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Is There A Difference Between How The Body Burns Carbs And Fat For Fuel?

This is a great question and one that I’ll try to answer without getting overly technical. Bear with me on some of the details but hopefully by the end of the article I’ll have simplified things for you. We’ll get into how your body uses both carbs and fat in energy production along with how this pertains to exercise.

I figure most people are interested in learning how stored body fat is used for energy compared to carbs with exercise. Understanding the differences will help you make more informed decisions with your pre and post workout nutrition.

Alright, let’s start with the first part of this question regarding whether or not there’s a difference between how the body burns carbs and fats for energy.

It’s important to understand the body really cannot use either carbohydrates or fat directly for energy. They both must be broken down into smaller units which the cells can use in the Krebs cycle.

There’s a conversion process for both carbs and fats in order to get a usable form of energy for the cells. Although the processes for digestion and conversion are different for fats and carbs, they both end up in the cells of lean muscle tissue for energy production or being stored in fat cells.

The body will determine which form of substrates to use depending on the energy system being used at the time (the intensity and nature of your physical activity).

To simply things I want you to think about your body being able to produce energy with or without the presence of readily available oxygen. There are several different pathways for energy production depending on the intensity of exercise, but it mostly comes down to the presence of readily available oxygen.

For the purpose of this discussion, the most important thing to know is the body cannot use fat for energy outside of the aerobic energy system. This means that while you’re doing exercise that is higher intensity in nature you’ll only be able to use glucose (sugar) or lactic acid (a by-product of anaerobic energy production) for fuel in the cells.

What happens after the physical activity is over is another story. I say this so you don’t get hung up on the idea of being in your “fat burning zone” all the time like we frequently hear from some so called experts. This whole concept is misleading to say the least.

It’s imperative that you look at what your body can use for fuel both during and after exercise. Often times it’s what you’ll burn for fuel after your workout that makes the most significant impact on body composition changes.

Carbohydrates as fuel sources for your body

Let’s briefly talk about how your body uses carbs for fuel. In essence, the potato or other carbohydrate food source you consumed must be digested and the starch broken down into the simple sugar form of glucose before it can be used by the cells. The digestive process actually starts with saliva in your mouth and continues in the stomach and gastrointestinal tract.

Most carbs with the exception of fibers are finished with the nutrient assimilation process in the small intestine. It’s here where the polysaccharide sugar chains are broken down into glucose which is transported through the membranes in the gastrointestinal tract and then sent via blood flow to the liver.

Some forms of carbohydrate like fructose and galactose actually have to go to the liver first before they’re converted to glucose. Either way it’s glucose that’s used as the base unit of carbohydrate for energy production in the cells.

Technically, glucose must be broken down into Acetyl CoA before it enters the Krebs Cycle (how energy is produced in the cells). The idea is that all carbs must get broken down into smaller units before they can be used in energy production.

If the body doesn’t need the glucose for immediate energy demands, it stores the excess glucose in the liver and muscles as glycogen. This glycogen can be broken back down to glucose and used for fuel later on when the body needs energy.

In the presence of readily available glucose the body will always use this for fuel preferentially over fats in the beginning of an exercise session.

During exercise while you’re in an aerobic state (in the presence of readily available oxygen) the body will use glucose first then gradually start shifting to fat for energy production. Without getting into too much detail your body will use both substrates for energy with low-moderate intensity exercise.

The longer you go with aerobic exercise, the more your body will call upon fats for energy production.

As I mentioned earlier, when the intensity of exercise increases or if you’re doing short bursts of all-out effort, your body won’t be able to continue using fat. It basically comes down to the fact that your body can’t use fat for energy production outside of the aerobic energy system.

Fats as fuel sources for your body

Dietary fat is digested and assimilated into smaller units much like carbohydrates although the processes are different. When fats reach the small intestine they’re broken down with the assistance of bile acids and salts. The smaller droplets of fat are then acted upon by lipases and ultimately converted into fatty acids and glycerol.

There are several other steps here but to cut to the chase the end products of both glycerol and fatty acids can end up being converted directly or indirectly to Acetyl CoA for use in the Krebs Cycle. So in a lot of ways, although the processes are different we’ll still be ending up at the same place.

I don’t want to turn this into a complicated dissertation on digestion and assimilation of nutrients so let’s just get to the point.

The body will use both dietary fat and carbohydrate for energy production and although various factors dictate which one gets used when, the idea is that they both go through processes to get into usable forms the body can use in the cells.

Ok, let’s get into the part that you’re probably most interested in learning about.

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How does the body use stored fat for energy production?

As you can see there’s always a physiological process for using any substrate for energy production, be it carbs, dietary fat, or stored body fat.

Contrary to what some slick marketing pitches would have you believe, fat doesn’t get “zapped, melted, or incinerated” on the spot. In other words, stored body fat does NOT get burned right there in the fat cell. It must be liberated and sent to a muscle cell.

The process for this liberation involves a somewhat complex hormonal/enzymatic pathway. Basically to simply things, an important enzyme called “hormone sensitive lipase” is used as the catalyst for the lipolysis or breakdown of fat in the cells to smaller forms of fatty acids and glycerol that the body can use.

So what triggers this process of lipolysis or the breaking down of stored fat in fat cells to be used by the body for energy? One word…DEMAND.

When your body needs more fuel to meet energy demands than is readily available from stored glucose (glycogen) in the muscles and liver, dietary fat, or sugars consumed in the diet, it goes looking for it elsewhere.  Hence, your fat cells open up and provide the needed energy due to demand.

Here’s the part you need to get your head around. While you need some carbohydrate in the diet to facilitate the process of liberating stored body fat for fuel, if you’re constantly meeting energy demands from sugars or dietary fat, there’s no need to go looking for it elsewhere.

In other words, there must be a deficit at some point with dietary intake of carbs and fats or an increased energy demand from exercise in order for stored body fat to get released. Just eating healthy and doing some regular exercise isn’t a guarantee for fat loss.

So how do you look at this in terms of optimizing stored body fat burning with exercise?

For starters, you want to make sure you restrict carbohydrate intake at times (key point: not ALL the time) so that you don’t always have enough dietary fuel to meet energy demands. Caloric shifts where you restrict for short periods of time (days not weeks), while going “lower-carb” followed by periodic re-feeds are going to work best for most people to create the necessary demand for body fat as a fuel source.

If you restrict energy (calories) too much, for too long, the body will simply down-regulate the metabolism through hormonal processes. Everything should work in short windows of time. Restrict for a while then put more calories, especially from carbs, back in to keep your metabolism humming along.

From an exercise viewpoint, you want to avoid consuming dietary fat prior to an exercise session. A little bit of glucose from say a piece of fruit is a better choice. This is to ensure you have some fuel to support the intensity of your workout. Not a lot though, around 20-25 grams of carbs from a simple sugar source will be enough to support most workouts.

The idea is you want to be able to provide your body with energy to support the demands of your workout while at the same time not overloading on fuel so there’s no need to tap into stored fat either during or after the workout.

Post-workout, I recommend to my Charleston personal training clients that they consume  a fast assimilating protein like whey combined with a small amount of simple sugar. This is going to be an ideal way to jump start the anabolic processes of muscle repair and rebuilding.

There is much debate on which type of exercise is best for fat loss, either low-intensity (fat burning zone) exercise or higher intensity interval or burst training routines?

The truth is BOTH are beneficial and optimal results will be seen by combining various forms of physical activity that work ALL the energy systems.

Sure, you won’t be able to burn fat for fuel with high intensity exercise but the stimulation of powerful fat burning hormones like adrenaline, nor-adrenaline, and growth hormone can help facilitate stored fat being used for energy post-workout (up to 24-36 hours afterwards).

With longer duration aerobic (low-moderate intensity exercise) you’ll be able to burn more total calories and more percentages of fat during the workout. Using both of these exercise strategies through the week, perhaps alternating between days, will help you get the best of both worlds and minimize the risks of over-training.

The take-away on all this is you have to create the need or demand for your body to tap into energy reserves (body fat stores). This is a complex hormonal process but the most important thing to grasp is the need to provide short-term restriction of dietary fuel sources.

If you always have enough dietary fat or sugars from carbs to meet energy demands there will be no need for the body to tap into energy reserves.

Just think of fat as fuel, which is all that it is. If you want to tap into it, you need demand and a facilitator for that demand….hence physical activity or exercise. Your body will ALWAYS use either carbs or dietary fat if readily available preferentially over stored body fat.

Create the demand to tap into reserves with a combination of periodic restriction and energy utilization with exercise. Support all the processes needed for this to happen with a diet that provides essential nutrients, enzymes, vitamins, and minerals.

In other words, eat clean, exercise often, and allow your body to figure out the rest. Don’t overload it with starches and excess sugars it doesn’t need for exercise and you’ll find yourself tapping into stored fat with some good old fashioned hard work.

Shane Doll is a certified Charleston fitness trainer, fat loss expert, speaker, and founder of Shaping Concepts. With a staff of over 10 certified fitness professionals, Shaping Concepts provides personal fitness programs with a specialty on weight loss and body transformation. Sign up for a FREE, no-obligations consultation today.

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Category: Fat Loss.