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Metabolic Effects of HIIT
Len Kravitz, Ph.D.

Introduction
High intensity interval training (HIIT) workouts are enormously popular 2014 exercise programs in the fitness industry. These training sessions typically include short bursts (from 6 seconds to 4 minutes) of intense exercise (greater than or equal to 90% maximal aerobic capacity) alternated with relief breaks of varying lengths (Kessler et al., 2012, Boutcher 2011). The workouts include a limitless variety of exercises including body weight movements, power lifting, plyometrics, sprints, playground physical activities (e.g., sled pushes and pulls), multiple mode training (e.g. cycle ergometer, treadmill, elliptical training, etc.), heavy rope drills and more. Characteristically, HIIT is depicted as an approach to training that provides several health, fitness and performance benefits in a time-efficient manner. Although many fitness professionals proclaim numerous health benefits of HIIT, an evidence-based review is necessary to validate these claims. The acute physiological responses of a HIIT workout are illustrated below, with a synthesis of the long-term health-related adaptations discussed in this column.

Acute or Immediate Physiological Responses of a HIIT Workout (Boutcher 2011)
1) Heart rate elevates significantly
2) Epinephrine and norepinephrine elevated 6.2 to 14.5 times greater than baseline
3) Initially blood glucose (from glycogen breakdown) is elevated (for exercise fuel) but may decline during HIIT session
4) ATP and phosphocreatine decline steadily (used to meet rapid fuel needs of contracting muscle)
5) Increased levels of blood glycerol and free fatty acids suggesting an early breakdown of triglycerides
6) Growth hormone may increase up to 10 times above baseline
7) Venous blood return to the heart enhanced, directly increasing stroke volume
8) Lactate levels may increase up to 10 times above baseline
9) Sympathetic nervous system (which speeds up neural signaling messages) is elevated
10) Parasympathetic nervous system (which slows neural signaling messages) is depressed

How Much Can HIIT Improve Cardiovascular Fitness?
Boutcher (2011) completed a recent comprehensive research review on high-intensity interval (which he describes as intermittent training) and reports that healthy young and older adult men and women can improve cardiorespiratory fitness (VO2max) 4% to 46% in training periods lasting from 2 to 15 weeks in length. Kessler et al. (2012) add that HIIT training appears to induce rather rapid changes in VO2max. Boutcher continues that the current scientific explanation for this increase in VO2max from HIIT training is proposed to be a consequential increase in stroke volume (volume of blood pumped by the hear per beat), which is induced by an increase in the heart muscles' contractile capability during near maximal exertion. As well, Boutcher summarizes that HIIT has also been shown to increase mitochondrial biogenesis (the size and number of mitochondria, the cells ATP synthesis factory organelle) that 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 et al., 2012). As shown in the large, pioneering Cooper Institute study on the benefits of cardiovascular exercise, Steven Blair et al. (1996) completed medical exams and VO2max tests on 25,341 men and 7080 women. Data from this study clearly indicated that regardless if individuals were unhealthy or healthy, smokers or nonsmokers, had normative or elevated cholesterol levels, were obese or normal weights, all seemed to have enhanced cardioprotection (i.e., protection from factors associated with causing cardiovascular disease) for being moderately or highly fit compared as compared to low-fit men and women.

Additionally, Swain and Franklin (2006) reviewed epidemiologic studies (studies that report incidence or prevalence data) that assessed the benefits of moderate versus vigorous physical activity as well as clinical investigations (studies that test and analyze results) that actually trained subjects at different intensities. All of the epidemiology and clinical studies showed very similar results, that the vigorous exercise intensity was more beneficial in positively altering one or more risk factors to coronary heart disease. Thus, for counteracting the effects of cardiovascular disease, the number one cause of death throughout the world (WHO, 2013), HIIT should be considered a most advantageous training intervention.

What is the Effect of HIIT on Insulin Resistance?
According to the American Diabetic Association (ADA, 2013), insulin sensitivity describes the ability of the body's cells to uptake glucose (also referred to as glucose metabolism). It is commonly increased during and after exercise. Boutcher (2011) and Kessler et al. (2012) comprehensively reviewed studies investigating the effect of HIIT on improving insulin sensitivity and glucose metabolism. Data indicate that insulin sensitivity can be improved 23% to 58%. Study lengths vary from 2 weeks to 16 weeks for these adaptations to occur. The mechanism for this improvement appears to be well documented, with the ability of the exercising muscle contractions to stimulate the glucose shuttle transporters (known as GLUT4 translocaters) to take up glucose into the working muscle (from the blood), whether insulin is available or not (ADA, 2013).

What is the Effect of HIIT on Cholesterol Levels?
Kessler et al., (2012) summarize 14 studies that have examined the effect of HIIT on cholesterol levels. The researchers elucidate that HIIT has been shown to improve HDL cholesterol after a minimum of 8 weeks of training. The scientists suggest that HIIT with an accompanying moderate decrease in body fat (or body weight) is needed to see improvements in total cholesterol, LDL cholesterol and blood triglycerides levels.
Cholesterol is a fat-soluble substance that is transported 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. The four main classes of lipoproteins are categorized as 1) chylomicron, which transports triglycerides and cholesterol from the small intestines, 2) very low density lipoprotein (VLDL), made in the liver for the transport of triglycerides (fat molecule in the human body), 3) low-density lipoprotein (LDL), a product of VLDL metabolism and the primary transporters of cholesterol, and 4) high-density lipoprotein (HDL), sometimes called the "good" or "healthy" cholesterol, because it is responsible for the transport of 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 most favorable in preventing or reversing heart disease.

What is the Effect of HIIT on Blood Pressure?
Kessler et al. (2012) summarize 12 studies that investigated the effect of HIIT on blood pressure. From their review the authors state that consistent HIIT for up to 10 weeks has not shown any measurable effects on resting blood pressure (in persons with hypertension). In fact, the researchers conclude that at least 12 weeks of HIIT training is needed for persons with elevated blood pressure (and not on any hypertension medicine) to have a positive and appreciable lowering (from 2% to 8% lower) in both systolic and diastolic blood pressure.

What is the Effect of HIIT on Impacting Fat Loss and Weight Management?
Boutcher (2011) observes that the striking surge in epinephrine and norepinheprine from HIIT training may serve a catalyst role for improving fat loss. Both of these fight or flight hormones (also known as adrenaline and noradrenaline) have been shown to drive lipolysis (the breakdown in fat), and are largely responsible for fat release from both subcutaneous and intramuscular fat stores (to be used for fuel in exercise). Specialized _-adrenergic receptors on fat cells are known to activate lipolysis (Boutcher). Interestingly, more _-adrenergic receptors have been found in deep abdominal (also called visceral fat because it surrounds the internal organs of the body) fat as compared to 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 et al. (2012) summarize that it appears that at least 12 weeks of HIIT is needed in order to have notable changes in body weight and or percent body fat. However, since body composition change is so reflective of the combination of dietary intake, behavior change and exercise, assessing specific results for clients from HIIT is somewhat speculative and more reflective on the entire weight management program design.

HIIT Summary
The most common adaptation from HIIT, and certainly highly significant is the improvement in VO2max which is directly related to enhanced cardiprotection from heart disease. Perhaps equally important to maximal aerobic capacity adaptations are the dramatic acute and chronic effects HITT has on improving insulin sensitivity. The effects of HIIT on visceral fat and subcutaneous fat loss are encouraging. And, the impact of 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!

References:
American Diabetic Association. (2013). Blood Glucose Control and Exercise
http://www.diabetes.org/food-and-fitness/fitness/get-started-safely/blood-glucose-control-and-exercise.html
Accessed March 1, 2014
Blair, S.N. et al. (1996). Influences of cardiorespiratory fitness and other precursors on cardiovascular disease and all-cause mortality in men and women, The Journal of the American Medical Association, 276(3), 205-210
Boutcher, S. H. (2011). High-intensity intermittent exercise and fat loss. Journal of Obesity, 2011, Article ID868305, 10 pages, doi: 10.1155/2011/868305
Kessler, H.S., Sisson, S. B., Short, K.R. (2012). The potential for high-intensity interval training to reduce cardiometabolic disease risk. Sports Medicine, 42(6), 489-509.
Swain, D.P. and Franklin, B.A. (2006). Comparison of cardioprotective benefits of vigorous versus moderate intensity aerobic exercise. American Journal of Cardiology, 97: 141-147.
World Health Organization. (2013). Cardiovascular Diseases (CVDs)
http://www.who.int/mediacentre/factsheets/fs317/en/
Accessed March 2, 2014

Bio:Len Kravitz, PhD, is the program coordinator of exercise science and a researcher at the University of New Mexico, Albuquerque, where he won the Outstanding Teacher of the Year award. He has received the prestigious Can-Fit-Pro Lifetime Achievement Award and American Council on Exercise Fitness Educator of the Year.
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