Do You Burn Fat When Your Glycogen Stores Are Full?

Just as your car needs gas to run, your body needs fuel to keep it going. That energy either comes from glucose (made from carbs) — which can be used immediately or stored in the body as glycogen — or from fat.

Here's how it works in a nutshell: When you eat bread, fruit, beans and other carb-containing foods, your body breaks them down into glucose (sugar). Glucose serves as the primary source of energy for your body, especially when it's very active, according to the Mayo Clinic.

Some of the glucose is used for energy right away, and the rest is converted into glycogen and stored for later use. An average adult is able to store approximately 100 grams of glycogen in the liver and approximately 400 grams in the muscle cells, with around 5 grams floating in the bloodstream, according to a March 2018 paper in ​Nutrients​. Small amounts are also stored in the brain, heart, kidney and fat cells.

Between meals, your body draws on these energy stores to keep blood glucose at a steady level and to keep your body running, per The Nemours Foundation's KidsHealth.

"Glycogen stored in the liver is used as a rapidly available energy source for glucose-dependent tissues such as the brain, red blood cells, retina and more," says registered dietitian and ACSM-certified personal trainer Jim White, owner of Jim White Fitness and Nutrition Studios. "As for glycogen stored in the muscle, this is used locally and cannot be mobilized to other tissues. Its primary function is to provide energy to the muscles during physical activity."

White explains that someone who is fasting will deplete their liver glycogen storage in approximately two days. But a trained, well-fed athlete who starts an intense two-hour workout with replenished glycogen storage may see a 50 percent muscle glycogen depletion, close to the threshold where muscle function is compromised.

Both! When you're at rest or doing every day activities, White explains that fat contributes around 80 percent of the body's energy while 20 percent comes from carbohydrates. When you're working out — even if the activity is mild — at about 25 percent of your VO₂ max, 80 percent or more energy expenditure comes from fat. (P.S. VO₂max is the greatest quantity of oxygen you're able to consume, per the National Academy of Sports Medicine).

That percentage decreases when the intensity of exercise increases. At about 65 percent of your VO₂ max, fat and carbs contribute equally to calorie burning (energy expenditure). In a high-intensity exercise, or about 85 percent of VO₂, the contribution of fat diminishes to 25 percent or less of energy expenditure.

In other words, lower-intensity workouts produce higher fat combustion while higher-intensity workout produce higher carb combustion and can potentially deplete glycogen stores, confirms the International Sports Sciences of Association.

Sounds like a dream come true, right? Walk 30 minutes a day and burn tons of fat! Well, walking is definitely good for weight loss, but it may not be enough to burn a ton of fat off your body quickly.

If you want to burn more fat, you should first rethink your diet. Be sure you're taking in fewer calories than you burn, as a calorie deficit encourages your body to burn fat as a fuel source. Just don't drop your calories too low. People assigned female at birth shouldn't eat fewer than 1,200 calories a day, while those assigned male at birth should stay above 1,500 calories, according to Harvard Health Publishing.

To determine your calorie needs for weight loss, use an online calorie calculator to figure your maintenance needs, then subtract 500 to 1,000 daily calories to safely lose 1 to 2 pounds a week, per the Mayo Clinic.

The Mayo Clinic also suggests that going low-carb with your diet will help support faster fat burn too. If you're not helping restore your glycogen stores with carbs, then your body has to burn fat for fuel.

Increasing the amount of time and intensity you work out is essential if your goal is to burn more fat.

"The best recommendation for most people who want to achieve a healthy body weight and improve their fitness is to exercise at the highest intensity for their age and fitness level," White says. "Using exercise to promote weight loss is most effective when individuals exercise as intensely as possible for a given timeframe."

White explains that if you walk for 30 minutes at a pace of 2 miles per hour, you'll burn approximately 100 calories — 80 percent will come from fat and 20 percent from carbs. But if you run for the same amount of time at 9 miles per hour and burn 450 calories, 20 percent will come from fat and 80 percent from carbs. Yes, the percentage of fat burning from running is significantly lower, but the total amount of calories coming from fat is higher than the low-intensity walk.

Also, keep in mind that factors such as sex, hormones, diet composition, the time of eating before exercising, nutritional status and the weather while exercising can affect the use of fat or carbohydrates as a source of energy. So, it's not so cut and dry.