Friday, July 27, 2012

Obesity Epidemic is Not Due to Less Exercise - New Study says

Hunter gatherer clue to obesity

By Helen Briggs
BBC News 
The idea that exercise is more important than diet in the fight against obesity has been contradicted by new research.

The Hadza live a hunter gatherer existence that has changed little in 10,000 years.  A study of the Hadza tribe, who still exist as hunter gatherers, suggests the amount of calories we need is a fixed human characteristic. This suggests Westerners are growing obese through over-eating rather than having inactive lifestyles, say scientists.

One in 10 people will be obese by 2015. And, nearly one in three of the worldwide population is expected to be overweight, according to figures from the World Health Organization.

The Western lifestyle is thought to be largely to blame for the obesity "epidemic".

Various factors are involved, including processed foods high in sugar and fat, large portion sizes, and a sedentary lifestyle where cars and machines do most of the daily physical work.

The relative balance of overeating to lack of exercise is a matter of debate, however.

Some experts have proposed that our need for calories has dropped drastically since the industrial revolution, and this is a bigger risk factor for obesity than changes in diet.

A study published in the PLoS ONE journal tested the theory, by looking at energy expenditure in the Hadza tribe of Tanzania.

The Hadza people, who still live as hunter gatherers, were used as a model of the ancient human lifestyle.

Members of the 1,000-strong population hunt animals and forage for berries, roots and fruit on foot, using bows, small axes, and digging sticks. They don't use modern tools or guns.

Diverse lifestyles

A team of scientists from the US, Tanzania and the UK, measured energy expenditure in 30 Hadza men and women aged between 18 and 75.  They found physical activity levels were much higher in the Hadza men and women, but when corrected for size and weight, their metabolic rate was no different to that of Westerners.

Dr Herman Pontzer of the department of anthropology at Hunter College, New York, said everyone had assumed that hunter gatherers would burn hundreds more calories a day than adults in the US and Europe. The data came as a surprise, he said, highlighting the complexity of energy expenditure.  But he stressed that physical exercise is nonetheless important for maintaining good health.

"This to me says that the big reason that Westerners are getting fat is because we eat too much - it's not because we exercise too little," said Dr Pontzer.  

"Being active is really important to your health but it won't keep you thin - we need to eat less to do that.  Daily energy expenditure might be an evolved trait that has been shaped by evolution and is common among all people and not some simple reflection of our diverse lifestyles."

Hunter-Gatherer Energetics and Human Obesity

Herman Pontzer1,2*David A. Raichlen3Brian M. Wood4,Audax Z. P. Mabulla5Susan B. Racette6Frank W. Marlowe7
1 Department of Anthropology, Hunter College, New York, New York, United States of America, 2 New York Consortium in Evolutionary Primatology, New York, New York, United States of America, 3 School of Anthropology, University of Arizona, Tucson, Arizona, United States of America, 4Department of Anthropology, Yale University, New Haven, Connecticut, United States of America, 5 Department of Archeology, University of Dar es Salaam, Dar es Salaam, Tanzania, 6 Washington University School of Medicine, St. Louis, Missouri, United States of America, 7 Division of Biological Anthropology, University of Cambridge, Cambridge, United Kingdom

Abstract Top

Western lifestyles differ markedly from those of our hunter-gatherer ancestors, and these differences in diet and activity level are often implicated in the global obesity pandemic. However, few physiological data for hunter-gatherer populations are available to test these models of obesity. In this study, we used the doubly-labeled water method to measure total daily energy expenditure (kCal/day) in Hadza hunter-gatherers to test whether foragers expend more energy each day than their Western counterparts. As expected, physical activity level, PAL, was greater among Hadza foragers than among Westerners. Nonetheless, average daily energy expenditure of traditional Hadza foragers was no different than that of Westerners after controlling for body size. The metabolic cost of walking (kcal kg−1 m−1) and resting (kcal kg−1 s−1) were also similar among Hadza and Western groups. The similarity in metabolic rates across a broad range of cultures challenges current models of obesity suggesting that Western lifestyles lead to decreased energy expenditure. We hypothesize that human daily energy expenditure may be an evolved physiological trait largely independent of cultural differences.

Introduction Top

By 2015, nearly one in every three people worldwide is projected to be overweight, and one in ten is expected to be obese [1]. The attendant health risks of being overweight or obese, including Type 2 diabetes, cardiovascular disease, and certain cancers, are well known [1]. The proximate cause of weight gain is energy imbalance, with food energy intake (kCal/day) exceeding total energy expenditure (kCal/day), but the societal causes of the global obesity pandemic remain a focus of debate [1][7]. Generally, rising obesity incidence is thought to result from the current Western lifestyle, in which activity levels and diet deviate substantially from the conditions under which our species’ metabolic physiology evolved [2][6]. Some propose that modern conveniences and mechanization lead to decreased physical activity and lower energy expenditure in industrialized societies [1][3]. Others hypothesize that changes in diet and energy intake are responsible, citing the relatively recent increase in energy dense foods, particularly processed foods high in fructose and other simple sugars that can depress energy expenditure and increase appetite and adiposity[4][7].
Determining which aspects of the Western lifestyle are truly aberrant for our species and pose the greatest risk of obesity is complicated by the conflicting and limited data on diet and metabolism in non-Western populations. For example, while Western diets are certainly more sugar-rich and energy-dense than more “traditional” diets and wild foods [4][8][9], many hunter-gatherers seasonally consume a large portion of their daily calories as honey [10][11] (Fig. S2), which has high concentrations of glucose and fructose [12]. Similarly, while high activity levels have been reported in some subsistence farming populations [13][15], a recent meta-analysis of 98 diverse populations worldwide found no effect of socioeconomic development – a rough index of mechanization and diet – on daily energy expenditure or activity level [16]. Notably, metabolic measurements are lacking for hunter-gatherer societies, whose diet and lifestyle provide the best models for studies of human evolution [10].
In this study, we examined daily energy expenditure and physical activity level in Hadza foragers to test the hypothesis that hunter-gatherers expend more energy each day than subjects in market and farming economies. The Hadza are a population of hunter-gatherers living in a savannah-woodland environment in Northern Tanzania; their traditional foraging lifestyle has been documented extensively in previous work [17]. While no living population is a perfect model of our species’ past, the Hadza lifestyle is similar in critical ways to those of our Pleistocene ancestors. The Hadza hunt and gather on foot with bows, small axes, and digging sticks, without the aid of modern tools or equipment (e.g., no vehicles or guns). As in many other forager societies [10], there is a sexual division of foraging effort; Hadza men hunt game and gather honey, while Hadza women gather plant foods. Men’s forays are typically longer than women’s, as reflected in their mean daily travel distances (see below). Women typically forage in groups, while men tend to hunt alone [17]. As is typical among traditional-living Hadza, over 95% of their calories during this study came from wild foods, including tubers, berries, small- and large-game, baobab fruit, and honey [17] (Fig. S2).
We compared energy expenditure and body composition among the Hadza, measured using the doubly labeled water method [18], to similar data from other populations taken from previous studies[19][26] and new measurements of U.S. adults (Methods). Given their traditional, physically active lifestyle, we expected the Hadza to have lower body fat than individuals in Western populations. Further, if current models for obesity are correct, the Hadza, with their natural diet and lack of mechanization, should expend more energy than individuals living in market economies with comparatively sedentary lifestyles and highly-processed, sugar-rich diets.
We also measured daily walking distances (km/day) using wearable GPS devices, and the cost of walking (kCal kg−1 m−1) and resting metabolic rate (RMR, kCal kg−1 s−1) using a portable respirometry system (Text S1). Because it was not feasible to measure basal metabolic rate (BMR, kCal/day), we calculated physical activity level (PAL) as TEE/estimated BMR (Methods). Institutional approval and informed consent were obtained prior to data collection.

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