Fats are generally known for leading to weight-gain and are often disliked by individuals sensitive about body image.
Fats have gained a bad rep over the years. This is partly due to the rates of obesity and heart disease that continue to rise worldwide. Yes, fats are a contributor to the obesity epidemic and also play a major role in heart disease. However, not all fats are bad.
There are different types of fat in the body, some of which are beneficial to health and well-being. Omega-3 fatty acids like DHA (Docosahexaenoic acid) and EPA (Eicosapentaenoic acid), for example, are major building blocks for the brain and support ocular function. Fats are also packed with energy and help fuel the body through exercise, daily activity, and even through starvation.
Roles in Physiology:
Fats have countless roles in the body. Once metabolized, fats can become building blocks for cells, chemical messengers, fuel for working muscles, antibacterials, and much much more. Fat tissue also provides insulation and protection for internal organs which helps maintain core temperature and contributes to homeostasis.
In skeletal muscle, fats store as triglycerides, which break down into free fatty acids (FAAs) used directly for the production of ATP and fueling the working muscle. ATP (Adenosine Triphosphate), is the energy currency molecule for cells in muscle and throughout the body.
The majority of energy stored as fat is found in adipose tissue. Together intramuscular triglycerides and adipose tissue represent the body’s greatest energy depo.
Energy of Fats:
Among the four macromolecules of biology, fats are the most energy dense. This means they contain the most calories. When compared to other metabolic fuel sources, like protein and carbohydrates, fat contains more than twice the energy amount. Approximately 9 kilocalories (kcal) of energy are harvested per gram of fat. In comparison, only 4 kcal per gram are harvested from either carbohydrates or proteins.
Due to its energy density, fat is the preferred form of energy storage in the body. The body harvests fat energy reserves for a variety of reasons including exercise and starvation. Because of their massive reserves, there is no risk of running out of fat during a single bout of exercise, as there often is with carbohydrates. Under the proper training adaptations, athletes and non-athletes alike can tap into the massive energy storages found in fat tissue.
At rest, most energy comes from the oxidation of FAAs and carbohydrates (Mul, 2015). Humans naturally burn fat to fuel daily activity. The preferred fuel type (fat, carbohydrate, or protein) of working muscle, however, can change depending on the type of physical activity and intensity.
An individual’s VO2max gives an assessment of exercise intensity. It does so by measuring volume oxygen intake with rising exercise intensity. For example, an individual exercising at about 25% of their VO2 max, is acquiring about 90% of all energy from the oxidation of fats (FAAs). At about 85% VO2 max, the preferred fuel type has shifted to carbohydrates, which brings forth an important point. As exercise intensity increases, the body relies more and more on carbohydrates for fuel and less on fats (Mazzeo). Fats are the preferred fuel type for low to medium intensity exercise (25%-65% of VO2max) (Horowitz, 2000).
Why does preferred fuel type shift with rising intensity? Well, at high-intensity exercise the body must quickly generate ATP to meet physical demands. Fats are a great source of ATP, but the process of beta-oxidation is much slower than that of carbohydrates. Carbohydrates readily oxidize to yield ATP, better fueling mechanical work.
Training Adaptations Relating to Fat Metabolism:
Training adaptations relating to an increase in fat metabolism are beneficial to individuals and athletes alike. This is because an increase in fat metabolism during exercise will enhance endurance and performance. When the body utilizes more fat as fuel, carbohydrates and glycogen (of which we have a limited amount) are spared (Mazzeo). This is significant because carbohydrate depletion associates with fatigue and decreased performance during exercise. By adapting skeletal muscle to use more fat, ATP is produced via a more effective pathway (meaning more ATP is produced).
The training adaptations in response to endurance training link to an increase in fat metabolism. A training adaptation is a physiological change in which a part (or parts) of the body undergoes a change in response to consistent long-term stimuli. Endurance training, also known as aerobic (oxygen dependent) exercise, consists of physical activity which stimulates the heart, lungs, and circulatory system (AHA, 2017). Walking, jogging, swimming, and cycling are all forms of aerobic exercise. These help lower the chance of chronic conditions like diabetes and heart disease.
Engaging in regular endurance training (aerobic exercise) for weeks to months will result in training adaptations related to an increase in fat metabolism. This is because the body is constantly challenged to sustain low to medium intensity exercise for long periods of time. The body will respond by producing ATP via pathways which are more effective and that spare carbohydrates to delay fatigue.
Adaptations to endurance training include an increase in FAA movement from triglycerides storage into the cell for fat (beta-) oxidation, and for the burning of carbohydrates via aerobic pathways. There is also an increase in the volume of cellular mitochondria present in the muscle fibers. Mitochondria are the cell organs responsible for energy harvesting reactions. An increase in mitochondria means an increase in ATP via beta-oxidation of fats and aerobic oxidation of carbohydrate.
Why are these adaptations significant?
Carbohydrate sparing is significant for lengthening athletic performance. By burning carbohydrates via the aerobic pathway, the body is generating 16 times more ATP than it would via the anaerobic pathway (38:2 ATP ration). As the body uses carbs more effectively, athletic performance increases.
Being able to use more fat to fuel exercise is also significant. There is a massive reserve of fat in the body. By tapping into this reserve a person can exercise for longer. The increase in the mitochondrial number found in muscle cells represents an increase in the body’s ability to burn fat and sustain endurance exercise.
Are you curious about how your body works? Visit CORE and speak to some of our very knowledgeable personal trainers or nutritionists! Whether you’re trying to lose fat, gain muscle, or are simply curious about how to improve your overall health, CORE is ready to help!
- Mul, Joram D., et al. “Exercise and Regulation of Carbohydrate Metabolism.” Progress in Molecular Biology and Translational Science, U.S. National Library of Medicine, 20 Aug. 2015, www.ncbi.nlm.nih.gov/pmc/articles/PMC4727532/.
- Horowitz, et al. “Lipid Metabolism during Endurance Exercise | The American Journal of Clinical Nutrition | Oxford Academic.” OUP Academic, Oxford University Press, 1 Aug. 2000, academic.oup.com/ajcn/article/72/2/558S/4729603.
- Mazzeo, Robert. “Fat Metabolism During Exercise – The Energetics of Exercise.” Coursera, University of Colorado Boulder, www.coursera.org/learn/science-exercise/lecture/LKsKa/7-fat-metabolism-during-exercise.
- AHA. “Endurance Exercise (Aerobic).” Heart.org, American Heart Association, 6 June 2017, www.heart.org/HEARTORG/HealthyLiving/PhysicalActivity/FitnessBasics/Endurance-Exercise-Aerobic_UCM_464004_Article.jsp.