Working Out in a Fasted State: Worthwhile or Not?

It’s an ongoing debate in the fitness community: is it better to workout on an empty stomach or eat a snack before hitting the gym? Whether your goal is weight loss or muscle building, you’re sure to find dozens of articles that encourage each strategy. So, when you’re losing weight with phentermine, is working out in a fasted state harmful, helpful, or somewhere in between? Here we’ll talk about how your body gets fuel in fed versus fasted conditions, as well as current recommendations about working out in a fasted state.

Food as Fuel: How the Body Gets Energy

Your body uses calories from food and drinks as fuel to provide energy and power everyday activities. These calories fall into four groups: carbohydrates, protein, fat and alcohol. The first three are nutritious and helpful to fuel your body throughout the day and night. Calories from alcohol provide no nutritional benefit (yet another reason to avoid alcohol while taking phentermine).

At Mealtime

Carbohydrate is the preferred source of fuel for our bodies when all else is equal. Even though it is one of the less energy-dense nutrients at just four calories per gram, carbs are easily digested and utilized for quick energy. However, the body can also process and use protein and fat as energy when carbohydrates are in short supply. Fat is the most energy-dense nutrient at nine calories per gram and the preferred source of energy for people who follow very low-carbohydrate, high-fat diets like Keto or Atkins.

Between Meals

Thankfully, our bodies are also designed to keep our cells fueled and happy between meals! This allows us to go about our daily lives without needing to stop and “top up” our stomachs every couple hours.

The first and most accessible energy source when circulating glucose runs low is glycogen. Glycogen is a form of carbohydrate that’s stored in your liver, and to a lesser extent in your muscles. It provides quick energy since it’s easily broken-down into glucose. We draw on glycogen in the morning when we haven’t eaten for many hours overnight, as well as in-between meals, to keep blood sugar stable.

However, individuals who follow very low carb diets don’t maintain large glycogen stores since this backup energy is a formed by storing excess carbohydrate from food. Instead, these people’s livers produce ketone bodies to fuel their cells between meals. Ketone bodies are derived from fatty acids, so proponents of these diets say that this alternative energy pathway helps eliminate more fat. One disadvantage of using fatty acids for energy, however, is that they require oxygen to be broken-down. As a result, they aren’t a very efficient fuel source during anaerobic activity like sprints or heavy weight lifting.


Activity and Energy Needs

When you move, you need more fuel than when you’re sedentary. It’s just like a car: when you drive all day it needs more gas than when it remains parked for hours at a time.

We burn more calories during and after exercise because our bodies need extra energy power increased activity and enable muscle repair. While you may not worry too much about which fuel source your body uses during hours of sitting in the office or classroom, fuel makes a bigger difference when you’re worried about performance and endurance in sport.

This is where the “to eat or not to eat” debate comes into play. Is it better to workout before or after breakfast? Should you have a quick snack before hitting the gym after work? When – and what – you eat largely determines which energy source your body will use for fuel.

Working Out in a Fasted State: The Benefits

If you’re an early-riser, working out in the morning may be the most productive time for you to fit in a sweat session. Or, if you’re running from work to an evening class you may not have time to sit down for a healthy snack. So, do you need to eat beforehand – even if it’s super early in the morning or just as you walk into the gym? Actually, it may be beneficial to hold off until after your workout.

Quick note: The term “fasted” can either refer to not eating at all, or to not eating any carbohydrates. The benefits below would apply to either use of the term.

More Fat Burn

Some research indicates that you should break a sweat before breakfast (or after fasting) to maximize fat loss.

The idea is that if you exercise after an extended fast (such as overnight, or at the end of your fast if you’re intermittent fasting), your body is already low on glucose and drawing on glycogen for energy. So, when you start exercising and energy needs increase, you will more quickly deplete your glycogen stores and switch to burning fatty acids for energy. Proponents of working out a fasted state say that this accelerated switch from glycogen to fatty acids as the main source of fuel means you’ll lose more fat.

One 2017 study supports this hypothesis. Subjects showed increased expression of genes that promote metabolic health (including blood sugar regulation) when they fasted before walking, as compared to after they’d eaten breakfast. While this study was very small, and the researchers found no major differences in short-term fat burning, the potential effect of fed vs. fasted exercise on metabolic hormones provides thought-provoking support for those who say fasted workouts improve long-term health.

Diversified Fuel Efficiency

Some sources also note that if you consistently workout after fasting (e.g. before breakfast or without an afternoon snack), you may be able to improve your body’s ability to utilize glycogen and fatty acids as complementary energy sources.

Still, biologically, fat is most effectively utilized when you’re exercising at about 60% of your max, while carbohydrates are the preferred energy source for any higher-intensity workouts. This is because glucose is more quickly and anaerobically available.


Eating Before a Workout: The Benefits

We can all agree that exercising after a big meal leads to nothing but misery and sluggishness, but is it smart to grab a small snack before breaking a sweat? Some experts respond with a resounding yes. Here are a few of the advantages of eating before your workout!

A Blood Glucose Boost

Eating 30 to 120 minutes before a workout gives your blood glucose a boost so that you’re not immediately reliant on glycogen to carry you through the workout. Glucose (blood sugar) is even more readily-available than glycogen and acts as your body’s preferred source of energy. So, eating (or drinking) a small, carbohydrate-rich snack before exercising tops-up your body’s energy stores and gives it the fuel it needs to perform!

This pre-workout snack is especially important for people with diabetes, or other conditions that affect blood glucose stability. If you already have trouble regulating blood sugar, it’s even more likely that fasted workouts will make you feel dizzy, lightheaded, nauseous or generally unwell. Better to grab a quick snack!

Better Performance in High-Intensity Sports

If you’re hoping for gains in strength, speed or performance, working out on an empty stomach probably isn’t the best strategy for you. Since fatty acids need oxygen to be broken-down, they’re not as helpful when you need quick energy for intense exercises (like sprinting, HIIT or power lifting). As a result, relying on already-depleted glycogen stores for energy can hurt your performance.

A healthy, fed adult stores enough glycogen for about 90 minutes of exercise. However, if your stores are already being eaten away to keep your blood sugar up during a fast, you may find yourself lacking in that extra burst of energy to finish the set, sprint or class.

Higher Overall Calorie Burn

If your ultimate goal is weight loss, this continued energy may help you burn more calories as well. Higher intensity (and longer) workouts require more energy, so they burn more calories. Even though you may burn a higher percentage of fat doing moderate-intensity workouts while fasted, you’ll burn more calories overall if you spend that same amount of time doing high-intensity (glucose-consuming) workouts, or spend more time exercising. So, if a pre-workout snack helps you perform better, this is a strong case for grabbing a bite to eat before hitting the gym.

So, whether you’re looking for more performance-driven results or weight loss, it may be beneficial to grab a small, healthy snack before your workout. To get the most benefit, focus on carbs and protein in the hours leading up to your workout, or just carbs if you’re eating right before you get to the gym.

Faster Muscle Building and Repair

When we strength train – whether it’s lifting weights or jogging uphill – we build muscle. This growth occurs through the tearing and subsequent repair of the tiny muscle fibers. This is part of why we’re usually so sore after an especially tough workout.

To repair and build muscles we need protein. So, eating a pre-workout snack that contains both carbs for immediate fuel and protein for muscle recovery undoubtedly has its benefits. If you prefer working out in a fasted state, make sure to refuel with protein after you finish.

So, is it better to eat before or after my workout?

Ultimately, it all comes down to what feels good for you and your body… and fits your workouts!

If you prefer to exercise on an empty stomach and tend to participate in lower-intensity workouts, you may do okay with working out in a fasted state. If, however, you like high-intensity or endurance sports it often proves beneficial to grab a quick, healthy snack before starting your workout.

In the end, do what feels best for you, and maximizes your ability to perform and enjoy your workout. There are benefits to both fasted workouts and fed workouts, so you should experiment with what feels best for you. At this time, no evidence definitely establishes that one strategy is better than the other for healthy, recreational athletes.

That being said, science does show that nutritious, portion-controlled pre-workout snacks are beneficial for some populations, including:

  • People with diabetes
  • Athletes who engage in high-intensity sports
  • Exercisers who plan to work out for more than 90 minutes


What to Eat Before and After a Workout to Lose Weight

Regardless of whether you choose to eat before exercise, after exercise, or both, it’s important to fuel your body with healthy food and drinks. Here are a few suggestions for what to eat before and after working out!

What to Eat Before a Workout

You should adjust what you eat depending on when you have time to fit in a snack. If you have time, eating a well-balanced meal 2-3 hours before your workout can give you the fuel you need to perform. If, instead, you prefer to snack just before hitting the gym, consider one of these healthy pre-workout snacks:

1-2 Hours Before

Aim for a medium-sized, 100-200 calorie snack with a mix of carbs and protein. This will give you the energy boost you need to perform your best, plus boost your protein to support muscle building and repair. Consider:

  • Whole-grain cereal and milk
  • Whole-wheat crackers with hummus
  • A banana and 1 Tbsp peanut butter

30 Minutes Before

Look for a small snack with about 25 grams of carbs to boost your glucose and give you that quick burst of energy. Healthy carb-based snacks include:

  • A small pack of crackers
  • Half a fruit smoothie
  • A piece of fruit (banana, apple, etc.)


What to Eat After a Workout

Whether or not you choose to eat before exercising, it’s important to always refuel after breaking a sweat. Recovering right helps your body heal and rebound faster so that you can reap the most benefits from your workout!

1. Water

After a good sweat session, hydration should be your first priority. Make sure to replace lost fluids with water and other low-calorie fluids.

2. Food

We sweat off a lot of fluid while working out, but we also burn through a lot of fuel. Even if you’re exercising for weight loss, it’s still important to grab a quick snack after your workout to facilitate muscle recovery and replenish glycogen.

If possible, eat within 30 minutes of finishing and go for a combination of complex carbs and lean protein (e.g. half a turkey sandwich on whole-wheat bread). Still, make sure to measure portions so that you don’t accidentally undo your whole workout with your post-workout snack!

What do you think? Do you like working out in a fasted state, or do you prefer to eat before hitting the gym? Let us know in the comments section below!

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One Comment

  1. When I m dieting for fat loss, I lift weights in a fasted state 5 days per week and do 25 to 30 minutes of HIIT cardio in a fasted state 3 to 4 times per week. The same thing you d eat after any workout 30 to 40 grams of protein and about the same amount of carbs. You can adjust those numbers based on your target calories and macros, but they re good starting places for most people.

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