When your cells run low on glucose during a workout, they pull fat from cells and break it down into usable energy. This results in fat loss; however, if you consume more calories than you burn, the excess energy will be converted to fat and redeposited in the fat cells.
Certain hormones signal fat cells to release fatty acids when your cells need an additional energy source. The fatty acids travel through your bloodstream and into your cells, where they are broken down.
Body Fat Function
When you consume more energy or calories than you expend, your body stores that extra energy in your fat cells in the form of triglycerides, explains Cleveland Clinic. When your body needs that energy for fuel, your body converts the triglycerides into a usable form of energy. According to Colby College, on average, individuals have enough energy stored in fat tissue to survive for two months without food.
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Body fat is more than a back-up fuel source. According to the University of California, Davis Health, body fat is also necessary for:
- Maintaining a consistent body temperature
- Making up a structural component of cell membranes
- Memory storage
- Regulating the body's water balance
- Maintaining the transmission of nerve impulses
- Regulating and producing certain hormones
While it is true that excess body fat and obesity pose serious and well-documented health concerns, the body is also at risk when the body fat is too low. Risks include decreased functioning of the immune system and lower estrogen levels in women, which can result in decreased bone mass, notes the University of California, Davis.
Your ideal body fat percentage varies based on your gender. These are the percent body fat norms for women according to the American Council on Exercise:
- Essential fat: 10 to 13 percent
- Athletes: 14 to 20 percent
- Fitness: 21 to 24 percent
- Acceptable: 25 to 31 percent
- Overweight: 32 percent or greater
And the percent body fat norms for men are:
- Essential fat: 2 to 5 percent
- Athletes: 6 to 13 percent
- Fitness: 14 to 17 percent
- Acceptable: 18 to 24 percent
- Overweight: 25 percent or greater
Physiology of Fat Loss
Your body's first source of energy is carbohydrates, which are stored in your body as glycogen. Glycogen is broken down into glucose, which can then be used by your cells to create energy, advises Colby College. Carbohydrates can be converted to energy by your cells using either aerobic or anaerobic respiration.
However, if your body needs additional energy, it will signal fat cells to release fatty acids to be used as energy. Two hormones may signal the release of fat from fat cells:
- Glucagon, which is a signal that the body's blood sugar is low
- Adrenalin, which is released as a part of the body's fight or flight response
Fats then travel through the body to reach the target cells, such as your skeletal muscles when you are working out. These muscles then break down the fats into usable energy using aerobic respiration, explains Colby College. The byproducts of this fat-burning process in the human body are expelled from your body through sweat, urine and your breath when you exhale carbon dioxide, according to Cleveland Clinic.
Wentworth Institute of Technology notes that when your body uses stored fat for energy, the fat cells shrink, but they do not disappear. This means that if you consume too many calories in the future, the fat cells will expand to store the extra energy.
Read more: The Ultimate Guide to Fat Loss
Fat Loss From Ketosis
The liver is another primary site where fat is burned. Cells in the liver breakdown fats to use for energy for itself, but also to release into the bloodstream in the form of ketones — a derivative of fat that other cells can easily access for additional energy — says Colby College.
This process is the basis of the ketogenic, or keto diet. When using keto to burn fat, individuals get most of their calories from fats and aim to consume fewer than 5 percent of calories from carbohydrates, advises the University of California, San Francisco. This means that individuals eliminate grains, fruits, starchy vegetables and legumes from their diet. By cutting carbs, the body resorts to using ketones as energy and burns more fat to make ketones.
The diet is beneficial for fat loss and its anti-inflammatory effects, which may promote healing and provide treatment options for stroke victims and individuals with epilepsy. The University of California, San Francisco notes that there are few human studies to confirm the benefits of keto and the long-term effects of the diet are unknown.
The keto diet is restrictive and can be tough to follow long term. Stanford Medicine also notes that keto may negatively affect cholesterol levels due to the high saturated fat content. Too, many individuals suffer temporary adverse effects when starting the diet, including headaches, fatigue and body aches.
Fat Burning Exercises and Diet
If your goal is to lose weight, focus on aerobic activities for your fat-burning workout as oxygen is necessary to break down fat. Low-intensity workouts and workouts lasting more than an hour use fat as a main source of energy, advises the State University of New York, Cortland. Your body relies primarily on carbohydrates during high-intensity workouts and a mix of carbs and fats during a medium-intensity workout.
High-intensity workouts and strength training are still beneficial to your health, but may not directly burn fat. In addition to exercise, your diet plays an important role in fat loss. If you eat too many calories, regardless of whether the calories come from fat, protein or carbs, the excess is converted to fat and stored in your fat tissue.
While eating fewer calories than you burn is an important component of any fat loss plan, don't overdo it. If your diet results in a severe calorie deficiency, your body's metabolic processes may start to change and work against you, decreasing your ability to effectively lose weight and keep it off, notes a study in the February 2014 edition of the Journal of the International Society of Sports Nutrition.
- Cleveland Clinic: "Where Does Body Fat Go When You Lose Weight?"
- Colby College: "Energy Production"
- University of California, San Francisco: "As the Keto Diet Gains Popularity, Scientists Explain What We Do and Don’t Know"
- Stanford Medicine: "A Skeptical Look at Popular Diets: How Ketogenic Should You Go?"
- Wentworth Institute of Technology: "How Do I Burn Fat?"
- University of California, Davis Health: "Body Composition"
- American Council on Exercise: "Percent Body Fat Calculator: Skinfold Method"
- State University of New York, Cortland: "Human Energy"
- Journal of the International Society of Sports Nutrition: "Metabolic Adaptation to Weight Loss: Implications for the Athlete"