By Sally Warner MA PhD and Kris Walker MD

What are fats?

Fats are organic compounds composed of a glycerol backbone with fatty acids attached to it. There are three primary types of fatty acids: saturated, monounsaturated (MUFA), and polyunsaturated (PUFA). Saturated fatty acids are those that only have single bonds between the carbon atoms and all remaining bonds are filled with hydrogen atoms. These are common in animal fats. Trans fatty acids fall under this category, although they are not naturally occurring. Instead, trans fatty acids are engineered by hydrogenating unsaturated fatty acids to create a solid, more stable fat product. The second type of naturally occurring fatty acid, MUFAs, has one double bonded carbon atom. MUFAs are found in olive oil, canola oil, peanut oil, as well as other products. Lastly, PUFAs have more than one double bond. The PUFAs can be further broken down into two major groups of essential fatty acids, the omega-6 series and the omega-3 (Kruger and Horrobin, 1997). Omega-6 fatty acids are found in corn oil, soybean oil, and most other vegetable oils, as well as pastries. Examples of where omega-3 fatty acids are found are: walnuts, canola oil, flaxseed oil, salmon, sardines, and other fish. Below is a diagram for clarification.

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

Studies have shown that dietary manipulation can alter substrate use as fuel during endurance exercise. Supplementing a high carbohydrate diet with additional high fat meals before exercise or eating a high fat diet for several days with carbohydrate restoration for one day will increase fat oxidation during exercise bouts. However, performance was not enhanced by these manipulations. (Zehnder, 2006 and Stellingworth, 2006) In fact, a high fat diet combined with vigorous exercise may increase oxidative stress enough to overwhelm the muscles’ antioxidant capacities, as demonstrated in a study done on male rats. (Greathouse, 2005) Dietary manipulation may not even be necessary to increase fat oxidation, as shown by the increased fat oxidation during exercise by trained males with decades of endurance training. (Boon, 2007)

Medium Chain Triglycerides: 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. Medium Chain Triglycerides (MCT’s) are medium chain (6-12 carbon) fatty esters of glycerol. Because MCT’s smaller molecule size and hence easier to absorb, its theorized that these fat-esters can pose a performance advantage. 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.

Two more studies that looked at medium-chain triglyceride ingestion before and during exercise. One study showed no glycogen sparing with ingestion of carbohydrates combined with medium chain triglycerides as compared to ingestion of carbohydrates alone. (Horowitz, 2000) Another study showed that medium-chain triacylglycerol taken prior to exercise and ingested along with carbohydrates during exercise caused gastrointestinal symptoms in half of the subjects, did not alter substrate metabolism, and significantly impaired sprint performance during prolonged cycling. (Goedecke, 2005)

Conjugated linoleic acid: Conjugated linoleic acid (CLA’s) is a trans fat, though not harmful in the same way as other trans fatty acids. It is an isomer of linoleic acid, a fatty acid which is found mostly in the meat and dairy products of ruminants. Conjugated linoleic acid is an increasingly popular supplement, with both strength and endurance athletes. Kangaroo meat is very high in CLA, and grass-fed ruminants contain more CLA than grain-fed animals. Eggs are also high in CLA. Flax seed oil and safflower oil also contain CLA. Conjugated linoleic acid is purported to reduce fat mass, improve strength and endurance, and increase muscle mass. A few studies have shown a reduction in fat mass with CLA, both alone, and with exercise, while others have shown no effect. (Colakoglu, 2006, Lambert 2007, and Kreider, 2002). A recent meta-analysis of the literature reported that supplementing with CLA at a dose of 3.2 grams per day may produce a modest loss of body fat in humans. The actual numbers are not that impressive: as compared to placebo, the reduction of fat mass was 0.09 +/- 0.08 kg/week. (Whigman, 2007) Conjugated linoleic acid combined with creatine improves strength and body composition when combined with resistance training. (Tarnopoisky, 2007) The implications of this are not clear for endurance exercise, however, and the study combined the 2 supplements, so it is not clear whether CLA contributed to the gains or if the gains were due to the creatine supplementation alone. Most of the studies thus far have been done in mice and rats and seem to show that CLA promotes fat oxidation and increases exercise capacity (Mizunoya, 2005), but may intensify the oxidative stress caused by exhaustive exercise. (Di Felice, 2007)

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).

Conclusion: Fat supplementation may increase the use of fat as a fuel during exercise, but does not offer any performance advantage over a high carbohydrate diet. Medium-chain triglycerides and triacyclycerol ingestion prior to and during exercise does not spare glycogen, may cause gastric distress, and may impair high intensity performance. Conjugated linoleic acid supplements are becoming popular and are purported to improve body composition, strength, and endurance. Studies in mice have shown an improvement in endurance performance, but these results have not been duplicated in human studies. CLA may be effective in improving body composition in humans, and more studies need to be done.

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:

Angus, D.J., Hargreaves, M., Dancey, J., & Febbraio, M.A. (2000). Effect of carbohydrate or carbohydrate plus medium-chain triglyceride ingestion on cycling time trial performance. Journal of Applied Physiology, 88, 113-9.

Achten, J., Halson, S.L., Moseley, L., Rayson, M.P., Casey, A., & Jeukendrup, A.E. (2004). Higher dietary carbohydrate content during intensified running training results in better maintenance of performance and mood state. Journal of Applied Physiology, 96, 1331-1340.

Boon H, Jonkers RA, Koopman R, Blaak EE, Saris Wh, Waenmakers AJ, Van Loon LJ. (2007).Substrate source use in older, trained males after decades of endurance training. Med Sci Sports Exerc, 39(12):2160-70.

Burke, L.M. & Hawley, J.A. (2002). Effects of short-term fat adaptation on metabolism and performance of prolonged exercise. Medicine and Science in Sports and Exercise, 34, 1492-1498.

Burke, L.M., Kiens, B., & Ivy, J.L. (2004). Carbohydrate and fat for training and recovery. Journal of Sports Sciences, 22, 15-30.

Burke, L.M. & Kiens, B. (2006). “Fat adaptation” for athletic performance: the nail in the coffin? Journal of Applied Physiology, 100, 7-8.

Colakoglu S, Colakoglu M, Taneli F, Cetinoz F, Turkmen M. (2006).Cumulative effects of conjugated linoleic acid and exercise on endurance development, body composition, serum leptin and insulin levels. J Sports Med Phys Fitness, 46(4):570-7.

Decombaz, J., Schmitt, B., Ith, M., Decarli, B., Diem, P., Kreis, R., Hoppeler, H., & Boesch, C. (2001). Postexercise fat intake repletes intramyocellular lipids but no faster in trained than in sedentary subjects. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 281, R760-R769.
Di Felice V, Macaluso F, Montalbano A, Gammazza AM, Palumbo D, Angelone T, Bellaflore M, Farina F. (2007).Effects ofconjugated linoleic acid and endurance training on peripheral blood and bone marrow of trained mice. J Strength Cond Res, 21(1):193-8.

Goedecke J.H., Christie C., Wilson, G., Dennis S.C., Noakes, T.D., Hopkins, W.G., & Lambert, E.V. (1999). Metabolic adaptations to a high-fat diet in endurance cyclists. Metabolism, 48, 1509-17.

Goedecke JH, Clark VR, Noakes TD, Lambert EV. (2005).The effects of medium-chain triacylglycerol and carbohydrate ingestion on ultra-endurance exercise performance. Int J Sport Nutr Exerc Metab, 15(1):15-27.

Greathous KL, Samuels M, DiMarco NM, Criswell DS.(2005).Effects of increased dietary fat and exercise on skeletal muscle lipid peroxidation and antioxidant capacity in male rats. Eur J Nutr, 44(7):429-35.

Hargreaves, M., Hawley, J.A., & Jeukendrup, A. (2004). Pre-exercise carbohydrate and fat ingestion: effects on metabolism and performance, 22, 31-38.

Helge, J.W. (2002). Long-term fat diet adaptation effects on performance, training capacity, and fat utilization. Medicine and Science in Sports and Exercise, 34, 1499-1504S.

Horowitz JF, Mora-Rodriguez R, Byerley LO, Coyle EF.(2000). Preexercise medium-chain triglyceride ingestion does not alter muscle glycogen use during exercise. J Appl Physiol, 88(1):219-25.

Horvath, P.J., Eagen, C.K., Fisher, N.M., Leddy, J.J., & Pendergast, D.R. (2000). The effects of varying dietary fat on performance and metabolism in trained male and female runners. Journal of the American College of Nutrition, 19, 52-60.

Jeukendrup AE, Aldred S. (2004). Fat supplementation, health, and endurance performance. Nutrition, 20(7-8):678-88.

Jeukendrup, A.E., Saris, W.J., Brouns, F., Halliday, D., & Wagenmakers, J.M. (1996). Effects of carbohydrate and fat supplementation on carbohydrate metabolism during prolonged exercise. Metabolism, 45, 915-21.

Johnson, N.A., Stannard, S. R., & Thompson, M.W. (2004). Muscle triglycerides and glycogen in endurance exercise. Sports Medicine, 34, 151-164.

Kreider RB, Ferreira MP, Greenwood M, Wilson M, Almada AL. (2002).Effects of conjugated linoleic acid supplementation during resistance training on body composition, bone density, strength, and selected hematological markers. J Strength Cond Res, 16(3):325-34.

Kruger, M.C. & Horrobin, D.F. (1997). Calcium metabolism, osteoporosis and essential fatty acids: a review. Progress in Lipid Research, 36, 131-151.

Lambert, E.V., Goedecke, J.H., Zyle, C., Murphy, K., Hawley, J.A., Dennis, S.C. & Noakes, T.D. (2001). High-fat diet versus habitual diet prior to carbohydrate loading: effects of exercise metabolism and cycling performance. International Journal of Sport Nutrition and Exercise Metabolism, 11, 209-25.

Lambert EV, Goedecke JH, Bluett , Heggie K, Claasen A, Rae DE, West S, Dugas J, Dugas L, Meltzeri S, Chariton K, Mohede I. (2007) Conjugated linoleic acid versus high-oleic acid sunflower oil: effects on energy metabolism, glucose tolerance, blood lipids, appetite and body composition in regularly exercising individuals. Br J Nutr, 97(5):1001-11.

Lowery, Lonnie M. (2004). Dietary fat and sports nutrition: a primer. Journal of Sports Science and Medicine, 3, 106-117.

Misell, L.M., Lagomarcino, N.C., Schuster, V., & Kern, M. (2001). Chronic medium –chain triacylglycerol consumption and endurance performance in trained runners. Journal of Sports Medicine and Physical Fitness, 41, 210-5.

Mizunoya W, Haramizu S, Shibakusa T, Okabe Y, Fushiki T. (2005) Dietary conjugated linoleic acid increases endurance capacity and fat oxidation in mice during exercise. Lipids, 40(3):265-71.

Ophardt, C.E. (2003). Virtual Chembook, Department of Chemistry, Elmhurst College, 190 Prospect Avenue, Elmhurst, IL 60126.

Pendergast, D.R., Leddy, J.J., & Venkatraman, J.T. (2000). A perspective on fat intake in athletes. Journal of the American College of Nutrition, 19, 345-350.

Simopoulos, A.P. & Cleland, L.G. (2003). Omega-6/omega-3 essential fatty acid ratio: the scientific evidence. World Review of Nutrition and Dietetics, 92, 1-22.

Spriet, L.L. & Gibala, M.J. (2004). Nutritional strategies to influence adaptations to training. Journal of Sports Sciences, 22, 127-141.

Stellingwerff T, Spriet LL Watt MJ, Kimber NE, Hargreaves M, Hawley JA, Burke LM. (2006) Decreased PDH activation and glycogenolysis during exercise following fat adaptation with carbohydrate restoration. Am J Physiol Endocrinol Metab, 290(2):E380-8.

Tarnopoisky M, Zimmer A, Paikin J, Safdar A, Aboud A, Pearce E, Roy B, Doherty T. (2007) Creatine monohydrate and conjugated linoleic acid improve strength and body composition following resistance training in older adults. PLoS ONE 2(10):e991.

Venkatraman, J.T., Leddy, J., & Pendergast, D. (2000). Dietary fats and immune status in athletes: clinical implications. Medicine and Science in Sports and Exercise, 32, S389-S395.

Van Zyl, C.G., Lambert, E.V., Hawley, J.A., Noakes, T.D., & Dennis, S.C. (1996). Effects of medium-chain triglyceride ingestion on fuel metabolism and cycling performance. Journal of Applied Physiology, 80, 2217-25.

Watkins, B.A., Lippman, H.E., Le Bouteiller, L. Li, Y., & Seifert, M.F. (2001). Bioactive fatty acids: role in bone biology and bone cell function. Progress in Lipid Research, 40, 125-148.

Watt, M.J., Heigenhauser, G.J.F., & Spriet, L.L. (2002). Intramuscular triacylglycerol utilization in human skeletal muscle during exercise: is there a controversy? Journal of Applied Physiology, 93, 1185-1195.

Whigham LD, Watras AC, Schoeller DA. (2007). Efficacy of conjugated linoleic acid for reducing fat mass: a meta-analysis in humans. American Journal of Clinical Nutrition, 85(5):1203-1211.

Zehnder M, Christ ER, Ith M, Acheson KJ, Pouteau E, Kreis R, Trepp R, Diem P, Boesch C, Decombaz J. (2006) Intramyocellular lipid stores increase markedly in athletes after 1.5 days lipid supplementation andareutilized during exerciseinproportionto their content. Eur J Appl Physiol, 98(4):341-54.