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Fats or Carbohydrates – What’s Your Body’s Preferred Fuel Source?

honey being poured on spoonMany people mistakenly believe that carbohydrates are the body’s preferred source of fuel. Their reasoning? When carbohydrates are present, your body uses the glucose they provide before it uses fat. But does that automatically make carbohydrates your body’s preferred fuel source? When alcohol is present in your body, it chooses to metabolize that before any other energy source. Does that mean we should be eating a diet of protein and alcohol? Obviously not.

Your Body Has a Nearly Endless Supply of Fat

While the amount of carbohydrates your body can store in the form of glycogen is limited to just 500 grams or so for a well-trained male, your body can store tens of thousands of calories worth of fat. If your body really preferred carbohydrates as its primary fuel source, shouldn’t those numbers be reversed?

The Majority of Your Energy Metabolism is Aerobic

Most of your day is spent in aerobic respiration. This is a period of low-intensity energy demand that predominately uses fat for fuel. When we work out, we start to shift our energy metabolism to anaerobic, which uses glucose for energy. We only spend a fraction of our day in anaerobic respiration.

What’s Actually Happening With a Carbohydrate Dominant Diet?

Let’s just assume you’re the typical person that might work out for an hour a day and then spends the rest of the day sedentary at a desk job or maybe even a little bit active. Athletes aside, you probably burned a few hundred calories during your high-intensity workout. Let’s be generous and say you burned 400 calories. If you were working out at full intensity that entire time (you aren’t), you would have burned 100 grams of glucose (100 grams * 4 calories/gram = 400 calories).

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You might be eating low-glycemic carbohydrates, at just the right intervals, at just the right amounts so as to prevent a rise in insulin and storage of excess glucose. More than likely though, you are taking the round-about way of using fat for fuel. Your body eats an excess amount of carbohydrates and then uses insulin to store the excess. If your muscle and liver glycogen stores are full (remember you have a limited storage capacity), this glucose is converted to triglycerides in the liver and then circulates the bloodstream. If the fat is not needed for energy, it is stored as body fat.

If the fat is needed, the triglycerides are broken down and used as fuel. However, why would you want to take this path to derive your energy from fat? Why not just eat fat and save your cells from the constant bombardment of insulin that can lead to decreased insulin sensitivity and possibly even type II diabetes?

No, I am Not Carb-Phobic

Before all the low-carb naysayers come out, let me say that even though carbohydrates are not an essential macronutrient, I still believe they play an important role. I don’t believe in a no-carbohydrate diet, or even a low-carbohydrate diet. I believe in a controlled-carbohydrate diet. One that provides enough glucose to power through high-intensity exercise, and provides enough glucose to your brain for optimal function. The rest of the calories you should come from protein and the source of fuel your body is actually needing – fat.

How many carbohydrates should you be eating? This number is obviously going to vary. For an individual that is trying to lose weight, and works out for an hour or so a day and then spends the rest of the day sedentary or close to it, a carbohydrate intake of 150-200 grams is a good starting point. For athletes and other highly active individuals, a higher carbohydrate intake will be warranted.

The point is, in a perfect world, you would be eating enough carbohydrates to fuel anaerobic respiration, and then eating your .8-1 gram of protein per pound of lean body weight to build and maintain muscle mass. The rest of your calories would be filled with healthy fats that provide all sorts of health benefits. Doing so will keep your body in a perfect hormonal state for efficient fatty acid mobilization, and will spare you from the negative effects of chronic high insulin levels. Enjoy your healthy fruits and vegetables, but eat your carbohydrates in proportion to your energy demands.

  • Wendy Bottrell

    Excellent! Thanks for this info.

  • Annie

    Hi Tony,
    I am thinking of trying to do this, but I am wondering if you have any easy cheat ways of estimating carbohydrate intake. For example, what would 100 grams of carbohydrates translate into food in a day. If I had some real life examples it would be easier than having to count carbs. Are you including all carbs, even fruits and veggies along with breads/starches? Links to any carb charts? Thanks much.

    • Coach Calorie

      I usually don’t count calories from high fibrous, low-carbohydrate veggies like broccoli, spinach, and cauliflower. Every calorie counts, but these veggies are so healthy and contain so few calories, that I eat them to my heart’s content. It’s very hard to overeat these foods.

      So, 100 grams of carbohydrates would translate into unlimited greens (as explained above), a sweet potato, some squash, and 3-4 pieces of fruit. Sounds healthy to me!

      All those carbohydrate sources count – fruit, tubers, grains, legumes, dairy, etc

      • Juanita Fouche

        Hi there! Any sample eating plans according to what your article says? It would really help!

  • Curt

    Interesting – I really knew all the science behind it. I just know that when I eat less processed foods and less carbohydrates, I lose fat and am happy :).

    Thanks for shedding some light on the science though. I didn’t realize that we only use those carbohydrates as fuel when we’re anaerobic (did I get that right?).

    With that being the case, when do you suggest the best time to eat carbs? Before a workout?

    • Coach Calorie

      Curt, perhaps I should clarify. Glucose is used in both aerobic and anaerobic respiration. However, during aerobic respiration, we can derive glucose from the breakdown of fats. The important thing to remember is that glucose is what’s necessary. Whether you get that from carbohydrates, fats, or protein depends on your diet and exercise intensity.

      Best times to eat carbs would be when your insulin sensitivity is at its highest – pre and post-workout, and your first meal of the day.

  • Kerri

    What “kind” of carbs should I eat before a high intensity heavy weights and HIT cardio mix workout? I guess I always thought it had to be WWbread but do you think I would get better results with just veggies? I’m down to the last 8 pounds and they are fighting me and so far they are winning! Help me beat them Tony and win the battle! :-)

    • Coach Calorie

      I don’t eat anything pre-workout because it doesn’t want to stay down with an intense workout. However, if I were to, I would have a protein fruit smoothie. Frozen banana, frozen berries, whey protein, and a little water and blend. You could also add some protein powder to a bowl of oatmeal. If you have a lot of time between you meal and workout, just have a typical meal.