I was originally going to write a blog about the different health benefits that endurance training (slow jogging) offered compared to High Intensity Interval Training (HIIT). I soon found out that each topic was so complex that I would have to write two separate blogs. I chose to write about HIIT first.
I heard of HIIT training years ago but never really tried it out. I recently grew bored doing incline walks and 25 minute jogs on the treadmill. Instead of walking or jogging, I decided to do sprints on the treadmill (not really all out sprints, more like fast jogs) where I would run quickly for 90 seconds followed by 2 minutes of rest continuously for 26 minutes. This is essentially what High Intensity Interval Training is.
If you do a quick google search for HIIT you will see dozens of websites talking about how HIIT burns more fat than endurance training, helps people lose weight quicker and causes your metabolism to increase so you burn calories throughout the day following the exercise. Are all of these true? Some of them? None?
The answer lies somewhere in between.
What is HIIT?
HIIT is basically short intervals of bursts of speed or any effort to complete exhaustion (treadmill, elliptical, bike, jump rope) followed by short rest periods. For simplicity sake I will only talk about HIIT involving sprinting or cycling, whether it be on a treadmill, a stationary bike or outside. If you chose other methods for HIIT the principles will be the same as for sprinting anyway.
What makes HIIT different than jogging?
I will keep this brief. There are two components to energy used in physical activity. The major component is the energy used during the activity while the other component is termed excess postexercise oxygen consumption (EPOC), which occurs after the activity is completed. The majority of calories burned from jogging fall into the energy used during exercise category, while a significant amount of calories are burned during and after exercise when performing HIIT (1). HIIT produces an increase in metabolic rate (raises your metabolism) which continues well beyond the end of the activity.
How to perform HIIT
There are various ways to engage in HIIT. The common theme among all of them is that you want to exercise at a pace where you are completely exhausted at or before 90 seconds (with the low end being 2-10 seconds). This should ensure that you are exercising at a pace ≥50% of maximum speed (2). Depending on how hard you exercise you can take anywhere from 50 seconds up to >5 times the exercise duration as your break (if you sprint for 30 seconds >5 times break will be >150 seconds). How hard you push yourself and how long you break will determine how many sets you complete in a set time.
Although some studies have produced conflicting results about the best approach to HIIT, it has been found that a 30 second all-out effort followed by a roughly 3 minute break is an extremely powerful and time efficient way to drastically improve your performance in intense short bout exercises (2). Surprisingly, these benefits were found even when the subjects decreased their exercise time every week, indicating that the volume (time) of training may not be as important as intensity.
HIIT uses fast twitch rather than slow twitch muscle fibers
Although not many studies addressed the relationship between muscle fibers and exercise, one study found that anaerobic exercise (anaerobic means with oxygen-think sprinting) produced a reduction in slow twitch muscle fibers while the proportion of fast twitch muscle fibers either increased or remained unchanged (2).
A six week study where fit males engaged in short bouts of sprinting (<10 seconds) found a significant increase in the percentage of type II muscle fibers and an increase in the area they occupied following the study (3). Long distance jogging works out mostly slow twitch muscle fibers. People with large amounts of fast twitch muscle fibers would be sprinters or in any sport that requires short bursts of speed. Explosive athletes use mostly fast twitch muscle fibers. Outside of improving athletic performance, I am not sure what additional benefits are offered by having a higher proportion of fast twitch muscle fibers. If you know, please leave a comment!
HIIT offers a lot of benefits in half the time
Okay, maybe half the time is exaggerating. But performing HIIT over endurance training will lower the amount of time you need to exercise. One study had participants go all out on a bicycle for 30 seconds followed by 4 minutes of rest. They did this for a total of 20 minutes, which equaled about 5-6 bursts of intense exercise per session. It was found that six sessions of this spread over two weeks (equaling about 15 minutes of actual exercise and a total time of 2.5 hours including recovery) led to an increased skeletal muscle oxidative capacity (improved ability to oxidize and use stored fat as energy) (4).
Theories about how HIIT increases Resting Energy Expenditure (REE)
- VO2 max = maximum rate of oxygen consumption. Determines your endurance capacity. A higher % of VO2 means that you are pushing yourself harder and consuming less oxygen
Resting Energy Expenditure (REE) is how much energy you burn at rest. It is basically the calories needed per day to maintain all bodily processes. Numerous studies have shown that high intensity exercise, >70% of VO2 max, increases the REE by 5-15%, up to 24-48 hours following the exercise but a similar rise is not seen following less intense exercise (5).
There are a few theories which aim to explain this sustained increase in REE. One theory is that high intensity exercise increases levels of serum norepinephrine, which have been found to stay elevated up to 24 hours following high intensity exercise but only for 2 hours following non intense exercise. Norepinephrine is involved in the fight or flight response and increases heart rate and the release of stored glucose.
It is also possible that high intensity exercise increases lipid oxidation rates. This means that more lipids are used as energy following intense exercise which is not seen in low Intensity exercise. It is thought to increase our resting lipid metabolism rates. Along these lines, it was found that following high intensity exercise our body increases our storage of glycogen. It is believed that since our body acts to replace the glucose we used to exercise, it switches its energy use from glucose to lipids, since it would not make sense to replace and use glucose at the same time. Once again, these are unproven theories, which may be partly, or not responsible at all, for the increased REE.
Extra calories burned from the increase in REE
It is hard to determine just how many extra calories are burned throughout the day after performing HIIT. One of the only studies I found showed that performing HIIT only led to an extra 30 calories being burned throughout the day when compared to low intensity endurance training. It seems that “greatly increasing metabolism,” with HIIT, may have been a bit of an exaggeration. If it does cause extra calories to be burned after the workout it would not be enough to lead to the fat loss seen in a few studies using HIIT.
Studies showing HIIT may burn significant calories after exercise
The most amazing thing about HIIT, at least to me, is the fact that you are sometimes burning 90% or less calories than an endurance training session (because of the shorter HIIT session) yet studies have found fat loss is sometimes greater in HIIT than longer sessions of endurance training (6). This shows that something other than calories burned is at work when examining the fat loss from HIIT. Calorie for calorie, HIIT produces greater weight loss and fat loss than endurance training. However, this does not really matter, as you can maintain endurance exercise much longer than HIIT, and will thus usually burn more calories. I was just pointing out how great of an effect such a small amount of work can produce with HIIT.
One study spanning 15 weeks with thirty four women assigned to a HIIT group, steady state exercise group and control group found that only the HIIT group experienced a decrease in total body mass, significant fat mass loss and decreased abdominal fat (7). The control group and steady state exercise group saw a gain in fat mass and central abdominal fat; although none of the gains reached statistical significance.
Another study found that HIIT induced more subcutaneous fat loss than an endurance training regimen which burned twice the calories as the HIIT program (8). When the energy cost of both training programs were compared calorie for calorie, the HIIT program produced a nine fold greater reduction in subcutaneous skin fold measurements. Once again, calorie for calorie does not mean much in the real world, as you will be burning significantly more calories doing endurance training.
Studies showing HIIT may NOT burn significant calories after exercise
So far, it seems like everything I have said about HIIT make it appear that it delivers everything promised. However, I came across a few reliable and newer studies showing that HIIT does not significantly increase post exercise oxygen consumption (EPOC-theorized to lead to additional calories burned throughout the day) or fat loss following the workout when compared to endurance training.
A very recent study found no difference in post exercise oxygen consumption (EPOC) between the endurance group and the HIIT group (6). Within 45 minutes of exercise, oxygen consumption levels returned to baseline (pre-exercise levels) in both groups. There was also no significant difference in post exercise fat oxidation between the two groups. They found no difference in concentrations of epinephrine and norepinephrine post exercise between the groups and levels returned to baseline in both groups within 180 minutes. They also found that the greater EPOC volume seen within 2 hours of exercise completion amounted to an extra 40 calories burned in both groups.
Another study involving sixteen subjects had their 24 hour energy expenditure and nutrient oxidation measured after daily activities. There were three days: Sedentary day (con) consisted of no exercise, low intensity day (LI) (40% of maximal oxygen consumption), or high intensity exercise day (HI). The exercise of choice for this study was cycling (9). The study standardized calories burned at 400 calories for both the low and high intensity days.
It was found that the 24 hour energy expenditure was similar for both low intensity and high intensity days. Within a few hours of exercise the energy expenditure returned to pre exercise levels. Surprisingly, during exercise fat oxidation was higher in the low intensity compared to the high intensity group. Within a few hours of exercise completion oxidation levels returned to pre exercise levels in both groups.
Looking at all past studies using HIIT, it appears that HIIT induces the greatest fat loss in subjects that are overweight (10). HIIT was not as effective at inducing abdominal fat loss in subjects that were already fit.
Calories burned during HIIT
HIIT, at least as performed in most of the studies I have read, does not result in a significant amount of calories burned during the actual exercise. Many of the subjects in the studies using HIIT only burned a few hundred calories at most. If you want to burn more than 200 calories using HIIT, you will have to do at least 8 or so sets of sprints with breaks in between. Plan on spending about 25 minutes or more (including breaks) if you plan on sprinting 8 sets. But be warned, pushing your body to the extreme can put a lot of stress on your body and end up being counterproductive.
Does HIIT reduce appetite?
One theory about HIIT leading to weight loss is that it suppresses subject’s appetites. Most of the studies linking HIIT and appetite reduction have been generalized from rat studies. A few studies have shown that rats engaging in hard exercise have reduced food intake throughout the remainder of the day. Scientists believe that an anorectic peptide called CRF causes the reduced appetite (10). The few studies in humans have shown a reduced appetite immediately following exercise but a normal appetite for the rest of the day. More research needs to be done in this area before any solid conclusions can be formed.
Limitations in most of the studies
Almost all studies examining HIIT use stationary bicycles and not treadmills. It is possible that sprinting has a different effect on our bodily systems than riding a stationary bicycle. Hopefully future studies utilize sprinting instead of riding a stationary bicycle, especially since running is one of the most common forms of cardiovascular activity.
Personal advice about HIIT
As I tend to do with many things, I took HIIT to the extreme. I figured the more the better, so instead of doing 4-5 sets of sprints I decided to do 10-12 with only 90 second breaks. By the end of the first week I noticed that my mood was down and I was more irritable and tired than usual. I did not connect the dots. By the second week I was even more irritable and my strength was noticeably diminished. I also gained two pounds. Combined with pushing my body too hard I was also not consuming enough calories. I got weaker, I did not lose any weight and my mood got worse. Luckily I figured out the cause before I continued to stress my body further.
Listen to your body. If you feel weak or irritable you may be pushing yourself too hard. When doing HIIT make sure you rest enough between sets and don’t do it every day of the week. They recommend doing it no more than three times per week.
From what I initially read and heard from other people I became excited about the possibility of spending less time on cardio while losing more fat, especially in the abdominal area. It turns out, like many other subjects, the hype surpasses the scientific basis. While older studies showed greater fat loss and increased post oxygen consumption with HIIT over endurance training, recent studies have found similar fat loss and little increase in post oxygen consumption in either HIIT or endurance training. A newer and better controlled study found that norepinephrine and post exercise oxygen consumption return to baseline within a few hours following HIIT. I would be lying if I said I was not a little disappointed in these findings.
Final word: HIIT vs. endurance training
After reading all of the studies, I think that both HIIT and endurance training have a place for weight loss. The problem with HIIT, one that I personally experienced, is that pushing yourself too hard can lead to wearing your body out really quick. The solution to this would be to reduce the amount of sprints per workout and to increase resting periods. The problem with that is you will only ending up burning about 200 calories for the whole workout. HIIT does raise your metabolism afterwards but not as much as previously thought. The main advantages with HIIT is the short time needed to do it and possibly increased loss of abdominal fat.
If you are overweight or obese, endurance training will end up burning more total calories. At this stage you just want to lose weight. Burning more calories would result in greater weight loss. In addition, performing sprints when severely overweight may be difficult or impossible, until you lose more weight or improve your cardio further by walking quickly or jogging. If you are already in great shape, adding HIIT to your weekly routine MAY help you burn that stubborn abdominal fat.
Has anyone or does anyone currently engage in HIIT? If so, do you prefer HIIT over endurance training? Which exercise prescription produced greater weight loss? Please feel free to leave comments about this article, share your stories about this subject or ask questions.
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