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

THE ADIPOCYTE

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 (see Figure 1).

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 (see Figure 2), 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)

The majority (80%) of fatty acids entering the muscle during exercise are oxidized for energy, while most fatty acids entering the muscle after a meal are repackaged into TAGs and stored in the muscle in lipid droplets (Shaw, Clark & Wagenmakers, 2010). Fatty acids stored in muscle are called intramyocellular triacylglycerols (IMTAGs) or intramuscular fat.

There are two to three times more IMTAGs stored in slow twitch muscle fibers (the slow oxidative fibers) than there are in fast-twitch muscle fibers (Shaw, Clark & Wagenmakers 2010). Shaw and colleagues note that even though this IMTAG supply makes up only a fraction (1%–2%) of the total fat stores within the body, it is of great interest to exercise physiologists because it is a metabolically active fatty-acid substrate especially used during periods of increased energy expenditure, such as endurance exercise.

Fatty Acids Burned for Energy

Fatty acids burned for energy (oxidized) in the muscle can come either directly from the blood or from IMTAG stores. For fatty acids to be oxidized, they must be transported into the cells’ mitochondria (see Figure 3). A mitochondrion is an organelle that functions like a cellular power plant; it processes fatty acids (and other fuels) to create a readily usable energy currency (ATP) in order to meet the energy needs of a muscle cell.

Most fatty acids are transported into the mitochondria via the carnitine shuttle (Holloway et al. 2008), which uses two enzymes and carnitine (an amino acid-like molecule) to do the transporting. One of these enzymes is called carnitine palmitoyltransferase I (CPT1). CPT1 may work with one of the same proteins (FAT/CD36) used to bring fatty acids into the muscle cells from the blood (Holloway et al. 2008). Once inside the mitochondria, fatty acids are broken down through several enzymatic pathways—including beta-oxidation, the tricarboxylic acid (TCA) cycle and the electron transport chain—to produce ATP.

Fatty-Acid Oxidation During a Single Bout of Exercise

At the start of exercise, more blood flows to adipose tissue and muscle (Horowitz & Klein 2000), releasing more fatty acids from adipose tissue and delivering more fatty acids to the muscle.

Exercise intensity has a great impact on fat oxidation.We burn the most fat when exercising at low to moderate intensity—that is, when oxygen consumption is between 25% and 60% of maximum (Horowitz & Klein 2000). At very low exercise intensities (25% VO2max), most of the fatty acids used during exercise come from the blood (Achten & Jeukendrup 2004). As exercise increases to moderate intensity (around 60% of VO2max), most of the fatty acids oxidized appear to come from IMTAG stores (Horowitz & Klein 2000).

At higher exercise intensities (>70% VO2max), total fat oxidation falls below the levels observed at moderate intensity (Horowitz & Klein 2000). This reduction in fatty-acid oxidation is coupled with an increase in carbohydrate breakdown to meet the energy demands of the exercise (Horowitz & Klein 2000).

We often overemphasize the fatty-acid contribution to calories burned during a bout of exercise. It’s also important to consider recovery from a bout of exercise, as well as training adaptations to repeated bouts, if you’re helping clients meet their fat-loss goals.

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