The impact of brief high-intensity exercise on blood glucose levels

Abstract

Background
Moderate-intensity exercise improves blood glucose (BG), but most people fail to achieve the required exercise volume. High-intensity exercise (HIE) protocols vary. Maximal cycle ergometer sprint interval training typically requires only 2.5 minutes of HIE and a total training time commitment (including rest and warm up) of 25 minutes per session. The effect of brief high-intensity exercise on blood glucose levels of people with and without diabetes is reviewed.

Methods
HIE (≥80% maximal oxygen uptake, VO2max) studies with ≤15 minutes HIE per session were reviewed.

Results
Six studies of nondiabetics (51 males, 14 females) requiring 7.5 to 20 minutes/week of HIE are reviewed. Two weeks of sprint interval training increased insulin sensitivity up to 3 days postintervention. Twelve weeks near maximal interval running (total exercise time 40 minutes/week) improved BG to a similar extent as running at 65% VO2max for 150 minutes/week. Eight studies of diabetics (41 type 1 and 22 type 2 subjects) were reviewed. Six were of a single exercise session with 44 seconds to 13 minutes of HIE, and the others were 2 and 7 weeks duration with 20 and 2 minutes/week HIE, respectively. With type 1 and 2 diabetes, BG was generally higher during and up to 2 hours after HIE compared to controls. With type 1 diabetics, BG decreased from midnight to 6 AM following HIE the previous morning. With type 2 diabetes, a single session improved postprandial BG for 24 hours, while a 2-week program reduced the average BG by 13% at 48 to 72 hours after exercise and also increased GLUT4 by 369%.

Conclusion
Very brief HIE improves BG 1 to 3 days postexercise in both diabetics and non-diabetics. HIE is unlikely to cause hypoglycemia during and immediately after exercise. Larger and longer randomized studies are needed to determine the safety, acceptability, long-term efficacy, and optimal exercise intensity and duration.

Full text

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Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy 2013:6 113–122
Diabetes, Metabolic Syndrome and Obesity: Targets and erapy
The impact of brief high-intensity exercise
on blood glucose levels
O Peter Adams
Faculty of Medical Sciences,
the University of the West Indies,
Cave Hill Campus, St Michael,
Barbados
Correspondence: O Peter Adams
Faculty of Medical Sciences, the
University of the West Indies,
Cave Hill Campus, St Michael, Barbados
Tel +24 641 741 18
Fax +24 642 967 38
Email peter.adams@cavehill.uwi.edu
Background: Moderate-intensity exercise improves blood glucose (BG), but most people fail
to achieve the required exercise volume. High-intensity exercise (HIE) protocols vary. Maximal
cycle ergometer sprint interval training typically requires only 2.5 minutes of HIE and a total
training time commitment (including rest and warm up) of 25 minutes per session. The effect
of brief high-intensity exercise on blood glucose levels of people with and without diabetes is
reviewed.
Methods: HIE ($80% maximal oxygen uptake, VO2max) studies with #15 minutes HIE per
session were reviewed.
Results: Six studies of nondiabetics (51 males, 14 females) requiring 7.5 to 20 minutes/week
of HIE are reviewed. Two weeks of sprint interval training increased insulin sensitivity up
to 3 days postintervention. Twelve weeks near maximal interval running (total exercise time
40 minutes/week) improved BG to a similar extent as running at 65% VO2max for 150 minutes/
week. Eight studies of diabetics (41 type 1 and 22 type 2 subjects) were reviewed. Six were
of a single exercise session with 44 seconds to 13 minutes of HIE, and the others were 2 and
7 weeks duration with 20 and 2 minutes/week HIE, respectively. With type 1 and 2 diabetes,
BG was generally higher during and up to 2 hours after HIE compared to controls. With type
1 diabetics, BG decreased from midnight to 6 AM following HIE the previous morning. With
type 2 diabetes, a single session improved postprandial BG for 24 hours, while a 2-week
program reduced the average BG by 13% at 48 to 72 hours after exercise and also increased
GLUT4 by 369%.
Conclusion: Very brief HIE improves BG 1 to 3 days postexercise in both diabetics and non-
diabetics. HIE is unlikely to cause hypoglycemia during and immediately after exercise. Larger
and longer randomized studies are needed to determine the safety, acceptability, long-term
efficacy, and optimal exercise intensity and duration.
Keywords: high-intensity interval training, sprint interval training, diabetes, glucose
The impact of brief high-intensity exercise
on blood glucose levels
Type 2 diabetes is a worldwide epidemic associated with obesity and a sedentary
lifestyle.1 The estimated lifetime risk of developing diabetes for a person born in the
United States in 2000 is 32.8% for males and 38.5% for females.2 Diabetes increases
morbidity and mortality due to heart disease, stroke, blindness, kidney failure, foot
problems, and periodontal disease,3 and has a significant impact on quality of life.4
In 2010 it accounted for US$376 billion or 12% of the global health expenditure. This
is approximately US$1330 per person per year.5
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Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy 2013:6
Treatment goals for patients with diabetes include achiev-
ing and maintaining optimal blood glucose, blood pressure,
and lipid levels in order to prevent or delay the progression
of chronic complications.6 Exercise, along with diet and
weight control, is considered essential for the prevention and
management of diabetes. Epidemiological studies suggest
that physical activity can reduce the risk of type 2 diabetes
by 30% to 50% in the general population.7 Exercise helps
treat the glucose, blood pressure, and lipid abnormalities
often found in people with diabetes, and assists with weight
loss maintenance.8 In the United States, only 39% of adults
with diabetes are active compared to 58% of those without
the condition.9
VO2max, the maximum amount of oxygen in milliliters that
can be used in one minute per Kg of body weight, is a mea-
surement of cardiovascular fitness. It correlates with insulin
sensitivity in people at risk of developing type 2 diabetes.10
Moderate aerobic exercise requires 40% to 60% of VO2max or
50% to 70% of the maximum heart rate. Aerobic exercise is
considered vigorous when it requires . 60% VO2max or .70%
of the maximum heart rate.11 For many persons with diabetes,
moderate aerobic exercise would be the equivalent of brisk
walking.
Endurance aerobic exercise is usually performed continu-
ously over a prolonged period of time at submaximal intensity.
Most recommendations are for 150 to 210 minutes per week
of moderate-intensity endurance aerobic exercise, plus some
resistance exercise, spread over three to five sessions.8,12–14
This time commitment is in addition to all of the other self-
care activities recommended for people with diabetes, and a
lack of time is often cited as a reason for not exercising.15 A
cardiac evaluation may be required especially when vigorous
physical activity is being contemplated and in the presence of
additional risk factors for coronary artery disease.16
Effect of aerobic and resistance
exercise training on glycemic control
Meta-analyses on the effects of exercise have estimated that
for people with type 2 diabetes, both aerobic and resistance
exercise improve glycemic control to an extent comparable to
some oral antidiabetic drugs.17–23 Exercise should theoretically
be an attractive option for people who prefer not to use drugs,
or wish to obtain additional blood glucose control benefits.
There is some evidence that both exercise duration
and intensity affect HbA1c levels. A meta-analysis of ran-
domized controlled trials of at least 12 weeks in duration
concluded that structured exercise training of more than
150 minutes of exercise per week resulted in greater HbA1c
reductions (−0.89%), than those with less weekly exercise
time (−0.36%).18 Another meta-analysis of aerobic exercise
studies concluded that not only did higher exercise intensity
tend to produce larger improvements in VO2max, but that exer-
cise intensity predicted postintervention HbA1c (r = −0.91,
P = 0.002) better than exercise volume (r = −0.46, P = 0.26).
Workouts were, on average, 49 minutes (including 10 to
15 minutes of warm-up and cool-down), with a mean of
3.4 sessions per week for 20 weeks.24 However, only one study
included in the meta-analysis approached high-intensity at
75% of VO2max.25 In another meta-analysis for studies involv-
ing aerobic, resistance, and combined training, the overall
reduction in HbA1c was 0.8% (90% CI ±0.3) with the effect
of exercise intensity being unclear.20
Glucose metabolism during
moderate-intensity exercise
Skeletal muscle is responsible for most of the uptake of glucose
after a meal, and transport of glucose into the muscle is consid-
ered the limiting step in glucose disposal.26,27 Glucose transport
occurs primarily by diffusion utilizing glucose transporter
carrier proteins (GLUT). Both exercise and insulin regulate
glucose transport mainly by the translocation of the GLUT4
isoform from an intracellular compartment to the plasma mem-
brane and transverse tubules.8,28 GLUT4 levels are considered
an important determinant of insulin sensitivity.26,27
At rest and postprandially, glucose uptake is insulin-
dependent, with the major purpose being the replenishment
of muscle glycogen stores.8 Insulin-stimulated GLUT4 trans-
location is generally impaired in type 2 diabetes.28 During
exercise, muscle utilizes glucose made available by intramus-
cular glycogenolysis and by increased glucose uptake. Both
aerobic and resistance exercises increase GLUT4 abundance
and translocation, and hence blood glucose uptake by a path-
way that is not dependent on insulin.8 Glucose uptake into
contracting muscle is therefore normal even in the presence
of type 2 diabetes.8,28,29 Following exercise, glucose uptake
remains elevated, with the contraction-mediated pathway
remaining active for several hours.8
During moderate-intensity exercise (60% VO2max) of short
duration in persons without diabetes, increased glucose uptake
by muscle is balanced by an equal rise in hepatic glucose
production, and blood glucose levels remain unchanged.8,30
There is a decrease in insulin level, which sensitizes the
liver to glucagon, thus increasing glucose production.30
Catecholamines play a role in increasing glucose production
only during moderate-intensity exercise greater than 2 hours
duration. With type 2 diabetes, blood glucose uptake by
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Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy 2013:6
muscles usually increases more than hepatic production.31
This is also normally accompanied by a decline in plasma
insulin levels, greatly reducing the risk of hypoglycemia in
diabetics not using insulin or insulin secretagogues.8 The
effects of aerobic exercise vary with duration and intensity,
but following a single exercise session there is generally an
increase in insulin action and hence glucose tolerance for
between 24 and 72 hours.8
High-intensity exercise
High-intensity interval training (HIT) consists of brief bursts
of very vigorous exercise separated by brief recovery periods.
Total exercise time is short. While there is no universal
definition of HIT, it often refers to exercise performed with
an “all out” effort, or at least to an intensity that approaches
VO2max ($90% VO2max).32
The classic form of “all out” HIT is the Wingate test.
After about 3 to 5 minutes of warm-up the subject cycles
for 30 seconds at maximum effort against a standardized
resistance. Typically four to six Wingate tests are performed
separated by 4 minutes of rest, for a total of 2 to 3 minutes
of maximal exercise spread over 15 to 30 minutes.33 This “all
out” cycle ergometer form of HIT is also referred to as sprint
interval training (SIT). When used in this paper, SIT will refer
only to Wingate tests as just described. Because of the intensity
and short duration of the Wingate test, most of the power
generated represents anaerobic as opposed to aerobic power,
with an aerobic power contribution of between 16% and
19.5%.34–36 The primary energy source is glucose derived from
muscle glycogen, and as aerobic capacity is exceeded, most of
this is converted to lactate to provide anaerobic ATP. The initial
30-second Wingate test can use almost a quarter of the stored
muscle glycogen, and although the rate of glycogenolysis is
reduced in subsequent bouts, significant amounts of lactate
accumulate.37,38 The exercise is extremely stressful with the
perceived exertion being very high. Reports of nausea and
light-headedness are not uncommon.39 It requires a high
level of motivation, and often sessions are supervised, with
significant verbal encouragement to exert maximal effort.40
The average person with diabetes is not likely to tolerate this
well. Specialized costly equipment, usually a cycle ergometer,
is also required. While time spent exercising intensely is very
short, training time commitment is longer as it will include
warm-up, cool-down, and rest periods.
Other forms of HIT may not require “all out” effort and
may be performed at only half the intensity of an “all out”
SIT protocol but with longer sessions, more repetitions,
and shorter rest periods, and are tolerated much better.41,42
High-intensity exercise may also be performed on a continu-
ous basis, but even very fit persons can usually maintain an
intensity of $80% VO2max for only 10 to 15 minutes.30 The
exercise load needed depends on the individual’s exercise
capacity. For people with a low VO2max of 20 mL/Kg per
minute, the necessary exercise load may be equivalent to
walking up a slight grade at 3 mph.42
Glucose metabolism during
high-intensity exercise
In intense exercise (.80% VO2max), unlike at lesser intensi-
ties, glucose is the exclusive muscle fuel.30 Catecholamine
levels rise markedly, causing glucose production to rise
seven- to eightfold while glucose utilization is only increased
three- to fourfold. In people without diabetes there is a small
blood glucose increase during intense exercise that increases
further immediately at exhaustion and persists for up to
1 hour. Plasma insulin levels rise, correcting the glucose level
and restoring muscle glycogen. This physiological response
would be absent in type 1 diabetics.
Aerobic endurance and high-intensity
exercise
HIT is effective in improving aerobic endurance. In one
study six “all out” SIT sessions over 2 weeks improved
the mean cycle endurance time to fatigue while cycling at
approximately 80% of pretraining VO2max by 100% (from 26
to 51 minutes).43 This required a total high-intensity exercise
time of only 15 minutes with a total training time commit-
ment of approximately 2.5 hours. In another study, a less
intense version of HIT (6–10 cycling bouts of 30 seconds
each at 125% of the power at VO2max with 2 minutes recovery)
produced a similar improvement in VO2max after 4 weeks of
training, as was seen in the more intense SIT group (three
to five “all out” 30-second cycling bouts with 4 minutes
of recovery).41 The less intense HIT required only half the
intensity but double the repetitions of the SIT, and may be
more practical for the nonathlete.
Many people do not exercise despite the proven benefit
of endurance exercise. An exercise program requiring less
time commitment may appeal to some people. The aim of
this paper is to review the impact of high-intensity exercise
of short duration on blood glucose levels in diabetic and
nondiabetic people.
Methods
A narrative review was done. PubMed was searched in
July 2012 using the following search terms: high-intensity
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Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy 2013:6
interval training, sprint interval training, and high-intensity
exercise each combined with glucose and/or diabetes. To
be included: (1) exercise intensity had to be at least 80%
VO2max or 90% maximum heart rate, or include maximal
cycle ergometer sprints; (2) the duration was no more than
15 minutes of high-intensity exercise and 30 minutes of
total exercise time per session; and (3) glycemic control was
assessed. Review articles and references retrieved were hand
searched for additional primary studies.
Results
High-intensity interval training and insulin
sensitivity in healthy nondiabetic adults
Six studies with a total of 51 male and 14 female partici-
pants in the high-intensity exercise groups are reviewed.
Four of these used SIT (maximal effort cycle ergometer)
exercise protocols of 2 to 6 weeks duration,39,44–46 and two
used near maximal running as the intervention (Table 1).47,48
Although near maximal running would not be as intense as
the cycle ergometer, it was not surprising that in the study
with the heavier subjects,47 overuse shin splint injuries
caused three out of eight participants in the intense run-
ning group to miss between one and four training sessions.
Three studies divided participants into an intervention and
comparison group,39,47,48 with two stating that allocation
was random.39,48
Both Richards et al39 and Babraj et al44 studied young
healthy subjects who were sedentary or recreationally active
and found that 2 to 3 days after a 2-week exercise program
consisting of six sessions of SIT, insulin sensitivity but not
fasting blood glucose improved compared to baseline. Whyte
et al46 studied overweight and obese sedentary men, and
after a similar SIT protocol found no improvement in fasting
blood glucose; moreover, insulin sensitivity was improved
compared to baseline at 24 but not 72 hours. Richards et al39
randomized subjects to one of three groups: (1) six sessions of
SIT; (2) a single session of SIT; and (3) no exercise. Insulin
sensitivity was estimated before and 72 hours after the inter-
vention by means of a hyperinsulinemic euglycemic clamp,
considered the gold standard test. It increased in the group
that did six sessions of SIT (mean change of glucose infusion
rate: +1.7 ± 0.6 mg/kg per minute, P = 0.04) but not in the
single session SIT and no exercise groups. The intervention
had no effect on the thermogenic response to beta-adrenergic
receptor stimulation, which is considered an important
determinant of energy expenditure and by extension a major
regulator of energy balance and body mass. Babraj et al44
estimated insulin sensitivity before and 48 to 72 hours after
the intervention by means of oral glucose tolerance tests and
the Cederholm index. While FBG and fasting insulin levels
were unchanged, both glucose area under the curve (AUC;
−12%), and insulin AUC (−37%), were significantly reduced
during the oral glucose tolerance tests. In addition, aerobic
cycling performance was improved by about 6% (P , 0.01)
compared to baseline. Endurance aerobic and strength
training studies of up to 16 months duration have generally
demonstrated only a reduction in insulin AUC in response
to a glucose load following training, without a concurrent
reduction in glucose AUC.44
Burgomaster et al45 demonstrated that SIT increased
muscle GLUT4 content, a determinant of insulin sensitivity,
by 20% compared to baseline after 1 week of exercise, and
that the levels remained elevated over the remaining 5 weeks
of training and a subsequent 6 weeks of detraining. Muscle
oxidative capacity, as estimated by the protein content of
cytochrome c oxidase subunit 4 (COX4) also increased by
35% after 1 week of HIT, and remained higher compared
with baseline after 6 weeks of detraining (P , 0.05).
Nybo et al47 found that 20 minutes of near maximal run-
ning (40 minutes of total exercise time) per week for 12 weeks
was as effective as 150 minutes of running at 65% VO2max
per week over the same period, in improving both fasting
blood glucose and blood glucose 2 hours after the ingestion
of 75 g of glucose. For the latter, blood glucose was improved
from a mean of 6.1 (standard error (SE) ± 0.6) mmol/L to 5.1
(SE ± 0.4) mmol/L (P , 0.05) in the maximal running group
and from 5.6 (SE ± 1.5) mmol/L to 4.9 (SE ± 1.1) mmol/L
(P , 0.05) in the prolonged running group. Sandvei et al48 had
similar findings, with only 7.5 to 15 minutes of near maximal
running per week, but a longer total exercise time when warm-
up and rest periods were included.
These studies therefore demonstrate that in young nondia-
betic adults, as little as 15 minutes of high-intensity exercise
spread over 2 weeks is enough to improve insulin sensitivity
without a change of body weight. Energy expended would be
equivalent to about 500 Kcal. In contrast, a typical aerobic
training program consumes 2000 to 3000 kcal/week with
guidelines recommending 150 minutes of training per week.
It was postulated that despite the negligible energy expen-
diture, HIT improved insulin action by depleting muscle
glycogen stores.44
High-intensity training and glucose
regulation in people with diabetes
There has been little testing of brief high-intensity exercise
in either type 1 or type 2 diabetic patients. Eight studies were
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Table 1 Characteristics of reviewed high-intensity exercise studies on healthy people without diabetes, and effects on insulin sensitivity and blood glucose
Study Gender Intervention group
Age (years)
BMI (Kg/m2)
Weight (Kg)
Mean ± SD
Number in
intervention
group
Intervention Study duration Number in
comparison
group
Effect on measure
of blood glucose
Richards
et al39
5 male
7 female (sedentary
or recreationally
active)
Age: 29 ± 3
BMI: 26.2 ± 1.3
Weight: 76 ± 6
12 SIT (4 to 7 × 30 sec maximal
cycle ergometer efforts
separated by 4 minutes of rest).
Six SIT sessions over
2 weeks. Eight minutes
of high-intensity
exercise/week.
Single SIT control: 9
Sedentary control: 10
Six SIT sessions increased insulin
sensitivity signicantly 3 days after
the last session compared to
baseline, and comparison groups.
No effect on FBG.
Babraj
et al44
Male (sedentary or
recreationally active)
Age: 21 ± 2
BMI: 23.7 ± 3.1
Weight: 82 ± 17
16 SIT (4 to 6 × 30 seconds
of maximal cycle ergometer
efforts separated by
4 minutes of rest). Total time
commitment of 17 to
26 minutes per session.
Six SIT sessions
over 2 weeks.
Average of 7.5 minutes
of high-intensity
exercise/week.
Compared to baseline 2 to 3 days after the last session,
insulin sensitivity improved 23%
(P , 0.01), and plasma glucose
area under the curve decreased
(P , 0.01) compared to baseline.
No effect on FBG.
Burgomaster
et al45
Male (active) Age: 22 ± 1
Weight: 80 ± 4
8SIT (4 to 6 × 30 seconds
maximal cycle ergometer
efforts separated by
4 minutes of rest).
Three sessions per
week for 6 weeks.
An average of
7.5 minutes of high-
intensity exercise
per week.
Compared to baseline Muscle GLUT4 increased 20%
after 1 week of SIT and remained
elevated 6 weeks postexercise.
Whyte
et al46
Male
Sedentary
Age: 32 ± 9
BMI: 31 ± 4
Weight: 94 ± 13
10 SIT (4 to 6 × 30 seconds
of maximal cycle ergometer
efforts separated by
4.5 minutes of rest).
Six sessions over
2 weeks.
Compared to baseline No change in FBG and glucose
area under the curve at 24 and
72 hours after exercise, but
insulin sensitivity index higher
at 24 hours (P = 0.027).
Nybo
et al47
Male
Sedentary
Age: 37 ± 3
Weight: 96 ± 3
8 5-minute warm-up, then
5 × 2-minute intervals
of running with heart rate 95%
of maximum at the end of the
interval (total exercise
time 40 minutes/week).
Two sessions per week
for 12 weeks.
Twenty minutes
of high-intensity
exercise/week.
9, performed 1-hour
continuous running
at 65% VO2max (about
150 minutes/week)
11, no exercise
Similar lowering of FBG and
blood glucose 2 hours after
a 75 g glucose tolerance test,
done 48 hours after the last
exercise session.
Sandvei
et al48
4 males
7 females
Sedentary to
moderately trained
Age: 18 to 35
BMI: 23 ± 1
Weight: 70 ± 3.5
11 10-minute warm-up, then
5 to 10 × 30 seconds near
maximal sprints with
3-minute rest periods.
Three sessions/week
for 8 weeks.
12 performed
continuous running
at 70% to 80% maximal
heart rate for 90 to 180
minutes/week
High-intensity running, but not
continuous running, improved
insulin sensitivity 60 hours after last
exercise session. FBG signicantly
improved in both groups.
Abbreviations: BMI, body mass index; SD, standard deviation; SIT, sprint interval training; FBG, fasting blood glucose; GLUT4, glucose transporter protein 4; VO2max, maximal oxygen uptake.
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identified with only three of these involving type 2 diabetics.
In addition to maximal cycle ergometer SIT, HIT protocols
included a less intense form of HIT not requiring “all out”
effort, and protocols requiring very short bursts of exercise
(as little as 4 seconds) interspersing up to 30 minutes of mod-
erate exercise. Noninterval high-intensity exercise protocols
included continuous exercise at 80% to 110% VO2max for up to
13 minutes. Six studies measured the effects of single exercise
sessions, and the other two studies were of 2 and 7 weeks
duration. All involved small numbers of subjects (up to eight
different subjects in an intervention group), and except for
three studies the mean age was ,35 years (Table 2).
Effects of high-intensity exercise on blood
glucose in type 2 diabetic patients
A single session of continuous high-intensity exercise
resulted in 60 minutes of postexercise hyperglycemia,49
while both a single session of HIT,50 and a 2-week training
program51 have been shown to improve postprandial glucose
control over a 24-hour period following exercise.
Little et al51 evaluated the effects of six sessions of HIT
over 2 weeks on glucose regulation 48 to 72 hours after the
last training session in people with type 2 diabetes. Most par-
ticipants engaged in 60 minutes or less of exercise per week
prior to entering the study. The HIT protocol required only
30 minutes of high-intensity exercise per week, with a total
time commitment (including warm-up, cool-down, and rest)
of 75 minutes. The exercise intensity was less and may be
more acceptable than “all out” SIT protocols. When asked how
enjoyable would engaging in HIT three times per week for the
next 4 weeks be, the mean response was 7.9 ± 1.0 on a scale
ranging from 1 (not enjoyable at all) to 9 (very enjoyable).
Additionally, it elicited ratings of perceived exertion of 4 to 8
on a 10-point scale. Before training and from 48 to 72 hours
after the last training session, glucose regulation was assessed
using 24-hour continuous glucose monitoring under standard-
ized dietary conditions. The average 24-hour blood glucose
concentration was reduced by 13%, from 7.6 mmol/L (SD ± 1)
to 6.6 mmol/L (SD ± 0.7) after training (P , 0.05). The sum
of the 3-hour postprandial glucose AUC for breakfast, lunch,
and dinner was reduced by 30% (P , 0.05). GLUT4 protein
content was 369% higher after 2 weeks of training.
Gillen et al,50 studying 7 of the 8 individuals who
participated in the study by Little et al,51 and using an identical
exercise protocol, demonstrated that a single exercise session
also reduced the sum of the 3-hour postprandial glucose AUC
(P = 0.01) and the proportion of time spent above 10 mmol/L
in the 24-hour postexercise period when compared to a
nonexercising control day. Average 24-hour blood glucose
was, however, not significantly reduced (P = 0.16). The
results of the two studies might be clinically significant, as
controlling postprandial hyperglycemia is a treatment goal
with type 2 diabetics.
Kjaer et al49 investigated the effect of 5 minutes of high-
intensity exercise on blood glucose control during and for
3 hours immediately following exercise in type 2 diabetic
patients (two on a sulfonylurea and five on diet only). There
was a greater and more sustained rise in glucose levels in
type 2 diabetics compared to controls. In type 2 diabetic
subjects, blood glucose increased from a pre-exercise level
of 147 mg/dL (SD ± 21) to a peak 30 minutes postexercise
at 169 mg/dL (SD ± 19). This value was maintained until
60 minutes postexercise, and then plasma levels decreased
over the remainder of the 180-minute recovery period. For the
controls, blood glucose increased from a pre-exercise level
of 90 mg/dL (SD ± 4) to a peak at 10 minutes postexercise at
100 mg/dL (SD ± 5). Glucose concentrations at 60 minutes
postexercise did not differ significantly from pre-exercise
levels. In both groups, plasma insulin levels increased after
exercise above pre-exercise levels, and returned to baseline
about 120 minutes postexercise. Plasma epinephrine and
glucagon responses to exercise were higher in type 2 diabetics
than in control subjects (P , 0.05). However, 24 hours after
exercise in the type 2 diabetic group and not the controls,
there was an increased effect of insulin on glucose uptake
compared to the pre-exercise state as estimated by the insulin
clamp technique. Other studies have found similar increases
in insulin-mediated glucose disposal after short-term high-
intensity exercise in insulin-resistant subjects.52,53 It was
concluded that because of exaggerated counter-regulatory
hormonal responses, maximal dynamic exercise results in a
60-minute period of postexercise hyperglycemia and hyper-
insulinemia in type 2 diabetics.49
Effects of high-intensity exercise on blood
glucose in type 1 diabetic patients
The high-intensity studies involving type 1 diabetics mainly
investigated blood glucose control during and in the 2 hours
after exercise.
Harmer et al54 studied the effects of 7 weeks of SIT. The
number of cycle bouts per training session was increased from
four in week 1, to six in week 2, eight in week 3, and ten in weeks
4–7. SIT resulted in a greater rise in plasma glucose during and
immediately after exercise (a 20-minute period) in diabetics
compared to nondiabetic controls. This increase was significantly
attenuated by 7 weeks of training. HbA1c was not altered.
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Table 2 Characteristics of the reviewed high-intensity exercise studies on people with diabetes, and changes in BG control
Study Type of
diabetes
Number in
HIE group
Gender
Activity status
Intervention group
Age (years)
BMI (Kg/m2)
HbA1c (%)
Mean ± SD
Intervention Study duration Number in
comparison
group
Effect on measure of BG
Little
et al51
Type 2 Number: 8
Gender: not stated
Activity: most
sedentary
Age: 63 ± 8
BMI: 32 ± 6
HbA1c: 6.9% ± 0.7%
Ten 60-second efforts at
90% maximal HR on a cycle
ergometer interspersed with
60 seconds of rest (allowed
to rest or pedal slowly)
Three 25-minute
sessions per week for
2 weeks. Each session
included 3 minutes
of warm-up and
2 minutes of cool-down.
Compared
to baseline
Average 24-hour BG reduced 13%
(P , 0.05), and sum of the 3-hour
postprandial glucose AUC reduced 30%
(P = 0.05) 2 to 3 days after HIT compared
to preintervention.
Gillen
et al50
Type 2 Number: 7
Gender: not stated
Activity: most
sedentary
Age: 62 ± 3
BMI: 31 ± 2
HbA1c: 6.9% ± 0.7%
Ten 60-second efforts at
90% maximal HR on a cycle
ergometer interspersed with
60 seconds of rest (allowed
to rest or pedal slowly)
Single 25-minute session
including 3 minutes of
warm-up and 2 minutes
of cool-down.
7 (same individuals
as intervention
group) not
exercising
Average 24-hour BG not signicantly
reduced (P = 0.16), but sum of the 3-hour
postprandial glucose AUC reduced for24
hours following HIT (P = 0.01) compared
to no exercise.
Kjaer
et al49
Type 2 Number: 7
Gender: male
Activity: sedentary
Age: 55 ± 4
BMI: 27 ± 2
Cycling 7 minutes at 60%
VO2max, 3 minutes at 100%
VO2max, and 2 minutes
at 110% VO2max
Single 12-minute
exercise session
with 5 minutes of HIE.
7 nondiabetic BG increased more during exercise in the
diabetic group, and peaked 30 minutes
postexercise.
Harmer
et al54
Type 1 Number: 8
Gender: not stated
Age: 25 ± 4
BMI: 25 ± 3
HbA1c: 8.6% ± 0.8%
Four to eight 30-second
maximal cycle ergometer
exercise separated by
4 minutes of rest
Thrice weekly sessions
for 7 weeks.
7 nondiabetic BG increased more during and 20 minutes
postexercise in the diabetic as compared
to nondiabetic group. The increase was less
after 7 weeks of training. HbA1c not altered.
Guel
et al55
Type 1 Number: 8
Gender: not stated
Age: 19 ± 2
BMI: 22 ± 2
HbA1c: 7% ± 0.4%
Eleven 4-second maximal cycle
ergometer sprints separated
by 2 minutes of rest
Single 20-minute
exercise session.
8 (same individuals
as in the intervention
group) not exercising
BG declined more rapidly during exercise
in the HIT group, but remained stable in
the 1-hour postexercise period.
Guel
et al56
Type 1 Number: 7
Gender: 4 males,
3 females
Age: 22 ± 4
BMI: 25 ± 4
HbA1c: 7.4% ± 1.5%
Cycling at 40% VO2max
interspersed by sixteen
4-second maximal cycle
ergometer sprints
Single 30-minute
exercise session.
7 (same individuals
as the intervention
group) cycling at
40% VO2max
BG fell less in the HIT group, and did not
continue to decline postexercise unlike
with controls.
Maran
et al57
Type1 Number: 8
Gender: male
Activity: active
Age: 34 ± 7
BMI: 24 ± 2
HbA1c: 7.1% ± 0.6%
Cycling at 40% VO2max
interspersed by 5 seconds
of maximal sprints every
2 minutes
Single 30-minute
session.
8 (same individuals
as in the intervention
group) cycling
at 40% VO2max
Between 12 AM and 6 AM, postexercise
BG lower in the HIT group compared
to the comparison group (BG AUC
147 versus 225 mg/dL, P , 0.05).
Mitchell
et al58
Type 1 Number: 10
(8 different subjects
with 2 studied twice at
different pre-exercise
BG levels) Gender:
5 males, 5 females
Age: 29 Cycle ergometer at 80%
VO2max until exhausted
Single 10 to 13-minute
exercise session.
8 nondiabetic BG increased more in the diabetic group
and remained high 2 hours postexercise
unlike normal controls.
Abbreviations: BMI, body mass index; HbA1c, glycosylated hemoglobin; SD, standard deviation; HR, heart rate; BG, blood glucose; AUC, area under the curve; HIT, high-intensity training.
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Brief high-intensity exercise and blood glucose

Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy 2013:6
All the other studies involved a single exercise session.
Guelfi et al55 studied the effects of HIT (repeated 4-second
cycle ergometer efforts separated by rest) on blood glucose
during exercise and in the immediate 1-hour postexercise
period. Participants injected their normal dose of insulin and
had breakfast. After the postprandial peak in blood glucose, on
alternate days, participants either exercised or rested. During
exercise blood glucose declined more rapidly as compared
to the nonexercising controls, indicating that high intensity
exercise may increase the risk of hypoglcemia. This finding is
not supported by the other studies reviewed. However during
the recovery period blood glucose levels continued to decline
in the controls while remaining stable in the exercise group
suggesting a decreased risk of postexercise hypoglycemia.
Guelfi et al56 also compared a HIT protocol that was com-
bined with moderate-intensity exercise (repeated 4-second
cycle ergometer efforts over 30 minutes separated by cycling
at 40% VO2max) to moderate exercise only (cycling at 40%
VO2max) for 30 minutes. Exercise commenced 3.5 hours
postprandially when the blood glucose was about 11 mmol/L.
Blood glucose fell to a greater extent in the moderate exercise
group compared to the HIT group, and remained stable in
the HIT group in the 1-hour recovery period while continu-
ing to fall in the moderate-exercise group. Blood glucose
at 1-hour postexercise was 3.3 mmol/L lower than the pre-
exercise level in the HIT group, and 6.3 mmol/L lower in
the moderate exercise group (P = 0.021). However, Maran
et al,56 using a similar exercise protocol, demonstrated that
following morning HIT, blood glucose was significantly lower
between midnight and 6 AM the next day compared to when
only moderate-intensity exercise was done.57 Mitchell et al58
showed that continuous noninterval exercise at 80% VO2max
until exhaustion (approximately 10 to 13 minutes) increased
blood glucose during and in the 2-hour postexercise period.
Thus, unlike moderate-intensity exercise, HIT is unlikely
to cause hypoglycemia during or immediately postexercise
in type 1 diabetics, but 14 to 20 hours later it may result in
lower glucose levels.
Summary
Clinical practice guidelines typically recommend that people
with type 2 diabetes perform moderate to vigorous aerobic
exercise and resistance exercise three to five times per week
for a total of at least 150 to 210 minutes per week. Many
persons do not achieve the recommended amounts of exercise
with lack of time being cited as a reason.15 Meta-analyses
have shown that non-HIT programs can produce HbA1c
improvements between 0.6% and 0.89%, an amount that is
clinically significant and comparable to some medication
regimens.
Classic SIT protocols can require as little as 2 to
3 minutes of maximal exercise spread over 15 to 30 minutes.
With healthy nondiabetic subjects, 2-week protocols have
resulted in improvements in muscle GLUT4 content, insulin
sensitivity, and FBG. Insulin sensitivity improvement has
been sustained up to 3 days post-intervention. Improvements
in VO2max were similar to that achieved by much longer ses-
sions of endurance aerobic exercise.
In type 2 diabetic patients, a low-volume 2-week HIT
program increased GLUT4 protein, a marker of insulin sen-
sitivity, and decreased average blood glucose 48 to 72 hours
postexercise. The protocol used was less intense than SIT and
was acceptable to study participants. However, unlike with
moderate-intensity exercise, blood glucose levels tend to be
higher in both type 1 and 2 diabetic patients during and in the
2 hours immediately following intense exercise due to rising
catecholamine levels promoting glycogenolysis, and these
levels may remain high in the 2-hour post-exercise period.
HIT depletes muscle glycogen and it is possible that after
catecholamine levels decrease in the post-exercise phase,
a period of increased peripheral uptake of glucose follows
as glycogen stores are replenished.
Conclusion
The optimal exercise strategy has not been determined,
but low volume SIT with as little as 7.5 minutes of high-
intensity exercise per week may be a time-efficient exercise
strategy to help control blood glucose in diabetic patients
and improve insulin sensitivity in nondiabetic adults.
Unlike moderate-intensity exercise, high-intensity exercise
decreases the risk of hypoglycemia during and immediately
after exercise in diabetic patients. Therefore, there may be
no need for well controlled patients on insulin or insulin
secretagogues to eat or decrease medication dosage shortly
before high-intensity exercise. However, the perceived exer-
tion associated with the “all out” version of HIT is very
high and the acceptability, feasibility, and safety for the
sedentary diabetic and nondiabetic population are in doubt.
Both the risks of musculoskeletal injury and cardiovascular
complications have to be considered. The cost of exercis-
ing and the provision of facilities (equipment, supervision,
and gyms) also have to be taken into account if this form
of exercise is to have a mass impact. The less strenuous
version of HIT used by Little et al51 might be preferred to
“all out” SIT as it was well accepted while still being of
short duration.
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Diabetes, Metabolic Syndrome and Obesity: Targets and Therapy 2013:6
Studies on the effect of high-intensity exercise on blood
glucose have been few and of short duration, and have
involved a small number of patients who were probably
not representative of the general diabetic population. With
diabetics, it is therefore uncertain if any improvements in
blood glucose achieved by a brief intervention would be
sustained over a longer period, reduce HbA1c levels, improve
health outcomes, and can be replicated in the general dia-
betic population. Similarly, in the nondiabetic population
it is not known whether improvements in insulin sensitiv-
ity would be sustained and result in a clinically important
endpoint such as diabetes prevention. Further studies are
needed to determine whether HIT programs, perhaps in the
less intense form or as an adjunct to moderate-intensity
exercise, would be effective in the long-term and have a
high enough adherence rate to be efficacious. Large scale
randomized trials lasting years may be necessary to show
whether HIT can prevent diabetes, and such trials may be
impractical.
Disclosure
The author reports no conflicts of interest in this work.
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