By now I’m sure you’ve heard about high-intensity interval training (HIIT). HIIT combines short periods of intense exercise with short periods of rest.
It is one of the best exercise protocols for fat loss. But do you know why this is the case? Below I lay out 10 physiological adaptations your body undergoes when you do HIIT so that you can completely understand why it’s so effective for fat loss.
Improved Insulin Sensitivity
Insulin sensitivity, or how well your cells respond to insulin, has a big impact on how well you tolerate carbohydrates, and whether those carbohydrates will affect your ability to mobilize fatty acids. Reduced insulin sensitivity means you need more and more insulin to do the same job. And since insulin is a storage hormone, when it’s high, it’s more difficult to lose fat.
Following just 2 weeks of HIIT, in which there was a total of only 15 minutes of exercise, insulin sensitivity was improved by 23% .
Produces the Afterburn Effect
Excess post-exercise oxygen consumption (EPOC), is the increased oxygen your body uses after an intense workout to erase its oxygen debt. It uses this oxygen to return the body to homeostasis.
That means it uses additional calories to perform tasks such as muscle repair and replenishment of fuel stores. EPOC is better known as the afterburn effect, which is the process of burning extra calories long after your workout is over.
When comparing HIIT to low-intensity exercise, your exercise intensity positively affects both the magnitude and duration of EPOC . In other words, the greater your intensity, the greater the afterburn effect.
Specifically Targets Stomach Fat
Yes, we all know that you can’t target fat loss per se. However, that’s not what we’re talking about here. Doing sit-ups isn’t going to target belly fat.
However, there are things you can do that will change the way you store and mobilize fat. The way you store fat is determined by many factors – genetics and hormones being two big ones. And the type of exercise you do affects your hormone profile.
Doing HIIT can create a metabolic environment that stimulates a higher proportional release of abdominal fat. You still lose fat all over, but a higher proportion comes off in the midsection.
High-intensity interval exercise three times per week for 15 weeks was compared to the same frequency of steady-state exercise, and only HIIT produced significant reductions in total body fat, subcutaneous leg and trunk fat, and insulin resistance .
Improved Vo2 Max
VO2 max is your body’s max capacity to transport and use oxygen during exercise. It is a great measure of physical fitness. Generally speaking, the higher your VO2 max, the better your fitness level. A higher VO2 max also means that you can exercise at greater intensities for longer periods of time.
Creation of New Mitochondria
Mitochondria are little cell powerhouses that produce energy (ATP). In simple terms, they take the fat and carbohydrates you either eat or store and convert them to usable energy. The more mitochondria you have, the more efficiently your body utilizes the calories you consume.
Boosts Favorable Hormones
High-intensity interval training does more than just burn calories. It primes your body for fat loss by creating a favorable metabolic environment.
Internally, your body undergoes many hormonal changes in response to intense training. Specifically, HIIT boosts growth hormone and testosterone levels after just 10 minutes, and the amount secreted is correlated to your exercise intensity   .
Growth hormone and testosterone are a potent combo for both fat loss and muscle growth. Engaging in HIIT will provide you with this amazing benefit.
Burns More Total Fat Compared to Endurance Training
Did you know you can burn more fat doing HIIT than your typical steady-state endurance training, even when burning a fraction of the calories? It’s true.
A study comparing a 15 week HIIT program to a 20 week endurance-training (ET) program showed that despite its lower energy cost, the HIIT program induced a more pronounced reduction in subcutaneous fat compared with the ET program.
When the scientists adjusted the numbers so the calorie burn was equal, the decrease in the sum of six subcutaneous skinfolds induced by the HIIT program was ninefold greater than by the ET program .
Builds Muscle While Losing Fat
Many people say you can’t build muscle and burn fat at the same time. While it can be difficult to put on a large amount of muscle mass while in a calorie deficit, you can certainly accomplish both goals concurrently.
In fact, a 12 week HIIT program has been shown to increase lean body mass, while at the same time reducing total body fat, abdominal and trunk fat, and visceral fat .
The additional muscle will pay dividends by increasing your resting metabolic rate so that you’re burning extra calories at all times of the day. Build muscle and lose fat, all while exercising for less time. Seems too good to be true, but it is.
Increased Capacity for Fat Oxidation
During exercise our bodies undergo all kinds of chemical reactions and stress adaptations. Our bodies literally change from the inside out. During HIIT specifically, we make changes to our physiology that enable us to burn more fat.
Just seven sessions of HIIT over 2 weeks induced marked increases in whole body and skeletal muscle capacity for fatty acid oxidation during exercise in moderately active women . HIIT causes changes to your body that increases its ability to burn fat.
I’ve listed out the science behind the benefits of HIIT, but that’s really just scratching the surface. High-intensity interval training also:
- Adds variety – there really is an endless combination of fitness activities that can be incorporated into a HIIT workout.
- Creates time efficiencies – you no longer have to slave away on the treadmill for an hour or more. The same benefits or more can be accomplished in under 20 minutes.
- No equipment necessary – no gym membership? No problem. All you need is your body to get a great workout.
As I mentioned earlier, I have a HIIT workout bonus to give you. There are 3 workouts – 1 that can be done at home, 1 that involves kettlebells, and 1 that uses weights. You can get your bonus workouts here.