Back to Fitness: Fat Burning


By John Andersen

A friend asked me in the store last week, “How do you run to burn fat?” This was actually a great question, with a very specific answer that may not be what you think.

I would like to start by saying there will always be endless debate on the best ways to exercise, live longer, lose weight, etc. The most important thing is that you find something you enjoy and can stick with regularly to make exercise an integral part of your life!

This month we’ll look at a science-based view on the optimal way to burn fat.

Let’s start with the fact that during exercise, our body has two main fuel sources to choose from–sugar and fat–and the intensity at which we exercise influences which fuel our body will use. As our friend Dr. Mark Cucuzzella puts it, our body is like a hybrid car, constantly burning a blend of gasoline (sugar) and electricity (fat). At higher intensity exercise, our hybrid engines are mostly utilizing sugar, which gives us 36-38 ATP per molecule of glucose (ATP is the body’s energy currency). We only have enough glucose stores to last for 1-2 hours before we crash! At lower intensity exercise, our hybrid engines are mostly utilizing fat, which gives us a whopping 460 ATP per molecule of fat! We all have enough fat stores to fuel the electric engine for days on end!

While burning glucose enables us to perform at a more intense level, it also produces more byproducts (such as lactic acid) and is more stressful on our body. Fat burning exercise on the other hand is very easy on our body and allows us to recover quickly and be less prone to injury.

So what is the best way to burn fat? Exercise at a lower intensity, but for extended periods of time, regularly, and on an empty stomach.

• Lower intensity: To be specific, we’re trying to exercise below our aerobic threshold heart rate. Without getting too technical, this tends to be running or walking at a “conversational pace” where it’s easy to chat with a friend without being out of breath. For the more technically inclined, you can use a heart rate monitor and find your aerobic threshold heart rate (ATHR) and just stay below it. There are many formulas to find your ATHR, the most simple of which (per Dr. Phil Maffetone) is to take 180 minus your age. I am 37, so my ATHR would be 143. If I am exercising below this heart rate, I am burning mostly fat. If I exercise above it, I start to use less fat and more sugar the higher my intensity goes.

• Extended periods of time: Because our fat burning system is so efficient, you do need to exercise for longer periods of time to burn significant amounts of fat. I recommend making an hour of exercise a goal. One hour three times a week is better than 30 minutes six times a week. Make the time for yourself! Running, walking, hiking, biking–any activity that you can enjoy is great. And runners, don’t forget that you’re allowed to walk from time to time! There is nothing wrong with walking to keep that heart rate down on our hills. You’ll be able to go farther and longer if you mix in some walking into your runs.

• Regularly: To really start to burn off some fat, you’ve got to make this a habit and get out there several times a week. Let’s face it. This is the hardest part. Life is too busy, I’m tired, it’s cold outside, etc. But I am convinced that anyone can carve out this time. It may be at 5 a.m. or 8 p.m., but you can find it. You’re worth it!

• Fasted: The beauty of exercising at a lower heart rate is that you simply do not need any food to fuel that exercise. So you can just wake up, get a cup of coffee (no sugar!) and go. There is a specific reason I say fasted. If you eat or drink right before exercise with foods containing any sugar or simple carbohydrates, your body will preferentially use this energy as well as release insulin. Insulin can inhibit fat breakdown (lipolysis), making it harder for our body to burn fat as an energy source. So ideally, exercising on an empty stomach will help your fat burning engine work with maximum efficiency. Remember to try to avoid sugars and limit simple carbohydrates throughout your day. These foods are what make fat in the first place! Avoid “fake healthy” exercise foods like sports drinks and nutrition bars that are loaded with sugar. Eat whole foods as much as possible.

So, should you never exercise at a high intensity? Will you not burn fat at a high heart rate? Again, it’s important that you find a form of exercise that you enjoy and can sustain. If that is a higher intensity activity, great! And yes, you will lose weight from this because exercise of any type is going to burn calories and increase your metabolism. However, this discussion is focusing on the BEST way to burn fat, and that would be lower intensity activity for a long period of time. This is probably the main reason that most distance runners tend to be thin. Most long runs are slow paced and runners out there are burning fat for hours at a time. But you don’t have to run. Just block out some time and walk, hike, or cycle. Just do it for an hour, three times a week, combined with a healthy diet low in sugar.


Dr. Mark Cucuzella’s articles on our endurance engine and heart rate zones:

Dr. Phil Maffetone on his 180 method: