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By Steve Born

Endurance and ultra-endurance athletes require all three forms of fuel that the human body uses for energy: carbohydrate, protein, and fat. A major factor for optimal performance is using the right fuel, at the right time, in the right amount. Like every aspect of success in endurance events, proper nutrition requires planning, practice, and training to reap the benefits on race day. This article will give you the background information you need about fueling.

As all athletes know, carbs are king when it comes to fueling the body for any endurance exercise. That does not mean, however, that any carbohydrate at any time will keep you going. Carbohydrates can either help or hinder performance, depending on what kind you use, how much you use, and when you use them. Far too many misinformed athletes continue to use energy products loaded with simple sugars, or they use complex carbs, a superior choice, but at the wrong time and in the wrong amounts. These practices will actually impair, not help, your performance.

Simple sugars, maltodextrin, and osmolality

Most dietary sugars are simple molecules known as monosaccharides and disaccharides. The shorter the chain length of a carbohydrate source, the higher it will raise a chemical measure known as osmolality when dissolved. In solution, simple sugars can only attain about 6-8% concentration or they will sit undigested in your stomach, as the osmolality will be incompatible with the digestive juices. Products containing simple sugars, typically sucrose, fructose, and/or glucose (dextrose), must be extremely dilute to match body fluid osmolality (280-303 mOsm). This weak of a concentration presents a problem to athletes because it cannot provide sufficient calories (perhaps only 100 cal/hour, at the most) to working muscles. To obtain enough calories from a weak 6-8% solution, an athlete would have to consume two or more bottles of fuel per hour, which means excess fluids, increasing the risk of fluid intoxication. Using simple sugar-based "energy drinks" is not a wise strategy.

"Well then," you might say, "I'll just mix a stronger concentration." But this approach also fails. Making a double or triple strength mixture from a simple sugar-based carbohydrate fuel won't work because the concentration of that mixture will exceed 6-8%, far too concentrated to match body fluid osmolality. It will remain in the stomach until sufficiently diluted, which may cause substantial stomach distress. Drinking more water to dilute your over-concentrated concoction puts you back in the original condition of increased risk of overhydration and all the problems that causes, so that's not a good option. But if you don't drink more, your body will draw fluids and electrolytes from other areas that critically need these fluids and electrolytes (like blood and muscle) and divert them to the digestive system to lower the osmolality of your over-concentrated simple sugar drink. This also will result in a variety of stomach distresses, not to mention increased cramping potential and other performance-trashing issues.

The same problem occurs when an athlete combines a simple sugar fuel with a complex carbohydrate fuel. Consumed together or within close proximity of each other, simple sugars and complex carbohydrates increase the solution concentration beyond the efficient digestion level for either component. This will compromise energy production and promote the likelihood of a variety of stomach issues. In the words of Dr. Bill Misner, Adding simple sugar fractions (any of the carbohydrates ending in ose) to complex carbohydrate fractions (maltodextrins) may double the osmolar pressure of the solution to hypertonic values. When a 6-8% simple sugar solution is added to a 15-18% complex carbohydrate solution, the osmolality of the combined solutions is simply not absorbable in the human gut. Molecules that contain many sugar units chained together are called polysaccharides, known familiarly as complex carbs and starches. One of these, maltodextrin, can make up to an 18% solution concentration and still match digestive system osmolality. This allows very efficient passage from the digestive tract to the liver, which converts some of the maltodextrin to glycogen for storage and some directly to glucose for immediate use by the muscles. With polysaccharides you get much more energy from stomach to liver, thus providing maximal amounts of energy to be produced in a form that your body can efficiently process.

Based on caloric delivery alone, complex carbohydrates such as maltodextrin are far superior to simple carbohydrates (simple sugars). But that's not all. Simple sugars, even in small amounts, can incite a condition known as insulin spike. This sudden recruitment of insulin causes a subsequent dramatic drop in blood sugar, which can take blood sugar levels even below the fasting level! This flash and crash type of energy typically results in the dreaded bonk, something every athlete wants to avoid. However, complex carbs, which enter the bloodstream at a 15-18% solution, do not promote this wild fluctuation in blood sugar levels. Even though a maltodextrin might have a high GI (see next page) and rapidly elevate blood sugar levels (a desirable effect), during exercise your body processes them with far less insulin fluctuation, most likely due to the steady release and breakdown of glucose from its polymeric source, and other hormonal factors. You never get the below-baseline drop in bloodsugars).

Some athletic nutritionists disregard osmolality, but we do not believe its importance can be overstated. As Dr. Misner states, when osmolality goes above 303 or below 280 mOsm, the gut must pull minerals and fluids . . . to mediate a narrow 280-303 mOsm range for immediate calorie absorption. Both simple sugars and complex carbohydrate maltodextrins are absorbed at equal rates if the solution concentration matches body fluid osmolality (280-303 mOsm). Simple sugars meet this criterion only when they are mixed in calorically weak 6-8% concentrations; digestion slows down or ceases at higher concentrations. When athletes make a double or triple strength simple sugar-based drink, trying to increase caloric input, they usually develop problems such as gastric distress, bloating, flatulence, vomiting, and muscle cramps.

On the other hand, the maltodextrins (complex carbohydrates) used in Hammer Nutrition fuels match body fluid osmolality even when mixed in concentrations as high as 15-18%. This presents a distinct advantage because your body is able to digest, and thus convert to energy, a greater volume of calories from complex carbohydrates than it can from simple sugars.

Glycemic Index

People often ask about the glycemic index (GI) of various carbohydrates and how those figures relate to fueling for endurance exercise. Here's the scoop: GI rates the speed at which the body breaks down a carbohydrate into glucose. The lower the GI, the slower the process, and therefore the more stable the energy release. For food eaten at times other than during exercise and recovery, GI is an important dietary factor and we recommend eating foods with a low-to-middle GI rating.

However, during and immediately following exercise, a high-GI carbohydrate, one that elevates blood sugar levels rapidly, is desirable, as long as you keep caloric intake within approximately 280 cal/hour, as hormones associated with sympathetic nervous system activity will inhibit GI impact on insulin release. Negative diet/ health-specific effects associated with consumption of high-GI carbohydrates are not a concern during and immediately after exercise; high-GI carbs actually perform better than low-GI carbs at these times.

Long-chain, high-GI maltodextrins have a GI value of about 130, compared to glucose (100) or sucrose (62). This means that maltodextrins raise blood insulin more effectively than simple sugars, but without the rapid and precipitous drop that is a common (and deleterious) effect of simple sugars. Also, as mentioned earlier, maltodextrins allow you to absorb a greater volume of calories than you can from simple sugars.

Don't complex carbs take longer to utilize?

Technically, this is true; all carbohydrates will eventually be broken down to glucose. However, the first fuel (sugar) the body will use when exercise commences is muscle-stored glycogen, which is a long-chain (complex) carbohydrate that, as Dr. Misner puts it: . . . is a form of starch which contains eight parts amylopectin to two parts a-amylose. Thus, wouldn't it make sense to say that if the body's first-used fuel is muscle glycogen and that its makeup is complex in nature, the body obviously is very efficient in breaking it down for rapid conversion to energy?

This particular athlete goes on to say, As the race progresses, your ability to cleave it [maltodextrin] into the absorbable form of carbohydrate (glucose) gets slower and slower. But maltodextrin is patient. It will sit in your stomach and wait for quite a while for something to come along and break it into glucose. This, my friend, is what causes that very undesirable bloating and eventual feeling like you want to hurl.

We could not disagree more. Our unflinching belief is that the time it takes from gut to muscle isn't nearly as long as some experts think it is, if there is any difference to begin with. And even if maltodextrin took slightly longer in breaking down in the gut as compared to glucose,and the difference, if any, would be fractional, the earlier-mentioned benefits of using complex carbohydrates only versus simple sugars (such as glucose) or combinations of carbohydrates (which we'll discuss shortly) more than justifies the use of complex carbohydrates.

Interestingly, the very company this athlete is affiliated with (at least to some degree) states the following on their website: Maltodextrin has a much lower osmolality than glucose and fructose and therefore can be mixed in much higher concentrations without any stomach issues. Molecules of maltodextrin are larger than glucose, so drinks with maltodextrin will have a few large particles compared to a drink with glucose. The number of particles determines how much water it will hold. The more molecules of smaller-sized glucose in the drink, the more water will be pulled into the intestine than the maltodextrin-based drink. Since maltodextrin-based products don't pull as much water into the intestine, it is absorbed faster into the bloodstream.

Bottom line: While the process is, of course, quite detailed, the truth is that the bonds that compose maltodextrin are very weak and readily broken apart in the stomach. As already mentioned a couple of times now (but worth repeating again), maltodextrin allows you to absorb a greater volume of calories for use as energy than you can from simple sugars.

Complex carbohydrates only or a combination of carbohydrate sources: Which is better for the endurance athlete?

Findings from research conducted by the Dutch sport scientist Asker Jeukendrup has caused quite a stir. In fact, a few companies produce fuels that contain the carbohydrate formulations used in the studies. In general, Jeukendrup found that a blend of carbohydrates increased oxidation rates, indicating higher energy production. In one study, cyclists who ingested a 2:1 mixture of maltodextrin to fructose oxidized carbohydrate up to 1.5 grams/minute. Another study used a mixture of glucose, fructose, and sucrose and had rates that peaked at 1.7 g/min. Both those results are pretty eye opening, considering that complex carbohydrates typically oxidize at a rate of about 1.0 g/min.

However, there's more to the results than what first meets the eye. Most of Jeukendrup's subjects cycled at low intensity, only 50-55% maximum power output, which I think we'd all agree is very much a recovery pace, if that.

To be blunt, at a leisurely 50% VO2 Max pace, athletes can digest cheeseburgers and pizza with no gastric issues. However, if the heart rate and core temperature are raised to only 70% VO2 Max, the body must divert core accumulated heat from central to peripheral. This reduces the blood volume available to absorb ingested carbohydrates or whatever the athlete has consumed. After over two decades of experience, we have found that in the overwhelming majority of the athletes we've worked with "athletes engaged in typical 75-85% efforts and/or in multi-hour endurance events" the combination of simple sugars and long chain carbohydrates, and in amounts higher than approximately 1.0-1.1 grams per minute (roughly 4.0-4.6 calories per minute), have not yielded positive results. They did, however, increase performance-inhibiting, stomach-related maladies.

Lowell Greib, MSc ND, explains that gastric emptying is a key limiting step in carbohydrate metabolism: If your stomach can't empty the product (no matter what it is) you are going to get nothing from it except a huge gut ache and possibly lots of vomiting! Unless there is new research that I am unaware of, gastric emptying is directly proportional to the osmolality of the solution in the stomach. Long chain carbohydrate (maltodextrin) contributes less to increasing the osmolality than do disaccharides (sucrose, lactose, maltose, etc.).

The question is not whether or not Jeukendrup's published studies are disputable, but rather if these studies apply to faster paced, longer duration bouts of exercise. We do not believe this to be the case, which is why we do not recommend the use of multiple carbohydrate sources during exercise.

Bottom line: Stick with complex carbohydrate fuels, don't consume simple sugars with or within close proximity of complex carbohydrates, and we guarantee you'll see better results.

Fatty acids for fuel

If we can't replace all of the calories we expend, then how do we keep going hour after hour? The answer is that we have an enormous supply of calories in body fat. The typical athlete can count on a reserve of up to 100,000 calories in the form of stored fatty acids, that's enough, if you could process it all, to fuel a run from Portland, OR to Los Angeles, CA, a distance of almost 1,000 miles! These fatty acids are the fuel of choice when exercise goes beyond about two hours, providing approximately 60-65% of your caloric expenditure. In other words, your body has a vast reservoir of calories available from body fat stores and it will use those liberally to satisfy energy requirements during lengthy workouts and races.

However, for this process to continue without compromise or interruption, you must not consume excess calories. If you try to match energy losses with caloric replacement from your fuel, you will not only cause a variety of stomach-related ailments, you will also inhibit the efficient utilization of fats for fuel. The bottom line is that caloric donation from consumed fuels must cooperate with your internal fat-to-fuel conversion system. Do not attempt to completely replace caloric expenditure. Your best strategy is to replenish calories in amounts that support efficient energy production and do not interfere with the use of fatty acids for fuel.

Protein for fuel

Aside from certain circumstances, which we'll discuss shortly, when exercise goes beyond two hours, you need to incorporate some protein into the fuel mix. After approximately 90-120 minutes, and continuing until you stop your activity, about 5-15% of your caloric utilization comes from protein. This process, called gluconeogenesis, is unavoidable, and if you don't supply the needed protein in your fuel, your body will literally scavenge it from your own muscle tissue. This is called catabolism (muscle breakdown), known informally, but quite accurately, as protein cannibalization. It can cause premature muscle fatigue (due to excess ammonia production from the protein breakdown process) as well as muscle depletion and post-exercise soreness. Protein cannibalization also compromises your immune system, leading to increased risk for colds, flu, and other diseases.

For exercise and competition that extends two hours or more, your primary fuel should incorporate protein in a ratio of about 8:1 (by weight) carbs to protein. Sustained Energy, Perpetuem, and Perpetuem Solids meet this requirement; they are your best choices for fueling during any long-duration exercise.

The benefits of soy protein during endurance exercise

As noted previously, it's good to have a little protein along with your complex carbs to avoid the negative effects of muscle catabolism, but you must have the right kind of protein. The preferred protein for use during prolonged exercise is soy, primarily because its metabolization does not readily produce ammonia. Whey protein, with its usually added amounts of glutamine, makes an excellent post-workout protein, but is not a good choice before or during exercise. You're already producing ammonia during exercise, so consuming glutamine supplements or glutamine-enhanced whey protein will only exacerbate the problem.

There is some confusion regarding the glutamine and ammonia build-up. Yes, glutamine does eventually scavenge ammonia. The key word, however, is eventually. When glutamine metabolizes, it increases ammonia initially, then scavenges more than originally induced, but it takes approximately three hours or so to accomplish this. You're already producing ammonia during endurance exercise, and since ammonia is a primary culprit in premature fatigue, it seems logical that you'd not want to increase ammonia levels even more. However, that's exactly what you'll do when you consume glutamine supplements or glutamine-enhanced whey protein during exercise. That's one reason why soy protein is preferable for use during prolonged exercise.


Soy protein has a couple of other great features, too. First, it is an easily digestible protein. Second, it has an excellent amino acid profile, with a substantial proportion of branched chain amino acids, or BCAAs, which your body readily converts for energy. During exercise, nitrogen is removed from BCAAs and used in the production of another amino acid, alanine, high amounts of which also occur naturally in soy protein. The liver converts alanine into glucose, which the bloodstream transports to the muscles for energy.

BCAAs and glutamic acid, another amino acid found in significant quantities in soy protein, also aid in the replenishing of glutamine within the body without the risk of ammonia production caused by orally ingested glutamine.

Soy's amino acid profile has high amounts of both alanine and histidine, which are the amino acid components of the dipeptide known as carnosine, a nutrient known for its antioxidant and acid buffering benefits. Soy protein also has a high level of aspartic acid, which plays an important role in energy production via the Krebs cycle. Additionally, soy protein has high levels of phenylalanine and tyrosine, both of which may aid in maintaining alertness during extreme ultra distance races.

Lastly, soy produces more uric acid than whey protein. This might not sound good, but uric acid is actually an antioxidant that helps neutralize the excessive free radicals produced during exercise. High uric acid levels, from soy's naturally occurring isoflavones, are another strong reason for preferring soy protein during endurance exercise.

The "gray area" of fueling

As discussed earlier, when exercise goes beyond two hours, we generally recommend that athletes use a carb + protein fuel (Sustained Energy or Perpetuem), either as their sole fuel from beginning to end, or as their primary fuel (roughly 2/3-3/4 of the time). The reason for this recommendation is that once you hit that second hour and beyond, a small percentage (roughly 5-15%) of energy requirements will be fulfilled from protein. If you don't provide some in the fuel mix, at least part of the time, your body has to cannibalize the lean muscle tissue to obtain the amino acids it needs to fulfill that small percentage of its energy requirements.

The last thing you want to do is have your body literally digest its own muscle tissue to make fuel. One reason is the increase in fatigue-causing ammonia; there is no doubt that excess ammonia is a primary culprit, perhaps THE primary culprit, in premature fatigue during endurance events. The other reason is that you'll have broken down a greater volume of muscle tissue, which will prolong recovery time.

Things may (key word may) be a little different come race day. We believe that a race that's in the 2-3 hour range, perhaps just slightly longer, is in a gray area so to speak, which means that you can use either a carb + protein fuel (Sustained Energy or Perpetuem) or a carb only fuel (HEED or Hammer Gel). The selection needs to be based on the following:

  • The type of race that you're doing.
    For example, running is a more impactive and thus a more digestively challenging type of exercise than cycling.
  • The intensity of the effort.
    It's a lot easier to digest calories when the pace is more relaxed, which it usually is during a training session rather than during a race. That's why, in THE TOP 10 *The biggest mistakes endurance athletes make* article, we suggest having a fueling game plan but to write it in pencil, not in ink. What is meant by that saying is that caloric intakes that worked during training may not be appropriate during a race; you may need to consume slightly less in a race than you did during training. Increased anxiety, increased pace, and increased potential for dehydration all contribute to the possibility of a less-than-optimally-functioning digestive system. In addition, at the increased pace during a race, more blood is diverted from digestion and directed toward maintaining muscle performance.
  • The weather and how well or poorly you're acclimated to it.
    The hotter the weather, the more compromised the digestive system becomes. During hot-weather racing, athletes usually find that they need to increase their water and Endurolytes intake while lowering their calorie intake.
  • The terrain
    For example, doing lots of climbing while on the bike or during a run usually diminishes digestive capabilities somewhat.

Our belief is that if the race is going to involve high intensity right from the gun, and/or if the weather is going to be very warm-to-hot, and/or if other factors such as hilly-to-mountainous terrain come into play, deference should be given to the fuel that is the quickest to digest, and that means HEED or Hammer Gel. Yes, some ammonia will be produced during the effort by not providing the body with some protein along with the carbs. However, if the race is in the 2-3 hour range, and perhaps just slightly longer, it will be over long before the issues involved with ammonia accumulation truly become problematic.

To summarize, we recommend a carb + protein drink (Sustained Energy or Perpetuem) when exercise goes beyond two or so hours. However, come race day, when a lot of variables need to be taken into consideration, you have a lot of options to choose from when the race is in the 2-3 hour range . . . you need to go with the fuel that makes the most sense, based on the aforementioned factors/variables. If those factors do come into play, we recommend the use of Hammer Gel or HEED for a high intensity race that's in the 2-3 hour range. If you know you're going to be out there for more than three hours we believe your body is going to perform better if Sustained Energy or Perpetuem is used as the primary-to-sole fuel, with the occasional use of Perpetuem Solids being perfectly acceptable as well.

All this said, this is not meant to be a set in stone rule. Everyone is different so your fuel selection may be different than another athlete's. The earlierlisted information is just a suggestion for you to consider when doing a race that is 2-3 hours in length, the gray area of fueling.

Endurance Amino - Where does it fit?

For these gray area-duration events, a HEED or Hammer Gel (or both), Endurolytes, and Endurance Amino combination is superb. You're supplying your body with high quality calories from two very easily digested fuel sources, you're taking care of electrolyte replenishment in ideal fashion via Endurolytes, and, with Endurance Amino, you're supplying your body with the primary amino acids (the three branched chain amino acids and alanine) that are used in the energy cycle. Plus, the BCAAs in Endurance Amino assist in replenishing depleted glutamine stores while also helping to prevent muscle tissue breakdown, the latter helping to prevent excess fatiguecausing ammonia from being produced and accumulating. In addition, the glutathione component in Endurance Amino provides a number of benefits, primarily powerful antioxidant support.

During a gray area- duration event, you could certainly use Sustained Energy, Perpetuem, and Perpetuem Solids but for events in that 2-3 hour range it may be more feasible to use Hammer Gel or HEED to cover your calorie requirements, augmented by a dose or two of Endurance Amino to cover some of the amino acid requirements. It's certainly worth testing in your training!

Now, in longer races (3+ hours or longer) the amino acids in Endurance Amino enhance the full-spectrum amino acid profile that naturally occurs from the protein component in Sustained Energy, Perpetuem, and Perpetuem Solids. However, with Endurance Amino we're only talking about a few specific amino acids, the three BCAAs, alanine, and glutathione (which is actually a tripeptide), so you're not fully replacing the full-spectrum amino acid profile that occurs in Sustained Energy, Perpetuem, and Perpetuem Solids. For example, by going solely with Endurance Amino, you're not receiving any histidine, aspartic acid, or phenylalanine (among other amino acids), which have some during exercise benefits.

What you are getting with a combination of Endurance Amino and Sustained Energy, Perpetuem, or Perpetuem Solids is more of some of the primary during exercise amino acids, which is not a bad thing at all. In fact, we believe it's highly beneficial because you're providing the body with even greater amounts of some key during exercise amino acids without oversupplying the body with more amounts of amino acids that it may not really require. Plus, with Endurance Amino, you're providing your body with a nice dose of multi-beneficial glutathione.

As you can see, there is a lot of information in this article to digest (no pun intended), but we're convinced that if you follow our recommendations you will no longer have to suffer with a number of performance-inhibiting problems, stomach issues included, that are the result of improper fueling.

When it comes to calorie replenishment, the amounts we recommend do not come anywhere near the replace what you lose figures that far too many so-called experts recommend. However, our recommendations more accurately reflect what your body can comfortably accept from you.

When considering your basic caloric needs, think complex carbohydrates such as the maltodextrin-based products, Hammer Gel and HEED, and, most of the time, a complex carbohydrate + soy protein fuel, Sustained Energy, Perpetuem, or Perpeteum Solids for exercise over two hours.

Please remember that the most important point about our calorie intake recommendations is to customize them to your own personal needs. In your training log, make sure you include fueling data too. We give you pretty close numbers to start with, and you might end up with them also, but we don't offer them as a onesize- fits-all remedy. Besides body weight, your needs will vary with a number of factors such as fitness level, exercise intensity, weather, altitude, type of sport, and innate physiological differences.

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