What is "fat burn" HR lower than "cardio" HR?

I don’t understand how a lower HR will cause more fat to be burned? I assume that if you are at a higher HR you are expending more energy and therefore must be burning more fat.

I can think of two explanations:

  1. The body uses a different energy source at a higher intensity level. However, I understood glucose to be the “quick” energy source and fat to be the long term one. Therefore, I would think that at a higher intensity level the glucose would burn off very quickly and fat would then be used.

  2. You do burn fat at the higher HR. It is just that at the lower HR (fat burn), you are ONLY burning fat, and not really getting a cardio workout. At the cardio level, you get both a cardio workout and fat burn.

So what is the real answer?

The way I understand it, higher intensity is better because you continue to burn more calories even after your workout than you would if you had exercised at a lower intensity. Also, you burn more calories during your workout. Where they come from is not necessarily as important as how many calories you burn, unless, of course, you’re a professional athlete with no body fat to spare, but that’s another thread entirely.

From what I’ve read, lower-intensity exercise does burn a higher percentage of fat than high-intensity (I think almost 10 percent); however, if you’re exercising at a low intensity and you stop, your body stops burning calories virtually as soon as you stop exercising, whereas exercising at a higher intensity helps your body continue burning more calories, even after you’ve stopped exercising. So basically, you’re burning fewer calories overall at a lower intensity than you would if you were exercising at a higher intensity. Again, for the average adult, the energy source when exercising is irrelevant - most people need to exercise to maintain a healthy weight and good cardiovascular fitness. The reason low-intensity exercise is often recommended for fat-burning, particularly to beginners, is that it is easy to do. The easier it is to exercise, the more likely people are to stick with an exercise program. Of course, to continue improving, once a beginner achieves a good level of aerobic fitness, he or she will have to increase their intensity to keep burning calories and increase their cardiovascular fitness even more.

Here’s an article on it that probably explains it better than I could: Exercise to Burn Fat

overlyverbose - I read the article you quoted, and it seems to be summed up as follows:

The body burns glucose for sustenance energy (heart beat, brain, etc.).

The body uses fat for low intensity workouts.

The body uses glucose for high intensity workout.

Just to be clear, though, I really am not looking for a discussion on the benefits of one over the other. I really was just curious why fat burn is a lower HR than cardio. Obviously I was mistaken that cardio burns fat. Thanks for pointing out that article.


Stating that a low HR burns more fat is predicated, of course, on a lower exercise intensity. The higher the intensity, the higher the HR. However, besides the reasons that overlyverbosestated, the statement is false for the above reason. In addition, further down in the site it is noted that the well-trained athlete uses more FFA than others, and a well-trained athlete would have a lower resting HR to begin with.

Sorry I didn’t answer your question. Here’s a link to another article that provides a little more technical explanation of how scientists have come to the conclusion that lower intensity exercise burns a higher ratio of fat than higher intensity exercise.

American Fitness: Who’s Getting Burned?

Here’s another article, from WebMD, that explains the fat/carb ratio in exercise: Which Exercise Burns the Most Fat?

Basically, it says that the ratio of fat and carbohydrates that you burn when exercising has a lot to do with the amount of oxygen your body requires to maintain the level at which you are working. When you’re working out at a level that is easy to maintain (translating to a lower heart rate, with less oxygen intake), your body doesn’t need to rely upon immediate energy stores like carbohydrates. It can use fat cells instead, and can obtain any necessary carbohydrates from those already circulating in the blood stream. However, the more intensity you add to your workout, the more oxygen your body needs, and the more ready-to-use energy you need. Carbohydrates are far easier for your body to break down than fats, which is why, during higher-intensity exercise, your body uses more carbs than fat.

This article might be even more helpful to you in explaining what I cannot: Fat Burning During Exercise

It’s written by Dr. John Hawley, a contributor to the publication “The Physician and Sports Medicine.” In it, he describes why fat is used more in lower intensity exercise than higher intensity exercise.

I hope this helps more than the earlier answer did.

Proportionately, at low intensities more fat than glycogen is indeed used. However, at higher intensities more overall energy is expended. It turns out that, for a set period of exercise time, higher intensity exercise actually uses more fat (in absolute terms) than low intensity exercise - simply because more energy overall is expended in the high intensity case. So, if you compare walking for an hour with running for an hour, when you’re doing the former, you’re using the greater proportion of fat. However, you will have actually used more fat in absolute terms when running for the hour as compared with walking.

One more thing I wanted to mention is that your body DOES use fat as fuel during cardiovascular exercise - the percentage of fat to carbohydrates just changes dependent upon intensity.

But upon preview, I see that Boldface Type has already pointed that out. So, what he and barbitu8 said.

So far, so good… I get that higher intensity exercise will burn more fat than a lower intensity exercise if they are done for the same length of time.

But now suppose it isn’t time that is equal, but calories burned. So, as an example, you workout for 30 minutes at a high intensity workout and burn 500 calories. The next day you workout at a low intensity for 60 minutes and again burn 500 calories.

So what happens when those calories are replaced? Does the body know where the calories came from and try to put them back? Or will it just start by replacing the glucose and then when that is “full” move onto storing the rest as fat?

Pretty much.

One other thing to keep in mind is that high-intensity exercise has an effect on your resting metabolism that low-intensity exercise just can’t match.

Well, the body will always replace glycogen stores (if possible), but it doesn’t necessarily ‘want’ to replace fat stores. Given an intake of sufficient carbs after exercise, the body will replace glycogen - glycogen is the muscle’s fuel of choice. In fact, there is a short window of opportunity immediately after exercise (<30min) to most effectively replace glycogen.

As to whether glycogen is replaced 100% first and only then excess carbs stored as fat … I don’t know, but I doubt it. I would guess that at least some of the initial carbs you eat would be stored as fat. However, once your glycogen stores are full, any remaining carb excess will be stored as fat.

However, in terms of fat gain or loss, basically all that matters is net calorie balance. (The Atkins diet attempts to force the use of fat as fuel by depleting the body of its glycogen store, but net calorie balance is still the important thing).

Covert Bailey covers this subject fairly well in his book Fit or Fat