Proper Caloric Intake During Endurance Events

Edition 7, 2005
By Steve Beaver Born

Back in the late 80's and early 90's when I competed in my first two Races Across America (RAAM) there wasn't much in the way of information… no, let me correct that, we competitors didn't have ANY seemingly reliable information regarding how to fuel the body properly. I can remember being told something to the effect of "with the amount of calories you'll be burning, you should eat as much as you can to avoid bonking or running out of fuel." And that's what I did. My support crew kept making me eat and eat and eat even when I was already full. It seemed to me that the trick was to eat as much as possible but not get sick and you'd have the best results. I remember being bloated for a good portion of the race and how my stomach just swelled to the point where I thought it would drape over the top tube. Sure, there were times when I just couldn't eat anymore and I demanded that my support crew stop feeding me. However, for the most part, we didn't know any better so we operated under the belief that said to eat as much as possible. Sometimes I wonder how I ever finished those two races! Oh, if I could go back and do those races again knowing what I know now.

Even if you never do anything remotely resembling RAAM, this article is an extremely important one. In it you'll read about what constitutes the proper fuel and how much you should be consuming. The answers may surprise you but I'll tell you truthfully, once I adopted and applied these fueling guidelines, my performances improved each and every year. You put so much effort into your training and equipment; make sure you put the right fuel, in the right amount, in your body. Your body will thank you and your performance will be proof.

Endurance and ultra-endurance athletes require all three forms of fuel 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 to know about fueling, and conclude with some recommendations about what and how much to use.

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. For example, far too many misinformed athletes continue to use energy products loaded with simple sugars, or they use complex carbs, a better choice, but at the wrong time and in the wrong amounts. These practices 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 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, glucose, and/or dextrose) must be extremely dilute to match body fluid osmolality. This weak of a concentration presents a problem to athletes because it cannot provide sufficient calories (perhaps only 100/hour) to working muscles. To obtain enough calories from a weak 6–8% solution, an athlete would have to consume excess fluid, which definitely increases the risk of fluid intoxication. So 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. As mentioned earlier in the "10 Biggest Mistakes" article, 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 over-hydration 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 deal with your over-concentrated simple sugar drink. This too will result in a variety of stomach-related distresses, not to mention increased cramping potential and other performance-trashing issues. The simple fact is that using simple sugar-based products is simply futile! Endurance athletes who try to fulfill calorie/energy requirements with sugar-based drinks and powder mixes usually end up with a variety of complaints and poor performances.

Molecules that contain many sugar units chained together are called polysaccharides, known familiarly as complex carbs and starches. One of these, maltodextrin, can make 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, and in a form you body can efficiently process.

Therefore, the "gold standard" carbohydrate source for energy drinks, bars, and gels is polysaccharides such as maltodextrin, as you get far more energy calories across the gastric lining—up to three times more than what a simple sugar carbohydrate source can provide—with no stomach distress.

Based on caloric delivery alone, complex carbohydrates such as maltodextrin are far superior to simple carbohydrates, such as sugars like glucose, sucrose, and fructose. 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 results in your familiar bonk. 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 below), during exercise your body processes them with far less insulin fluctuation, probably 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 blood glucose that simple sugars cause.

Simple Sugars = Ineffective Fuel

The bottom line is that simple sugars are a very inefficient fuel source. Using them to fuel your body is like trying to heat your house by burning newspapers in your stove. You get a fast heat, but it burns out quickly, and you have to continually feed the fire. Not good! Complex carbohydrates, on the other hand, are similar to putting a nice big log on the fire in that they burn longer and more evenly, with the declination in "heat" (energy levels) being much more gradual. That is why no Hammer Nutrition fuel (Hammer Gel, HEED, Sustained Energy, or Perpetuem) ever contains any added 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 general health purposes. However, the numeric differences between high GI sugars and high GI maltodextrins (complex carbohydrates) are negligible. They are both high and raise insulin levels to balance and transport excess blood sugar. The advantage of long-chain, high-GI complex carbohydrates during exercise or immediately afterwards is their higher caloric value and easy absorption in the body. This means that you can absorb a greater volume of calories for energy production or glycogen replenishment.

GI is an important dietary factor, but too much can be made of it. GI acts in concert with many other factors. For food eaten at times other than exercise and recovery, you should take it into account. After exercise begins, however, sympathetic nervous system hormones inhibit GI impact on insulin release. Unless your caloric intake exceeds the maximum that the liver can return to the body in the form of glycogen (about 280 calories per hour), glycemic index is not a major factor in choosing energy sources. During exercise, it's a nutritional error to value GI above saccharide profile.

How Much To Consume

Now that you know what kind of carbohydrate to use, the next question is, "How much?"

With some allowances provided for very large athletes the human body can only return (from the liver to muscle tissue) about 4.1–4.6 carbohydrate calories per minute, or about 250–280 calories per hour. When an athlete consumes more than 280 calories per hour from carbohydrates during an event, the excess remains undigested in the stomach, or passes unused into the bowel, where, in the unmincing words of Dr. Bill Misner, "they accumulate in gastric or intestinal channels in 100-degree temperatures and putrefy in time."

Yes, you may be burning up to 800 calories per hour, but your body cannot replace that amount during exercise. Trying to replenish calories at the same rate as depletion only causes problems. Instead of having more energy available, you'll have a bloated stomach, and perhaps even nausea and vomiting. You've seen it happen, but it's not a necessary aspect of intense competition; more likely it's the result of improper caloric intake.

Fatty Acids For Fuel

If we can't replace all the calories we expend, then how do we keep going hour after hour? The answer is the practically limitless 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, Oregon to Los Angeles, California—a distance of almost 1000 miles! These fatty acids are the fuel of choice when exercise goes beyond about two hours, providing about 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. As mentioned in "The 10 Biggest Mistakes" article, 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. For what that means in real life training and racing, see the chart at the end of this article.

Protein For Fuel - Gluconeogenesis

When exercise goes beyond 90 minutes, you need to incorporate some protein into the fuel mix. After about 90 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 about two hours or more, your primary fuel should incorporate protein in a ratio of about 8:1 (by weight) carbs to protein. Both Sustained Energy and Perpetuem meet this requirement; they are your best choices for fueling any endurance activity.

The Benefits Of Soy Protein During Endurance Exercise

As noted above, 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 high glutamine content, makes an excellent post-workout protein, but is not a good choice before or during exercise.

Soy protein has a couple 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 quantity in soy protein, also aid in the replenishing of glutamine within the body without the risk of ammonia production caused by orally ingested glutamine.

Also, 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.

Suggested Amounts To Consider

If you've read this far, you might be thinking, "Enough with the biochemistry lessons! Just tell me how much to take!" Now is where we sum up all the above into hard numbers. Please remember, however, the most important point about these figures 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, too, but we don't offer them as a one-size fits all remedy. Your needs will vary with a number of factors besides body weight, such as fitness level, exercise intensity, weather, altitude, type of sport, and innate physiological differences.

When considering your basic caloric needs, think complex carbohydrates such as a maltodextrin-based product, and supplemental protein for exercise over 90 minutes. To give you a practical application of these numbers, we've "translated" the data into servings of Hammer Gel, and scoops of HEED, Sustained Energy, and Perpetuem. No matter what your sport or length of exercise, these fuels that give your body exactly what it needs to operate at maximum efficiency.

Suggested Amounts by Body Weight*


  • Up to 120 pounds: 2 servings/hour
  • 120–155 pounds: 2.5 servings/hour
  • 155–190 pounds: 3 servings/hour
  • 190+ pounds: 3 to 3.5 servings/hour


  • Up to 120 pounds: 1 to 1.5 scoops/hour
  • 120–155 pounds: 2 to 2.5 scoops/hour
  • 155–190 pounds: 2.5 to 2.75 scoops/hour
  • 190+ pounds: 3 scoops/hour


  • Up to 120 pounds: 1.5 scoops/hour
  • 120–155 pounds: 1.75 to 2 scoops/hour
  • 155–190 pounds: 2.25 to 2.5 scoops/hour
  • 190+ pounds: 2.5 to 3 scoops/hour


  • Up to 120 pounds: 1 scoop/hour
  • 120–155 pounds: 1 to 1.5 scoops/hour
  • 155–190 pounds: 2 scoops/hour
  • 190+ pounds: 2.25 to 2.5 scoops/hour

*These are estimated doses. Each athlete should determine in training, under a variety of conditions, their personal optimum.

Steve Born is a technical advisor for Hammer Nutrition with over a decade of involvement in the health food industry. He has worked with hundreds of athletes—ranging from the recreational athlete to world-class professional athlete—regarding their supplement/fueling program. Steve is a three-time RAAM finisher, the 1994 Furnace Creek 508 Champion, 1999 runner-up, the only cyclist in history to complete a Double Furnace Creek 508, and is the holder of two Ultra Marathon Cycling records. In February 2004 Steve was inducted into the Ultra Marathon Cycling Hall of Fame.

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