DIETARY FAT AND PHYSICAL ACTIVITY: FUELING THE CONTROVERSY

RT# 25 / Volume 7 (1996), Number 3

---

Participants:

Andrew Coggan, Ph.D. Metabolism Unit Shriners Burns Hospital Assistant Professor University of Texas Medical Branch Galveston, Texas	Will Hopkins, Ph.D. Senior Lecturer Schools of Medical Science (Dept. of Physiology) and Physical Education University of Otago Dunedin, New Zealand
Ellen Coleman, R.D., M.A., M.P.H. Member, SCAN/Gatorade Speakers Bureau Nutrition Consultant The Sport Clinic Riverside, California	Lawrence Spriet, Ph.D. Dept. of Human Biology and Nutritional Sciences University of Guelph Guelph, Ontario, Canada Member, GSSI Sports Medicine Review Board

Introduction

Through the years, the Gatorade Sports Science Institute has focused a number of its articles and roundtable discussions on various nutrients and pseudo-nutrients with purported benefits for physically active people. Recently, the macronutrient needs of the active person has been discussed at great lengths in lay magazines and in the scientific literature, and the results of some recent studies have suggested that active people might require more fat in their diets than previously thought. We asked the opinions of four prominent physiologists and/or nutritionists who have studied the athlete's diet at length. Their answers to our questions follow.

A few research studies and popular books have suggested that high fat diets might help to improve physical performance. What are your thoughts on this issue?

Coggan: Despite the recent hullabaloo, there is really no good evidence that performance can be improved by increasing dietary fat intake (or by decreasing carbohydrate intake, which is actually what has been proposed). The stories presented in the popular press are simply anecdotal, while the handful of laboratory studies purporting to show a benefit suffer from serious methodological flaws. For example, in one recent study the diets were not presented in a random order, whereas in an earlier study the authors failed to adequately control for changes in body weight. By comparison, there is a wealth of solid scientific data accumulated over many years indicating that an adequate carbohydrate intake is necessary for optimal sports performance.

Hopkins: I am of the belief that ultra-endurance athletes, or those involved in events lasting a few hours or more might benefit from higher fat intake. From what I've seen in the literature, and on the basis of the underlying physiology, I would advise such athletes to adapt to a high-fat diet (60-70% of calories) for several weeks before a big event, and switch to a high-carbohydrate diet about two days before the event. During an event, I might suggest that they consume a easily digestible fat supplement. Recent papers showing positive effects of some of these strategies include those by Lambert et al., Lapachet et al., Muoio et al. and Van Zyl et al.

Coleman: Although fat can contribute significant energy during prolonged low- to moderate-intensity endurance exercise, athletes should not eat a high-fat diet or rely on high-fat supplements. Fat oxidation cannot support exercise intensities greater than about 60% of VO2 max. Eating too much fat (over 30% of total calories) also tends to decrease carbohydrate intake; therefore, muscle glycogen stores cannot be maintained on a high-fat diet. A high-fat diet also contributes to obesity and increases the risk of developing coronary heart disease, stroke, and certain cancers.

Spriet: I agree with Dr. Coggan's assertion that the majority of the support for high-fat diets is based on personal experience and anecdotal information. The studies cited by Dr. Hopkins suffer serious methodological flaws and therefore do not constitute sound scientific evidence. I could never advocate that any athlete consume 60-70% of the day's caloric intake as fat. Much of the information in popular books and magazines has not undergone the peer-review process, so it is difficult to support the validity of these claims.

What is the physiological premise behind the suggestion that eating high fat meals might benefit athletes?

Coleman: It has been claimed that "fat loading" enables athletes to burn fat, rather than glycogen, as the major fuel source. Whereas total glycogen storage (in liver and muscle) is equivalent to only about 2000 calories, every pound of fat supplies 3500 calories. Since fat stores are plentiful and glycogen stores are limited, fat loading has been purported to enhance performance.

Hopkins: That's correct. It all comes down to the idea that fuel stores are a limiting factor in ultra endurance events. When you exercise at high intensity, the active muscle fibers use their stored fuel first. For an athlete on a normal diet or high-carbohydrate diet, the main fuel is carbohydrate, which is stored in muscle as glycogen. When muscle glycogen levels are low, the muscle fibers start to use blood borne glucose. However, it's likely that you can't burn blood borne glucose as fast as intramuscular glycogen, so you have to slow down when your muscle stores are low. The strategies that I outlined earlier are all aimed at decreasing the utilization of muscle glycogen by allowing the body to adapt to burning more fat over several weeks, thus increasing fat utilization during an event.

Spriet: The premise that providing more fat to the muscle will increase fat utilization and decrease carbohydrate use is theoretically sound. However, achieving a significant increase in fat availability through an increase in dietary fat intake does not appear to be physiologically possible. Caffeine ingestion appears to stimulate the release of free fatty acids from fat stores, at least in well trained athletes, thus increasing fat availability. Aerobic training also increases one's ability to use fat as a fuel. Increasing dietary fat intake does not appear to work in a similar manner.

Coggan: As I understand it, the idea behind reducing dietary carbohydrate intake and replacing the calories with something else (e.g., fat) is to reduce insulin levels, thus favoring the use of stored body fat as a fuel. While this idea is correct in a general sense, it is overly simplistic. The mobilization and utilization of fat is controlled by numerous factors, of which insulin is only one. In addition, the diet must contain sufficient carbohydrate to replenish muscle glycogen stores, which is the most important fuel during high-intensity exercise. Finally, as Dr. Spriet suggested, the goals that these lower carbohydrate diets are supposed to achieve (i.e., lower insulin levels and increased fat oxidation) are already furthered by simply exercising regularly.

Habitual physical exercise has been shown to improve a person's ability to use fat as a fuel. Will habitually eating a high fat diet provide similar adaptations?

Hopkins: I know that training allows you to use a greater proportion of fat for exercise at the same absolute intensity, but can it promote greater fat usage during exercise at the same relative intensity? For example, at maximal steady state, or the anaerobic threshold, it wouldn't surprise me if training makes little difference to the mix of fuel that you burn. Therefore, I'm not sure how much of an effect training really has on your ability to burn fat. I think that changing the diet is the primary way to have an impact on the fuel mix during exercise.

Spriet: Eating a high-fat diet has not been studied to any great extent in humans due to the related health concerns. High-fat diets and obesity are risk factors for cardiovascular diseases and several other diseases. Based on the limited research available, it appears that a chronic high-fat diet will increase the ability to use fat at moderate intensities of exercise, although exercise time to exhaustion is not improved. It is also difficult for people on a high-fat diet to exercise at high intensities typical of athletic events because their body stores of carbohydrate are often compromised. The issue is complicated as well by the data that exists in the animal literature, which indicate that eating a chronic high-fat diet can produce increased running time to exhaustion in rats exercising at moderate intensities. However, rodents tend to rely more on fuels from outside the muscle cell (glucose and fat) for energy during exercise, and less on muscle stores (glycogen and triglycerides) than humans. Therefore, extrapolation from rodents to humans in this case is not warranted.

Coleman: A 1983 study by Phinney suggested that humans can adapt to high-fat diets. Cyclists who ate a high-fat diet (85% of calories) for one month used three times less muscle glycogen and four times less blood glucose during exercise at 63% VO2 max. Their utilization of fat rose to make up the difference. However, the high-fat diet did not improve performance compared to a diet containing 50% of calories as carbohydrate.

Adaptation to a high-fat diet takes at least two weeks. As Dr. Spriet suggested, exercise during this time will be difficult and unpleasant due to low glycogen stores. Even when adaptation is complete, the athlete's ability to exercise hard (>60% VO2 max) will be impaired. Further, a high fat diet is hard to digest, which is one of the reasons that fat should be limited in the pre-exercise meal. Lastly, high-fat diets lack the variety needed for optimum nutrient intake.

Coggan: In a general sense, eating a high-fat diet can promote similar adaptations as physical training. For example, the quantity of fat-metabolizing enzymes in muscle does rise with a low-carbohydrate, high-fat diet, as is true with training. The magnitude of this increase, however, is much smaller when induced by diet than when induced by training. More importantly, increasing the amount of fat in the diet to create such adaptations has the disadvantage of limiting the amount of carbohydrate stored as muscle and liver glycogen. The body therefore relies more on fat as a fuel not really as a preference, as is the case with training, but simply out of need. In other words, fat (and, importantly, also protein) is used more heavily only because the preferred fuel, carbohydrate, isn't available.

If eating a high fat diet is not the answer, what types of dietary interventions would you recommend to improve performance?

Coleman: In general, the athlete's diet must, at a minimum, meet the criteria for a "healthy diet" as defined by the 1995 U.S. Dietary Guidelines. These recommendations establish a good foundation for athletes desiring optimum performance. In particular, athletes should consume: 1) adequate carbohydrate ( 6 to 10 g/kg/day) to maintain muscle glycogen stores; 2) adequate fluid and electrolytes to maintain proper water and electrolyte balance; 3) adequate energy to prevent depletion of fuel stores; 4) adequate protein (1.2 to 1.8 g/kg/day) to preserve fat-free mass; 5) adequate vitamins and minerals in relation to energy expenditure.

Coggan: Generally, a "good" diet should be viewed as a necessary but not a sufficient condition for optimal performance. In other words, the diet provides the foundation upon which the athlete builds, but by itself really doesn't affect performance unless it is nutritionally inadequate. However, what is nutritionally adequate for an inactive person may not be so for an athlete involved in training. Obviously, most athletes need more calories than do most inactive individuals; they may also need slightly more protein. Most apparent, though, is an increased need for carbohydrates, particularly when the training load is very heavy and/or when tapering for important competitions. In fact, the "carbo loading" regimen and other types of carbohydrate feedings before and during exercise are the only dietary manipulations scientifically shown to have a significant positive impact on endurance performance.

On the other hand, with the exception of a small requirement for essential fatty acids (3-5% of calories), there is really no need for fat in the diet of either non-athletes or athletes; only a need for the energy and the fat-soluble vitamins it contains. In terms of health, however, it is clear that a diet that is too high in fat can have a tremendous negative impact.

Hopkins: I think that a look at the quality of the fat in the diet is warranted as well. A French group reported that masters athletes who supplemented with borage oil (mostly omega-6 polyunsaturated fatty acids) for six weeks had better oxygenation of their blood during high-intensity exercise (see Aguilaniu et al.). That would theoretically translate into better endurance performance. Similar findings were demonstrated in a recent study of rats fed fish oil (mostly omega-3 polyunsaturated fatty acids) (see Lortet and Verger). I also think there is a place in the athlete's diet for antioxidants as a possible means of reducing muscle damage during overtraining. A case can also be made for athletes eating more high-quality protein (~30% of calories) for its anabolic potential, as well as supplements like creatine, which have been shown to have a positive effect on performance in a number of studies.

Spriet: I continue to recommend that carbohydrate comprise 65-70% of the diets, proteins about 15% of calories, and fat ~20% of total calories for people who exercise on a near-daily basis. This diet will generally provide the necessary fuels for exercise and help to maintain a more favorable blood lipid profile. Adequate carbohydrate consumption in the hours prior to activity will also ensure adequate liver glycogen stores. It is clear that adhering to this dietary regimen will be more beneficial than increasing the fat and protein contents at the expense of carbohydrate. The prior response regarding the quality of fat and protein in the diet is speculative, at best.

How can coaches, athletes, and consumers sort through all of the science and pseudo-science to determine for themselves if claims for nutritional products or regimens are valid?

Coleman: Nutrition quackery is the promotion of false and/or unproved nutrition products or services for a profit. Athletes can avoid being victims of a nutrition rip-off by learning to recognize some of the techniques that "quacks" tend to use, and by following certain common sense guidelines. The athlete should beware if: 1) the claim sounds too good to be true; 2) the suspected "quack" encourages distrust of reputable health professionals such as medical doctors, exercise physiologists, and registered dietitians; 3) a preponderance of case histories, testimonials, and subjective evidence are used to justify exaggerated claims.

Coggan: Other important questions to ask yourself are, "Is somebody trying to sell me something, e.g., a product, book, or "system?", and "Is the claim or product being promoted as "revolutionary", with sweeping and widespread benefits?" True scientific progress is slow and is generally conducted in piece-meal fashion. I think that if people are properly skeptical and ask themselves these and similar questions, they can avoid most of the traps being laid out there for consumers.

Spriet: I would further recommend staying away from some of the fad magazines and books that abound in today's society. Rather, look more to the science and educational materials that contain opinions of several researchers in a particular area. Especially useful are forums where it is clear that several participants are involved or where the material has been reviewed by several knowledgeable people in an area. It must be remembered that the driving force behind many of these books and magazines is making a profit, with very little regard for accuracy.

Hopkins: As Drs. Coggan and Spriet suggested, an understanding of the important issues related to proper research design is helpful in deciphering nutritional claims. This generally requires becoming an expert in the area, reading the original literature, and then deciding for yourself. This, of course, is not a practical option for most people. Therefore, I think that you have to be conservative. Don't rush off and try all of the promising nutritional aids that come along. Advertising hype can be deceptive. Besides, do you really want to be committed to taking these products for the rest of your sporting career? In addition, even if a research study suggests an overall positive effect of a nutritional aid, it is possible that some individuals will be adversely affected. You have to see if a product works for you, but make sure to try it well in advance of any important competition. Keep good records of your training and performance, and see if performance is affected one way or the other when you commence using the product.

Finally, be aware that we humans are experts at self-deception. If we try something new and believe it will work, we'll generally convince ourselves that it has worked by subconsciously putting in some extra effort-but only for a while. Try to find a more rational, cost-effective and lasting means to motivate yourself to train and compete.

Selected Readings:

Aguilaniu, B. Flore, P., Perrault, H., Page, J.E., Payan, E., and J. Lacour (1995) Exercise-induced hypoxemia in master athletes-effects of polyunsaturated fatty-acid diet. Eur. J. Appl. Physiol. 72:44-50,

Coggan, A.R. and B.D. Williams (1995) Metabolic adaptations to endurance training: substrate metabolism during exercise. In: Hargreaves, M. ed. Exercise Metabolism, Champaign, IL: Human Kinetics: 177-210

Coggan, A.R. (1996) Effect of endurance training on glucose metabolism during exercise: stable isotope studies. In: Maughan, R.J.; Shirreffs, S.M. eds. Biochemistry of Exercise IX. Champaign, IL: Human Kinetics: 26-36

Coyle, E.F. (1995) Fat metabolism during exercise. Sports Sci. Exchange 8(6),

Lapachet, R.A.B., Miller, W.C., and D.A. Arnall (1996) Body fat and exercise endurance in trained rats adapted to a high-fat and /or high-carbohydrate diet. J. Appl. Physiol. 80:1173-1179,

Lortet, S. and P. Verger (1995) Alteration of cardiovascular function in trained rats fed with fish oil. Int. J. Sports Med. 16:519-521

Phinney, S.D., Bistrian, B., Evans, W.J., Gervino, E., and G.L. Blackburn (1983) The human metabolic response to chronic ketosis without caloric restriction: preservation of submaximal exercise capability with reduced carbohydrate oxidation. Metabolism 32:769-776

Sherman, W.M. and N. Leenders (1995) Fat loading: the next magic bullet? Int. J. Sport Nutr. 5(supple):S1-S12

Van Zyl, C.G., Lambert, E.V., Hawley, J.A., Noakes, T.D., and S.C. Dennis (1996) Effects of medium-chain triglyceride ingestion on fuel metabolism and cycling performance. J. Appl. Physiol. 80:2217-2225