Should athletes use fat or carbohydrate as fuel?

The fat ‘fuel tank’ is much bigger than the carbohydrate one but that doesn’t mean it should be filled more often

Many runners, for instance, are talking about the possibility that increased dietary fat might help them run faster, and some have actually boosted their fat intakes in hopes of trimming their 10-K and marathon times.Why all the fuss about fat? For one thing, some very skillful, entrepreneurial types have successfully pitched the idea that endurance athletes’ traditional high-carbohydrate, low-fat diet is actually bad for performance. Their reasoning is that carbos increase pancreatic production of a key hormone called insulin, which inhibits fat metabolism, while fattier eating ‘teaches’ muscle cells to use more fat during exercise.

Coaxing muscles to burn more fat is an attractive idea, since even the very skinniest runner has enough fat to fuel over 100 miles of running, while few runners have enough carbohydrate to run more than 20 miles or so. To put it simply, the fat ‘fuel tank’ is much bigger than the carbohydrate one. Increased fat-burning is thought to be good in another way: it helps conserve whatever carbohydrate is present in the muscles – a good idea since very low carbo concentrations are associated with extreme fatigue, and since some carbo needs to be around in order for fat itself to be used for fuel (that’s the old, still-honoured principle that fat ‘must burn in a carbohydrate flame’). At least in theory, then, the athlete who eats more fat has greater fuel stores to draw on and is less likely to ‘bonk’ during long workouts and competitions.

Some athletes with an evolutionary perspective also clamour for higher-fat eating, claiming that our ancestors were carnivorous types with a lust for meat and fat. Human digestion and metabolism, they say, is thus better adapted to high-fat and high-protein foods, rather than leafy and root vegetables.

Of course, such arguments ignore a few realities. First of all, insulin is usually elevated for no more than a few hours after a meal. Since most athletes don’t exercise right after eating, insulin’s suppression of fat metabolism becomes academic. In addition, a high-fat diet, by crowding carbos off the plate, can sometimes make it difficult for athletes to cram enough glycogen into their leg muscles. Thus, even though fattier diets might conserve carbohydrate, they could also decrease the total amount of carbohydrate available for exercise (a bit like starting a long trip with only a half-tank of petrol), which would not be optimal (to be fair, though, even the most dedicated fat disciples often call for a few days of carbo-loading just before a critical workout or competition).

What our ancestors actually ate

The evolutionary argument for fat also tends to fall apart when scrutinised closely. While it’s true that Paleolithic human ‘hunter-gatherers’ consumed wild game, the animals used for food were undoubtedly quite lean, probably averaging no more than 4 to 5 per cent body fat (they didn’t have an opportunity to stand in pens all day eating as much as they wanted, in contrast to our present-day domesticated livestock).

Furthermore, the wild plant foods selected by our ancestors typically contained less fat, more protein, more fibre, and more actual nutrients per unit weight than those plant edibles prominently displayed on the shelves of your local grocery store. As a result, it’s likely that no more than 22 per cent of the calories in the typical Paleolithic diet came from fat, and only 5 percent of calories or so originated in saturated fat (‘Paleolithic Nutrition Revisited: A Twelve-Year Retrospective on its Nature and Implications,’ European Journal of Clinical Nutrition, vol. 51, pp. 207-216, 1997).

Those who are in favour of fat do like to point out that some very successful athletes – like the Russian female cross-country skiers who recently won the World Championships – follow high-fat diets (however, the failure of a Russian gold medallist – Lyubov Egorova – to pass a subsequent drug test revealed that fat was perhaps not the only thing the Russian women were taking on board; Egorova, by the way, is a six-time Olympic champion). In the scientific realm, a higher-fat diet has been correlated with higher testosterone levels in male athletes, an effect which might enhance muscle mass and improve recoverytime between tough workouts, and several studies have linked increased fat intakes with better endurance.

The argument for fat

Is fat really a better fuel for endurance performance? Well, its fans are particularly fond of mentioning a study carried out at MIT several years ago. In that investigation, five elite endurance cyclists followed a high-carbohydrate diet for one week (60 per cent of calories came from carbohydrate, 30 per cent from fat, and 10 per cent from protein), and then shifted over to a diet which supplied the same number of daily calories but contained much more fat. In fact, on this four-week ‘ketogenic’ diet, fat supplied about 85 per cent of total calories (key sources of fat included mayonnaise, heavy cream, sour cream, and cream cheese), while carbs chipped in just 2 per cent (that’s not a misprint; the athletes swallowed no more than 20 grams – 80 calories – of carbohydrate per day, less than the quantity in one average banana).

VO2max and endurance capacity were assessed before and after the athletes embarked on the high-fat diets. The endurance test was a simple one: the athletes attempted to cycle for as long as possible at an intensity of 62 to 64% VO2max, or about 75 per cent of maximal heart rate (‘The Human Metabolic Response to Chronic Ketosis without Caloric Restriction: Preservation of Submaximal Exercise Capability with Reduced Carbohydrate Oxidation,’ Metabolism, vol. 32 (8), pp. 769-776, 1983).

Fat fanatics sometimes contend that endurance capacity increased with the higher-fat diet, and it’s true that the high-fat pedallers lasted 151 minutes during the endurance test, compared to 147 minutes when carbo-loaded. However, this small disparity was not statistically significant. Four weeks of fat gobbling also had no significant impact on VO2max, which held steady at about 69 ml/kg.min. So, the truth is that high-fat diets had little effect on aerobic capacity or performance in this study.

The only major difference between the high-fat and moderate-fat situations was that the athletes metabolised much more fat – and much less carbohydrate – during exercise when they were on the high-fat diets. In fact, fat degradation during exercise doubled, while utilisation of blood glucose for energy declined by 67 per cent and muscle-glycogen breakdown plummeted by 78 per cent after the four weeks of high-fat fare. As we mentioned earlier, that’s a potential positive, since the increased conservation of blood glucose might decrease mental fatigue, and the preservation of precious and rather limited muscle glycogen could expand endurance. ‘Potential’ is the key word, however, since the fatty feasting didn’t actually improve athletic capacity. And remember that when you’re on a high-fat diet, it can be more difficult to stockpile much carbohydrate in your body in the first place. High-fat diets conserve carbohydrate, which is good, but lead to little carbohydrate storage, which can be very bad.

And while you might interpret this MIT study to mean that you can perform just as well with fat as you can with carbos, bear in mind that the exercise intensity chosen for the endurance test – 62 to 64 percent of VO2max – was a very low one, far below the intensities most runners utilise during even their most slowly paced races – the half-marathon and marathon. A key point is that muscles can ‘get by’ on fat metabolism at such low intensities, while higher intensities increase their thirst for carbohydrate and downgrade their reliance on fat. Thus, if the MIT researchers had asked their athletes to blast along at more realistic competitive intensities (say at 85 to 90% VO2max), a high fat-burning capacity would not have been very useful, and the carbo-fed athletes might have fared better.

Does fat really increase your risk of heart problems?

Many athletes completely reject the idea that higher-fat eating might be a good idea, thinking that even if fat turned out to be better for performance, they would still avoid the stuff because of worries about cardiovascular disease. However, a recent 12-week study completed at the State University of New York at Buffalo found that when a group of 12 male and 13 female runners who ran about 35 miles per week boosted fat intake from 16 to 30 per cent of total calories for four weeks, their blood-fat profiles improved (‘Effect of a High or a Low-Fat Diet on Cardiovascular Risk Factors in Male and Female Runners,’ Medicine and Science in Sports and Exercise, vol. 29(1), pp. 17-25, 1997). When the runners increased their fat intake to 30 per cent of calories, total cholesterol remained constant, while HDL-cholesterol (aka ‘good cholesterol’) increased, as did a key chemical called Apolipoprotein A1, which is thought to be protective against cardiovascular upsets.

In this study, six male and six female runners then advanced fat intake to 42 per cent of total calories for four more weeks – and increased HDL-cholesterol even further, even though the number of grams of fat in their daily diet had tripled, compared to the beginning of the study. This is the research which prompted many newspapers and magazines to trumpet the headline ‘High-Fat Diet Heart-Safe for Runners.’
But the Buffalo study has been criticised somewhat, for a couple of reasons. One frequently heard charge is that the results are simply a fluke – a chance occurrence in a fairly small group of runners. We’ll deal with that criticism in a moment.

Another charge is that the Buffalo results might only apply to serious athletes who log at least 35 miles of road pounding per week. Runners scooting along for just 15 to 20 weekly miles or so might not get the same boost in HDL if they increased their fat intakes, for example. In fact, they might just get fat – or raise their LDL-cholesterol (aka ‘bad cholesterol).

This second point is not really a criticism of the Buffalo work, however, since the researchers never claimed that everyone who laced up running shoes could eat a lot of fat and improve their blood-fat profiles. A more serious quibble is that the Buffalo runners might have been training more intensely and getting fitter over the course of the study as they increased their fat ingestion (increased fitness does tend to turn HDL up a few notches). The runners did start with a 16-per cent fat diet for four weeks, shifted to 30-per cent fat for four more weeks, and then changed to 42-per cent fat for the final four weeks of the study. HDL rose from period to period, making it look as though increased fat were promoting the climb, but if fitness were also rising over the same time frame, the correlation between fat and HDL could be spurious (the researchers didn’t publish the runners’ actual VO2max values). It would have been useful if the researchers had started some people on high fat and then gradually reduced fat and increased carbohydrate over the course of the study. If more fat is really linked to better HDL, then HDL should have fallen with this scenario.

This New Zealand study agrees

However, we can’t just make the seemingly puzzling Buffalo study go away, especially since recent research carried out at the University of Otago in New Zealand has also linked high fat intakes with good blood-fat set-ups. In the New Zealand work, 34 experienced endurance cyclists were divided into two groups. One group followed a high-carbohydrate diet (60 per cent of total energy from carbos, 20 per cent from fat, and 20 per cent from protein), while the second collection of cyclists pledged allegiance to a high-fat diet (45 per cent of calories from fat, 40 per cent from carbos, and 15 per cent from protein). One of the great things about this study was that the diets were followed for a relatively long time – three months (‘The Effects of High-Carbohydrate Versus High-Fat Dietary Advice on Plasma Lipids, Lipoproteins, Apolipoproteins, and Performance in Endurance Trained Cyclists,’ Nutrition, Metabolism, and Cardiovascular Disease, June, 1996).

HDL changed nary a tick in this New Zealand study, but both groups – high-fat and high-carbohydrate – decreased total cholesterol, reduced LDL-cholesterol, and increased Apolipoprotein A1 over the course of 12 weeks. In other words, high-fat and high-carbohydrate diets both led to equivalent, positive changes in blood-fat profiles. Amazingly enough, most of the fat consumed by the Otago cyclists was in the form of saturated fat – the kind believed to be most damaging to blood-fat profiles!

What’s going on? Although we’ve been told for more than a decade that higher fat intakes may increase the risk of heart problems and that high-carbo eating is ‘heart smart’, could the truth be quite different? Well, consider this important investigation carried out by Paul Thompson, M.D., president-elect of the august American College of Sports Medicine and a noted cardiovascular researcher. Back in the early 1980s, Thompson and colleagues at Brown University studied 24 fit runners (VO2max = 56). Initially, the runners were following a fairly standard diet, with 53 per cent of calories from carbohydrate, 32 per cent from fat, and 15 per cent from protein. However, some of the runners embarked on a 14-day high-carbohydrate diet, with 69 per cent of calories from carbs, while the others started up a 14-day high-fat diet, with 69 per cent of calories from fat (‘The Effects of High-Carbohydrate and High-Fat Diets on the Serum Lipid and Lipoprotein Concentrations of Endurance Athletes,’ Metabolism, vol. 33(11), pp. 1003-1010, 1984).

And what happened? Well, the ‘heart-smart’, high-carb diet produced a number of negatives, including a 9-per cent drop in HDL, a significant increase in LDL, and a 30- to 50-per cent rise in blood triglycerides. Meanwhile, the heart-stupid, high-fat diet produced a number of positives, including a tendency for both HDL and Apolipoprotein A1 to increase and a 10- to 20-per cent drop-off in serum triglycerides!
Hello! This study has been largely ignored in the rush toward high-carbohydrate diets, but it provides support for the Buffalo research mentioned earlier and is itself supported by a study carried out at the National Institute of Health in Bethesda, Maryland. In that investigation, when individuals who had been eating a 40-per cent carbohydrate diet were placed on an 80-per cent carbo one, the rate of HDL synthesis remained the same, but the rate of HDL destruction increased by almost 40 per cent, leading to an overall drop in serum HDL (‘High Density Lipoprotein Metabolism in Man,’ The Journal of Clinical Investigation, vol. 60, pp. 795-807, 1977).

And we could actually drone on for quite awhile about studies linking more fat in the diet with better blood fats in athletes. For example, in a study carried out in Sweden, individuals who ingested a high-fat (52-per cent fat) diet during a rugged, eight-day, cross-country ski expedition in the mountains lowered LDL and boosted HDL by 19 per cent, while skiers on low-fat diets (21- to 26-per cent fat) failed to nudge HDL upward (‘Influence of Dietary Fat, Cholesterol and Energy on Serum Lipids at Vigorous Physical Exercise,’ Scandinavian Journal of Clinical and Laboratory Investigations, vol. 51, pp. 437-442, 1991).

In yet another study, when individuals engaged in strenuous physical training (walking 37 kilometres per day) while consuming a high-carbohydrate diet (85 per cent of calories from carbs), VLDL (another ‘bad’ cholesterol) increased and HDL decreased. However, when the same subjects utilised a high-fat diet (75 per cent of calories from fat), VLDL decreased while HDL went up (‘The Acute Effect of Prolonged Walking and Dietary Changes on Plasma Lipoprotein Concentrations and High-Density Lipoprotein Subfractions,’ Metabolism, vol. 37 (6), pp. 535-541, 1988).

Should we turn the train around?

And on and on. If you’re an endurance athlete, there’s lots of evidence to support the idea that if you beef up your fat intake, your blood-fat profile will be consistent with a lower risk of cardiovascular disease. And there’s evidence, too, that if you step up the carbohydrate content of your diet, your blood-fat profile will actually look worse, even though carbos are the macronutrients which are recommended for heart-disease prevention. So, I repeat, what is going on?

‘Sometimes the train is going in the wrong direction, and we need to step off and turn it around,’ says Paul Thompson. ‘It may be that the way we’ve been thinking about diet and blood fats in athletes is just plain wrong.’

As Thompson points out, we shouldn’t be surprised that increased carbohydrate intakes can lead to higher LDLs. Once extra carbohydrate is absorbed, the liver has to do something with it, and often the liver will simply convert the carbos into LDLs.

To understand these studies, we also need to think about what HDL actually is. Thompson calls HDL ‘a marker of fat tolerance’. In other words, if your HDL is high, that usually means you can handle dietary fat pretty well; if you eat a load of fat, it gets metabolised pretty quickly, instead of being tucked into an artery wall or added on to your existing fat stores. ‘We’ve actually done studies in which fat was infused into individuals intravenously, and the people with high HDL were almost always the ones who could clear the fat out of their blood pretty quickly,’ says Thompson.

Lots of people think that HDL is a great predictor of heart disease (and of course the popular press reinforces this idea), but the link between HDL and cardiovascular maladies can be tenuous. For example, the Tarahumara Indians of Mexico, who are noted for their prodigious feats of athleticism (they may run hundreds of miles in just a few days to visit relatives in a distant village), have average HDL levels of 27 milligrams per decilitre. Many cardiovascular-disease experts believe that HDLs below 35 are strongly predictive of heart disease, but the Tarahumaras have almost no heart disease at all!
The reason the Tarahumaras have such low HDLs is that their diets are so low in fat. Existing on chillies, cactus, and squash, they never have to process much fat internally, so HDL levels stay low. If they increased their fat intake, their HDLs would probably come right up, but that wouldn’t mean that their heart-attack risk would be any lower. It’s already at near-zero.

How are dietary fat and HDL related?

Once the fat from your food is absorbed, it tends to move into and around your bloodstream as large spherical structures called VLDLs (very low density lipoproteins). These VLDLs bob through your bloodstream like beach balls in a fast-moving stream. At the core of each of these VLDL spheres is fat (triglyceride), while the outer shell of the sphere is composed of protein and cholesterol.

Now, when this balloon-shaped structure passes through a capillary (a tiny blood vessel) near a muscle cell, a critically important enzyme located in the wall of the blood vessel called lipoprotein lipase can break down the fatty interior of the sphere, allowing fatty acids to diffuse into the muscle to be used as fuel. Meanwhile, the cholesterol on the outer surface of the balloon is used to form HDL-cholesterol (‘good cholesterol’).

Runners and other endurance athletes tend to end up with high HDL readings, because their training increases their concentrations of lipoprotein lipase. That’s a key adaptation to training, and the increased quantity of lipoprotein lipase of course allows the muscles to use more fat for fuel during exercise, conserving glycogen. However, a sedentary person has not taught his muscles to burn fat at such high rates, and therefore will have less lipoprotein lipase and HDL. Worse yet, the fat they’ve eaten will tend to stay in their blood, where it can be deposited into the walls of the coronary arteries, increase the risk of ischaemic heart disease, or onto the insides of the renal arteries, heightening the risk of high blood pressure.Overall, high HDL is a sign that you’re good at metabolising fats, while low HDL may mean that you’re poor at it. As Thompson says, ‘If your HDL is low and you’re eating a high-fat diet, you’ve got troubles’.

Remember that endurance activity increases one’s concentrations of lipoprotein lipase, because training tends to increase the muscles’ demands for fat. More lipoprotein lipase ordinarily leads to higher HDL, which is why endurance athletes tend to have HDL levels that are 40- to 50-per cent higher than those of sedentary people. If you take an endurance athlete who is on a moderate-fat diet and give him/her more fat, HDL tends to go up because the athlete is ‘clearing’ more fat from the blood and metabolising more in the muscles. Whether the athlete is really at lower risk for heart disease is debatable; the HDL is up because more fat is being taken on board. If you give the endurance athlete less fat and more carbohydrate, then HDL may go down, because there’s just not enough fat to charge up the VLDL-lipoprotein lipase-HDL system. However, it’s not clear that this means the athlete is now at greater risk of heart trouble.

‘The link between HDL and fat processing means that it’s a little crazy to take HDL measurements in the fasted state, which is what is almost always done,’ says Thompson. ‘When we check for the possibility of diabetes, we don’t measure blood sugar in the fasted state; we challenge the patient with a dose of glucose. Similarly, when we check a person for fat tolerance, we shouldn’t keep him away from food – we should challenge him with a cheeseburger, fries, and onion rings to see how fast he can get that fat out of his blood. If the fat lingers for a long time, then the person could be in trouble.’ Most endurance athletes wouldn’t be in much trouble, because they should have enough lipoprotein lipase from their training to break the fat down – and a lot of HDL to look good on their blood tests. While some might believe that the HDL is protecting the athlete, what’s actually protective is the muscles’ appetite for fat and the high concentrations of lipoprotein lipase. The HDL is just a by-product of the fat-clearing system.

One caveat, though, is that lipoprotein lipase and HDL production are probably subject to genetic control. If you’re an endurance athlete, you can’t just automatically assume that you can feast on fat without difficulties; you should get your HDL checked to see if you’re a good ‘fat tolerator’.

However, as mentioned, having a low HDL doesn’t automatically mean you have an impending encounter with the grim reaper. According to Thompson, ‘You can’t just say, ‘Oh, your HDL is low, you’re at risk’. After all, look at the Tarahumaras. You might simply be on a very high-carbohydrate – and therefore low-HDL-forming – diet. And raising HDL is not a guarantee that your heart-attack risk is lower; you might just be clearing more fat out of your system. The truth is that we’re not so sure about the effect of diet on cardiovascular-disease risk in endurance athletes.’

Back to the original question..

But before you interpret all this to mean that you can slop a big spoonful of gravy onto your mashed potatoes or drop a high-fat ‘sports bar’ into your gym bag, let’s get back to the performance question. Is there any evidence that higher-fat eating can actually help you run, cycle, or swim faster?
Well, we should mention that there are studies linking increased fat with improved endurance, but many of these investigations have been flawed. They have either glycogen-depleted their subjects prior to testing, thus creating a very unrealistic situation (who burns up all their glycogen just before competing?), or else have used very low, non-competitive exercise intensities, as in the MIT study (remember that fat tends to do very well as long as exercise speeds are kept slow).

So, we’ll start with the studies in which fat did well, turn to those in which carbos ruled, and then compare the conflicting evidence. First of all, in the high-quality University of Otago study mentioned above (the one in which experienced competitive cyclists followed either a high-fat or high-carbohydrate diet for three months), the high-fat athletes performed just as well as the high-carbohydrate riders after 12 weeks. Both groups improved time to exhaustion during rigorous exercise by about 10 per cent over the three-month period, and both groups had no change in VO2max over the study period. So, this study suggests that endurance cyclists can choose either high-fat or high-carbohydrate diets without suffering any negative performance consequences. That’s not a ringing endorsement of high-fat (or high-carbo) diets, but it does suggest that athletes have some choice about what to eat.

In a study carried out by one of the big guns of exercise science – Tim Noakes, M. D. – and his colleagues at the University of Cape Town Medical School, five experienced cyclists followed a high-fat diet (70-per cent fat and just 7-per cent carbohydrate) for two weeks and – on a separate occasion – a high-carbohydrate diet (74-per cent carbohydrate, 12-per cent fat). After each of these dietary regimes, they engaged in a variety of exercise tests ranging from moderate to peak intensities (‘Enhanced Endurance in Trained Cyclists during Moderate-Intensity Exercise following 2 Weeks Adaptation to a High-Fat Diet,’ European Journal of Applied Physiology, vol. 69, pp. 287-293, 1994).

Performance of high-intensity exercise was similar on the two diets, but the cyclists were able to exercise significantly longer at a medium intensity (60-per cent VO2max) after the high-fat diet. In fact, exercise time at the medium intensity nearly doubled in the high-fat cyclists, from 43 to 80 minutes. However, this study has very little application for serious athletes, because the medium-intensity rides were completed after the cyclists had already glycogen-depleted their leg muscles. Who would ever be stupid enough to enter a competition in a glycogen-bereft state?

For the same reason, we can ignore the oft-cited research carried out at the State University of New York at Buffalo in which higher-fat diets were linked with up to 40-per cent improvements in endurance in trained runners (‘Effect of Dietary Fat on Metabolic Adjustments to Maximal VO2 and Endurance in Runners,’ Medicine and Science in Sports and Exercise, vol. 26, pp. 81-88, 1994). As in Noakes’ work with the cyclists, these runners were nearly glycogen-naked before they began their tests of endurance, so it was no surprise to learn that the runners who had fed their muscles a steady diet of fat fared better during the trial exertions.

A much-cooler piece of research showing some possible benefits for fat was carried out by Noakes and his colleagues using a somewhat different protocol. In this second study, cyclists again followed high-fat or high-carbo regimes for two weeks, but this time they followed each two-week banquet with three days of ‘carbo-loading,’ taking in a ton of carbohydrate. Just to jazz things up a bit, the cyclists also consumed a fat-containing sports beverage (it was actually a 4.5-per cent medium-chain triglyceride drink) before and during their exertions, which consisted of 150 minutes at 70 per cent VO2max, followed by a 20-K time trial as fast as they could pedal (‘Effects of a Low-Carbohydrate High-Fat Diet Prior to ‘Carbohydrate Loading’ on Endurance Cycling Performance,’ Clinical Science, vol. 87, Supplement, pp. 32-33, 1994).

The key finding was that the high-fat diet plus carbo-loading improved the 20-K time by about 4.5 per cent, from 30.9 to 29.5 minutes, compared to the high-carb diet plus carbo-loading scheme. The reason for this was that the former diet was linked with a lower use of glycogen during the 150-minute, pre-20-K exertion, leaving more glycogen available for the 20-K test. In other words, the high-fat diet had taught leg muscles to be a little more glycogen-conserving (it’s also possible that the high-fat scheme had led to greater glycogen storage during the three-day carbo loading).

This study can still be criticised pretty strenuously, since again the athletes were basically glycogen-depleted when they started the 20-K trial, and the 70-per cent VO2max attained during the 150-minute build-up is not really a competitive intensity unless the race is an ultra-endurance event. However, the whole idea of stoking in fat for a while and then shifting to carbs for three days or so before a big event is still an intriguing one. In fact, research carried out with rodents strongly supports the scheme (the little fellows almost always store more glycogen and do better on endurance tests when they first follow a high-fat, rather than high-carbohydrate, dietary regime, and then fuel up on carbs).

It’s okay for ultras

At any rate, it’s reasonable to say that if you are getting ready for an ultra-endurance event (50K or more of running), a couple of weeks on a high-fat diet, followed by three days or so of carbo-loading just before competition, may be better nutritional preparation than carbo-loading straight-through. The reasoning here is that the fat feasting will increase your muscles’ abilities to oxidise fat, giving you a greater pool of available energy during your prolonged race, while the last three days of carbo-loading will super-concentrate your muscle-glycogen levels, permitting perhaps greater intensity during your race and increased resistance to fatigue. The only difficulty is that when you first switch to higher-fat eating, it can be hard to sustain your usual training loads – because your muscles are used to glycogen for fuel and your muscle glycogen levels are now depressed because of the shift to fat. That being true, it might be better to gradually build up your fat intake, instead of plunging precipitously into a hot bath of fat ingestion a couple of weeks before your ultrathon.

It’s still far too early to say that a similar strategy would be optimal for the marathon, however (especially if you run the race in three hours or less). Although the marathon is long, it’s still short enough so that race intensity is rather high – and carbohydrate is presumably your muscles’ preferred fuel. If your marathon takes longer than four hours, on the other hand, and your intensity is modest (a low percentage of your VO2max), the high-fat-followed-by-carbs protocol would seem to have a better chance of working. There’s very little evidence that it would be good for a 5K or 10K, though.

To summarise, there’s no convincing evidence to suggest that a high-fat diet is better than a high-carbohydrate diet prior to running events ranging in distance all the way up to the marathon. The studies which do show a fat ‘edge’ are just too poorly done. However, we do have Charlotte Cox’s University of Otago work showing that high fat is just as good a dietary strategy as high carbohydrate for competitive cyclists over a three-month period. This three-month period may be key, because as you’ll see in a moment, the studies giving carbohydrate an edge have usually been conducted over pretty short time periods.

The case for carbo

What evidence supports carbohydrate? Well, in a recent study carried out at Loughborough University in England, 18 runners (12 men and six women) were first asked to run a 30-K time trial. During the following week, nine of the runners increased their carbohydrate consumption, while the other nine stoked in more protein and fat (total calories per day were the same in the two groups). When a second 30-K trial was completed at the end of the week, eight of the nine ‘carbo-fuelled’ runners improved their times, while none of the fatty competitors got better. Average improvement in the carbo group was a rather useful 2.6 minutes, a 2-per cent improvement. This study was a good one, because – unlike the fat-dominated studies – it was carried out at a realistic, competitive intensity with athletes who had not depleted their muscles of energy prior to testing (‘The Effect of a High-Carbohydrate Diet on Running Performance during a 30-Km Treadmill Time Trial,’ European Journal of Applied Physiology, vol. 65, pp 18-24, 1992).

And that upswing in performance was no surprise to Swedish scientists at the Karolinska Institute in Stockholm, who had previously shown that increased carbohydrates could bolster 30-K performances by as much as 8 per cent (‘Diet, Muscle Glycogen, and Endurance,’ Journal of Applied Physiology, vol. 31, pp. 203-206, 1971). Close analysis of the two studies revealed that carbo stockpiling was especially beneficial in races or tough workouts longer than about 20K on flat terrain – or just 10 to 11K on hilly running routes.

These studies have been criticised for failing to give athletes enough time to truly adapt to high-fat diets, but a more recent investigation carried out at the University of Copenhagen found that high-carbohydrate diets produced better performances than high-fat diets, even when the experimental period was extended to seven weeks. In that work, 10 men followed a high-carbohydrate diet (65-per cent carbs) for seven weeks, while 10 others relied on high-fat fare (62-per cent fat). Both groups trained three to four times a week for 60 to 75 minutes at up to 85 per cent VO2max (‘Interaction of Training and Diet on Metabolism and Endurance during Exercise in Man,’ Journal of Physiology, vol. 492(1), pp. 293-296, 1996).

VO2max increased by 11 percent in both groups after seven weeks, but time to exhaustion on a strenuous exercise trial (at 81 per cent VO2max) was considerably higher in the high-carbohydrate group (102 versus 65 minutes). The researchers weren’t sure exactly why the carb group did better (muscle glycogen was not depleted in either group at exhaustion), but they reasonably concluded that ‘ingesting a fat-rich diet during an endurance training programme is detrimental to improvement in performance’.

However, this Copenhagen study was conducted with initially untrained individuals, so one could argue that the results might be far different with more experienced athletes with greater experience at (and more metabolic adaptations to) ‘fat burning’. The bottom line is that almost all of the studies which show carbohydrate beating fat in experienced athletes have been carried out over pretty short time periods – and thus may not have given competitors a chance to truly adapt to high-fat diets. Longer-term studies are needed. And we shouldn’t fail to mention that some studies have observed no advantage at all for higher carbs. For example, in a fairly well-known study, carbo-loading failed to improved the half-marathon times of experienced runners at all (‘Effect of Exercise-Diet Manipulation on Muscle Glycogen and Its Subsequent Utilisation during Performance,’ International Journal of Sports Medicine, vol. 2, pp. 114-118, 1981).

What the Kenyans do

How about looking at what real-live athletes are actually doing? Well, the best, most consistent endurance athletes in the world, the Kenyan runners, follow a high-carbohydrate, low-fat diet year-round. They are extremely scrupulous about taking in copious amounts of carbohydrate and moderate quantities of protein, while avoiding fatty foods. However, the sceptic can point out that no one has ever really given a group of Kenyans a trial run on higher-fat eating to see what would happen. Maybe they would run even faster (now there’s a scary thought!).

Here are the key points to take away from this article:

If your HDL is low (below 35 or so) and you’re on a high-fat diet, you’ve got troubles from a health standpoint. You should probably shift over to lower-fat fare.

Contrary to what you’ve been told, if you regularly engage in endurance exercise eating more fat is not necessarily bad for your blood-fat profile; in fact, it might even perk it up a bit by raising HDL and Apolipoprotein A1, although there’s no guarantee that such ‘perking’ would actually decrease your risk of heart disease. However, most endurance athletes who engage in regular, strenuous training do not need to fear fat, nor should they attempt to eradicate it from their diets.

Since fat is such an energy-rich substance, higher-fat diets can be useful for the athlete who is having trouble taking in enough energy during the day to support his/her training load. In female athletes troubled by loss of menstruation (and thus increased risk of stress fractures), higher-fat diets may mean a return to normal caloric intakes, normal menstruation, and better bone health. Fat is necessary to absorb the key vitamins D, E, A, and K, and dietary fat also helps maintain normal levels of intramuscular fat, which is burned at moderate rates by your leg muscles when you exercise, thus helping to conserve precious glycogen.

It’s a mistake to think that fat makes you fat. What makes you fat is eating excess calories, and it matters not at all whether the calories come from fat, carbos, or protein. So, don’t try to throw away all fat thinking that it will increase your chances of becoming a lean, mean, fighting machine. You become that lean machine by exercising regularly and watching your total caloric intake.

If you do decide to step up your fat intake, monitor your blood-fat status closely with your doctor as time goes by (and please report to us about what happens). If you decide to increase your carbos, please do the same, since we now know that high-carb eating sometimes makes those profiles look worse.

Fat is abundant in the body and can serve as a nearly endless source of energy during low-intensity exercise, but the key problem is that the maximal-possible rate of energy production from fat can be quite modest, compared to the rate associated with carbohydrate. That’s why you can literally walk for days using fat as your primary fuel, but as soon as you change over to fairly fast running, your leg muscles shun fat and begin to treat carbohydrate like a long-lost relative. That suggests that most endurance athletes should worry about getting enough carbohydrate – not fat – into their muscles, but fat proponents contend that fat can do better with high intensities if muscles are given enough time to adapt. The three-month Otago research supports this latter idea, but more evidence is needed.

The ‘best’ diet for performance is now hotly debated, with carbohydrate feasting still holding an edge. However, there’s enough evidence to suggest that higher-fat diets may be superior in certain situations. For example, ultra-endurance athletes (runners who compete at 50K or more) probably need to maximise their ‘lipolytic aerobic power’ (their ability to utilise fat as a primary source of energy) and may be better off with increased fat intakes. In addition, the idea of gradually boosting fat intake for a couple of weeks, and then pouring in the carbos for a few days before a major competition with a duration of 25 to 45K, is still an intriguing one which needs to be researched further. There’s no evidence at all to support the idea that high fat is advantageous for shorter running events, such as 5-K and 10-K races. In fact, if you’re primarily a 5- and 10-K runner, it’s very difficult to imagine why eating more fat would make you faster.

The following guidelines will work well for most endurance athletes:

  1. Eat enough total calories to sustain your daily energy needs (including routine activities and your training).
  2. To make sure your muscles don’t run low on glycogen, take in three to four grams of carbo- hydrate per pound of body weight per day (shoot for the latter number if you train fairly strenuously for an hour or more each day).
  3. Ingest enough protein to repair and maintain your body tissues, plus adequate amounts of vitamins and minerals.
  4. TIME your intakes appropriately, making sure you ingest rich lodes of carbohydrate and protein during the critical two-hour ‘window’ following your workouts, when your muscles are most willing to take nutrients on board.
  5. If your HDL level is okay, don’t be fearful of fat; the stuff can contribute to the pleasure of eating and in certain cases may help you get enough calories for recovery from tough training.

If you do all of these things, you will ensure that your body will get the most from your training and that you will have the best chance of reaching your performance goals.

Owen Anderson