Calories.  Talk about having a love-hate relationship.

When you’re trying to build muscle and improve performance, calories are your best friend.  When you’re drowning your sorrows in a pint of ice cream, they’re a shoulder to cry on.  And when you’re trying to undo the damage from all that rocky road?  Calories can feel like your worst enemy.

So yeah, when it comes to calories, it's complicated.

But your metabolism and the way your body uses calories for fuel shouldn’t feel like some mystery process.  With some basic understanding of the processes at play, you can leverage your body’s calorie-burning engine to get the results you want.  So, let’s take a closer look at exactly what’s going on in your body’s constantly-burning engine— your metabolism.

It might not seem like much, but BMR actually makes up the majority of the calories your body burns each day, coming in at a whopping 60-70%.  Your BMR doesn’t include any of the calories your body burns through activity or exercise.

2. Food Thermogenesis— What it takes to digest food

Your body burns about 10% of it’s daily calories digesting and absorbing the food you eat.  But don’t make the mistake of thinking all food is equal when it comes to your digestion— your body actually has to work harder to digest certain macronutrients (thus, burning more calories).  The heavy hitter in the macro department is protein, which uses about 20-30% of its calories in the digestive process. Carbs come in next at 5-10%, followed by fat at 0-3%.

3. Physical Activity— What it takes to move your body

Exercise and other activities account for about 20% of your daily burn, although this number can be lower or higher, based on the intensity of your movements.  Keep in mind that this number doesn’t just reflect your exercise calories, everything from shopping, to dancing, to vacuuming your living room takes energy— that’s why it's so important to get moving anyway you can.

How Your Body Uses Fuel

When you eat, your body is harvesting energy to use as fuel from the three macronutrients—  protein, carbs, and fat.  But it’s important to understand that these three fuel sources can’t be used by the body interchangeably.  Different types of activities and states require different fuel sources to keep your body going.  Here are four examples of how your body fuels itself differently, depending on the situation.

1. Intense Exercise— Above 70% of your Max Heart Rate (MHR)  

Have you ever thought about why you find it difficult to talk during intense exercise?  It’s because your body focuses on breathing in order to deliver oxygen to those muscles you’re taxing so hard.  And that’s the type of exercise we’re talking about here.  At this intensity, your body tries to combat it’s low-oxygen state by turning to carbs as a quick-burning and easily-accessible fuel source for your oxygen-deprived muscles.

2. Moderate Exercise— Below 70% of your MHR

At this level of intensity, you can probably manage to speak pretty comfortably during your workout.  Because your muscles are getting plenty of oxygen, your body turns to a slower-burning fuel source — fat — to help you power your way through your workout.

3. Overnight Fasting

Your metabolism might slow down during sleep, but it doesn’t stop—  that’s because your body is busy with its vital functions, as well as repairing the cellular damage that has occurred throughout the day.  Here, your body first turns to fat for fueling most tissue, and carbs (in the form of glucose which is stored as glycogen) to fuel your brain.  Because you’re not eating for the 8-12 hours you’re asleep, your body enters a fasted state.  If your body doesn’t have enough glycogen stored during this period, your blood sugar will drop, forcing your body to convert protein to glucose in order to stabilize your blood sugar level.

4. Starvation

When your body begins to starve— whether by outside factors or through a low-calorie diet— your metabolism begins to slow down to conserve what fuel you have left.  It’s not unusual to feel tired, irritable, or anxious when this happens.  At this point, protein and fat become your primary sources of fuel.  After about 48 hours with no food, your body runs out of glycogen completely, and begins to break down muscles, organs, and fat deposits in order to continue feeding your brain and blood cells.

It’s important to note that it's possible to shift the type of fuel your body uses during exercise through training.  As your physical fitness improves, your body becomes better at using oxygen, making it possible for you to train more efficiently at higher levels.  When this happens, your body begins to burn more fat during higher-intensity training.  Not only can this process aid in the reduction of body fat, it also allows you to perform for longer periods of time without feeling fatigue.  

2. Amp up the intensity of your workouts.

Adding higher-intensity intervals to your training creates an afterburn effect referred to as excessive post-exercise oxygen consumption or EPOC.  The result of this process is that not only are you burning calories during your workout, but you continue to burn calories at a higher level in the hours (and even days) after your workout is completed.  Check out my article on HIIT vs. LISS Cardio for everything you need to know about EPOC and how to make it work for you.

3. Focus on your daily protein goal.

You already know that increasing your muscle mass is the best way to ramp up your metabolism and burn more calories, but you won’t get those muscles to grow unless you feed them.  A diet rich in high-quality protein sources will give you everything you need to speed up muscle repair and generate growth.  For a great-tasting source of quick and easy, high-quality protein look no further than Isothority— the flavors will blow your mind!

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