Weight Loss: Fat Burning vs Sugar Burning

Posted by cannonbeach February 14th, 2011

By Dr Tim Krupa

When I started my nutritional practice over 30 years ago, my patients were very interested in weight loss programs so I spent considerable time in weight-loss management.  But I have to confess that over the years my enthusiasm for the effort faded.  It was tough.  It took a great amount of one-on-one time and patient compliance was usually pretty low.  I wasn’t too surprised to read some time later that a person’s odds of beating cancer are better than successfully completing a weight loss program. Sad but true.

But a few things have come to light in recent years that have helped the odds.


Your body has a choice of fuels depending on the situation.  If it’s an emergency or if the body needs energy fast, it chooses SUGAR as its fuel.  Sugar is stored mostly in the muscles and the blood stream so it’s readily available.  In any physical activity the body is trained to always burn sugar for approximately the first 12 minutes.  At that 12 minute mark the body then decides whether to keep burning sugar or to switch fuels and to start burning fat. 

How does the body make the decision to switch fuels?  It’s a function of your activity level, and the body uses your heart rate to make the decision.  If you are working or exercising at an aerobic level, the body will always choose to burn sugar.  If you are working or exercising at a slower, SUB-aerobic level, the body can take it easy and switch to burning fat. The reality is that fat burns more slowly than sugar.

So you can see what happens.  If you’re doing high energy aerobics (high heart rate) you will only burn sugar! This perfectly explains why people can do aerobics forever and still have an unhealthy percentage of body fat.  The high energy, high heart rate aerobics is only burning sugar, no fat.

The ideal heart rate for burning fat can be generated by a vigorous walk when you get about half way between your aerobic zone and your resting heart rate.  If you’re in your 30s or 40s that’s about 120, about 100 if you’re older.  There are charts that show the aerobic level and the resting level for your age that can help you determine your ideal heart rate for optimum fat burning.

And don’t forget about those first 12 minutes!  You can’t change that.  The body will always burn only sugar in that time period.  If you were to exercise for only 12 minutes at a time every hour on the hour, you would never burn an ounce of fat.  You must get beyond the 12 minute mark for the fat burning to kick in.  That’s why a long walk for an hour is a great way to get into a sustained fat burning mode.

There’s more to fat burning and weight loss, but I’ll save that for next time when I address hormone imbalance, why it causes weight gain and what you can do about it naturally.

9 Responses

  1. Jean says:

    I never knew that about burning sugar or fat. Good to know. Thank you.

  2. This is so refreshing to read from a Dr. I respect. I struggle with instructors who push us so hard. I was tested and know that I need to keep my heart rate about 94-103 to burn fat. That means walking slowly. Yea! Thank you for confirming that. Now all I have to do is do it! kb

  3. Malia says:

    Doesn’t it seem that we sometimes make things more difficult then they need to be. Most of the answers are so simple. With this its just walking…. Great blog

  4. Geo Sheig says:

    hi-ya, I like all your posts, keep them coming.

  5. Aron Blemberg says:

    Great post

    Regards, Aron

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  9. Susan G says:

    Good article. Thank you.

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