Per request of a client, I am going to address heart rate and training zones today. The takeaways from this post will be:

1) Is there really a "fat burning zone" and should I aim for that?
2) Why are carbs an important element in my diet if I’m exercising?
3) How does heart rate give me clues for how my body is utilizing energy?

In order to understand your zones, you first need to understand how the body creates energy.

Phosphocreatine system: "Fight or Flight"
Without getting into the organic chemistry, this is a simple reaction that takes place in the muscles and nets 1 ATP (adenosine triphosphate - the body's energy molecule).  This system is that 10 second burst of energy you get as your "fight or flight" response or at the beginning of a speed or power move in your workout.  There is no way to fuel this system with food and it occurs anaerobically (without oxygen); PCr molecules are just held "in the wings" in your muscles to create this reaction when your brain tells your body you need to GO!  If you run the 100m dash, you can probably fuel your body with this system for the 10-12 seconds it takes to run the race.  It then takes your body several minutes to refuel the system.

Glycolysis
This system takes place in the muscles as the breakdown of muscle glycogen.  Glycogen is chemically the same as glucose, except it is stored within the muscles and liver instead of floating free in the plasma.  The average person can store about 1400 Calories of glycogen in their muscles.  In order to store/replenish this system, ingested carbohydrates are broken down into glucose, and the hormone insulin aids in "packing" it in the muscles.  The reaction that takes place is anaerobic and each reaction nets 2 ATP.  During any kind of workout, whether it is a long slow distance run, or a sprint, weight training, or intervals, there is some amount of glycolysis going on.  The more intense the workout, the higher percentage of energy is dependent on carbohydrate.  (I'll talk about that a little more soon!)

Aerobic Fat Breakdown
The only way to breakdown fat is through the aerobic pathway that you might still be having nightmares about from High School Chemistry, called the Krebs Cycle.  This reaction takes place in the mitochondria of the cells and yields 38 ATP, in the presence of oxygen.  It metabolizes Free Fatty Acids (FFA's) and triglycerides in the muscles (those are fancy terms for body fat).  Since even elite athletes have almost an unlimited amount of fat for energy, this is the most efficient system for endurance sports.

Training Zones and Energy Expenditure
I am now going to explain the energy breakdown based on how you are training.

Aerobic Workouts
Let’s say you go for a run, a brisk walk, or a light cardio workout, like hitting the elliptical.  Most likely, you will workout for 40-60 minutes (or longer) and your heart rate will be around 65-75% it’s max.  Most people know what this “feels” like, you are working moderately, you can probably carry on a conversation, but you aren’t completely wearing yourself out.  You probably couldn’t sing while working out though!  Initially, you will create energy through glycolysis, but if you stay in this heart rate zone, you will eventually settle so that about 75% of your energy is coming from the breakdown of fat, and 25% from glycolysis.  Perhaps a small percentage comes from the breakdown of amino acids, but that is negligible, especially if there is plenty of carbohydrate to breakdown.

Anaerobic Workouts
Power and speed workouts are almost always anaerobic – they require short bursts of energy to complete advanced tasks.  Weight training, interval training, sprints and mid-distance running, and plyometrics are all examples of anaerobic training.  Traditionally, you are above 85% max heart rate during these exercises.  During anaerobic bursts, you are likely using as much as 70-75% of your energy through glycolysis, the remaining is through fat burn, and a small amount as amino acids.  When in this type of state, you will be creating lactic acid as a byproduct and your body will not be able to keep up in filtering it out – this is the burning sensation you have in your muscles during an anaerobic workout.  Because of this, traditional anaerobic training involves 1-3 minutes of hard work followed by moderate work, or complete rest.  If your heart rate gets in the 90-95% max range, don’t worry, this isn’t necessarily unsafe, but it means you will likely go past your lactic acid threshold sooner and need to take a break.

A couple things to add:
1) The breakdown of fat is necessary in the presence of carbohydrate.  Remember, I said earlier that both glycolysis and fat breakdown occur simultaneously no matter how you are training.  If you are low or nearly out of glycogen, you will go into ketosis.  Ketosis is not a good place to be, your performance will suffer, you will likely be breaking down additional amino acids/muscle during the workout, and your body will be working to balance pH, as it is releases small acidic byproducts called ketones.  Another term for this is “carb crash” or “hitting the wall.”  No matter how much you think this is good because you are “burning fat, not carbs” – that just isn’t true.  You will likely injure yourself if you carry on for too long, and also fewer reps = fewer calories burned, so you want to be at your peak level of performance during a workout.  This is related to why marathon runners will often take a carb gel at least once during the event, by the 20 mile mark most people are running really low on muscle glycogen and performance suffers, so replenishment during the run is necessary.

2) As you build your endurance from aerobic and anaerobic training, what actually happens in your body is the increase of density of mitochondria in your cells, so you are essentially increasing your ability to metabolize fat as an energy source, and therefore preserve carbohydrate, and last longer before going into ketosis.  Both types of exercise will increase your endurance.  My advice for people who are just getting started is to stick with it, it will take your body a long time to build up a base to make it through longer workouts because your lactic acid threshold will be so low.  Just imagine, every time you push yourself a little bit farther, your body is responding by making more of those little “engines” in your cells, you will improve over time!

3) If you ONLY train in your aerobic zone, you will be training your body to get really efficient at using fat as its main metabolic energy source.  This seems good, but in fact you will also be training your body to STORE fat as well, since you are consistently choosing it as your main source of energy.  Since aerobic training typically doesn’t stimulate the muscles for strength and the nerves for speed and agility, you will be training one dimensionally by only working on long and slow.  This is the surest way to plateau.

4) It is also easy to get caught up in percentages versus total calories burned.  When working out anaerobically, you burn many more total calories, so even though the percentage of calories as fat is lower, you may end up equaling or even surpassing the total amount of fat burned during your workout versus an aerobic workout.  Let me say this another way, "the fat burning zone" of 65-75% max heart rate is a false term.  You burn more PERCENTAGE of calories as fat, but often fewer total calories as fat.

5) Finally, there is the process known as EPOC – excess post-exercise oxygen consumption.  After a workout, your body goes through several processes to get back to homeostasis.  This includes replenishing muscle glycogen that has been used, rehydrating and balancing electrolytes, reducing core temperature, and rebuilding the small microtears from stress on the muscles.  EPOC specifically refers to the lag time between when your central nervous system signals the heart and lungs to increase volume of oxygen/blood circulation from the beginning of an exercise.  At the end of a workout, the body is in "oxygen debt" because of this lag.  It even turns out that the amount of oxygen needed at the end of the workout is greater than the amount that was in short supply at the beginning of exercise.  EPOC can account for some increase in the burning of kilocalories, but perhaps not a huge amount.  However, combined with an elevated metabolism and the amount of energy that goes into repairing muscle tissue and returning the body to homeostasis, it is possible that EPOC and other post-workout processes after an anaerobic workout can lead to more calories burned by the end of the day.  The, quote, "afterburn effect."

So clearly anaerobic training is the more versatile approach, even though we have been told for years that the “fat burning zone” is only when we are around 65% our max heart rate.  With anaerobic training you will not only get faster and stronger and burn more calories during the workout (perhaps the greatest benefit), but you will potentially burn more calories in the recovery. Generally the recovery processes involve the Krebs Cycle/aerobic metabolism which uses fat for fuel.

There is a saying “you store what you stimulate.”  If you only stimulate fat during your workouts, you will store fat.  If you stimulate carbohydrates during your workout, you will be more likely to store food after to replenish your carbohydrates, and will burn fat along the way anyway.  Another way to think about this is to picture the average marathon runner versus the average short distance runner.  I’m not saying marathon runners are “fat” that is often not the case, but there are many people that are able to complete a marathon even though they have a beer gut.  Seriously though, even elite marathon runners, while extremely slender, are not very muscular.  Sprinters and mid-distance runners tend to have lower percent body fat and therefore more muscle.  Also think of any animal that needs to be fast, like a cheetah.  A cheetah is a lean, anaerobic machine, as is an athlete like sprinter Maurice Greene.

Conclusions
1) If you are going to be exercising, you can’t eliminate carbohydrates from your diet.  You can lower them, and you can be smart about choosing low glycemic carbohydrates, but no matter how you train, you need them to be healthy and have energy!  If you are injured and can’t exercise, perhaps you should look into a low-carb diet, but since I am a firm believer that weight loss and health includes diet AND exercise you all should be eating carbs.  Don’t let Dr. Atkins, R.I.P., scare you away from them!  

2) Any athletic training program should include some anaerobic training, whether this is intervals, strength training, or ideally both.  Even if you are getting ready for long distance events, it is invaluable to fold some cross training into your running schedule, in my opinion.  If you are trying to lose weight, it is the closest thing to a “magic pill.”  Working out anaerobically you not only burn more calories during the workout, but you build muscle and increase coordination, and raise your metabolism for the rest of the day as your body resets itself to normal levels, therefore kicking in what some people call the “afterburn effect.”  The "afterburn" may not be as big a factor as some people make it out to be, but combined with the other benefits of anaerobic training, it is an added bonus!

3) Know what it feels like to be in different zones.  Can you sing?  You’re dogging it.  Can you carry on a conversation?  You are probably in your aerobic zone.  Are you working so hard you can’t talk and you tire after a couple minutes?  You are likely in your anaerobic zone.  Do you see Jesus when you are working out?  You are probably in your “red zone” in the 95% range!!   If you don’t know what it feels like yet, wear a heart rate monitor. 

(To estimate your max heart rate, subtract your age from 220, knowing for those of you who train harder, your actual MHR will be higher than this number.  Than play with math, take your heart rate in 15 sec intervals and multiply by 4 to see where you are in bpm.)

One more fun thing to try!
Do you want to know what ATP feels like?  Here is a funny way to activate your phosphocreatine system (as long as no one else is in the house!)  Straighten your arms and start clapping your hands in front of you.  Slowly increase intensity for 30 seconds so that by 30 seconds you are clapping uncontrollably fast!  Then at the 30 second mark pump your fist in the air with maximal effort and yell “YES!!.”   You will probably feel a tingling feeling through your whole body – this is your “fight or flight” response!  In turn you will probably burn about 100 more calories in the next hour than if you hadn’t done that exercise :).  You’re welcome.