Yes, Virginia, there really is a fat-burn zone

Way back when, some personal trainers came up with the concept of a "fat-burn zone" training intensity; a work-rate designed to match your body's optimum ability to burn fat for muscle fuel. I think it was intended to make workouts easier for their lazy clients. Like usual, some others disparaged that concept and ridiculed it saying there's no such thing as "fat-burn zone." So what should we believe?

I stopped believing in the fat-burn zone after reading articles from the naysayers, like Marc Perry's "The Fat Burning Zone Myth: Don’t Be Fooled." To me it seemed like the answer was closer to a simple "calories in vs. calories out" formula; ride harder to burn more calories. Certainly if you have limited time then that makes sense; even if you believe in the fat-burn zone, who has the time to ride for hours at a moderate pace?

Then I was on a group ride with USA Cycling Pro Road Race National Champion "Fast" Freddie Rodriguez in which he mentioned several times that most of his riding involves lower-intensity fat-burn riding. Eh, a seasoned Pro believing in these myths? OK, let's first agree that being a Pro doesn't mean he knows everything (and may mean he has lots of time available for longer rides). But still, clearly whatever he was doing seemed to work well for him and maybe I needed to revisit the subject (something that I had turned over in my mind countless times anyway).

So, the naysayers will tell you that working harder burns more calories per hour, which is true enough and not in dispute. But if you visit Marc's article above and read the chart there, do some calculations and ponder it a bit you may start believing in the fat-burn zone after all.

According to Marc's chart a 30-minute workout at 75% intensity burns 140 calories from fat, while a 50% intensity burns 120 calories from fat; OK, true enough, but that's almost the same results with far less suffering! And we can ride at 50% much longer without depleting ourselves. Heck, we might even have some muscle glycogen left afterward so we can do some higher intensity training the next day. Note that I wouldn't advise only doing moderate "fat-burn" rides regardless... since we all want to get faster we need the higher intensities too.

But, hold on, what's this "afterburn effect" he mentions later? It seems higher-intensity workouts cause our bodies to burn more calories after the workout is done; perhaps more than we burn during the workout. This reflects the energy we burn later on to repair and otherwise recover from the workout itself. According to the article "39 calories burned for the cycling group [from 3.5 minutes] vs. 65 calories burned for the sprinting group [from 45 seconds]."

One problem with the afterburn effect cited in his article is that he cites two different studies to reach his conclusion: One studies 30-minute workouts, another studies 45-second and 3.5-minute workouts... I don't think it is logical to apply that to the typical hours-long rides we cyclists use for even higher-intensity training. The afterburn effect is far more applicable to weight-training, I would think, as it reflects the massive amount of energy the muscles require for fiber repairs and the extra energy required to replace the spent glycogen (your body doesn't replace burned fat unless you eat too much, so I bet that also explains the lower afterburn from a lower-intensity workout).

The article refers to another article which states “If you're working your muscle to the point where you're causing damage at the microscopic level, it's going to take energy to repair that." And "We have a long way to go before we understand this [afterburn effect]." In fact, I would say Marc's article is severely flawed as a guide for most cyclists, as nobody can do 100% sprints for more than a couple of minutes total. And we don't usually train sprints more than once a week (if at all).

When we cyclists do high-intensity workouts we need to burn lots of glycogen. Many cyclists eat gels and drink energy drinks during their rides to avoid depleting their glycogen... and then wonder why they aren't lean. Perhaps it's because if we deplete our glycogen and then replace it immediately with carbs, can we really lose much fat? And the afterburn effect from our typical rides, even the higher intensity ones, can't possibly come close to what the article cites (unless you're doing standing-start track sprints, e.g.).

Even the most ardent cyclists I know (including some State and National Champions) don't ride more than about 3.5 hours a week at higher intensities. And that's still far more than the 45 seconds Marc uses for proof of his conclusion.

  • If you ride 8 hours a week in the 50% zone you will burn 1920 calories from fat (about 1/2 pound). 
  • If you ride 3.5 hours at 75% you will burn 980 calories from fat (about 1/4 pound). 

Never one to rely on one source, another article, on, shows that upping the intensity from 60-65% to 80-85% only raises the fat burn from 73 calories to 82 calories per 30 minutes.

  • If you ride 8 hours a week in the 60-65% zone you will burn 1168 calories from fat (about 1/3 pound). 
  • If you ride 3.5 hours at 80-85% you will burn 574 calories from fat (about 1/8 pound). 

So these articles don't agree on the exact amounts, but they do show the same trend of the lower fat-burn from shorter, higher-intensity rides. Plus, you will burn some fat from the afterburn effect either way: Both intensities do have an afterburn effect, it's just exactly how much that's in dispute. I suspect the amount isn't that different for cyclists at typical intensity levels. Now, weight-trainers would have a larger effect, as I read it.

So, to burn fat cyclists need to work for long periods at an intensity they can maintain for that period. It has worked well for me; I spend almost all my time in lower intensities and consistently measure between 6% and 9% body fat. Even my tougher workouts are typically 90 minutes sub-threshhold ("recovery" and "endurance"), and perhaps 10 minutes at higher intensities.

OK, I'm no expert, but read the articles above and decide for yourself. I think it's likely Marc really had weight-trainers in mind, and we readers may be guilty of applying the results from a set of studies to cover sports they weren't even addressing.