High-intensity interval training, commonly called HIIT, was in 2015 the no.2 fitness trend in the world, according to a survey by the American College of Sports Medicine.

It is a style of exercising during which you alternate periods of (almost) all-out and low effort.

During the high-intensity phase you push yourself as hard as you can which place high amounts of metabolic stress upon your body and during the low-intensity phases you give your body the time to (partly) recover.

This has many benefits, which is why it is gaining popularity over traditional steady state cardio.

Let's take a quick look at what current research tells us about HIIT.

8 Reasons Why High Intensity Interval Training Makes Life More Awesome

1. Fat Loss


Are you seeking to lose some extra weight?

HIIT might be the solution. It is namely one of the most effective things you can do for fat loss.

How powerful was first shown back in 1994 by a study from the Laval University. They had 27 individuals who would either follow a steady state cardio program for 20 weeks or a HIIT program for 15 weeks consisting of 15 sprints lasting 30-seconds. 

Their finding was that participant who followed the HIIT program lost 9 times more body fat and 12% more visceral belly fat compared to the steady state cardio group.

Similar results were shown by researchers from the University of Western Ontario where 20 subjects followed an exercise routine 3 times per week for a duration of 6 weeks.

The first group performed 4-6 treadmill sprints lasting 30-second with 4-6 minutes rest in between sets while the Second group performed 30-60 minutes of steady state cardio at 65% of their V02-max.

The results were remarkable.

4-6 sprints of 30 seconds burned more fat than 60 minutes of incline treadmill walking

Researchers believe HIIT is superior for fat loss due to 5 reasons:

  • HIIT increases metabolic rate up to 24 hours after the workout
  • HIIT improves insulin sensitivity
  • HIIT results in higher levels of fat oxidation
  • HIIT significantly spikes the powerful fat burning human growth hormone and the catecholamine hormones.
  • HIIT suppresses appetite post-workout

2. HIIT Does Your Hearth Good


It has long been taught that long-distance running is good for you including your heart. That's why for years steady state cardio has been described by cardiologists to improve heart health.

But if running would really be good for your  heart, marathon runners should have lowered risk of all heart diseases. This, however, is far from the truth.

In fact, runners who often complete marathons have disproportionately high rate of heart muscle scarring.

Dr. James O'Keefe explains best why this is so.

“The hearth pumps about 5 quarts per minute when we're sitting. When we're running it goes up to 25 to 30 quarts. The heart wasn't meant to do that for hours, day in and day out. You end up overstretching the heart and tearing muscle fibers. Up to 30 percent of those who finish marathons have raised troponin levels, which is a marker for heart damage. That's the marker we look for to see is someone's having a heart attack – its irrefutable evidence of heart damage”

This is why it's been shown that people who run more than 20 miles per week at an average speed of 7.5 mph or faster are more likely to have shortened lifespans compared to those who run slower over shorter distances.

But the good news is that HIIT training seems not to increase risk of cardiovascular diseases and might actually lower it according to researchers from the University of Georgia.

They showed that 3 days of HIIT performed for a duration of 4 weeks increased stroke volume and reduced hearth rate both at rest and during training, factors that decrease the risk of heart diseases.

Other studies show lower blood pressure, lowered cholesterol level, and better arterial structure, in response to HIIT.

3. HIIT Elevates The Fat Burning Hormone


Hormones are one of the most important factors that decide whether or whether not you feel good, perform good, and look good.

One of the most potent hormones for body composition is human growth hormone (HGH), also called “the fountain of youth.”

HGH is a peptide hormone produced by the pituitary gland; a pea-sized endocrine gland at the bottom of the hypothalamus (brain).

This hormone is known to increases fat oxidation, muscle mass, and strength, as well as much more awesome stuff.

And this is exactly why HIIT is such a good choice, it significantly elevates human growth hormone (HGH).

One study even found that a single maximal sprint 30 second is enough to elevate growth hormone levels by 530%.

This elevation creates a fat burning environment which lasts for hours after the workout.

But to get an adequate HGH response from training you have to go balls to the wall, since the HGH boost is linear related to training intensity. The harder your train, the greater the HGH boost.

One interesting thing to know is that the growth hormone response is greater in women compared to men.

Men, however, receive a stronger increase in the anabolic hormones testosterone and dihydrotestosterone, who in turn increase muscle-building and decrease fat mass.

4. You're More Likely To Stick With HIIT


As with everything in life; consistency is key. 

Just take a look at new years resolutions. Many people promise to keep a training routine, but how many do actually accomplish them? Not many, because the have too much difficulties sticking to their routines.

This is where HIIT might help. In one study, the majority of participants surveyed indicated that the preferred HIIT over more traditional training modalities.

This might have to do with the fact that HIIT usually involves multiple different exercises and thereby prevents boredom. Compare this to the typical 60-minute cardio session during which is generally performed in only one movement pattern.

Another benefit is that HIIT requires much less time but produces similar aerobic results.

When back in 2006 a group of researchers from Canada compared the difference between HIIT and steady state cardio they found something remarkable.

They found equal improvements in aerobic capacity when comparing 90-120 min of steady-state cycling at 65% intensity, with 4-6 intermittent sprints of 30-seconds and a 4 min recovery between sets. 

If you compare the time difference over 2 weeks it would represent 8 hours less exercise for the HIIT group, with equal results.

5. HITT Spares / Increases Muscle Mass


One of the many downsides of steady state cardio is muscle loss, especially during periods when calories are suppressed (think fat loss).

In fact, it is not unusual that the great majority of weight loss from steady state cardio comes in the form of muscle tissue.

That's because steady state cardio significantly elevates the “stress hormone” cortisol. Besides a long list of negative health consequences (cognitive decline, bone loss, altered immune function, damage reproductive health, etc.) promotes excess cortisol levels fat storage and cause muscle loss.

That's why many marathon runners end up “skinny fat”. Even though their weight on the scale looks healthy, their physique and blood markers show otherwise.

When performing steady state cardio, this stress hormone starts to rise until the workout is completed. Thus the longer the duration of your running session the higher the levels of this hormone.

This was shown by a study from 1976 in the Journal of Applied Physiology. The researchers found no increase in cortisol secretion after 10 min steady state cardio (at 75%), yet after 30 minutes it was doubled.

A better option would be HIIT, which increases cortisol much less and thereby is much more muscle sparring.

In fact, you might even build muscle by performing HIIT.

One study, in which young men performed 20 minutes of bike interval training for 3 months, increased subjects their leg musculature by 0.4-kg and trunk musculature by 0.7-kg, while losing 2 kg of body fat.

6. HIIT Makes You Tougher


Let’s be honest here, HIIIT is not like a walk in the park.

When done properly your heart is racing like crazy, your muscles fill with lactic acid, providing a burning sensation, and your mind is begging you to stop.

But by pushing past your discomfort, you get a feeling of accomplishment difficult to describe. It develops your character and build confidence both in your physical and emotional ability.

This confidence, in turn, spreads out and affects every area of your life. 

7. HIIT Decreases Hunger


Most people know that to lose weight you have to burn more calories compared to the amount you consume. This is called the law of thermodynamics which can not be escaped.

And that's why so many follow a steady state cardio routine, to burn high amounts of calories during exercise.

The problem, however, is that steady state cardio can cause an insatiable appetite that can hold on for hours after a workout session. This is phenomenon commonly called “runger” by long-distance runners.

When the runners give in to their cravings, they very well may have nullified all the work they've just put into their training session.

But with HIIT, there’s evidence to believe that the opposite is true. Studies have shown that a HIIT workout suppresses the appetite.

A small study by researchers from the University of Western Australia tested the difference of 30 minutes HIIT versus traditional steady state cardio on the effects of appetite after a workout.

They found that the interval training group unconsciously consumed fewer calories after their workouts. They simply were less hungry.

The reason this happens may be because HIIT reduces the amount of ghrelin, an appetite-stimulating hormone. Interval training also increases blood lactate levels and blood glucose levels, which cut short-term appetite.

Another benefit is that HIIT may cause changes in the reward-related regions of the brain that decrease hunger and the want to eat while increasing the fullness from food eaten. This makes it easier in the long run to control calorie intake and thereby body weight.

8. HITT Makes Your Smarter


I hope you by now see why HIIT does the body good. But did you know it does your brain good as well?

This was shown by new research from Brazil, in which 22 men performed sprints on a bike. After their workout was over, the subjects were asked to do a cognitive test to assess brain function.

You would think that HIIT would make an individual so tired that performing this cognitive test would be a real struggle, but the results showed otherwise. The HIIT group was able to perform the cognitive noticeably faster compared to a control group.

Similar results were found in another study in which the difference in recall of novel vocabulary was tested between HIIT, low-intensity training, and no exercise. 

After the workout, researchers directly performed their testing in which they found out that the HIIT group was 20% faster compared to the other 2 groups. 

The researchers believed that HIIT makes you smarter because it raises the catecholamine levels and brain-derived neurotrophic factor (BDNF). The later is a protein vital for promoting the growth of your neurons.

It is believed that the amount of BDNF released is in proportion with the intensity of your training. The higher your training intensity the more BDNF gets produced.

So….. Sound pretty good this interval training isn't it?

Do you have experience with HIIT? And if so, how is does it work out for you?

Please let us know in the comment section below.

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logo-muscleforealAuthor Bio: Stefan de Kort

Stefan de Kort is a (personal) trainer from the Netherlands and creator of Muscleforreal, a website devoted to helping fitness fanatics build muscle, lose fat, gain athleticism, and be healthy. He does this through a combination of personal experience and reviewing current scientific literature. Stefan is also available for online personal coaching to help you reach your fitness and health goals.

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