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HIIT is an approach to exercise training that provides many health and fitness benefits in a notably time-efficient manner. Numerous studies have been conducted and published about the effects of this unique form of training on different populations, such as athletes, healthy men and women enthusiasts, obese people, and individuals with disease. In this section I am going to highlight HIIT research as it relates to cardiorespiratory fitness, cholesterol levels, blood pressure, and weight management. Things get a bit technical in this part of this book, but bear with me. I really want you to realize how healthy this exercise program may be for you!

How Much Can HIIT Improve Cardiorespiratory Fitness?

Cardiorespiratory fitness, also called aerobic fitness, refers to the ability of the heart, blood vessels, and lungs to supply oxygen-rich blood to exercising muscle tissues and the ability of the muscles to use the supplied oxygen to produce energy for a workout. That’s a lot to absorb in one sentence, so let’s break it down: cardiorespiratory fitness is a measure of your ability to consume oxygen, deliver it to your exercising muscles, and take and use the oxygen for energy production. Hopefully this makes better sense.

When it comes to improving cardiorespiratory fitness, HIIT is a marvel. In a comprehensive 2011 review in the Journal of Obesity, it was reported that healthy young and older adult men and women can improve their cardiorespiratory fitness up to 46% after 8 to 15 weeks of HIIT training. For the record, that is a staggering improvement in a relatively short period of time. Of course, the degree of improvement a person will actually achieve is directly related to her/his fitness level. For instance, the lower a person’s baseline fitness level, the greater the gains she/he will potentially make in cardiorespiratory fitness. Thus, individuals who lead sedentary lifestyles and then begin an exercise program will likely see the largest gains in cardiorespiratory fitness with a progressive and consistent training program. Conversely, people who have been regularly training for years will have smaller, incremental gains that are clearly not as large as those made by their sedentary counterparts.

According to current scientific research, this impressively swift increase in cardiorespiratory fitness is due to a major improvement in the heart’s blood pumping capacity, referred to as “stroke volume.” Stroke volume is the amount of blood pumped by the heart each heartbeat. Your heart, which is the size of your fist, pumps blood through its heart chambers in a way similar to how you quickly clench and open your fist. And, as the heart gets stronger, it pumps stronger, ejecting more blood (i.e., stroke volume) with each heartbeat.

Your ability to sustain aerobic exercise depends on your heart’s ability to continually pump oxygen-rich blood to the working muscles. The oxygen is needed to harvest the energy compound that powers movement, ATP. In fact, the presence of oxygen in the body’s cells actually boosts all of the major cell reactions that lead to the formation of ATP. So, the more oxygen present at the exercising muscle, the longer your muscles can last during the workout. Simply put, a stronger heart pumps more blood carrying oxygen to the working muscles.

Another fascinating cardiorespiratory adaptation of the body—one well observed during HIIT workouts—can be seen in your muscles. Your muscles move your body and are fueled by incredible bean-shaped, energy-producing organelles known as “mitochondria.” Mitochondria are often referred to as the energy factories of your cells. Depending on a person’s training and genetics, each individual muscle cell may have four hundred to two thousand of these powerhouse mitochondria. With regular HIIT workouts, mitochondria get bigger and the body makes more of them in the muscle cells. Isn’t that amazing? This occurrence is seen in women and men of all ages, fitness levels, and ethnic backgrounds. Therefore, the size and number of mitochondria directly translates into enhanced cardiorespiratory fitness at any level of exercise intensity.

From a health perspective, this cardiorespiratory improvement is invaluable, because low aerobic fitness is directly linked to heart attack and other health-related causes of mortality. In a large, 1996 pioneering study at the famous Cooper Institute in Dallas, Texas, researchers studied the medical exams and cardiorespiratory tests of 25,341 men and 7,080 women. The results of this landmark study indicated that adults who are moderately or highly fit had enhanced cardioprotection. “Cardioprotection” is a relatively new term in science. It means those women and men who were fitter had better defenses against heart disease and heart attacks, compared to women and men who were less fit. This finding was found to be true regardless of whether the individuals were smokers or nonsmokers, had normal or elevated cholesterol levels, or were obese or had average body weights. Another prominent study, this one published in the American Journal of Cardiology in 2006, concurs with the results of the Cooper Institute investigation. In this study, the researchers found that vigorous exercise intensity was more beneficial (i.e., more cardioprotective) in positively altering one or more risk factors to heart disease.

What do these results tell us? They remind us that regular HIIT workouts should be considered to be among the most effective ways of counteracting the effects of heart disease, the number one cause of death throughout the world. Here’s your take-home message: Keep striving to incrementally improve your cardiorespiratory fitness. The fitter your heart and lungs, the healthier and better equipped you are to fight off disease. As the old saying goes, keep moving briskly for enhanced cardioprotection!

What Is the Effect of HIIT on Insulin Sensitivity?

The term “insulin sensitivity” refers to the ability of the muscles to successfully utilize glucose for fuel. Insulin works very much like a key in a doorknob: it helps to unlock transporter proteins that bring glucose into the muscles for fuel. So, having high insulin sensitivity is very healthy, because it means insulin is working effectively to help bring glucose into the muscles. Several research teams have investigated the effect of HIIT on improving insulin sensitivity. Taken together, the results indicate that HIIT can increase insulin sensitivity by 23% to 58%. That is quite impressive! This great news means that HIIT training increases the ability of your body to take up glucose into the muscle cells to use for energy. This helps to prevent high blood glucose levels, a precursor to type 2 diabetes. Studies show this increase in insulin sensitivity begins around week two of your training and progressively improves over the next 16 weeks of your HIIT workouts. Your body rapidly adapts to your new exercise program by more effectively taking up glucose from the blood.

The reason for this improvement in insulin sensitivity from HIIT training is well understood. Scientists have discovered that exercising your muscles actually activates the body’s glucose transporter proteins, called “GLUT4 proteins.” Your HIIT workouts prompt these proteins to take glucose into the muscles for fuel.

You may be asking: why is this important? Here’s why. Under normal conditions, your muscles use insulin to activate the GLUT4 proteins to take glucose into the muscles (as explained). However, for people with insulin resistance, also called low insulin sensitivity, and type 2 diabetes, insulin is not able to effectively stimulate these GLUT4 proteins. What does this mean in real-world terms? It tells us that HIIT workouts are an excellent line of defense toward the prevention and/or management of type 2 diabetes, one of the fastest growing diseases throughout the world.

What Is the Effect of HIIT on Cholesterol Levels?

Cholesterol is a waxy, fat-like substance that’s found in all cells of the body. One of its most important functions is to aid in the production of hormones, and it also plays a chief role in constructing our cell membranes. It’s transported in the blood by special protein packages called “lipoproteins.” The two most important lipoproteins to know about are low-density lipoproteins (LDL) and high-density lipoproteins (HDL). LDL cholesterol is often called the “lousy” cholesterol because an excess of LDL in the blood builds up on the artery walls, creating the plaque that leads to heart disease. On the flip side, HDL cholesterol is frequently called good or healthy cholesterol because it’s responsible for removing and transporting cholesterol from the artery walls to the liver, where it is disposed of by the body. Having an elevated level of HDL is believed to be favorably associated with preventing heart disease, that is, having enhanced cardioprotection.

In 2012, an article in the journal Sports Medicine reviewed fourteen studies that examined the effect of HIIT workouts on cholesterol levels. The researchers found that HIIT has been shown to elevate your body’s “good” (HDL) cholesterol in as little as 8 weeks of training. This is excellent news, considering that HDL cholesterol helps clear out the dangerous plaque that accumulates on the inside of arteries and blood vessels. Increased levels of HDL cholesterol mean your blood has more plaque-devouring scavengers to help get rid of risky plaque build-up. In addition, the scientists who conducted the study believe that HIIT workouts, in combination with a decrease in body fat (or body weight), lower the “lousy” (LDL) cholesterol and other circulating blood fats in your blood. To summarize, there is solid evidence that HIIT workouts are a direct line of defense in combatting heart disease.

What Is the Effect of HIIT on Blood Pressure?

Additionally, the same 2012 article in Sports Medicine concluded people with elevated blood pressure (who were not on any hypertension medicine) positively and appreciably lowered their blood pressure after at least 12 weeks of HIIT training. The research shows that progressive and consistent HIIT workouts may lower systolic and diastolic blood pressure by about 2% to 8%—no small deal. As you may know, systolic blood pressure measures the pressure on your heart when it is ejecting blood and diastolic blood pressure measures it when your heart is refilling with blood. Left untreated for years, elevated blood pressure increases your risk of heart attack and heart failure. Over time, the strain on the heart caused by high blood pressure weakens your heart muscle, causing it to lose pumping strength, and the higher blood pressure damages the inner linings of arteries. It literally scrapes away on the crucial linings of arteries, causing the arteries to lose their elasticity. You want your arteries to be flexible and able to stretch; if they are not able to, your blood circulation will be impeded. HIIT workouts help to counteract high blood pressure and keep your arteries flexible.

How Does HIIT Impact Fat Loss and Weight Management?

There are many ways in which HIIT workouts help reduce body fat and promote healthy weight management goals. In the previously referenced 2011 research review published in the Journal of Obesity, it was found that HIIT workouts cause a striking surge in epinephrine and norepinephrine, also known as adrenaline and noradrenaline, respectively. Adrenaline and noradrenaline are your fight-or-flight hormones. These hormones are also known to ignite the breakdown or burning of fat by your exercising muscles. In addition, the surge of fight-or-flight hormones stimulates fat cells to release some of the body’s stored fat, which then becomes fuel for exercise. A 2017 study in the Journal of Diabetes Research also found that HIIT workouts have the potential to help reduce your abdominal fat stores, a process that has positive health implications. Fat around the middle of the body is highly associated with the development of heart disease and type 2 diabetes, so gradually depleting this abdominal fat is an important step in reducing a major health risk. Plus, reducing abdominal fat is one of the most common goals for people trying to improve their health and fitness.

I am often asked, and you are probably wondering, how long does it take for your body to start improving its fat burning capabilities from HIIT workouts? In a 2007 study published in the Journal of Applied Physiology, researchers determined that moderately active women in the study significantly improved their muscles’ fat burning capacity in as little as seven HIIT training sessions spread out over 2 weeks. This is a remarkable finding, and one definitely worth repeating for emphasis: it took subjects only 2 weeks and seven HIIT workouts to significantly improve their fat-burning capability. This is compelling evidence that HIIT training helps the body rapidly improve its fat-burning capacity. Combined with a sound weight management plan, HIIT training can be the key to optimal weight loss. (A comprehensive, state-of-the art weight management plan is provided in the second section of the book.)

Summary of the Major Health Benefits of HIIT

Without a doubt, improvement in cardiorespiratory fitness is one of the most life-changing benefits of HIIT workouts. This improvement is directly related to enhanced cardioprotection from heart disease and a number of other health enhancements, such as improving insulin sensitivity. HIIT workouts stimulate GLUT4 proteins, helping the body better utilize blood sugar and keep it from rising, which in turn helps prevent type 2 diabetes (or, helps diabetics better manage it). Also, HIIT’s effect on fat loss and fat utilization are particularly encouraging, as is the impact of HIIT on reducing systolic and diastolic blood pressure (when elevated) and improving HDL cholesterol. The evidence is in, and it shows that HIIT is a health-packed winner workout for all. Let’s keep moving, people!