Fat, or adipose tissue, serves many purposes in the body including insulation against ambient temperature, cushioning in and around the joints, hormone production, and, as fuel. Let’s explore the physiological processes when the body utilizes fat as fuel.
Fats & Health
Chemically, fats (or lipids), are hydrophobic (‘water fearing’) because they contain multiple carbon atoms bound to each other and are surrounded by hydrogen. This helps explain why oil doesn’t mix readily with water. Fat or adipose tissue is contained in many parts of our body.
The location of the adipose tissue can actually be good or bad for someone’s health. Some adipose tissue is located just beneath the skin, and is therefore known as subcutaneous fat.
Some adipose tissue can be located close to organs in the middle of the body, which is known as visceral adipose tissue (VAT).
Finally, some fats in the muscle are known as intramuscular triacylglycerol (IMTG) or intramuscular adipose tissue. The IMTG can be used as an immediate fuel source for muscle metabolism. Visceral fat has been found to increase a person’s risk of developing heart disease.
Fat as Fuel: The Slow & Steady Energy Source
Let us first recall that catabolism is the set of metabolic pathways that breaks down molecules into smaller units in order to release energy. With that said, fats require a comparatively involved handling process by the body. That is, they must be transported by means of carriers, they must have oxygen in order to be catabolized, and that catabolization must occur in the mitochondria. Oxidization is the term often used to describe the catabolism of fatty acids to produce energy.
The whole process can be likened to the routine of a person going to work: The person needs to go to work to earn money. He or she often is taken there by means of a vehicle (carrier), and upon arrival, the person must physically be in a certain part of the workplace in order to do the job. While there, the worker must have certain raw materials or a particular environment in order to perform the task.
The steps of fat oxidization include:
1) Emulsification of fat into smaller globules for absorption.
2) Lipolysis of triacylglycerols into fatty acids and glycerol
3) Removal of fatty acids from cells
4) Transport of fat by the bloodstream. The release of the fat is hydrolyzed (broken off) by lipoprotein lipase.
5) Transport of fats into the muscle cell- performed by a Fatty Acid Binding Protein (FABP) and a fatty acid transporter (FAT/CD36) protein.
6) Transport of fatty acids into the mitochondria, which is first turned into an acyl-CoA complex by acyl CoA synthetase and then transported across the mitochondrial membrane.
7) Oxidation of fatty acids in the Beta-oxidation pathway and the tricarboxylic acid (TCA, or Kreb’s) cycle.
As can be seen, the above steps involve special transporters and special enzymes and the availability of certain precursors and cellular structures. The process takes time, but it is a very efficient energy production process and the supply is essentially unlimited. Roughly 111,000 kcals are available in a man who weighs 80 kg (176 lbs) with 15% body fat.
Fat oxidation is based on several factors including the intensity and duration of exercise, the aerobic condition of the individual, dietary intake over the previous 48 hours, and carbohydrate intake before and during exercise. The two main sources of fatty acids in the body are plasma fatty acids and intramuscular fatty acids. Plasma fatty acids will be replenished by the process of lipolysis. It is worth noting that advances in technology have allowed scientists to trace sources, and thereby distinguish the relative contributions of each source at various intensities and durations of exercise.
When the body is at rest and someone begins exercise, the plasma fatty acids levels will decrease because the rate of lipolysis does not keep up with the use of plasma fatty acid. After about 15 minutes of exercise, the plasma fatty acid concentration will begin to return and peak at around 60 minutes of exercise–depending on the intensity. If the intensity for those 60 minutes is moderate, the plasma fatty acid concentrations will increase, but if the intensity is high, this increase will be minor or non-existent.
The Fat Burning “Zone” & Weight Loss
Studies have shown the maximum rate of fat oxidation occurs at about 63% VO2 max. The relative and absolute amounts of fat oxidation both increase at low exercise intensities (>25% VO2 max) as exercise intensity increases up to about 63% the relative proportion decreases, but the absolute amount increases. After 63% VO2 max (moderate intensity), the absolute amount decreases quickly from 65% to about 0 at 85% VO2 max. Glycogen and existing plasma glucose become increasingly important sources as intensity increases.
Thus, many people claim they need to be in their “fat-burning zone” to lose weight because they have been told they burn more fat at that intensity. This statement is valid; however it should be noted that the person is not burning more calories and this is what leads to maximum (or some instances, minimal) fat loss.
The body is constantly using both glycogen and fat as fuel and it doesn’t need our “guidance” in choosing which source to tap into to lose fat. It does, however, need our direction to get moving in the first place.
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References
1. A.E. Jeukendrup, 2002, “Regulation of skeletal muscle fat metabolism.” NY Academy of Sciences 967: 1-19.
2. Brooks, George A. “Mammalian fuel utilization during sustained exercise.” Comparative Biochemistry and Physiology Part B: Biochemistry and Molecular Biology 120.1 (1998): 89-107.
3. Jeukendrup, Asker, and Michael Gleeson. Sport nutrition: an introduction to energy production and performance. No. Ed. 2. Human Kinetics, 2010.
4. Guo, Zengkui, Bartolome Burguera, and Michael D. Jensen. “Kinetics of intramuscular triglyceride fatty acids in exercising humans.” Journal of applied physiology 89.5 (2000): 2057-2064.
5. van Loon, Luc JC, et al. “The effects of increasing exercise intensity on muscle fuel utilisation in humans.” The Journal of Physiology 536.1 (2001): 295-304.
These resources are for the purpose of personal trainer growth and development through Continuing Education which advances the knowledge of fitness professionals. This article is written for NFPT Certified Personal Trainers to receive Continuing Education Credit (CEC). Please contact NFPT at 800.729.6378 or info@nfpt.com with questions or for more information.
