth of children (…) is a wonderfully good gauge of living conditions and the relative prosperity of different groups in a population” (3:96)
Height has been used extensively as an economic variable that reflects health and human capital, especially when other economic indicators are not available.
I started this post with a quote from our most recent paper (1), stating that the Maya are, on average, the shortest non-pigmy people in the world. Being “non-pygmy”, in this context, is an important factor because it means that there are not any known genomic and/or hormonal factors that code for the Maya short stature. However, for a long time, they were considered the “pygmies” of Latin America” (4) . This was damaging because it assumed that being so short was “genetic” and “adaptive”, that everything was fine with this group of people, and nothing needed to change. However, our research showed that, when the Maya started migrating to the US, and benefited from clean drinking water, basic health care, much lower rates of infection load, extended education, enough food, and a peaceful environment, the average stature of the school children increased almost 11cm, in less than one decade. From these, 7cm were due to the increase in the relative length of the legs, in proportion to total stature (5). Genetic/genomic expressions do not change in such a brief period of time, therefore, all evidence suggest that the short stature of the Maya is due to a combination of environmental and epigenetic factors.
I must emphasize, here and now, that no human group “adapts” to poverty, segregation, racism, deprivation, infection, heavy workloads, and shortage of nutrients. They all suffer and many die. The ones who survive to adulthood, do so at the expense of their health and productivity, in some way or other. These are trade-offs that may maximize survival but come at a cost that, sooner or later, will be painfully paid with interest (6).
Very short stature-for-age, or stunting, is an indicator of chronic undernutrition and has long lasting health implications (7). Stunting is usually defined as the individual’s height-for-age being below the 5th percentile of the references, although slightly different cut-off points can be used (8). This makes stunting a sort of moving target. Depending what references and what cut-off points are used, a stunted child may or may not be classified as such. Stunting affects all body systems and, if not curbed early in life, will leave permanent and damaging physical, cognitive, and developmental traits.
In his paper “Nutritional status and physical work capacity”, Spurr (9) summarizes the effects of stunting on physical work capacity, maximal oxygen consumption, aerobic power, heart rate response to exercise, and endurance at submaximal work loads. Other studies have shown that no body system is spared of the nefarious consequences of stunting and that the longer it lingers, the worse the outcomes will be (10-12).
High levels of stunting have been consistently reported, for more than two decades, among the Maya in Belize (13-15), Guatemala (16-19), and Mexico (20-21). Fast and steep increases in height and leg-length, among the Maya migrants to the US, have been mentioned already in this post (5) but this positive outcome needs to be addressed in the context of the whole physical and health changes that characterize this group. Increments in height were accompanied by even steeper increments in weight, and body-mass-index (BMI) (22). The height of the Maya-American children averaged at the 25th percentile of the height of the US children, but their weight and BMI averaged at the 85th percentile. In summary, although the average height of these Maya-American children increased, and the percentage of stunted children decreased, they were still significantly shorter, but much heavier, than the references.
The nutritional dual-burden paradox
The nutritional dual-burden is defined as the coexistence of stunting and overweight/obesity in the same person (individual dual-burden), in the same household (stunted child, overweight/obese mother), or within the same population (8,23,24).
Nutritional dual-burden at the individual level is not common among Maya children, but is frequent among Maya adults, who tend to be very short and very fat. This means that they were stunted children who grew up to become stunted, overweight/obese adults, and accumulate in their bodies the negative effects of both of these conditions (25). Nutritional dual-burdened adult individuals, and specially women, are at higher risk of producing offspring who will also be stunted and, therefore, will perpetuate the short-and-fat legacy throughout generations. This is a non-genomic mechanism that may lead to low birth weight and elevated cardiovascular risk in the subsequent generations (26).
Intergenerational effects of stunting on health and disease
The intergenerational influences hypothesis (IIH) was proposed by Irving Emanuel (27:35) as, ‘‘… those factors, conditions, exposures and environments experienced by one generation that relate to the health, growth and development of the next generation.’’ The IIH has been further discussed elsewhere (26, 28–30), and put in perspective with related concepts such as “foetal programming”, “life history trade-offs”, and developmental origins of health and disease (DOHaD) (6,31–36). We have tested the IIH with the Maya communities and found out that Maya-Mexican children with a stunted mother were more likely to be stunted by 4-6 years of age (30). Also, in another Maya-Mexican three-generational sample, stunting in children was directly associated with the short stature of the mothers; and the children’s levels of fatness were also directly associated with the short stature of the mothers and the maternal grandmothers (20,37).
Stunting, muscle mass, and energy expenditure
Associations between stunting, body composition, and energy expenditure (or work capacity) have been studied for decades, showing that stunted individuals have less muscle, and much less energy for voluntary physical activity (9,11,12,38,39). In the last 10 years, this area of research has benefited considerably due to the commercialisation of accurate portable technology that allows free-living assessment of all components of energy expenditure, metabolism, and respiratory and cardiac function. Usually, the sample sizes are small and the studies tend to focus on only one or two parameters, making it difficult to establish more complex associations.
We conducted a study with a sample of 37 Maya-Mexican children and found that a lower height-for-age z-score, as a continuous variable (but not stunting as a categorical variable) significantly predicted lower activity energy expenditure (40). This study was innovative because the children wore a combined heart-rate and uni-axial accelerometer for 7 days (The Actiheart), but it was challenging because the device, when used in the field, performed differently than under laboratorial conditions (41), and required a much greater deal of maintenance than initially predicted. The continuous advance of portable, wearable technology, for the estimation of energy expenditure and other physiological parameters, is promising. This line of research must be pursued to further disentangle the associations between stunting, energy expenditure, body composition, and other health outcomes. The state of Yucatan has one of the highest rates of childhood stunting and, at the same time, the highest mortality rates for diabetes, cardiovascular diseases and stroke in Mexico as a whole. Among these, the Maya – being the poorest of the poor – tend to suffer more than the rest of the population (42).
Nutrition and globalization
The nutritional patterns of the Maya, in the last 20 years, have moved away from a diet high in fibre, vegetables, and pulses to a globalised diet with a very high content of salt, sugar, fat, processed foods, and carbonated beverages. In a study conducted in 2013 (43) the most common diet among the Maya in Mexico was characterized by a low consumption of fruits and vegetables, a medium consumption of pork, eggs, oil and lard, and a high consumption of soda and whole milk. The Maya health and culture has deteriorated as a result of this nutritional transition which is affecting the physical growth and health of the Maya families. The summary illustrates clearly the shifts in food consumption, and emphasises the role of globalization in the exacerbation of negative health outcomes among the Maya (44).
A photo that summarizes it all
The photo below was taken in 2010, in the Colonia San Jose Tecoh, Merida, Yucatan, Mexico. I am with a Maya grandmother and her 8 year old granddaughter. In this photo, I am the tallest person which is very unusual for me, because I am only 159cm (5ft 2inches). This photo illustrates the height gap of the Maya much better than any written description. Therefore, I will say no more!
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