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treadmill

I wish there was an easy way for us to know whether we are burning carbohydrates or fat when working out.

Don’t get me wrong, the technology exists, but most of us don’t have access to it. This type of testing equipment is usually found at university research facilities. In case you’re wondering, read on to understand how it works. Oh, and if you’re not wondering, I’m going to tell you anyway!


Listen to Dr. Neal address this topic on Episode 825 of the podcast  Optimal Health Daily.


Burning Fat vs Burning Carbs

If you want an idea of whether you’re burning mostly carbs or fat, the researcher will have you hop on a stationary bike or treadmill. They’ll then hook you up to this fancy machine. They’ll place a breathing apparatus around your head.

Do you remember Bane from the film, The Dark Knight Rises? Do you remember his mask? It kind of looks like that but less cool.

breathing_apparatus

LEFT: Bane from “The Dark Knight Rises” | RIGHT: A breathing apparatus test facility in the 1960's.

That’s because the breathing apparatus the researcher will place on you has a tube at the end of it which connects to a computer. The purpose of this equipment is to analyze your breathing: specifically, how much oxygen your taking in versus carbon dioxide your exhaling.

What’s amazing is that by analyzing your oxygen intake and carbon dioxide production, the computer can determine whether you’re burning mostly fat or mostly carbohydrate. This is because when the body burns fat, the stuff that’s left over is water and carbon dioxide. The water gets excreted in your urine and the carbon dioxide is exhaled when you breathe. So, the more carbon dioxide the machine senses you exhaling, it can estimate whether you’re burning mostly fat or carbohydrates.

Now, I should mention I’ve been choosing my words carefully. You may notice that I keep saying, “burning mostly carbohydrates” and “burning mostly fat.” This is because, most of the time, we’re using both for energy. But, we may be using one for energy more than the other. Whether we’re using mostly carbs or fat for energy is based on a number of different factors: your current level of fitness, your gender, your age, the composition of your last meal or snack and so on. When it comes to which one you’re burning during exercise, there are some general rules we can use.

Please know that these rules aren’t perfect. I’ll explain why. Ready? Here we go.

How Long Does it Take to Start Burning Fat?

burning_carbs

Whether or not we’re burning fat or carbohydrate during exercise is based on 2 factors: the intensity of the exercise and the duration (meaning, how long your workout lasts).

Let me give you some examples. Imagine, you’re going to perform a barbell bench press and you’re going to lift it 8 times (meaning, you’re going to complete 8 repetitions or reps). For most, completing these 8 reps will take about 30 seconds. Exercise physiologists have discovered that when exercises last 30 seconds or less, we actually use creatine as our main form of fuel. Our bodies naturally make creatine, so supplementing is not necessary. So, for really short bursts of exercise, like lifting a weight 8 times, we’re not using carbohydrate or fat. The body is using a different form of fuel altogether: creatine.

Let’s use a different example. Let’s say, instead of bench pressing for 8 reps, you decide you’re going to decrease the weight on the barbell and try and perform as many repetitions as you can for 1 full minute. When exercises last longer than 30 seconds, like lifting a weight for a full minute, the body switches from using creatine as its fuel source to carbohydrate. Carbohydrates are a better resource for energy when compared to creatine, so that’s why the switch happens. But, carbs can only sustain us for so long… about 3 to 5 minutes max.

What happens when an exercise lasts longer than 5 minutes? Well, that’s when the body turns to its best source of energy — fat. So, theoretically, when you’re going out for a jog, or a swim, or a bike ride that lasts longer than 5 minutes, the body will likely turn to burning mostly fat for fuel. But, as I mentioned: there are exceptions to this. We’re learning that when performing high intensity interval training, the body may not burn fat during the workout, but possibly may burn more fat AFTER the workout. It’s like that “afterburn” effect I talked about on an earlier Q&A episode. As you stay consistent with your workouts and add more muscle, you will probably find that the body will burn more fat all the time, during workouts of course but even when you’re watching TV or sleeping.

So, here’s the thing: don’t stress too much about which type of fuel you’re using and when. Focus on mixing up your routines so that you have some heavy lifting days, some high intensity interval days, and some light cardio days. From there, the body kind of figures out which fuel source to use.

How Many Carbs Should I Eat Per Day?

As far as how many carbohydrates you need to consume, especially for refueling purposes, the American College of Sports Medicine has some recommendations. They recommend we consume 2.7 to 4.5 grams of carbohydrate per pound of body weight (or, 6 to 10 grams of carbohydrate per kilogram body weight per day).

Let’s take a 125 lb. client as an example. They would need to consume about 338 grams of carbohydrate per day, minimum, up to about 560 grams. The American College of Sports Medicine also recommends that these carbohydrates come from mostly whole, minimally-processed foods.

Tags: health
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Neal Malik

Dr. Neal Malik leads the Master of Science in Nutrition for Wellness program and teaches core courses at Bastyr University California. He has a Doctorate in Public Health from Loma Linda University and is a Certified Exercise Physiologist from the American College of Sports Medicine.Send in your health related question and Dr. Neal will answer it on the Optimal Health Daily show!
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