hdl cholesterol

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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

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