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Interindividual Variation in Posture Allocation: Possible Role in Human Obesity

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Science  28 Jan 2005:
Vol. 307, Issue 5709, pp. 584-586
DOI: 10.1126/science.1106561

Abstract

Obesity occurs when energy intake exceeds energy expenditure. Humans expend energy through purposeful exercise and through changes in posture and movement that are associated with the routines of daily life [called nonexercise activity thermogenesis (NEAT)]. To examine NEAT's role in obesity, we recruited 10 lean and 10 mildly obese sedentary volunteers and measured their body postures and movements every half-second for 10 days. Obese individuals were seated, on average, 2 hours longer per day than lean individuals. Posture allocation did not change when the obese individuals lost weight or when lean individuals gained weight, suggesting that it is biologically determined. If obese individuals adopted the NEAT-enhanced behaviors of their lean counterparts, they might expend an additional 350 calories (kcal) per day.

Obesity is epidemic in high-income countries. In the United States alone poor diet and physical inactivity are associated with 400,000 deaths per year (1) and obesity-related medical expenditures in 2003 approximated $75 billion (2). Obesity is also an emerging problem in middle- and low-income countries, where the health and fiscal costs are likely to be devastating (3).

As the impact of obesity on health escalates, so too does the need to understand its pathogenesis. Weight gain and obesity occur when energy intake exceeds energy expenditure. We are interested in a specific component of energy expenditure called NEAT and the role it might play in human obesity. NEAT is distinct from purposeful exercise and includes the energy expenditure of daily activities such as sitting, standing, walking, and talking.

We have previously shown that when humans overeat, activation of NEAT helps to prevent weight gain (4). To better understand NEAT and its role in obesity, we separated NEAT into the thermogenesis associated with posture (standing, sitting, and lying) and that associated with movement (ambulation).

To investigate whether the obese state has an effect on NEAT, we first developed and validated a sensitive and reliable technology for measuring the postural allocation of NEAT in human volunteers (5, 6). This physical activity monitoring system uses inclinometers and triaxial accelerometers to capture data on body position and motion 120 times each minute. By combining these measurements with laboratory measures of energy expenditure, we can summate NEAT and define its components (7).

To compare body posture and body motion in lean and obese people, we recruited 20 healthy volunteers who were self-proclaimed “couch potatoes.” Ten participants (five females and five males) were lean [body mass index (BMI) 23 ± 2 kg/m2] and 10 participants (five females and five males) were mildly obese (BMI 33 ± 2 kg/m2) (8) (table S1). We deliberately selected mildly obese subjects who were not incapacitated by their obesity and who had no joint problems or other medical complications of obesity. The volunteers agreed to have all of their movements measured for 10 days and to have their total NEAT measured with the use of a stable isotope technique (9). They were instructed to continue their usual daily activities and occupations and not to adopt new exercise practices. Over the 10-day period, we collected ∼25 million data points on posture and movement for each volunteer.

Our analysis revealed that obese participants were seated for 164 min longer per day than were lean participants (Fig. 1A). Correspondingly, lean participants were upright for 152 min longer per day than obese participants. Sleep times (lying) were almost identical between the groups. Total body movement, 89% of which was ambulation, was negatively correlated with fat mass (Fig. 1, B and C). Notably, if the obese subjects had the same posture allocation as the lean subjects, they would have expended an additional 352 ± 65 (±SD) (range, 269 to 477) calories (kcal) per day (Fig. 1C).

Fig. 1.

(A) Time allocation for different postures for 10 obese and 10 lean sedentary subjects. Data are shown as mean + SEM. Significant differences between lean and obese are indicated: *, P = 0.001; **, P = 0.0005. There were no statistically significant differences between females (n = 10) and males (n = 10): Females stood 470 ± 35 min/day and males stood 429 ± 40 min/day. (B) Relationship between total body movement and body fat content. Body fat, determined from dual x-ray absorptiometry, is expressed as a percentage (left) and mass (right) plotted against the total 10-day accelerometer output [accelerometer units (AU)] for 20 (10 obese and 10 lean) sedentary subjects. The open diamonds are data for females and the filled diamonds are data for males. There was no significant relationship between fat-free mass and accelerometer output (fig. S1). The relationship between NEAT by doubly labeled water adjusted by weight versus accelerometer output is shown in fig. S2. (C) (Left) Total daily energy expenditure and (right) NEAT in 10 obese and 10 lean sedentary subjects. The uppermost segments of the bars for obese individuals (vertical arrows) represent the additional energy that could be expended if these subjects were ambulatory for the same amount of time as lean subjects. BMR, basal metabolic rate; TEF, thermic effect of food. There was no significant difference in sleeping time between the lean group (423 ± 15 min) and the obese group (434 ± 17 min). The energy expenditure data and standard deviations appear in table S2. The relationship between NEAT measured with doubly labeled water and NEAT measured with the instruments is shown in fig. S3.

To investigate whether these differences in posture allocation are a cause or consequence of obesity, we asked seven of the original obese volunteers (four females and three males, BMI 33 ± 2 kg/m2) to undergo supervised weight loss over a period of 8 weeks. The average weight loss was 8 kg. Likewise, we recruited nine of the original lean volunteers and one additional lean volunteer (six females and four males, BMI 23 ± 2 kg/m2) to undergo supervised overfeeding over a period of 8 weeks. The average weight gain was 4 kg. After these weight perturbations, we studied posture allocation in these subjects for another 10 days. Interestingly, both the obese subjects losing weight and the lean subjects gaining weight maintained their original posture allocation (Fig. 2). Thus, it appears that interindividual differences in posture allocation are biologically determined.

Fig. 2.

(A) Posture allocation in seven obese sedentary subjects who underwent caloric restriction (8). (Left) Posture allocation data at baseline and after weight loss of 8 ± 2 kg. (Right) The time the subjects spent standing/ambulating at baseline is plotted against the time the subjects spent standing/ambulating after weight loss. (B) Posture allocation in 10 lean sedentary subjects who underwent experimental weight gain (8). (Left) The posture allocation data for baseline and after weight gain of 4 ± 2 kg. (Right) The time the subjects spent standing/ambulating at baseline is plotted against the time the subjects spent standing/ambulating after weight gain. Data are shown as mean + SEM.

It should be emphasized that this was a pilot study and that the results need to be confirmed in larger studies. Nevertheless, the current data may be important for understanding the biology of obesity and how best to treat it. The propensity of obese persons to sit more than lean individuals has several potential explanations. Rodent studies support the concept that there are central and humoral mediators of NEAT (10, 11). For example, we have shown that a neuropeptide associated with arousal, orexin (12), increases NEAT in rats when injected into the paraventricular nucleus (PVN) of the hypothalamus. Preliminary data suggest that PVN injections of orexin also cause dose-dependent increases in standing posture allocation in rats (13). Thus, there may be central and humoral mediators that drive the sedentary behavior of obese individuals. The negative relationship between fat mass and movement (Fig. 1B) raises the intriguing possibility that body fat releases a factor that slows physical activity in obesity. However, these data also demonstrate that posture allocation is not the mechanism by which NEAT is modulated with short-term overfeeding. One hypothesis is that this occurs through modulation of energy efficiency; this is an area worthy of future investigation.

These data may also have implications for obesity intervention. One could argue that obese individuals have a biologically determined posture allocation and therefore are destined to become obese. If this were true, obesity would have been as common 50 years ago as it is today. However, obesity rates have increased and continue to do so (14). We speculate that obese and lean individuals respond differently to the environmental cues that promote sedentary behavior. If the obese volunteers adopted the NEAT-enhanced behavior of their lean counterparts, they could expend an additional 350 kcal per day. Over a year, this alone could result in a weight loss of ∼15 kg, if energy intake remained unchanged. Herein lies the rationale behind nationwide approaches to promote NEAT in small increments (15). For example, in Rochester, Minnesota, in 1920 before car use was commonplace the average walk to and from work was 1.6 miles (16). If walking this distance to work were reinstituted by our obese subjects, all of whom currently drive to work, an extra 150 kcal per day could be expended. We will need to use similar measures to promote NEAT as an impetus to create an active and dynamic environment in which, for example, dancing supersedes television as a leisure activity. Approaches that succeed in getting people out of their chairs and moving could have substantial impact on the obesity epidemic.

Supporting Online Material

www.sciencemag.org/cgi/content/full/307/5709/584/DC1

Materials and Methods

Figs. S1 to S4

Tables S1 and S2

References

References and Notes

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