Fat may seem like the enemy of civilized people—especially sedentary ones. Yet we cannot live without it. Fat plays a key role in the structure and flexibility of cell membranes, and it helps regulate the movement of substances through those membranes. Special types of fat, known as eicosanoids, send hormone-like signals that exert intricate control over many bodily systems, mostly those affecting inflammation or immune function.
Of course, the best-known function of fat is as an energy reserve. Fat has more than twice the energy-storage capacity of carbohydrate (9 calories per gram vs. 4 calories per gram). It has been estimated that lean adult men store about 131,000 calories in fat (Horowitz & Klein 2000), enough energy to keep the average person alive for about 65 days.
For fitness professionals, the prime concern arises when the body’s fat-storage function works too well, hoarding unwanted fat that makes people unhealthy and self-conscious about their appearance. Understanding how fat travels through the body can help personal trainers work with clients to reduce excess body fat and improve athletic performance.
The Journey of a Fatty Acid to Muscle
Fat resides primarily in designated fat-storage cells called adipocytes. Most adipocytes are just under the skin (subcutaneous fat) and in regions surrounding (and protecting) vital organs (visceral fat). Nearly all fat in adipocytes exists in the form of triacylglycerols (TAGs or triglycerides). Each TAG consists of a backbone (glycerol) with three fatty-acid tails.
Depending on energy supply and demand, adipocytes can either store fat from the blood or release fat back to the blood. After we eat, when the energy supply is high, the hormone insulin keeps fatty acids inside the adipocytes (Duncan et al. 2007). After a few hours of fasting or (especially) during exercise, insulin levels tend to drop, while levels of other hormones—such as epinephrine (adrenaline)—increase.
When epinephrine binds to adipocytes, TAG stores go through a process called lipolysis (Duncan et al. 2007), which separates fatty acids from their glycerol backbone. After lipolysis, fatty acids and glycerol can leave the adipocytes and enter the blood.
Fatty Acids in the Blood
Because fat does not easily dissolve in water, it needs a carrier protein to keep it evenly suspended in the water-based environment of the blood. The primary protein carrier for fat in the blood is albumin (Holloway et. al. 2008). One albumin protein can carry multiple fatty acids through the blood to muscle cells (Horowitz & Klein 2000). In the very small blood vessels (capillaries) surrounding the muscle, fatty acids can be removed from albumin and taken into the muscle (Holloway et al. 2008).
Fatty Acids Going From the Blood Into Muscle
Fatty acids must cross two barriers to get from the blood into the muscle. The first is the cell lining of the capillary (called the endothelium), and the second is the muscle-cell membrane (known as the sarcolemma). Fatty-acid movement across these barriers was once thought to be extremely rapid and unregulated (Holloway et al. 2008). More recent research has shown that this process is not nearly as fast as once thought and that the presence of special binding proteins is required at the endothelium and sarcolemma for fatty acids to pass through (Holloway et al. 2008). Two proteins that are important for fatty-acid transport into the muscle cells are FAT/CD36 and FABPpm.
Two Fates of Fat Inside Muscle
Once fat is inside the muscle, a molecule called coenzyme A (CoA) is added to the fatty acids (Holloway et al. 2008). CoA is a transport protein that maintains the inward flow of fatty acids entering the muscle and prepares the fatty acid for one of two fates:
- oxidation (in which electrons are removed from a molecule) to produce energy or
- storage within the muscle (Holloway et al. 2008; Shaw, Clark & Wagenmakers 2010)
For much more on the science of fat burning, including a section on how exercise adaptations improve fat usage, plus helpful illustrations, please see “The Physiology of Fat Loss” in the online IDEA Library or in the January 2014 print issue of IDEA Fitness Journal. If you cannot access the full article and would like to, please contact the IDEA Inspired Service Team at (800) 999-4332, ext. 7.