I’ve often been asked about what makes the body use carbs or fats for fuel both during and after exercise, so today I’ll try to answer this question without hopefully getting overly technical. Bear with me on some of the details, but hopefully by the end of the article I’ll have provided clarity. We’ll get into how your body uses both carbs and fat in energy production along with how this pertains to exercise and weight loss. Understanding the differences will help you make more informed decisions with your pre and post workout nutrition if your goals include changing your body composition.
Alright, let’s start with discussing whether there’s a difference between how the body burns carbs or fats for energy. It’s important to first understand the body really cannot use either carbohydrates or fat directly (in their original form) for energy production. They both must be broken down into smaller units which the cells can then use for energy. There’s a conversion process for both in order to get a usable form of energy. Although the processes are different for fats and carbs, they both end up in the cells of lean muscle tissue for energy production or being stored in the liver or fat cells. The body will determine which form of substrates to use depending on the energy system being used at that particular 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 (carbs) 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 always being in your “fat burning zone” like we often hear preached. This whole concept is somewhat 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.
Using carbohydrates for energy during exercise
Let’s briefly talk about how your body uses carbs for fuel. In essence, the carbohydrate food source you consumed must be digested and 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 during aerobic states). 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.
When you start to use fat for fuel during exercise
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.
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 a lot of folks are probably most interested in learning about.
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 the 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, 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 “more than enough” dietary intake of carbs and fat, there’s no need for your body to go looking for it elsewhere.
In other words, there must be a deficit at some point with dietary intake of carbs and fats in order for stored body fat to get released. Just eating healthy and doing “some” regular exercise is far from a guaranteed recipe 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 calorie and carbohydrate intake at times (key point: not ALL the time) so that you don’t always have enough incoming fuel readily available to meet energy demands. Caloric intake shifts where you create deficits for short periods of time (day or day(s) and not weeks), followed by a brief period where you stopped the deficit are going to work best for most people to create the necessary demand for body fat as a fuel source.
There are several ways to do this from caloric staggers, carb cycling, intermittent fasting, etc. It’s not that one way is superior to another. You need to experiment to find what strategy will work best for BOTH your body and your lifestyle. If you struggle on the strategy, it’s not likely you’ll be consistent enough with it to benefit.
Remember, 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. If it was just as simple as not eating much and doing a bunch of exercise you wouldn’t need to be reading this.
Listen, your body is designed to adapt, it’s designed to survive, you’re not going to get the lean, toned, healthy body you want by starving yourself. It’s all about short term periods of restriction and deficits caused by both a reduction in calories and expenditure with exercise followed by periods where calories are brought back up to meet energy demands that conditions the body to release the “unneeded” fat stores. It releases on demand what it needs for fuel but will be miserly to say the least with giving up fat when it thinks you’re in a famine.
What to consume prior to a workout?
From an exercise viewpoint, you don’t necessarily always need to eat something prior to a workout. It depends on how long it’s been since you’ve eaten and what type of exercise you’ll be doing. Generally speaking, higher intensity exercise will necessitate you having some glucose in your system or you’re going to really struggle. If you’re doing moderate intensity exercise and it’s been a while since you’ve eaten anything you can have something very light and easy to digest before the workout.
A good rule of thumb to remember is if you want to lose fat, you’ll want to avoid consuming dietary fat prior to an exercise session.
A little bit of carb from a low-sugar content fruit for example would be 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 something like a small apple would be suffice. You could also do something like a whey protein shake. 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 providing too much so there’s no need to tap into stored fat either during or after the workout.
Again, a lot of this depends on timing. I’m not a proponent of doing moderate to higher intensity workouts in fasted state. That’s why I typically recommend our early morning clients to have a piece of fruit, a protein shake with water, or something light before they train. The reasoning is they’ve been in a fasted state overnight and in the presence of low blood sugar, they’re likely to suffer from poor performance during their workout or worse suffer from dizziness, lightheadedness, or nausea.
If you were getting up early and going for a walk or doing low to moderate intensity aerobic exercise, then you could possibly wait until after your exercise session to consume something. I will say however, there used to be a lot of hype about doing fasted cardio in the morning, made popular by bodybuilders back in the day, but the research has simply not conclusively shown this to be more effective for fat loss. Here’s a research study from 2014 that showed little difference in body composition changes between individuals doing fasted and non-fasted aerobic exercise. So basically, my advice would be don’t do fasted cardio because you think you’re getting some significant edge with fat loss, but rather do it if it’s more convenient, or it simply makes you feel better.
What to consume following a workout?
Post-workout, using a fast-assimilating protein like whey combined with a small amount of carbs is going to be ideal to jump start the anabolic processes of muscle repair and rebuilding. This is a good choice for someone looking to maintain or build lean muscle while also dropping body fat. Someone not looking at dropping body fat would add more carbs to their post-workout nutrition along with simply consuming more protein and carbs throughout the day. Do you a common denominator? In either situation, there’s not much benefit in consuming dietary fats right after a workout. Those should come with the next meal, not immediately post workout.
What is the most effective type of exercise for fat loss?
As you already know 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 type routines? But let me share a little secret with you….
The truth is there’s no holy grail with using one particular form of exercise to optimize fat loss.
My recommendations for middle age and older adults
Ideally, you’ll want to include various forms of physical activity that work ALL the energy systems. The majority of energy expenditure should come from aerobic exercise (think low to moderate intensity exercise like walking where you could still carry on a conversation while doing it). Combining that with some strength training and occasionally (meaning once or twice a week) doing some short bursts of higher intensity effort (to get you out of breath) would be an example of covering different energy systems and having a balanced approach to exercise.
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 deficit of dietary fuel sources. If you always have enough dietary fat or 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 and dietary restriction. 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 regularly, manage stress, get plenty of sleep and allow your body to figure out the rest.
If you’re a middle age or older adult and you’re struggling to get dialed in with a fitness and nutrition program that produces the results you want, you’re not alone! Look, the things you may have done in your twenties and thirties to drop weight and get in shape, heck even in your early forties, simply don’t work the same as get older.
There’s a lot of reasons for this with changes in protein synthesis, insulin sensitivity, hormonal levels, inflammation levels, and lean muscle mass (just to name a few) resulting in the need for a modified approach.
You don’t have to keep guessing. At Shaping Concepts, we specialize in coaching middle age and older adults and have experience working with the unique needs of these populations. Don’t hesitate to contact us if we can be of assistance. We work with individuals both in our private Mt. Pleasant gym and through remote training and coaching programs.
Shane Doll CPT, CSCS is a fitness professional and expert on exercise and body transformation for middle age and mature adults. He seeks to make a difference in the lives of others by providing instruction and coaching with a servant-based attitude. Since 2004 his Charleston personal training programs have helped over 3,000 Lowcountry residents.
Subscribe to get weekly updates on all my latest content and news. I value your privacy and will never share your information with a third party. Give you my word to deliver valuable content but you can opt out at any time.