I was recently asked about the role of anaerobic metabolism in fat loss, and I figured that’s as good a topic as any to talk about.

For those unaware, your body has two basic ways of providing energy to your cells – one is oxygen-dependent, while the other can occur without oxygen. We know these are aerobic and anaerobic metabolism, respectively. We can further subdivide anaerobic metabolism into the phosphagen (alactic) pathway and the glycolytic (lactic-acid) pathway. Aerobic metabolism can occur both in oxygen-dependent or oxygen-independent modes; the latter tends to overlap with glycolytic metabolism.

As you might be aware, aerobic activity is associated with the “fat-burning zone”, where circulating fatty acids (triglycerides) are used for fuel. In the presence of oxygen, triglycerides can be oxidized to provide energy to the cell in the form of ATP – which is the “energy currency” of living cells.

Anaerobic activity tends to either use up existing reserves of ATP and creatine phosphate (the phosphagen system), or as people more commonly know it, reserves of glycogen stored in the muscles (the glycolytic system). Glycogen is nothing more than stored glucose taken in from the bloodstream, where glycolysis means “breaking down of glycogen”. Your body uses the breakdown of glycogen to replenish ATP levels.

In reality, any activity you do is going to see some contribution from all three pathways. The difference is the relative contribution from each. Instead of saying that an activity is aerobic or anaerobic, it’d be better to say that an activity is anaerobically-dominant.

The difference in the relative contribution is determined by the intensity and the duration of the exercise. The higher the required output, the faster your body requires energy. That’s why very heavy weight training or maximal sprints will tend to be capped at around 30 seconds – that’s the limit of what the phosphagen system can provide. These activities tend to represent maximal output from the body; they require lots of energy, and they require it quickly. Unfortunately phosphagen levels can only sustain very short bouts of activity, and none of the other pathways work quickly enough to keep you going.

As you go down the ladder of intensity, the duration can increase. Glycolysis can sustain upwards of 60-120 seconds of continuous activity, while the various forms of aerobic activity can sustain activity times measured in hours. It’s a matter of how quickly your body needs the energy and how quickly the metabolic pathway can provide it.

Oxygen Debt and EPOC

In times of yore, exercise scientists often spoke of the “oxygen debt” accrued by exercise. When you train hard, you build up a deficit of oxygen, which is why you breath hard after a short, hard bout of activity. Research has since discounted this idea in favor of the EPOC concept – Excess Post-exercise Oxygen Consumption. This is basically nothing more than increasing your oxygen intake, and it’s sometimes associated with increased fat-burning. This would be the mythical “afterburn” effect. The thing is, if you look at the research, the actual calorie burn from EPOC is rather small – we’re talking double digits.

So if EPOC isn’t really chewing up that much more calories in a 24-hour period, where is the effect coming from? There’s obviously something to the idea that anaerobically-dominant activity helps with body composition. If it’s not coming from the calorie burn, then what are we looking at here?

Nutrient Partitioning and AMPK

First we need to look at how and why calories get shuttled around the body. Generally speaking, nutrients are roughly split between muscle tissue and fat (discounting for a moment the needs of organs; we’re talking excesses and shortfalls here). This is where the concept of nutrient partitioning enters the picture – the ratio of nutrients that are sent to muscle vs. fat. Likewise, in a calorie deficit, partitioning affects how much comes out of muscle vs. fat.

This is largely out of our hands, sadly. Training and diet can affect it to some degree, though this is relatively minor. This means that if you happen to gain muscle and fat in a 1:1 ratio, then you’ll probably lose them in the same proportion on a diet.

That said, you can exercise in a way that can make this as favorable as it’s going to get – which is probably where anaerobic activity becomes a useful tool. The effect would seem to come from a funny little molecule called AMPK and all the effects it has on cellular activity.

AMPK is basically a cellular thermostat for energy levels, based on ATP levels and current glycogen stores. AMPK has some powerful effects, too – it’s connected to not only energy-regulating pathways, but it can also influence protein synthesis and muscle atrophy factors. The key thing we’re worried about is the effect from exercise – when you deplete glycogen during the course of a workout, AMPK swings into full effect. The use of glycogen as a fuel source is effectively suspended, and the muscle starts soaking up a lot more glucose. Further, any glucose that’s absorbed is sent straight back into the storehouse. During this period, fatty acid oxidation is sharply increased to make up for the energy short-fall, along with a release of fatty acids from fat cells (triggered by the hormonal environment).

Effectively the muscle becomes a calorie sink, eating up blood glucose and triglycerides – storing the former, burning the latter. This behavior explains a few things. After anaerobic exercise, insulin sensitivity appears to favorably increase in muscle but not fat, which improves partitioning – the affected muscles will basically soak up any available blood glucose. The effects on cellular metabolism explain why fat oxidation is observed to stay high even if you eat carbs. All the glucose is being stored away as glycogen while fats are used to fuel the cell. Finally we see that anaerobic cardio tends to increase certain enzymes associated with fat oxidation, as well as changes in the mitochondria (energy producers in the cell) that seem to correlate with endurance adaptations – yet without some of the negative side-effects of endurance training.

Taken together, anaerobic exercise seems to favorably shift partitioning towards muscles, by increasing the muscle’s rate of fat oxidation and in general acting as a calorie sink, while avoiding some of the muscle-eating adaptations that come from doing large amounts of cardio. Additionally, while the EPOC “afterburn” effect may not be eating up that much calories, it could well be causing a greater proportion of the calories that are burned to come from fatty acids – both by increasing fat oxidation in the muscles and by triggering a larger release of triglycerides from your fat cells.

Of course, this is just the micro-level slice of behavior – there’s going to be central/full-body issues that will be affected by your diet and current body-fat level – but that’s the cliff’s notes of why anaerobic activity seems to create favorable body-composition changes.

Criticisms and Potential Problems

This might seem like a mandate to go out and start doing nothing but interval training, the most widely-known form of anaerobic conditioning. Of course, it might not be this easy. While I think there is something to be said for emphasizing anaerobic training methods, we need a little more context. Mainly, we need to look at what constitutes anaerobic training and how these methods fit into an overall fat-loss strategy (which includes both diet and exercise).

Rather than labor the point, I’ll just direct you to a recent summary in Lyle’s blog where he examined a lot of the issues with interval training. If you don’t want to read all of it, the gist of it is that intervals in research aren’t controlled in the context of a larger diet/exercise strategy; the arguments used to support intervals (i.e., sprinters vs. marathoners) are intellectually dishonest; and the differences between intervals and steady-state/endurance training is minimal if you look at the absolute changes in body composition and calorie burn. There also may be other reasons that intervals show efficacy that aren’t controlled for, such as altered appetite and people not working hard to begin with.

Many of these issues are specific to the intervals vs. steady-state argument – the idea that intervals are flat-out superior to any sort of endurance-type steady-state work. The idea that you must burn yourself out with high-intensity cardio is a bit of a brain-bug in the fitness industry these days. The thing is, you can easily create a greater calorie burn with a slightly lower intensity but a larger duration of work. Additionally, you’re dealing with a false dilemma between polar extremes; it’s not a matter of doing either maximal sprints or running 10 miles. There is a middle ground that most people ignore.

I will say also that people often don’t distinguish between maximal power and maximal anaerobic output. Maximal power is what you see is a short 15-30 second sprint, say something like 100-200m sprints on the track, relying heavily on the phosphagen/alactic pathway. Maximal anaerobic output on the other hand is the point where you’re challenging glycolytic metabolism – and we see that this is going to happen in a range of 60-90 seconds of work. That’s where you’d want focus your efforts, as that’s what will burn up the most glycogen and trigger AMPK’s effects.

Now maximal bouts can challenge this system as well if you minimize the rest intervals, but you’re still running into other issues. Namely, this can burn you out in a hurry in the context of a weekly program, due to central stress. Milder “glycolytic training” is more of a peripheral or tissue-based effect and thus can be done more often.

I’ve often suggested tempo runs to people that want the benefits of both. I first read about these in Charlie Francis’s book The Charlie Francis Training System, which details his philosophy on training sprinters. Tempo runs can be thought of as moderate intensity intervals, which Charlie said he used to keep his sprinters in shape and keep body fat in check. While the high-intensity sprint training might have been limited to 2-3 sessions a week, tempo runs could be done much more frequently and for greater weekly volumes. The idea was to do a 100-200m run at roughly 75% of your best speed, alternated with lighter jogging, for up to several kilometers of total distance. By keeping managed rest periods and only a moderate pace, you end up training the anaerobic metabolism without the neurological/central stress – which is the goal.

Contrast this to what’s usually suggested for sprint intervals: maximum exertion for 15-30 seconds. Yeah, that’ll work the phosphagen system, and it’s certainly maximal power output, but that’s training something entirely different from the metabolic effect you want to train. It’s also a very neurologically-intensive training method – it empties out the “recovery sink” much faster than more moderate intensities.

In reality, if you’re doing intervals in a way that will best influence body composition, then it’s going to end up looking more like steady-state than (what most people think of as) intervals. Even tempo runs, while technically a form of interval training, have a greater work:rest ratio than 400m sprints or acceleration runs or anything like that.

What I’m talking about here is a middle ground – just as you don’t need to go out and do acceleration sprints four days a week on top of a hard strength-training program, there’s also not much justification for going out and racking up 10+ miles a day if your goal is to improve body composition. There’s a middle ground between the two, and that’s where you need to be – just hard enough to maximize calorie burn and activation of the anaerobic stuff, but not so hard you can’t get a decent amount of work done and get in a decent frequency of sessions.