Saturated and monounsaturated fatty acids are primarily made in the body. However, polyunsaturated fatty acids (PUFAs) cannot be made by the body and therefore must be ingested. Because of this, PUFAs are called essential fatty acids.

Medium-chain triglycerides (MCT’s) are a special category of fatty acids. Normal fats contain long chain fatty acids (LCT’s) whereas MCT’s have shorter carbon chains. This physical difference causes MCT’s to be digested and metabolized more quickly and easily than LCT’s. Found naturally in milk fat, palm oil and coconut oil, MCT’s are more water soluble, able to enter the blood stream faster and more easily converted to energy. Due to their unique structure MCT’s have been studied as a potential ergogenic aid for endurance exercise.

Fat Metabolism Defined: Following ingestion, dietary fat digestion begins in the mouth where the fat-digesting enzyme, lipase, initiates its break down. Lipase works by breaking the bonds between the glycerol and fatty acids. It is produced in the mouth (lingual lipase), pancreas (pancreatic lipase), and small intestine (intestinal lipase). Pancreatic and intestinal lipases operate in the duodenum of the small intestine, where the majority of fat digestion and absorption occur. Additionally, bile (which is produced in the liver and stored in the gall bladder) works to emulsify fats and break them down into smaller globules for absorption. Bile also increases absorption in the gut by helping to transport the fat globules to the intestinal lining. Following absorption, the globules are converted into triacylglycerols and are transported by the lymphatic system to the bloodstream. Once in the bloodstream, triacylglycerols are broken down again into free fatty acids and glycerols so that muscle, adipose (fat), or other tissue types can absorb the fatty acids. Once inside these cells, they can either be oxidized (“burned”) in the mitochondria’s citric acid cycle to produce ATP (energy) or are stored as triglycerides for later use (http://muscle.ucsd.edu/musintro/fattyacid.shtml). Because lipid molecules are more compact, the body can store larger amounts of lipids than glycogen or protein and therefore fats mainly function as an energy reserve (Ophardt, C.E., 2003).

Additionally, in comparison to carbohydrates and proteins, which yield 4 kcal of energy per gram, lipids yield 9 kcal of energy. This is because fatty acids have a much greater number of carbons (up to approximately 22) than carbohydrates. Yet the absorption time for fats is much slower than that for carbohydrates.

Fat and Endurance Performance: Since our primary sources for energy during exercise and rest are carbohydrate and fat, most studies examining exercise and fuel sources compare high carbohydrate and high fat diets. More specifically with endurance performance, the diets are composed of either a carbohydrate or fat intake of 40% or greater of total calories (energy intake), with many exceeding 60% (Helge, 2002). It is well accepted that high fat diets result in decreased resting muscle glycogen content and an increased rate of fat oxidation while exercising when compared with high carbohydrates diets (Helge, 2002). Additionally, endurance trained athletes on high fat diets demonstrate reduced utilization of glycogen stores during submaximal exercise (Burke & Kiens, 2006). However, increased fat oxidation and glycogen sparing is also an exercise adaptation observed in trained athletes, implying that these adaptations may not be exclusively due to diet (Horvath et al., 2000).

After reviewing the literature it is apparent that a majority of researchers agree that acute and chronic high fat diets (defined as 60% or greater) do not enhance endurance performance and they are not recommended for such a purpose (Burke et al., 2004 & Helge, 2002). In fact they seem to impair performance or simply function to maintain exercise levels compared to those seen with high carbohydrate diets. In multiple studies, subjects reported higher ratings of perceived exertion during exercise bouts while on the high fat diets and an impaired ability to maintain training (Burke & Hawley, 2002 & Helge, 2002).

There are a small number of studies that report improved performance on high fat diets (Pendergast et al. 2000, Helge, 2002 Van Zyl CG et al. 1996, Lambert EV et al. 2001). However, upon scrutiny, these are subject to great criticism. Very often in these studies it appears that the high fat diets increased caloric intake for otherwise hypocaloric individuals (Horvath et al., 2000 & Pendergast et al., 2000). Therefore the improved performance could be a result of more optimal caloric intake and thus balanced energy input and output, not the increased dietary fat content. Another problem with some of these studies is that by making the percentage of dietary fat so high the result is a carbohydrate-poor diet. Diets that are low in carbohydrates have been shown to impair performance as well as mood state during longer training periods (Achten et al., 2004). In addition, acute and chronic high fat dieting is generally associated with decreased muscle glycogen availability, which is deemed the primary fuel for maintaining moderate to high intensity exercise (Johnson et al., 2004). Decreased glycogen availability results in muscle fatigue, which is what we are trying to avoid (Pendergast et al., 2000).

With the help of newer technology and greater knowledge, some researchers have focused on dietary fat and endurance performance on a cellular level by examining intramuscular triacylglyceride (IMTG) stores as a lipid fuel source. The reason this is of such great interest is two fold: 1) fat used as a fuel source is glycogen sparing (glycogen depletion is correlated with fatigue) and 2) fat is a more plentiful fuel source. Several studies demonstrate that IMTG stores decrease with prolonged exercise (Watt et al., 2002). Additionally, higher fat diets may allow for greater fat oxidation during prolonged submaximal exercise while sparing glycogen stores. However, there are several problems associated with this research. Foremost is the fact that research techniques are limited. In fact, various estimations based on two techniques determining the percentage of IMTG oxidation to total fat oxidation range from 0-80% (Johnson et al., 2004). So, researchers disagree on the amount IMTG stores even contribute to exercising muscles. Studies that report declining IMTG stores with exercise provide widely variable decreases, making our understanding less clear (Watt et al., 2002). Also, the intensity of exercise affects the degree to which we rely on IMTG stores. At higher intensities our bodies rely more heavily on glycogen stores, so much of this data is only relevant for lower intensity work and not racing situations. A second limitation with the findings thus far is that although higher fat diets may have an effect on exercise metabolism, there are no reports on exercise performance benefits (Hargreaves et al., 2004). Unlike muscle glycogen depletion, currently there is no conclusive evidence that IMTG depletion is limiting to exercise performance in training or competition (Johnson et al., 2004 & Spriet & Gibala, 2004).

The medium chained triglycerides (MCT’s) have fallen into their own niche of research. Although they are defined as fats, MCT’s act similarly to carbohydrates because they are metabolized for immediate energy. This fact has lead to the notion that MCT’s could be used as a glycogen sparing fuel source. However, the research studies on MCT’s have shown mixed results at best. Similar to the high fat diet studies, only those which increased total caloric consumption found improved cycling performance and improved substrate utilization (Lambert, E.V. et al., 2001 & Van Zyl, C.G. et al., 1996). Isocaloric studies that either substituted MCT’s for carbohydrates or matched calories with a mix of MCT’s and carbohydrates and compared these to carbohydrate ingestion alone found no significant performance improvements (Misell, L.M. et al. 2001, Jeukendrup, A.E. et al., 1996, Goedecke, J.H. et al., 1999, Angus, D.J. et al., 2000). Therefore, the current research does not offer convincing evidence that MCT’s are an ergogenic aid for endurance performance.

Recommendations for endurance athletes:
1. Fat intake is important for holistic health and energy balance. For athletes, a prime nutrition concern is to maintain an energy balance between caloric intake and energy expenditure. Often athletes consume too few calories, which may lead to impaired performance and impaired fatigue resistance. While maintaining energy balance, it is important that the ingested calories are from healthy, physiologically beneficial sources. With that in mind, dietary fat is essential and must be ingested. Although it is not recommended to eat a high fat diet similar to those used in many studies, it is recommended to ingest a moderate amount of healthy fats, which can help to maintain strong immune and neurological systems as well as stave off potential micronutrient deficiencies.

2. Choose healthy sources of dietary fats. When consuming fats, it is important to be knowledgeable about the types of fats discussed previously. Ingestion of saturated fats should be very limited. It is recommended that trans fats be avoided as they can have negative health effects (Kruger & Horrobin, 1997). Monounsaturated fatty acids (MUFAs) and polyunsaturated fatty acids (PUFAs) have a multitude of health benefits, including promoting a healthy heart by reducing cardiovascular risks, improved immune function, sex hormone mediation, and improved bone health (Lowery, 2004, Venkatraman et al., 2000 & Watkins et al., 2001). For PUFAs, it has been established that the ratio of omega-6 to omega-3 fatty acids is key to healthful living and reaping the benefits of ingesting these fats. Research has demonstrated that in a typical American diet, individuals eat anywhere between a 12-50:1 of omega-6s to -3s. Although the exact desirable ratio is still unknown, it has been suggested that the ratio should be in the range of 5-10:1 or even 2:1 (Lowery, 2004, Watkins et al., 2001 & Simopoulos, 2003). The latter assessment is based on research that focuses on the changing American diet over thousands of years.

Summary: The bottom line is that fats must be ingested. The current research suggests that there are no performance enhancing effects from fat ingestion. However, there are health benefits from maintaining an energy balance, eating fats in moderation, and eating the right types of fats.

References:
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