HIIT (on bikes) - why it results in both more fat reduction, and Spot Fat Reduction at That, too than Hearty Steady State

There are lots of folks espousing the value of HIIT as an effective calorie burner when trying to burn fat. But is there really a special role for HIIT in the fat burning lexicon, or should we just strive to work harder - like 50-75% VO2max - throughout a cardio session? Some more recent work suggests there may be in terms of metabolic activity and even spot fat reduction. But before we go there, let's refresh a bit about HIIT.

The old argument that HIIT burns more calories than Steady State and so is beter has been given a good walloping by Lyle McDonald. Especially in his head to head of steady state with HIIT he pretty persuasively shows that "The intervals only come out a TINY bit ahead if you compare workouts of identical length and even there the difference is absolutely insignificant."

Review: What's HIIT supposed to Do?
Indeed, work from 2008 lead by Shannan E. Gormley comparing intervals to a decent level of steady state effort did show a benefit for intervals of a sort, but the question for the researchers is what's the optimal time to spend at that peak intensity in an interval to elicit this effect? As cited previously here, the authors state:

It should be noted that although interval training groups spend some of their training time at a very high intensity, a similar amount of time is spent at a lower intensity, and therefore the mean intensity of training may not be any higher than that of a continuous training program. In the current study, the interval training group used 5 min each for the work and the recovery phases of the intervals and had an average intensity of 72% HRR, which is slightly less than the 75% HRR of the vigorous [the steady state -mc] group. The work-recovery periods of Helgerud et al.^[16] were 4 min at ∼93% HR_max and 3 min at 70% HR_max, for a mean intensity of 83% HR_max in the interval group, whereas one of the continuous groups used 85% HR_max. Warburton et al.^[37] used 2 min at 90% HRR and 2 min at 40% HRR for the work and the recovery phases, yielding a mean intensity of 65% HRR in the interval group, and had the continuous training group use 65% HRR. Wisloff et al.^[38] used 4-min work phases at ∼93% HR_max and 3-min recovery phases at 60% HR_max, for a mean intensity of 79% HR_max in the interval group, and used ∼73% HR_max in the continuous training group. Despite the similarity of mean intensity between the interval and the continuous training groups, the interval groups in all of these studies experienced greater improvements in aerobic fitness after training. Therefore, although intensity is a key variable in cardiorespiratory training (as shown by comparing the two continuous training groups in this study), the mean intensity may not be as important as the highest intensity that is used for a significant portion of the training. A topic for future research is to determine what portion of training should be done at high intensities and using what work-recovery periods to obtain the greatest results

The above is looking not at fat loss effects of intervals, but training to enhance oxidative capacity for performance. More recently even really brief intense bouts of exercise (like 6 mins a week of effort compared with hours of steady state for the same physiological effect as hours of 60% MaxHR), there are similar kinds of performance benefits. So, there seem to be some performance optimization benefits from (a) looking at finding the right balance of peak intensity to recovery for work sessions and (b) looking at supramaximal efforts that may have similar effects in less time. Again, that's performance, not fat loss, and in the former case, we are talking really small degrees of difference.

So what about fat loss & HIIT, then?
HIIT means high intensity interval protocol. But what is the best HIIT to do if you're tuning it for fat lost first, and anything else second? And does it make a difference if you're dealing with elite athletes or people who are just well enough conditioned so their hearts won't explode if you ask them to go "really hard" for a bit? Is it 60 secs on? 30 off? 60:60? The infamous tabatta on for 20 off for 10 - and remember that was not primarily a fat burning study but an anaerobic/aerobic capacity study.

In 2007 & 2008 a couple studies came out on HIIT from New South Wales as part of some cool PhD work lead by Gail Trapp that i have come to cite frequently about a great HIIT protocol for fat loss and other cool, related benefits that looked explicitly at intervals for fat burning and effects between conditioned and less conditioned participants. Here's the first one, looking at what different HIIT intervals stir up metabolically.

Am J Physiol Regul Integr Comp Physiol. 2007 Dec;293(6):R2370-5. Epub 2007 Sep 26.
Metabolic response of trained and untrained women during high-intensity intermittent cycle exercise.

Trapp EG, Chisholm DJ, Boutcher SH.

School of Medical Sciences, Faculty of Medicine, Univ. of New South Wales, Sydney 2052, Australia. e.trapp@unsw.edu.au

The metabolic response to two different forms of high-intensity intermittent cycle exercise was investigated in young women. Subjects (8 trained and 8 untrained) performed two bouts of high-intensity intermittent exercise: short sprint (SS) (8-s sprint, 12-s recovery) and long sprint (LS) (24-s sprint, 36-s recovery) for 20 min on two separate occasions. Both workload and oxygen uptake were greater in the trained subjects but were not significantly different for SS and LS. Plasma glycerol concentrations significantly increased during exercise. Lactate concentrations rose over the 20 min and were higher for the trained women. Catecholamine concentration was also higher postexercise compared with preexercise for both groups. Both SS and LS produced similar metabolic response although both lactate and catecholamines were higher after the 24-s sprint. In conclusion, these results show that high-intensity intermittent exercise resulted in significant elevations in catecholamines that appear to be related to increased venous glycerol concentrations. The trained compared with the untrained women tended to show an earlier increase in plasma glycerol concentrations during high-intensity exercise.

Fat Mobilization: Freed for the Burning. Ah ha you say, there's no fat loss measured here. Right. But what IS measured here is catecholamine activation. Those threat response fight or flight hormones are what mobilize fat to get burned, baby burned. And from these the authors suggest a correlation to the level of catecholamine released and the level of glycerol to be found in the blood stream. In other words, higher degree of intensity, greater catecholamine release, more fat mobilised to be used for fuel.

Any type of heart rate elevation triggers some catecholamine response - so does drinking green tea. But what the authors show here is that both long and short high intensity efforts - sprints in this case - can be effective to trigger greater catecholomine release, but the longer 24 sec sprint with its equivalent longer recovery seems to be better overall for fat mobilization. That's cool. That's actually less work/minute at 24 secs on 36 off than the shorter burst of 8on/12off (24sec vs 40secs of work). Wow. So longer more intense intervals - not necessarily more work - yields higher levels of fat release for fuel - but both the shorties and the longies are good.

Applying these Inervals to Fat Loss. Trapp and Co. then took this finding to a larger cohort of 45 participants (up from 8 and 8), and went longitudinal running a 15 week study. 15 weeks is *good* for 45 people to hang in there.

Int J Obes (Lond). 2008 Apr;32(4):684-91. Epub 2008 Jan 15.
The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women.

Trapp EG, Chisholm DJ, Freund J, Boutcher SH.

Faculty of Medicine, University of New South Wales, Sydney, New South Wales, Australia. e.trapp@unsw.edu.au
OBJECTIVE: To determine the effects of a 15-week high-intensity intermittent exercise (HIIE) program on subcutaneous and trunk fat and insulin resistance of young women. DESIGN AND PROCEDURES: Subjects were randomly assigned to one of the three groups: HIIE (n=15), steady-state exercise (SSE; n=15) or control (CONT; n=15). HIIE and SSE groups underwent a 15-week exercise intervention. SUBJECTS: Forty-five women with a mean BMI of 23.2+/-2.0 kg m(-2) and age of 20.2+/-2.0 years. RESULTS: Both exercise groups demonstrated a significant improvement (P<0.05) in cardiovascular fitness. However, only the HIIE group had a significant reduction in total body mass (TBM), fat mass (FM), trunk fat and fasting plasma insulin levels. There was significant fat loss (P<0.05) in legs compared to arms in the HIIE group only. Lean compared to overweight women lost less fat after HIIE. Decreases in leptin concentrations were negatively correlated with increases in VO(2peak) (r=-0.57, P<0.05) and positively correlated with decreases in TBM (r=0.47; P<0.0001). There was no significant change in adiponectin levels after training. CONCLUSIONS: HIIE three times per week for 15 weeks compared to the same frequency of SSE exercise was associated with significant reductions in total body fat, subcutaneous leg and trunk fat, and insulin resistance in young women.

To be clear, in the second study, gals in the HIIT group did a five minute warm up, followed by 20mins of 8sec sprint followed by 12s of 20-30rpm recovery. The load was continually adjusted over the course of 15 weeks, starting at everyone getting to 20mins at .5kg of resistance. Based on heart rate, the load was upped by .5kg so that the heart effect was consistent as folks got stronger.

The steady state group worked at 60%V02peak - that's a good clip - about 75% maxHR so no slouching there. They worked up from 10mins to 40mins. The mean heart rate of the groups was 168.6 for the HIIT group; 155.7 for the steady state group (participants were 18-30 years old).

As to the fat loss: There was significant FM loss (P less than 0.05) r="−0.58,">This last point is not surprising, based on energy available for fuel from fat relative to bodyfat % (discussed here). The authors come back to this point stating:

High-intensity intermittent exercise training had a marked effect on fat levels for some individuals and a moderate effect for others. The correlation (r=0.58, P less than 0.01)43, 44 The four moderate fat loss responders in the HIIE group (women who had a 3% or less decrease in total fat) possessed significantly lower initial FM than the other women. With the four lean women removed, the mean fat loss in the HIIE group was 3.94±0.91 kg resulting in a 4.3% decrease in body mass and a 14.7% decrease in total FM. This 3.94-kg fat loss compares favorably to the 1.15-kg weight loss reported in a recent meta-analysis of the effects of SEE on weight loss.

That all sounds good and reasonable and wonderful, but then comes the particularly interesting bits - gosh what would almost seem like spot fat reductions:

High-intensity intermittent exercise led to a significant decrease (P less than 0.05) in central abdominal fat (−0.15plus or minus 0.07 kg), whereas the SSE and CONT groups had nonsignificant increases in central abdominal fat (SSE group, +0.1 plus or minus 0.08 kg; CONT group, +0.03 plus or minus 0.04 kg).

So more weight off the gut area in HIIT, legs and trunk (other newer work (like this one Nov 08 lead by Irving, and this one Aug 09 lead by Coker ) has seen similar results with gut fat). Intriguingly all groups put on fat in the arms (but not a lot). Indeed, the gut fat loss the authors cite as THE finding of the study. Even more, they state

Despite exercising half the time, HIIE subjects in the present study lost 11.2% of total FM with SSE subjects experiencing no fat loss.

That's a pretty big difference between the two groups

Discussion of Findings - Cautious optimism for Intense Intervals
The authors in true geek science-ness don't overegg the results:

Collectively, these results demonstrate that intermittent sprinting compared to SEE is a more effective and efficient way of controlling body composition. However, our estimates of energy expenditure and intake lack sufficient precision to comfortably conclude that energy balance was unaffected in the HIIE condition. Thus, it is feasible that the change in FM that occurred in HIIE may have been influenced by unreported changes in diet. Indeed, HIIE-induced suppressed diet intake may be one of a number of possible factors underlying the fat loss effect of HIIE.¹¹ For example, HIIE may have suppressed appetite or decreased attraction for energy-dense foods.^{24, 25} Another explanation for the HIIE fat loss effects is that this type of exercise may result in enhanced lipid utilization. Prior research in our laboratory has shown that lipid release, as indicated by blood glycerol levels, gradually increased over 20 min of HIIE.²⁰ Catecholamine levels in this study were also found to be significantly elevated after HIIE.²⁰

Free Fat. I love this! Because of that catecholomine hit we saw earlier, and because there's more fat available as fuel in the blood ready to be used, Trapp's crew hypothesizes, maybe people doing HIIT just aren't homeostatically tweaked to reach for calorically dense foods - their bodies know they have that covered. That's a really intersting idea. I wonder if doing HIIT closer to meal times enhances this effect, if that's what's going on.

What about this seeming spot reduction? But even if you want to say there are interesting side effects going on with HIIT that are causing these fat loss responses, the authors' key result is this abdominal fat difference. HIIT took OFF some ab fat; Steady state, i'm sorry to say, put some on. Dang.

Here's where exercise type may play an important role in whether or not this spot effect is achieved. The authors postulate the following:

It is considered that spot reduction (that is, deliberately reducing fat stores in specific areas of the body) is not possible, and the body will mobilize preferentially those stores with the highest concentrations of adipose cells.^{36, 37, 38} There is evidence in the current study that this principle may not apply to every exercise modality. In HIIE, where work is done primarily by the musculature of the legs and the trunk muscles act as stabilizers, there was a decrease in FM and an increase in lean mass, which summated to a significant change in percentage of fat in these two regions. This was not the case with the SSE group.

So where work triggers core stabilizers to get that extra intensity, there may be a seeming spot fat loss effect. It's also interesting to note that only the HIIT group had lean body mass increase.

Translating Results to Other Modes? Do these findings translate to other modalities for HIIT - like oh i dunno, maybe kettlebells? Don't know. Perhaps that would be an interesting comparison for bike, hardstyle with it's tension at the top of the swing say, and that hip/core/lat activation, and GS with its more relaxed swing. Do GS/HS differences fall away as the bell weight gets heavier?

In the meantime, the authors offer the following:

In conclusion, 20 min of HIIE [on a bike - mc] , performed three times per week for 15 weeks compared to the same frequency of 40min of SSE exercise was associated with significant reductions in fasting insulin, total body fat, subcutaneous leg fat and abdominal fat.

While the authors tested their participants with the 8/12 interval, their earlier work with the 24/36 suggests the benefits might be even greater - on a bike, but maybe with a kettlebell or a rowing machine, too.

Take Away: the Skinny on the Fat & HIIT
There are at least two ways to talk about HIIT - in the performance arena, and in fat burning. In performance, there is a small but not insignificant edge to interval work over intense steady state. In fat burning there is a really significant effect. Here's what i think it is.

While the authors make much of the spot fat reduction - and that's not nothing - the more intriguing thing is that *only* the HIIE group lost fat & had their lean body mass go up.

I've cited before work to show that without diet, any weight loss changes, even over 12 weeks of working out are small. In this case, there was no deliberate dietary intervention. So that there was such fat loss without more or less trying dietarily as well is really kinda eye openingly "what the heck?"

So i am intrigued by the authors' speculation about that catecholamine effect and glycerol release and potential effect on let's say homeostasis - a reduced reach for high cal foods, naturally. Wow. That makes HIIE worth looking at from a whole other point that has a whole lot less to do with the calories burned on the bike and the effect of those intervals throughout every other day of the week. And that's only 3*15. What would 3days at 20 or 30 or 40 minutes do? More is not always better - and intervals can be fatiguing but. Hmm.




Related Posts

• Kettlebells and Cardio - it doesn't have to be VO2max all the time
• Viking Warrior Conditioning, the Review with Kenneth Jay
• Does Cardio interfer with Strength work? How 'bout No
• Running the bells - hill workouts with kettlebells
• Weight loss doesn't work without Diet - really [story 1, story 2]
• Fat, the amazing fuel.
• Getting a handle on the habits of thinner peace (as martha beck calls it)

main refs

Trapp, E., Chisholm, D., Freund, J., & Boutcher, S. (2008). The effects of high-intensity intermittent exercise training on fat loss and fasting insulin levels of young women International Journal of Obesity, 32 (4), 684-691 DOI: 10.1038/sj.ijo.0803781

GORMLEY, S., SWAIN, D., HIGH, R., SPINA, R., DOWLING, E., KOTIPALLI, U., & GANDRAKOTA, R. (2008). Effect of Intensity of Aerobic Training on VO2max Medicine & Science in Sports & Exercise, 40 (7), 1336-1343 DOI: 10.1249/MSS.0b013e31816c4839

Trapp, E., Chisholm, D., & Boutcher, S. (2007). Metabolic response of trained and untrained women during high-intensity intermittent cycle exercise AJP: Regulatory, Integrative and Comparative Physiology, 293 (6) DOI: 10.1152/ajpregu.00780.2006