Metabolic
Effects of HIIT
Research:
Studies find high-intensity interval training protects against heart disease
and shows promise for improving insulin sensitivity and controlling blood
pressure.
High-intensity
interval training is enormously popular in the fitness industry this year. HIIT
workouts typically include short bursts (6 seconds to 4 minutes) of intense
exercise (≥ 90% maximal aerobic capacity) alternating with relief breaks of
varying lengths. The workouts include a limitless variety of exercises,
including
Body weight Movements, Power lifting, Plyometrics,
Sprints, Playground activities, such as sled pushes and pulls, Multiple-mode
training on equipment such as cycle ergometers, treadmills and ellipiticals and
heavy rope drills.
Lots of fitness
professionals are advocating HIIT as a time-efficient way to improve health,
fitness and performance—but how much evidence validates their claims? Read on
to find out.
Figure 1
illustrates acute physiological responses to a HIIT workout, and long-term
health-related adaptations are discussed in this column.
How Much Can HIIT Improve
Cardiovascular Fitness?
Boutcher (2011)
completed a comprehensive research review on high-intensity interval training
(which he describes as intermittent training) and reports that healthy young
and older men and women can improve cardiorespiratory fitness (VO2max) 4%–46%
in training periods lasting 2–15 weeks. Kessler, Sisson & Short (2012) add
that HIIT appears to induce rather rapid changes in VO2max.
Boutcher says
the current scientific explanation for this increase in VO2max from a HIIT
program is that it causes an increase in stroke volume (volume of blood pumped
by the heart per beat), which happens chiefly because the heart muscle’s
contractility increases at near-maximal exertion. Boutcher adds that HIIT
increases mitochondrial biogenesis (the size and number of mitochondria
increase, allowing the cells’ energy-producing organelles to make more ATP),
which readily translates into improved cardiovascular capacity at any level of
exercise intensity.
From a health
perspective, this VO2max improvement is quite noteworthy, because low VO2max is
predictive of cardiovascular fatalities and all- cause mortality (Kessler,
Sisson & Short 2012). As shown in the large, pioneering Cooper Institute
study on the benefits of cardiovascular exercise, Blair et al. (1996) completed
medical exams and VO2max tests on 25,341 men and 7,080 women. Data from this
study clearly showed that in comparison with men and women of low fitness,
people who were moderately or highly fit—regardless of whether they were sick
or healthy, smokers or nonsmokers, had normative or elevated cholesterol
levels, or were obese or normal-weight—seemed to have enhanced protection from
factors associated with causing cardiovascular disease.
Additionally,
Swain and Franklin (2006) reviewed epidemiologic studies (which report incidence
or prevalence data) that assessed the benefits of moderate versus vigorous
physical activity, as well as clinical investigations (which test and analyze
results) that trained subjects at different intensities. All of these studies
found vigorous exercise intensity was more beneficial in improving one or more
risk factors for coronary heart dis- ease. Thus, HIIT should be considered a
most advantageous method for counteracting the effects of cardiovascular
disease, the number-one cause of death throughout the world (WHO 2013).
How Does HIIT Influence Fat
Loss and Weight Management?
Boutcher (2011)
says the striking surge in epinephrine and norepinephrine from HIIT training
may be a catalyst for improving fat loss. Both of these fight-or-flight
hormones (also known as adrenaline and noradrenaline) have been shown to drive lipolysis—fat
breakdown—and are largely responsible for the release of fat from both
subcutaneous and intramuscular fat stores for use as fuel during exercise.
Specialized β-adrenergic
receptors on fat cells are known to activate lipolysis (Boutcher 2011).
Interestingly, more β-adrenergic receptors have been found in deep abdominal
fat (also called visceral fat because it surrounds the internal organs of the
body) than in subcutaneous fat. Boutcher asserts that HIIT may thus have the
potential to specifically lower abdominal fat stores, which have been
implicated as the unhealthy fat depots in the human body.
Kessler, Sisson
& Short (2012) say it appears that notable changes in body weight/percent
body fat require at least 12 weeks of HIIT. However, assessing specific HIIT
results for clients is somewhat speculative because changing people’s body
composition depends on a weight management program that finds an optimum mix of
exercise, dietary intake and behavior changes.
HIIT Summary
The most common
benefit from HIIT— and certainly highly significant—is the improvement in
VO2max linked to enhanced protection from heart disease. Perhaps equally
important are HIIT’s dramatic acute and chronic effects on improving insulin
sensitivity.
The effects of
HIIT on visceral fat and subcutaneous fat loss are encouraging. And the impact
HIIT has on reducing systolic and diastolic blood pressure (when elevated) and
improving HDL cholesterol is promising. The evidence is in, and it appears that
HIIT is a metabolic health effects winner!
