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Research Article

Introduction Top

Doubly labeled water studies show that physical activity induced energy expenditure decreases on average with more than 50% between the age of 20–30 and over 65 [1]. Exercise training does not seem to prevent the age-associated decline in physical activity because of compensation by a decrease in non-training physical activity [1]. Thus, a physically active lifestyle inevitably results in a larger decrease of daily energy expenditure at later age than a sedentary lifestyle. Here, longitudinal measurements were performed to observe the consequence of the age-associated reduction in physical activity for the maintenance of energy balance as reflected in the fat store of the body.

Methods Top

Healthy, non-obese adults (17 women and 23 men; 27±5 yr; 22.8±2.0 kg m⁻²) participated in the study. All subjects gave written informed consent to participate in the study, which was approved by the Ethics Committee of the Maastricht University Medical Centre. Follow-up measurements were performed after 11±4 (range 6–16) yr. Total energy expenditure was measured by monitoring their metabolism with doubly labeled water over a two-week period, the optimal observation interval for the biological half-lives of the isotopes [2]. The physical activity level (PAL) is defined as the factor by which total energy expenditure exceeds resting energy expenditure, measured in the early morning with indirect calorimetry. All initial measurements and follow-up measurements in 22 subjects were performed with doubly labeled water. Additionally, all follow-up measurements of PAL, including the remaining 18 subjects, were performed over the same period with a doubly labeled water validated triaxial accelerometer [3]. Body fat was assessed with deuterium dilution.

Results Top

Body mass index, as a general indicator of body fatness, increased from 22.8±2.0 kg/m² at baseline to 24.3±2.6 kg/m² at follow-up (P<0.01, Table 1). Total energy expenditure showed a non-significant decrease and resting energy expenditure showed a non-significant increase, in combination resulting in a significant decrease of activity energy expenditure from 4.21±1.05 MJ/d to 3.92±1.19 MJ/d (P<0.05). The PAL value decreased significantly (P<0.01) from 1.81±0.16 (range 1.51–2.15) to 1.75±0.11 (range 1.58–2.03). A similar reduction (P<0.05), from 1.84±0.17 (range 1.51–2.15) to 1.74±0.16 (range 1.43–2.08), was observed in the subgroup of 22 subjects where both PAL measurements were performed with doubly labeled water. The decrease in PAL was related to the initial PAL value (r² = 0.60, P<0.0001), where physically active subjects showed a larger reduction. At baseline, body mass index was negatively related to PAL (r² = 0.25, P<0.001; Figure 1A), while at follow-up the relation had disappeared. (Figure 1B). The majority of the subjects showed an increase in body fat with a mean value of 0.34±0.30 kg yr⁻¹ (P<0.0001). The rate of change of fat mass was positively related with baseline PAL (P<0.05) and inversely related to the difference in PAL between baseline and follow-up (P = 0.01; Figure 2). Thus, subjects with a higher PAL at baseline gained more fat.

Figure 1. Body mass index as a function of the physical activity level at baseline (A) and at follow up (B).

Linear regression analysis shows an inverse relationship at baseline (A, continuous line) and no relationship at follow-up (B, discontinuous line), 11±4 yr later, in 40 subjects.

doi:10.1371/journal.pone.0004745.g001

Figure 2. Change in fat mass as a function of the difference in physical activity level.

Physical activity level at baseline (PAL₁) and at follow up (PAL₂), 11±4 yr later, in 40 healthy subjects with body-mass indices within the normal range. Linear regression shows an inverse relationship between the change in PAL and the rate of fat mass change.

doi:10.1371/journal.pone.0004745.g002

Table 1. Subject characteristics and energy expenditure at baseline and follow-up.

doi:10.1371/journal.pone.0004745.t001

Discussion Top

The PAL of the subjects was typical for the general population, where doubly labeled water assessed PAL ranges between 1.5 and 2.1 for sedentary and very active people [4]. The earlier reported age associated reduction of PAL and the gain in fat mass was deducted from cross-sectional data, comparing young adults with elderly, and the two phenomena could not be linked together [1]. The current analysis shows a significant gain in fat mass in adults already from the age of 27±5 yr onwards, observed again after 11±4 (range 6–16) yr, and a close link with a change in PAL. Fat mass change ranged from a loss of 0.2 kg/yr to a gain of 1.1 kg/yr with an average gain of about 0.2 kg/yr in subjects not changing the PAL between baseline and follow-up, reflecting the fattening with increasing age corrected for the change in PAL.

In a laboratory study, Stubbs et al. showed that reducing the PAL from 1.8 to 1.4 over 7 days markedly affected energy balance. A change to a sedentary routine did not induce a compensatory reduction of energy intake and most of the excess energy was stored as fat [5]. Similarly, weight gain was observed in runners because of reductions in weekly exercise and was not reversed by resuming prior activity [6], showing that intake follows more an increase than a decrease in activity induced changes in daily energy expenditure. Here, a change from an active to a more sedentary life-style resulted in fat storage reflecting insufficient adaptation of food intake to a reduced energy requirement, even in the long term.

References Top

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2. Speakman JR (1997) Doubly-labelled water: theory and practice. London: Chapman & Hall.
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4. Westerterp KR, Speakman JR (2008) Physical activity energy expenditure has not declined since the 1980s and matches energy expenditures of wild mammals. Int J Obes 32: 1256–1263. Find this article online
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