Metabolic Effects of HIIT

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 (Kessler, Sisson & Short 2012; Boutcher 2011).

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 ellipticals; 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 Affect Insulin Resistance?

Insulin sensitivity, which describes the ability of the body’s cells to take up or metabolize glucose (ADA 2013), typically increases during and after exercise. Boutcher (2011) and Kessler, Sisson & Short (2012) comprehensively reviewed studies investigating the effect of HIIT on insulin sensitivity and glucose metabolism.

The studies found that insulin sensitivity can improve by 23%–58% over the course of 2–16 weeks of HIIT. The mechanism for this improvement appears to be well documented: Exercising muscle contractions can stimulate glucose shuttle transporters (known as GLUT-4 transporters) to take up glucose into the working muscle from the blood, whether insulin is available or not (ADA 2013).

How Does HIIT Affect Cholesterol Levels?

Kessler, Sisson & Short (2012) summarized 14 studies examining the effect of HIIT on cholesterol levels. The researchers said HIIT improves HDL cholesterol after a minimum of 8 weeks of training. The scientists suggest that a moderate decrease in body fat (or body weight) needs to accompany HIIT for improvements in total cholesterol, LDL cholesterol and blood triglycerides to occur.

Cholesterol is a fat-soluble substance carried in the blood by special transporters called lipoproteins. Lipoproteins are a necessary part of the intricate transport system that exchanges triglycerides among the liver, the intestine and peripheral tissues. There are four main classes of lipoproteins:

• chylomicron, which transports triglycerides and cholesterol from the small intestines
• very low-density lipoprotein (VLDL), which is made in the liver for the transport of triglycerides, the fat molecules in the human body
• low-density lipoprotein (LDL), a product of VLDL metabolism and the primary transporter of cholesterol
• high-density lipoprotein (HDL), sometimes called the “good” or “healthy” cholesterol, because it transports cholesterol from the artery walls to the liver, where it is converted to bile and used for digestion or disposed of by the body (This “reverse cholesterol transport process” of HDL lipoproteins is believed to be most favorable in preventing or reversing heart disease.)

How Does HIIT Affect Blood Pressure?

Kessler, Sisson & Short (2012) reviewed 12 studies that examined the effect of HIIT on blood pressure. The authors report that consistent HIIT for up to 10 weeks has not shown any measurable effects on resting blood pressure in people with hypertension. The researchers conclude that people with elevated blood pressure (and not hypertension medicine) need at least 12 weeks of HIIT training to see an appreciable reduction (2%–8%) in systolic and diastolic blood pressure.

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!