Reduced Activity & Insulin Sensitivity: A Physiology Study

A 2-wk reduction of ambulatory activity attenuates
peripheral insulin sensitivity
Rikke Krogh-Madsen, John P. Thyfault, Christa Broholm, Ole Hartvig
Mortensen, Rasmus H. Olsen, Remi Mounier, Peter Plomgaard, Gerrit van Hall,
Frank W. Booth and Bente K. Pedersen
J Appl Physiol 108:1034-1040, 2010. First published 31 December 2009;
doi:10.1152/japplphysiol.00977.2009
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Physical Activity, Sedentary Behavior, and Health: Paradigm Paralysis or Paradigm Shift?
Peter T. Katzmarzyk
Diabetes, November , 2010; 59 (11): 2717-2725.
[Full Text] [PDF]
Muscle specific microRNAs are regulated by endurance exercise in human skeletal muscle
Søren Nielsen, Camilla Scheele, Christina Yfanti, Thorbjörn Åkerström, Anders R. Nielsen,
Bente K. Pedersen and Matthew Laye
J Physiol, October 15, 2010; 588 (20): 4029-4037.
[Abstract] [Full Text] [PDF]
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Variability in training-induced skeletal muscle adaptation
James A. Timmons
J Appl Physiol, March , 2011; 110 (3): 846-853.
[Abstract] [Full Text] [PDF]
J Appl Physiol 108: 1034–1040, 2010.
First published December 31, 2009; doi:10.1152/japplphysiol.00977.2009.
A 2-wk reduction of ambulatory activity attenuates peripheral insulin
sensitivity
Rikke Krogh-Madsen,1 John P. Thyfault,2 Christa Broholm,1 Ole Hartvig Mortensen,1 Rasmus H. Olsen,1
Remi Mounier,1 Peter Plomgaard,1 Gerrit van Hall,1 Frank W. Booth,3 and Bente K. Pedersen1
1
Centre of Inflammation and Metabolism at Department of Infectious Diseases and Copenhagen Muscle Research Centre,
Rigshospitalet, Faculty of Health Sciences, University of Copenhagen, Copenhagen, Denmark; 2Harry S. Truman Memorial
Veterans Hospital, Health Activity Center, Departments of Nutrition and Exercise Physiology and Internal Medicine,
University of Missouri, Columbia, Missouri; and 3Health Activity Center, Departments of Biomedical Sciences and of Medical
Pharmacology and Physiology, University of Missouri, Columbia, Missouri
Submitted 30 August 2009; accepted in final form 29 December 2009
insulin resistance; clamp; insulin signaling
and at the low range may increase risk for developing chronic
metabolic disease(s) (4). We postulated that healthy, nonexercising subjects who transitioned from a high to low level of
ambulatory activity (from ⬎10,000 to ⬍2,000) would quickly
display metabolic alterations. Our initial findings showed that
healthy young men who reduced their daily steps from an
average of 10,501 ⫾ 808 to 1,344 ⫾ 33 for a 2-wk period
displayed a clustering of metabolic alterations including increased insulin response to an oral glucose tolerance test
(OGTT), increased plasma triglyceride response to an oral fat
tolerance test, and a 7% increase in visceral fat (21). These
findings led us to question if the loss of insulin sensitivity
induced by reduced ambulatory activity was due to peripheral
or hepatic decreases in insulin sensitivity. Moreover, if there
was a decline in peripheral insulin sensitivity we questioned if
it was associated with reduced activation of the insulin signaling pathway in skeletal muscle as has been shown in rodent
models in which daily physical activity was ceased for acute
periods of time (16). We additionally wished to determine if
reduced ambulatory activity would negatively impact lean
body mass or cardiorespiratory fitness [maximal oxygen consumption (V̇O2 max)], two factors strongly linked to metabolic
health and mortality risk. Herein we provide evidence that 2
wk of reduced daily ambulatory activity in healthy young,
nonexercising subjects decreases peripheral insulin sensitivity
and insulin stimulation of Akt in skeletal muscle without
change in rate of glucose appearance during a hyperinsulinemic-euglycemic clamp. In addition, we show evidence that
reduced ambulatory activity significantly lowers both lean
body mass and cardiorespiratory fitness.
METHODS
EPIDEMIOLOGICAL EVIDENCE indicates that low physical activity
plays a causal role in the development of Type 2 diabetes (11,
12, 17). Our longer-term hypothesis is that reductions in daily
ambulatory activity levels (reduced steps taken throughout the
day) initiate attenuation in skeletal muscle insulin sensitivity
that precedes a later development of overt insulin resistance. It
is estimated that most free-living, nonexercising US adults take
between ⬃2,000 and ⬃12,000 steps per day, a wide range of
ambulatory activity that at the high range may decrease risk
Address for reprint requests and other correspondence: R. Krogh-Madsen,
Centre of Inflammation and Metabolism, Rigshospitalet-Section 7641, Blegdamsvej 9, DK-2100 Copenhagen, Denmark (e-mail: [email protected]).
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Ethical approval. All subjects gave oral and written consent, and
the study was approved by the Scientific-Ethics Committee of Copenhagen and Frederiksberg Municipalities (file no. KF01268925) in
accordance with the Helsinki Declaration.
Subjects. Ten healthy human males (mean age 23.8 ⫾ 1.5; body
mass index of 22.1 ⫾ 0.7 kg/m2) participated in the study [reported in
a preliminary report (21)]. Before the study all 10 subjects underwent
a thorough clinical examination. All subjects were nonsmokers,
asymptomatic, with no family history of diabetes, did not take medications, and revealed no physical abnormalities during examination.
All subjects had normal resting values of blood pressure, normal
plasma levels of glucose, insulin, total cholesterol, low-density lipoprotein (LDL) cholesterol, high-density lipoprotein (HDL) cholesterol, and triglycerides (TG), normal hematological parameters including leukocyte counts, and normal hepatic, thyroid, and renal
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Krogh-Madsen R, Thyfault JP, Broholm C, Mortensen OH, Olsen
RH, Mounier R, Plomgaard P, van Hall G, Booth FW, Pedersen BK.
A 2-wk reduction of ambulatory activity attenuates peripheral insulin
sensitivity. J Appl Physiol 108: 1034 –1040, 2010. First published December 31, 2009; doi:10.1152/japplphysiol.00977.2009.—US adults
take between ⬃2,000 and ⬃12,000 steps per day, a wide range of
ambulatory activity that at the low range could increase risk for
developing chronic metabolic diseases. Dramatic reductions in physical activity induce insulin resistance; however, it is uncertain if and
how low ambulatory activity would influence peripheral insulin sensitivity. We aimed to explore if healthy, nonexercising subjects who
went from a normal to a low level of ambulatory activity for 2 wk
would display metabolic alterations including reduced peripheral
insulin sensitivity. To do this, ten healthy young men decreased their
daily activity level from a mean of 10,501 ⫾ 808 to 1,344 ⫾ 33
steps/day for 2 wk. Hyperinsulinemic-euglycemic clamps with stable
isotopes and muscle biopsies, maximal oxygen consumption
(V̇O2 max) tests, and blood samples were performed pre- and postintervention. A reduced number of daily steps induced a significant
reduction of 17% in the glucose infusion rate (GIR) during the clamp.
This reduction was due to a decline in peripheral insulin sensitivity
with no effect on hepatic endogenous glucose production. The insulinstimulated ratio of pAktthr308/total Akt decreased after step reduction,
with a post hoc analysis revealing the most pronounced effect after 4
h of insulin infusion. In addition, the 2-wk period induced a 7%
decline in V̇O2 max (ml/min; cardiovascular fitness). Lean mass of legs,
but not arms and trunk, decreased concurrently. Taken together, one
possible biological cause for the public health problem of Type 2
diabetes has been identified. Reduced ambulatory activity for 2 wk in
healthy, nonexercising young men significantly reduced peripheral
insulin sensitivity, cardiovascular fitness, and lean leg mass.
AMBULATORY ACTIVITY AND INSULIN SENSITIVITY
HR variability, and ECG magnitude for epoch settings of 15, 30, and
60 s and can be recorded for a set time. Data on interbeat intervals (IBI
logging) and ECG waveforms can also be recorded. Acceleration is
measured by a piezoelectric element within the Actiheart with a
frequency range of 1–7 Hz (3 dB). For every participant, the Actiheart
monitor was tested for adequate HR pickup by recording ECG
waveforms for ⬃30 s before the rest test. If pickup was adequate, the
Actiheart was set up to record HR and movement continuously for 1
wk, after which it was reloaded for the rest of the study period.
Fat and fat-free tissue masses for the whole body, trunk, and
extremities were measured using a dual-energy X-ray absorptiometry
(DXA) scanner, Lunar Prodigy Advance (GE Healthcare, Madison,
WI) (21).
Hyperinsulinemic-euglycemic clamps were performed after an
overnight fast (Fig. 1). Peripheral catheters were placed in an antecubital vein for blood sampling, in the contralateral antecubital vein
for infusion of glucose, insulin, and stable isotopes, and in a dorsal
hand vein for blood sampling; the catheterized hand was wrapped in
a heating blanket to obtain arterialized venous blood for measurement
of glucose and potassium during the clamp. The experiment was
commenced by priming the glucose pool by administrating a bolus of
17.5 ␮mol/kg of [6,6-2H2]glucose (Cambridge Isotopes Laboratories).
This was followed by a continuous infusion (rate 0.4 ␮mol·kg⫺1·min⫺1)
for 2 h before the start of the clamp.
Insulin (Actrapid, Novo Nordisk Insulin, 100 IE/ml) was infused
continuously at a rate of 40.0 mU·min⫺1·m⫺2, and the plasma glucose
concentration was kept at 5.0 mM by a coinfusion of glucose (200
g/1,000 ml, enriched with [6,6-2H2]glucose to 2.5%) at a variable rate.
During the 4-h clamps the infusion rate of [6,6-2H2]glucose was
reduced to 0.08 ␮mol·kg⫺1·min⫺1. Arterialized blood was analyzed
Fig. 1. Experimental design. Time lines of
the study (A) and of the hyperinsulinemiceuglycemic clamp (B). V̇O2 max, maximal oxygen consumption; DXA, dual-energy X-ray
absorptiometry.
J Appl Physiol • VOL
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parameters. None of the participants performed planned exercise
sessions of ⬎2 h/wk or walked ⬍3,500 steps/day.
Study design. In a free-living environment, participants were instructed to nightly record their daily steps (cycling not included) for 1
inclusion-week and then reduce steps to 1,500 per day for 14 days
(cycling not allowed). Step number was measured using a simple
pedometer (Yamax Digi-Walker SW-200, London, UK); daily physical activity was also monitored by an Actiheart monitor (Cambridge,
UK). Subjects did not exercise more than 2 h/wk on a recreational
level. Subjects who walked less than 3,500 steps per day or performed
regular exercise were excluded. All subjects agreed to refrain from
vigorous physical activity and only performed physical activity corresponding to their normal routines, for 2 days preceding the first test
day. The number of steps was registered every night at bedtime.
Dietary records were taken during the inclusion-week. The subjects
carefully noticed what they consumed day by day, and they maintained their usual dietary habits during the whole study. Subjects were
not allowed to consume alcohol 2 days preceding the first test and
during the study period. Participants agreed to weigh and register
consumed food the day before pre- and post-insulin clamp and the
V̇O2 max tests. Figure 1 illustrates testing times.
Determinations. An incremental exercise test was performed on a
cycle ergometer (Monark 839E, Monark Ltd, Varberg, Sweden) to
obtain V̇O2 max for each participant with indirect calorimetry system
(Moxus modular V̇O2 system, AEI Technologies, Pittsburgh, PA) as
previously described (1). Preceding the study, subjects performed a
familiarization test.
The Actiheart monitor [used to measure daily energy expenditure
(7)] is attached to the chest with two standard electrocardiogram
(ECG) electrodes and is able to measure acceleration, heart rate (HR),
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AMBULATORY ACTIVITY AND INSULIN SENSITIVITY
for glucose and potassium concentrations at intervals of 5 min during
the first hour, and every 10 min during the last 3 h of the clamp. Blood
samples were collected in heparin-containing tubes and immediately
centrifuged for 15 min at 3,500 rpm, and the plasma was stored at
⫺20°C until further analysis.
The plasma glucose enrichment was measured as previously described (23). The glucose turnover rate of appearance (Ra) and rate of
disappearance (Rd) were calculated assuming steady state:
baseline: Ra⫽Rd⫽
Finfusion
Eglucose
clamp: Ra共endogenous兲⫽
Ftotal
Eglucose
⫺ GIR, Rd⫽
Ftotal
Eglucose
RESULTS
Pedometer and Actiheart data. For a 2-wk period, participants reduced the number of daily steps by more than 85%
J Appl Physiol • VOL
Table 1. Correlation coefficients
⌬Daily Steps
⌬Physiological Variables
r
P value
⌬Daily energy expenditure, kJ/day
⌬V̇O2max, ml·kg⫺1·min⫺1
⌬V̇O2max, ml/min
⌬Leg lean mass. kg
⌬Glucose infusion rate, mg·kg⫺1·min⫺1
0.922
0.921
0.960
0.262
⫺0.225
0.001
⬍0.01
0.001
NS
NS
Pearson correlation coefficients (r) are given between ⌬ in daily steps and ⌬
in physiological variables, where ⌬ represents a decrease from pre- to postintervention. No other but the mentioned factors correlated. V̇O2max, maximal
oxygen consumption; NS, nonsignificant; n ⫽ 10.
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where Finfusion is the infusion rate of glucose tracer (␮mol·kg⫺1·min⫺1)
in terms of lean body mass; Eglucose is the enrichment of glucose in
plasma; Ftotal is the sum of Finfusion; and the [6,6-2H2]glucose infused by
the clamp; GIR is the glucose infusion rate; and kilograms refers to lean
body mass.
Baseline blood samples were obtained before initiation of the stable
isotope infusion (time point ⫺2 h, Fig. 1). Glucose, triglyceride (TG),
and free fatty acids (FFA) were measured with an automatic analyzer
(Cobas Fara, Roche, Basel, Switzerland); insulin and C-peptide by
ELISA (DAKO, Glostrup, Denmark); tumor necrosis factor-␣ (TNF-␣),
interleukin (IL)-6, IL-15, and leptin by ELISA (R&D Systems); and
adiponectin by a RIA kit (LINCO Research).
Vastus lateralis muscle biopsies were taken before and 1 and 4 h
after the starting of the clamp. Muscle was quickly frozen in liquid
nitrogen and stored at ⫺80°C until later analysis. Muscle tissue was
homogenized in ice-cold buffer containing protease inhibitors and
phosphatase inhibitors as previously referenced (13, 29). Insulin
receptor ␤ (IR␤), phosphorylation of Akt at threonine 308 (pAktthr308), total
Akt, phosphorylation of AS160 (pAS160), and AS160 total content
were performed as referenced previously (29). Regarding tyrosine
phosphorylation of IR␤ (pTyIR␤), muscle homogenate was incubated
overnight with anti-phosphotyrosine agarose beads (Sigma). Both
immunoprecipitates and muscle homogenates were separated by SDSPAGE and transferred to PVDF membranes before probing with
appropriate antibodies. The protein bands were detected using Supersignal West femto (Pierce, Rockford, IL, USA) or enhanced chemiluminescence (Amersham Biosciences, UK) and quantified using a
CCD image sensor and software (Quantity One, Bio-Rad, CA).
Total RNA was extracted and analyzed from skeletal muscle using
previously referenced methods (15). The primers and probes for
TNF-␣ and 18S rRNA were predeveloped TaqMan probes and primer
sets from Applied Biosystems (Foster City, CA). The relative expression of TNF-␣ was normalized to endogenous 18S.
Statistical analyses. Data were tested for normal distribution by the
Kolmogorov-Smirnov analysis. Variables were analyzed using parametric methods (paired t-tests; pre vs. post) on the absolute data, and
reported values are means ⫾ SE. Correlations were determined with
a Pearson correlation test. All Western blots were normally distributed
when log-transformed before analysis and then analyzed using parametric methods and expressed as geometric mean (95% confidence
interval). Within-subject variation over time and variation between
trials were analyzed using repeated-measures (2-way ANOVA, timeby-trial). If a significant interaction (time ⫻ trial) was found, Bonferroni-corrected P values were obtained as appropriate to identify
significant differences between trials and to identify significant differences from baseline values. P ⬍ 0.05 was considered statistically
significant. Analysis was performed using a statistical software package (SAS version 9.1.3 SAS Institute).
from a baseline of ⬃10,000 per day, as published previously
(21), resulting in reduced energy expenditure (kJ/day; estimated by Actiheart) from a mean of 5,020 ⫾ 515 to 3,010 ⫾
294 kJ/day. The decline in number of steps from baseline to 2 wk
significantly correlated with the decline in daily energy expenditure
(kJ/day) (r ⫽ 0.922; P ⫽ 0.001), and decline in V̇O2 max (ml/min)
(r ⫽ 0.960; P ⫽ 0.001) or V̇O2 max (ml·min⫺1·kg⫺1) (r ⫽ 0.921; P ⬍
0.01) (Table. 1).
Cardiovascular fitness and body composition (DXA). A
significant 7.2% and 6.6% decline was observed in V̇O2 max
(ml/min and ml·min⫺1·kg⫺1, respectively) following reduced
stepping (Table 2). Lean mass of legs, but not arms or trunk,
also decreased significantly after reduced ambulatory activity
(Table 2). The decline in number of steps from baseline to 2 wk
did not correlate with the decline in lean mass of the legs
(Table 1). There was no change in total fat mass (21).
Metabolism. GIR during the 240-min clamp significantly
declined 17% after 2 wk of reduced stepping (pretesting ⫽
6.29 ⫾ 0.62 mg·kg⫺1·min⫺1; posttesting ⫽ 5.22 ⫾ 0.54
mg·kg⫺1·min⫺1) (Fig. 2A). Plasma glucose was kept at ⬃5.0
mM during the clamps (pretesting ⫽ 4.96 ⫾ 0.05 mM; posttesting 4.92 ⫾ 0.08 mM) (Fig. 2B). Reduced stepping also
resulted in reduced insulin-stimulated glucose disappearance
rate (Rd) (an index of decreased peripheral insulin sensitivity),
but did not change endogenous hepatic glucose production rate
(Ra) (Fig. 2C) or basal glucose turnover (data not shown).
Calculations for glucose metabolism were performed during
steady-state euglycemia (120 –240 min of clamp).
There was no significant difference in baseline values preand postintervention with regard to plasma levels of TG, FFA,
glucose, insulin, and C-peptide (Table 2). The decline in
number of steps from baseline to 2 wk did not correlate with
the decline in GIR (Table 1).
Insulin signaling. Insulin stimulation increased ratios of
phosphorylated to total protein for both Akt protein (pAktthr308/
total Akt) and AS160 protein (pAS160/total AS160) in the
vastus lateralis muscle at 1 and 4 h of the clamp compared with
baseline (time point 0 h) (P ⬍ 0.05 for all comparisons), but
not for IR␤ protein (pTyIR␤/total IR␤) (Fig. 3). Reduced
stepping led to a significant reduction in insulin-stimulated
pAktthr308/total Akt (pre- vs. postintervention) at the 4th hour
of clamp (P ⫽ 0.03) (Fig. 3) and although pAS160/total AS160
and pTyIR␤/total IR␤ signaling tended to decrease from prevs. postintervention, the changes were not significant (Fig. 3).
Inflammation. Baseline (time point ⫺2 h) plasma levels of
TNF, IL-6, IL-15, leptin, and adiponectin (Table 2) and skel-
AMBULATORY ACTIVITY AND INSULIN SENSITIVITY
Table 2. Pre- and post-2-wk intervention results
Measurement
Postintervention
Mean
SE
Mean
3,435
47.7
259
194
149
1.3
11
3
3,194
45.1
243
193
70.9
26.3
6.7
18.6
1.9
0.8
0.3
0.5
69.7
25.6
6.7
18.1
SE
130
1
11
4
2
0.6
0.3
0.5
5.39
0.11
5.26
0.11
31.9
3.7
31.8
3.6
337.6
27.5
335.4
22.6
1,001
199
1,266
430
617
56
647
47
1.31
0.13
1.38
0.16
0.79
0.22
1.02
0.19
1.29
0.1
1.27
0.2
5,056
803
4,841
812
2,334
389
2,676
410
P Value
⬍0.01
⬍0.01
⬍0.01
NS
⬍0.001
NS
NS
⬍0.001
NS
NS
NS
NS
NS
NS
NS
NS
NS
NS
V̇O2max test results, dual-energy X-ray absorptiometry (DXA) scans, and
plasma concentrations on lipid and glucose metabolism as well as inflammation are shown. Total lean mass decreased significantly post-2 wk intervention
vs. preintervention (see Ref. 21). HR, heart rate, FFA, free fatty acids; TG,
triglycerides. n ⫽ 10.
etal muscle TNF mRNA and protein expression (data not
shown) were not affected by 2 wk of reduced daily stepping.
DISCUSSION
Major novel findings of this study were that 2 wk of reduced
daily ambulatory activity in a free-living condition resulted in
decreases in insulin-stimulated muscle Akt phosphorylation,
peripheral insulin sensitivity, leg lean mass, and V̇O2 max in
young healthy men who had not been previously exercise
trained. As shown by others in previous studies, the later three
aforementioned decreases are primary decrements that can lead
to chronic metabolic disorders and premature mortality.
Importantly, the experimental design, used here, models the
extremes [⬃2,000 to ⬃12,000 (4)] in numbers of daily steps in
the majority of free-living adults in developed countries, most
of whom do not partake in structured exercise programs (4).
Therefore, the present study is distinctly different from previous studies in which nontypical and extreme changes in activity levels were deployed including continuous bed rest in
which subjects are not allowed to stand or walk (8) and
detraining studies in which intense structured-exercise programs ceased (2, 18). Furthermore, subjects served as their
own control, eliminating potential gene differences that might
occur by comparing separate groups of active and less active
subjects.
The attenuation of peripheral insulin sensitivity after 2 wk of
the decreased ambulatory activity occurred without any detectable alteration in endogenous hepatic glucose production, implying that a peripheral decrement in insulin sensitivity is
primary to hepatic insulin resistance on reductions in daily
stepping. These findings further reinforce the dogma that
glucose entry is exquisitely matched with substrate usage in
skeletal muscle, in part by modulating muscle insulin sensitivJ Appl Physiol • VOL
ity. A remaining question to be answered is whether reduced
ambulatory activity either increases susceptibility to or is a
necessary component of insulin resistance. There is strong
support for the latter suggestion as high-fat diet-induced insulin
resistance or obesity-associated insulin resistance does not
normally occur in skeletal muscle when a sufficient level of
increased physical activity or acute exercise is imposed (19, 22,
26, 29), and epidemiological evidence clearly shows that reduced physical activity is a major risk for Type 2 diabetes
while high activity levels is protective (11, 12, 17).
Previous studies have shown that acute, 7-day exercise
protocols dramatically improve insulin sensitivity (10, 27).
However, the ability of increased physical activity (7 days of
exercise) to improve insulin sensitivity is blocked if subjects
are maintained in an energy balance by consuming postworkout calories that match the amount of calories expended in each
daily exercise bout (3). This evidence strongly suggests that the
ability of increased activity to improve insulin action is linked
to acutely induced energy deficits. Because the subjects in the
present study maintained the same caloric intake but dramatically lowered their daily activity it is likely that they were in a
state of positive energy balance. This concept is contradicted
by the evidence that subjects had a 1.6% reduction in total
body mass. That being said, if the subjects were in a positive
energy balance, it is likely that this excess energy played a role
in the loss of peripheral insulin sensitivity after reduced stepping. A followup study could examine this link by having one
group of subjects consume a reduced caloric intake that
matches the reduction in energy expenditure that occurs after
reducing daily stepping from 12,000 to 2,000 steps. This type
of design would not only help to define the role of energy
balance on insulin resistance induced by reduced stepping but
would also help to determine the impact of positive energy
balance on the loss of total body mass and lean body mass that
was witnessed.
Although we provide novel evidence that acutely reducing
ambulatory activity negatively impacts insulin sensitivity, previous examinations have found that exercise cessation in endurance athletes has a similar impact. Heath et al. (9) showed
that 10 days of exercise cessation in endurance athletes dramatically increased insulin responses to an OGTT. A similar
result was witnessed in master endurance athletes (⬃61 yr old)
who ceased exercise training for 10 days (25). King et al. (14)
showed that endurance athletes who cease training for 10 days
have a reduction in peripheral insulin sensitivity measured by
hyperinsulinemic-euglycemic clamp. They further determined
that this was due to a loss of insulin sensitivity and not a total
loss of insulin responsiveness as a much larger infusion of
insulin actually sustained insulin sensitivity after 10 days of
exercise cessation. In a study from another group, the cessation
of exercise in endurance athletes for only 60 h lowered insulin
sensitivity to what was measured in sedentary age-matched
controls (6). Similar results have been found in rodents. Kump
and Booth (16) have shown that rats allowed daily access to
voluntary running wheels for 3 wk (average daily run distance
equaled 5.7 km) display rapid losses of insulin-stimulated
glucose uptake and insulin signaling in skeletal muscle after
their daily running was inhibited by wheel lock for only ⬃2
days (53 h).
Although the evidence that exercise cessation or reduced
ambulatory activity leads to a rapid decline in skeletal muscle
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V̇O2max test
V̇O2max, ml/min
V̇O2max, ml·min⫺1·kg⫺1
Maximal power, W
Maximal HR, beats/min
DXA scan
Total body mass, kg
Trunk lean mass, kg
Arm lean mass, kg
Leg lean mass, kg
Plasma
Glucose, mmol/l
Insulin, pmol/l
C-peptide, pmol/l
FFA, mmol/l
TG, ␮mol/l
TNF, pg/ml
IL-6, pg/ml
IL-15, pg/ml
Adiponectin, ng/ml
Leptin, pg/ml
Preintervention
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AMBULATORY ACTIVITY AND INSULIN SENSITIVITY
insulin sensitivity in human and animal models is very strong,
it remains underappreciated as an initiator of metabolic disease
and an underused model to study initial declines in insulin
sensitivity that likely precede overt insulin resistance. The
most common experimental model for studying insulin resistance is acute and long-term high-fat diets, which, as we have
already stated, appear to be effective in sedentary conditions
only. A recent study by Brons et al. (5) showed that a 5-day
Fig. 3. Insulin signaling in skeletal muscle during
hyperinsulinemic-euglycemic clamp. Akt, AS160, and
insulin receptor (IR)-␤ protein content and phosphorylation (p) were examined in biopsies taken from
vastus lateralis muscle pre- and post-euglycemic
clamp. Muscle biopsies were obtained at time points 0
h (before insulin clamp) and 1 h and 4 h (after initiation
of insulin clamp); labels on x-axis indicate these times.
Labels on y-axis indicate: phosphorylation of IR␤ beta
expressed relative to IR␤ protein (A); phosphorylation
of Akt at threonine308 expressed relative to Akt protein (B); phosphorylation of AS160 expressed relative
to AS160 protein (C); and representative immunoblots
of signaling proteins. pTYIR␤, tyrosine phosphorylated IR␤. Data are expressed as geometric means
(95% confidence interval). *Post hoc analysis, P ⫽
0.03 (difference between pre- and postintervention). **Post
hoc analysis, P ⬍ 0.05 (different from 0 h). n ⫽ 10.
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Fig. 2. Glucose values during hyperinsulinemic-euglycemic clamp. A: glucose infusion rate
(GIR) during the euglycemic clamp. B: blood
glucose concentration just before (time point 0)
and during the euglycemic clamp. C: area under the curve (AUC) for glucose rate of appearance (Ra) and rate of disappearance (Rd) during
the euglycemic clamp (120 –240 min). *Significant difference between preintervention (Pre)
and postintervention (Post) (paired t-test, P ⫽
0.01) Preintervention is before reduced daily
steps; postintervention is final 3 days of 14-day
period of reduced daily steps. , Preintervention determination; , postintervention determination; filled bars, preintervention determination; open bars, postintervention determination.
Data are presented as mean ⫾ SEM. n ⫽ 10.
AMBULATORY ACTIVITY AND INSULIN SENSITIVITY
ACKNOWLEDGMENTS
All authors have contributed to the conception and design, or the analysis
and interpretation of data; the drafting of the article or revising of the article;
and the final approval of the version of the article to be published.
Ruth Rousing and Hanne Villumsen are acknowledged for technical assistance.
GRANTS
The study was supported by the Commission of the European Communities
(contract no. LSHM-CT-2004-005272 EXGENESIS) and by grants from the
Augustinus Foundation, The Novo Nordisk Foundation, and from Unilever.
R. H. Olsen received a scholarship from the Danish Research Council. The
Centre of Inflammation and Metabolism is supported by a grant from the
Danish National Research Foundation (DG 02-512-555).
DISCLOSURES
No conflicts of interest are declared by the authors.
J Appl Physiol • VOL
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high-fat/high-calorie diet in young healthy men (daily activity
levels were not reported) induced hepatic but not peripheral
insulin resistance measured by a hyperinsulinemic-euglycemic
clamp. This suggests that diet-induced insulin resistance may
initially be activated through hepatic disturbances while reductions in activity lead to declines in skeletal muscle insulin
sensitivity. Type 2 diabetes has been linked to both increased
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plausible that both of these lifestyle factors work in parallel to
impair both hepatic and skeletal muscle insulin sensitivity and
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The loss of 2.8% in leg lean mass with 2 wk of reduced daily
ambulatory activity was unanticipated as most literature suggests that reduced muscle loading must be drastic, such as limb
immobilization or bed rest, to observe muscle atrophy. Thus it
is likely that decreases in leg lean mass and in the rate of
glucose disappearance per kilogram of whole body lean mass
(Fig. 2C) both contribute to decreased glucose infusion rate in
a hyperinsulinemic-euglycemic clamp (Fig. 2A) after 2 wk of
reduced ambulatory activity.
A 7% suppression of cardiovascular fitness (V̇O2 max) after
lowered ambulatory activity was also unanticipated based on
our notion that the loss in ambulatory activity would be of
insufficient magnitude and duration to produce a loss. Because
of the previously established strong connection between reduced cardiorespiratory fitness and premature mortality, the
rapid decline in V̇O2 max is clinically significant in these young,
healthy subjects after only 2 wk of reduced stepping.
The parallel losses of peripheral insulin sensitivity, leg lean
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the best predictor of mortality (28), low skeletal muscle
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believed to be a primary initiator of many chronic diseases
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predispose to other environmental-gene interactions, especially
in at-risk populations such as overweight, elderly, or genetically predisposed humans, that lead to the early development
of chronic diseases and/or premature mortality. Chronic diseases take years to become overtly clinical. Our observations
suggest that reductions in daily steps or physical activity may
initiate or contribute to progressive declines in metabolic
function.
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