High Intensity Interval Training: Maybe The Fastest Way to Fat Loss and Fitness Gains -

Kaitlin Little

Exercise programs designed for weight loss have traditionally focused on moderate-intensity continuous exercise; such as walking or jogging. However, a growing body of evidence now shows that high-intensity interval training (HIIT) delivers similar—or even superior—results.

What is High-Intensity Interval Training (HIIT)?

HIIT is a form of cardiovascular exercise that involves repeated short (< 45 sec) or long (1-4 min) bouts of high-intensity exercise interspersed with low-intensity recovery periods. Elite athletes have been using HIIT since the turn of the 20th century to improve their endurance by alternating between sprints and jogging. However, these days a HIIT workout can involve cycling, running, swimming, or even resistance-based training, such as a circuit of bodyweight exercises. As long as you work really hard, rest, and then work really hard again, it’s a HIIT session.

When programming a HIIT session fitness professionals often use work-to-rest ratios to describe the length of each interval. A work-to-rest ratio of 1:2 simply means the recovery interval is double the duration of the work interval. For example, 30 seconds of high-intensity exercise (such as fast skipping) followed by 60 seconds of recovery (such as walking). The entire length of a HIIT workout can range from 5 to 45 minutes, including recovery periods. In the previous example, you would repeat each interval 10 times for a 15-minute workout. Although, aiming for at least 20 minutes is a good place to start for beginners.

How Effective is HIIT for Weight Loss?

HIIT is gaining a great deal of recognition as the most effective way to lose weight, but just how much evidence is there to support this claim? According to a 2019 meta-analysis published in the British Journal of Sports Medicine, engaging in regular HIIT sessions does indeed result in almost 30% greater fat loss compared to traditional continuous exercise. [1] Other clinical trials in both obese men and women have come to similar conclusions [2] [3].

But why is HIIT so effective for weight loss?

HIIT Burns More Calories in Less Time

If you ask most people why they don’t exercise, the common answer is “I don’t have time.” Fortunately, by using brief bursts of intense exercise, HIIT makes it possible to burn up to 50% more calories in the same amount of time. [4] Essentially, the theory is the harder you work, the more oxygen you consume. And the more you oxygen consume, the more calories you burn.

If you’re still not convinced, consider the following study. In 2015, researchers compared the total calories burned during four different exercise sessions all lasting 30 minutes, but focused on either HIIT, resistance training, running or cycling. Interestingly, the researchers found that the HIIT session resulted in a 20-35% greater calorie burn when compared to resistance training, and the two forms of steady-state exercise. [5]

In this study, the HIIT session followed a 1:2 work-to-rest ratio involving repeated intervals of 20 seconds at a maximum effort followed by 40 seconds rest. This means that the HIIT group only exercised for 10 minutes compared to 30 minutes in the running and cycling groups. What’s more, the HIIT groups reported greater enjoyment and attendance, suggesting that HIIT is not only a more time-efficient way to workout, but can also motivate you to workout more often.

Increases Resting Metabolic Rate Through EPOC

Once you finish an intense bout of exercise, you know you’ve burned a significant amount of calories. However, did you know your body continues to burn calories for several hours after your workout? This is due to excess post-exercise oxygen consumption (EPOC). Also known as oxygen debt, EPOC refers to the amount of oxygen required after exercise to restore the body back to its resting state. During EPOC you expend energy on:

Restoring oxygen levels
Removal of metabolic by-products, such as lactate and hydrogen ions
Restoring fuels, such as ATP, PC, and glycogen

Performing these processes results in an increased metabolic rate for up to 24 hours after exercise. Interval training, due to its strenuous nature, produces a greater EPOC compared to continuous endurance training. One study showed that just 20 minutes of interval training leads to the same 24-hour energy expenditure as a 50-minute continuous run after accounting for EPOC. [6] However, scientists have yet to confirm whether increases in EPOC resulting from HIIT are significant enough to lead to greater weight loss, and have suggested that hormonal changes may play a larger role. [7]

Improves Insulin Resistance

Insulin resistance occurs when the body stops responding to insulin, a hormone that plays an important role in the metabolism of carbohydrates and fats. Along with being a very serious condition that precedes type 2 diabetes, insulin resistance leads to chronically high insulin levels, which promotes fat storage and makes it very difficult to lose weight. However, regularly engaging in HIIT is shown to significantly reduce insulin resistance, helping to increase fat metabolism and maintain a healthy weight. [8]

One study demonstrated a 58% improvement in insulin resistance, accompanied by a 44% decrease in abdominal fat in diabetic men who performed one HIIT session a week for eight weeks. [9] Another two-week study, involving healthy adults, found that 30-second sprint intervals improved insulin sensitivity by 17% more than the steady-state cycling. [10]

Targets Abdominal Fat

Compared to continuous exercise, HIIT is shown to induce greater increases in catecholamine hormones, including dopamine, epinephrine, and norepinephrine. This is important because catecholamines, especially epinephrine, are drivers of fat metabolism and play a key role in releasing triglycerides from stored body fat. [11] [12]

One study found that HIIT and steady state continuous exercise both resulted in a 10% reduction in fat mass, in particular abdominal visceral fat. [13] Visceral fat, also known as belly fat, is found inside your abdomen and tends to accumulate around organs. As a result, this type of fat is considered extremely harmful and can increase your risk of type 2 diabetes, heart disease, and even certain cancers. [14] [15]

Preserves Lean Muscle Mass

In addition to accelerating fat loss, HIIT training may also promote the formation of lean muscle mass. In one study, ice hockey athletes, assigned to a 4-week interval training program, demonstrated greater improvements in muscle thickness, muscular power and speed, compared to players assigned to a training program focused on moderate-intensity continuous skating. [16]

These changes may relate to an increased production of growth hormone (GH), which is seen after high-intensity exercise, but not low-intensity exercise. One study showed that 30-second sprint intervals produced a marked response in GH, which remained ten times higher than baseline levels for an hour after exercise. [17]

However, increases in muscle mass are more common among those who are physically inactive prior to commencing HIIT. A 2017 study involving sedentary older men (>60 years old) showed that incorporating one session of HIIT each week significantly reduced body fat while preserving lean muscle mass and improving muscular power. [18]

What are the Health Benefits of HIIT?

High-intensity interval training offers numerous health benefits. The greatest amount of evidence exists for improved cardiorespiratory fitness, which is one of the strongest predictors of a person’s overall health. In fact, research shows HIIT may be twice as good for cardiorespiratory fitness as continuous endurance exercise. [19]

In 2017, researchers from Canada assigned healthy male participants to either a HIIT program of 30-second sprints with 1-minute rest repeated 12 times, or a traditional endurance training program of 20 minutes continuous steady-state running. The HIIT group increased their VO2max by 3.4% over the course of four weeks, while the endurance group only increased theirs by 1.7%. [20] VO2max is a measurement of how much oxygen a person can use during intense exercise and is the gold standard for evaluating cardiorespiratory fitness. The healthier the heart, lungs and blood vessels, the more oxygen working muscles receive and the quicker by-products (like lactic acid) can be removed.

Why Does HIIT Improve Cardiorespiratory Fitness?

HIIT improves cardiorespiratory fitness over time by triggering adaptations within the body, which can be broadly categorized as either central ( having to do with the heart) or peripheral (having to do with skeletal muscle).

Two important central adaptations resulting from HIIT include an increase in cardiac output, which is the amount of blood the heart can pump out per minute, and an increase in stroke volume, which is the amount of blood the heart pumps out per beat. [21] Just like any other muscle, the heart muscle can be trained to become stronger and more powerful. However, researchers have yet to determine exactly why HIIT increases these measures. Some believe HIIT triggers an increase in blood volume, while others suggest it increases the thickness and volume on the left ventricle, an important chamber within the heart.

Interval training also elicits peripheral adaptations, including increases in skeletal muscle capillaries. More capillaries within the major muscles of the lower and upper limbs increases blood and oxygen supply during exercise. Another peripheral adaptation is increased skeletal muscle mitochondria. Mitochondria are specialized organelles, which play an important role in converting oxygen and nutrients into energy for muscle contraction. More mitochondria promotes a greater reliance on fats, compared to carbohydrates. Together these two skeletal muscle adaptations help reduce the accumulation of by-products (like hydrogen ions and lactic acid), ultimately enabling you to perform at higher intensities for longer. [22]

Reduced Risk of Cardiovascular Disease

Although moderate-intensity continuous exercise is routinely recommended for the prevention of cardiovascular disease, HIIT may be more effective at reducing your risk. Prospective population-based studies have shown that individuals who participate in regular high-intensity exercise are less likely to suffer a cardiac event, such as a heart attack or stroke, than those who engage in low-to-moderate intensity exercise. [23]

HIIT is also shown to reduce risk factors, including high blood pressure and high cholesterol. In a recent clinical trial involving sedentary men, participants performed either 3 HIIT sessions, or 3 continuous exercise sessions per week for 24 weeks. At follow-up, significant improvements in total cholesterol, LDL ‘bad’ cholesterol, and HDL ‘good’ cholesterol were only seen in the HIIT group. [24] In another clinical trial involving hypertensive patients, HIIT proved more effective at reducing blood pressure compared to continuous exercise over the course of 12 weeks. [25]

Is HIIT Safe?

HIIT has proven very safe and highly beneficial for healthy individuals. However, musculoskeletal injuries are a concern when first starting out, particularly in elderly and previously sedentary patients. To prevent injuries, a conditioning phase of continuous moderate-intensity exercise and resistance training is recommended for at least 4 weeks prior to commencing any high-intensity exercise. Gradually increasing intensity will provide a beneficial base level of aerobic fitness, strengthen muscles, bones and joints, as well as increase flexibility. Practitioners can also prevent injuries through careful exercise selection, such as using a stationary bike instead of a treadmill, in addition to including a thorough warm-up and cool-down.

Although safe for most people, HIIT is a strenuous form of exercise and therefore poses safety concerns for certain high-risk populations, such as those with heart disease, high blood pressure, or other chronic health problems. If you are 70 or older, or have a history of heart disease or any other chronic health problem, it is advised that you check with your doctor before commencing HIIT or any exercise program—especially if you haven’t been physically active recently.

With that said, HIIT has been used successfully by patients suffering from a range of cardiac and metabolic conditions, including diabetes and heart failure. [26] Furthermore, no adverse events have occurred during cardiac rehabilitation programs that incorporate HIIT [27]. However, these findings should be interpreted with caution and more research is required to determine the risks and benefits of HIIT in this setting.

How to Get Started with HIIT?

When it comes to getting started with HIIT, the key is to recognize that exercising at “high intensity” is different for everyone. For example, an elite athlete may deem strenuous sprints as high intensity, while someone with a lower level of fitness may find brief periods of light jogging quite intense.

Whether an activity is classed as low, moderate or high intensity depends on your fitness level. Use the following guide to help determine your intensity during a workout.

Low Intensity: you can easily hold a conversation. If you were to rate your exertion level out of 10, you’d be working at around 2-4. Your heart rate remains around 40-55% of your maximum heart rate (MHR).
Moderate Intensity: you can speak a full sentence, but you can’t sing. Your exertion level is around 5-7 out of 10. Your heart rate increases to 55-69% of your MHR.
High Intensity: you’re too out of breath to speak a full sentence. Your working close to your maximum, at about 8-9 out of 10. Your heart is beating rapidly at around 70%-85% of your MHR.

Below you’ll find a 30-minute Beginners HIIT Workout, which has a 1:1 work-to-rest ratio and involves 10 intervals of jogging for 1-minute followed by walking for 1-minute. Once your fitness improves, you could try a 2:1 ratio by reducing the recovery periods to 30 seconds, or simply try ramping up the speed of your jog. If running is to stressful on your joints, you could adapt the program to include low-impact exercises, such as swimming, cycling or even rowing.

Although HIIT may be the most time-efficient way to exercise for both weight loss and overall health, incorporating interval training into your weekly routine won’t necessarily lead to weight loss by itself. It will certainly help by increasing your daily energy expenditure, boosting your metabolic rate and improving your insulin sensitivity, but you will also need to maintain a reasonable, well-balanced diet.

If you’re already active, and planning to add interval training to your current program, keep in mind that HIIT is both physically and mentally demanding. It is therefore, a good idea to incorporate other forms of exercise, including flexibility and resistance training to minimize your risk of injury and allow for adequate recovery.

References

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