Optimising Fat Burning on Non-HIIT days

We can't HIIT all the time, nor can we work steady state at the top end of our aerobic capacit all the time. Our central nervous system would come up and strangle us. That's another word for overtraining. But if we still want to make sure that we're optimizing our non-HIIT time for both endurance capacity and fat mobilization, can we do both at the same time? Outlook looks good that there's a sweet spot for such work in the 60-80% MaxHR zone.

Last week, we looked at a successful protocol for fat mobilization and showed fat loss when comparing steady state to HIIT. We might recall that of the two groups - steady state and HIIT - when there was no other change to regimen (no diet change for instance) only the HIIT group changed BF%, dropping 5 pounds plus of fat over 15 weeks. One would seem to see in this that HIIT is the best approach to get the fat off then, but there is a related question, as authors on a 2009 Journal of Strength and Conditioning Research paper puts it:

‘‘Should I exercise at a level that optimizes fat oxidation, or is total caloric expenditure the ultimate determinant of fat loss?’’Surprisingly, this fundamental question has not been answered to date, probably because of the difficulty of precisely controlling caloric intake and expenditure.

From the HIIT work, we saw that pushing anaerobically for sprints, and then recovering aerobically was great for fat mobilization. The authors, without unfortunately citing any specifics, suggest that there may be some issues with this conclusion

Those studies that have been completed generally have controlled for exercise dose, comparing highintensity, short-duration exercise with low-intensity, longduration exercise of equivalent caloric expenditure. However, nonexercise physical activity and caloric intake were not controlled, and no definitive conclusion could be reached.

I'm not sure about that and the authors get away without substantiating that claim that "no definitive conclusion could be reached". Indeed, they rather punt to say that they're not tring to figure out the optimal training intensity for fat loss. Ok, then what are they doing? They want to know

Can improvement in aerobic capacity and optimization of fat oxidation be attained
simultaneously, or are these objectives distinctly different and require different intensities of training for their attainment?

In other words if you're working out to be able to do more work at a level that is "glycogen spairing" - uses fat for fuel rather than precious muscle and blood sugar - can training to get that effect optimized also connect with fat burning? And so to get at this question, the authors say

The purpose of this study is to compare the FBZ [fat burning zone] and AZ [aerobic zone] in a group of competitive endurance athletes (runners). To the best of our knowledge, this is the first study to directly compare these 2 training zones in the same group of subjects. To the best of our knowledge, this is the first study to directly compare these 2 training zones in the same group of subjects.

As a first step, the authors make clear that it's really straight ahead to see the points at which one goes from using fat as the primary fuel to using sugars. The calcutation looks at the respiratory exchange ratio (RER). For Carbs being 100% of fuel, the RER is 1. For fat, it's .7. This can be tested by hooking a person up to a cart that collects respiratory gasses. Cool.

The authors also do a really nice job of saying what things impact on fat mobilization, too. Eg, eating carbs before exercise surpresses fat mobilization (hence the Precision Nutrition heuristic to say carb-dense foods for post-workout).

The nice thing is here, that while few of us have access to a metabolic cart (and using one isn't a lot of fun), we can use heart rate instead as a pretty good indicator of fuel type metabolized.

TO calculate Aerobic Zone, the authors used the ACSM measure: 50% of HRR (heart rate reserve) for the lower end, and anaerobic heat rate threshold for the upper end. While Anaerobic threshold is becoming an increasing question of debate (while lactate threshold has been totally toasted), the authors are just using points established by a Body, and testing against these. Good idea. There are known methods to determine AT in the lab - or what passes for tha threshold, so what the heck.

Fat burning zone was determined by watching the gas exchange readings on the cart: as soon as the ratio flipped beyond .7, one is beyond the FBZ.

Here's what the AZ/FBZ's look like:



To cut to the chase, the main result is that

for Maximum Fat Oxidation, 32 of the 36 subjects (89.0%) fell in a range of 60.2–80.0% of maximal heart rate.

And there is overlap between these two zones:


The authors' discussion of these results describes the following observations and potential uses for the findings:

the upper limit of exercise intensity for FBZ (80.0% max heart rate) is mid-range for the AZ (67.6–87.1% max heart rate). In addition, the upper limit for calories per minute (11.5) for FBZ is mid-range for AZ (9.15–-14.2 cal /min). The upper limit of fat calories per minute for FBZ (3.45) was not significantly different (t = 1.23, p = 0.225) from the lower limit of AZ (3.11). The biggest discrepancy between the 2 zones occurs when comparing fat calories expended at the upper limit of FBZ (3.45 fat cal/min) with the upper limit of AZ (1.68 fat cal/min).If the objective is metabolism of fat calories, training at the upper limits of AZ should not be recommended. If total caloric expenditure is the objective, the upper limits of AZ will be the most efficient.

One might think, well there you go: fat mobilization is below the top of the AZ and so back off from that edge. Umm, no, apparently not so fast:

In addition to more calories being expended during exercise, caloric expenditure during recovery from high-intensity exercise is greater than recovery caloric expenditure from low-intensity exercise because of the additional energy requirement of ventilation, restoration of adenosine triphosphate phosphocreatine, replenishment of glycogen stores, and body temperature elevation. Also, prolonged exercise at high intensity, as in marathon running, has shown a gradual decrease in carbohydrate oxidation and gradual increase in fat oxidation as glycogen stores become depleted. If fat calories, and not total calories, were a better predictor of weight control, we would expect endurance athletes, who spend a rather large volume of training above FBZ, to have weight control problems. This is clearly not the case.

In other words, almost as with hypertrophy training, volume has a pretty important role to play for fat mobilization, and volume here can be accrued by time. What's this mean: a few weeks ago, we looked at CoQ10's effect on endurance, and there we saw that after repeats of wingates that are pretty carb stealing, the oxidative system kicks in because those carb sources have been depleted. What this seems to suggest is that if you can stand it, working at the AZ top end for long enough will burn out the carbs and push into the fat as primary fuel source out of necessity.

That's potentially great to know for endurance athletes. But what about strength athletes working on their body comp?

Recommendations:
HIIT is generally recommended only a few times a week. And from last week's studies we saw that the bouts are only 15 mins of work long. These ranges are for the benefit of one's central nervous system.

If you still want to do fat burning work on non-HIIT days, or just more work on your HIIT days, it seems that one has a wide range of efforts to play in: 60-80% of MaxHR. So for CARDIO days when you've HIIT yourself to death and your CNS is crying for mercy, you can still do cutting by working in the 60-80% zone. Likewise, if you still have more sauce on HIIT days after 15 mins of work, you might want to add on some effort of steady state at your optimal fat burning zone.

Caveat on 60-80%. That's a heck of a big range. In some interesting reflections on their results, the authors note that we still don't know WHY there is such a range. Dietary practices alone apparently don't account for it. And in this group, we're talking really fit people too, and even in such a closely matched cadre, there was still this rather large range of values for where the OPTIMAL fat mobilization occured.

In other words, there is a LOT we don't know yet about individual variation within fat mobilization. It's not a 1:1 relationship of work this hard and get these results. The best the authors can say is with a 90% result of participants in this range, there's a good probability that work in this range will have the best likelihood of fat mobilization.

Example: Combos Applied.
All caveats considered, I had a note from a person last week responding to the HIIT work saying that he does a similar interval protocol on his bike of the 24/36 protocol doing hills as hard as he can, and then finishes up with 30 mins of work at 70% max heart rate - for him, that's his sweet spot, and fat loss has never been so fast for him as when he hit this combo.

Personally i think this is really cool to see how we can mix and match various optimizations for our goals.

Related Posts

• Respect the Fat
• 6 mins of work a week
• Does Cardio interfer with strength training? how 'bout no?

main ref:
Carey, DG (2009). Quantifying Differences in the "Fat Burning" Zone and the Aerobic Zone: Implications For Training Journal of Strength and Conditioning Research: , 23 (7), 2090-2095 : 10.1519/JSC.0b013e3181bac5c5