At 50% of maximal aerobic capacity, about 45–55% of calories come from fat; at 75% of max this drops to about 10–30%; and at max (100%), none of our calories come from fat.

[In reference to glycogen stores] On a moderate-carbohydrate diet (40–50% of total intake) an athlete will have about 1,000 Calories available for lower body exercise, and on a high-carbohydrate diet (60–70% of total intake) those stores can double to about 2,000 Calories.

As a general rule of thumb, for activities lasting more than 2 hours, if you eat at least half the calories you burn each hour, you’ll almost always be consuming an adequate number of calories to keep you going.

Efficiency describes the relationship between how many calories we burn and how many of those calories actually get converted to real work. As an example, if you are 22 percent efficient and you burn 100 Calories, only 22 Calories actually go into the pedals to make the bicycle move. The other 78 Calories get wasted as heat.

Our two primary forms of energy storage are the fat stored in adipose tissue and the carbohydrate stored as glycogen in muscles and the liver. Even very lean athletes have plenty of fat stores, so fat is not thought of as a rate-limiting fuel source.

Glycogen is the limiting factor; when it’s depleted it can severely limit our exercise capacity. This is because our nervous system and brain rely solely on glucose—the most basic unit of carbohydrate in the body. When glycogen stores are low and exercise intensity is high, we risk becoming hypoglycemic or having low blood sugar, which can bring us to a standstill. In addition, we need a little bit of carbohydrate to keep some of the metabolic pathways responsible for fat utilization functioning.

The amount of glycogen we have stored in our bodies can vary greatly and is affected by a number of factors, particularly how much carbohydrate we eat, how active we are, and our extent of lean muscle mass. As a simple rule of thumb, about 1 or 2 percent of the weight in lean muscle is glycogen.

While it may be simple to think that the total glycogen stored in the body can be calculated by simply multiplying the figures above by one’s body weight or lean muscle mass, calculating the available glycogen for a given activity is a little more complex. This is because a particular muscle can only access the glycogen inside of it, not the glycogen in other muscles. Your quads, for example, can’t tap into the glycogen stored in your biceps.

For a person who weighs 70 kg (154 lb.) and has 10 percent body fat, about 630 grams of glycogen are stored in muscle (1 percent of 63 kg of lean mass). During lower-leg exercise like cycling or running, however, only about 30–35 percent of the total muscle mass is being used, so the actual glycogen available for cycling in this example is closer to 221 grams (at 35 percent). At 4 Calories per gram of carbohydrate, the total stored carbohydrate calories available for lower-leg exercise is about 882 Calories. In addition to glycogen stored in muscle, about 80 to 120 grams of glycogen can be stored in the liver, which can account for an additional 320 to 480 Calories available to help maintain blood sugar and to fuel any muscle.

If you’ve ever reached a point during long-term exercise where you begin to smell ammonia in your sweat or urine, then you’ve actually pushed yourself to a point where you’re breaking down protein and using it as a fuel source.

In the same way that we can live without food for weeks and survive only days without water, performance depends more on hydration than it does on food.

After a lot of trial and error, I learned that a combination of sucrose and glucose worked best for maximizing gastric emptying and intestinal absorption. Sucrose is a simple sugar made of glucose and fructose. As other researchers have found, because glucose and fructose each have their own transporter in the small intestine, using these two sugars together optimizes the transport of fuel into the body. I also learned that the drink was optimal at no more than a 4 percent carbohydrate concentration (4 grams per 100 ml or 80 Calories per half-liter), which agreed with existing literature on exercise hydration and gastric emptying rates. Next, I settled on 300 mg of sodium per half-liter serving—almost triple the sodium found in most sports drinks.

So we started using sodium citrate in the drink mix, which eliminated the problem. Citrate is essentially citric acid or lime juice with the acid removed, leaving a negative ion that pairs easily with sodium. It’s easy on the stomach, is a strong acid buffer, and has a good taste, so we used citrate for all of our electrolytes, which also included potassium, calcium, and magnesium at concentrations similar to sweat.

The GI tract is not technically the inside of our body—it’s basically just a hollow tube going through us that is exposed on both ends to the outside world.

The length of time it takes for a bolus or liquid to pass through the stomach into the small intestine is referred to as the gastric emptying rate (GER). Because digested contents need to be emptied from the stomach before any absorption can take place, the gastric emptying rate is the first rate-limiting step in fueling and hydration.

Ultimately, of all the factors that affect the gastric emptying rate, the three most important are all related to hydration. A low water volume entering the stomach, high caloric density, and a body that is dehydrated will all slow gastric emptying and hamper the speed at which water and nutrients can be absorbed by the body.

When the osmolality of fluid reaching the small intestine is significantly greater than the osmolality of blood, the water inside the body will initially flow across the membrane into the gut, which can temporarily cause bloating and gastrointestinal distress. This means you’re effectively dehydrating before rehydrating.

In some cases, the osmolality of fluid reaching a person’s small intestine may be so great and so sudden that an excessive amount of water pours across the intestinal membrane back into the gastrointestinal tract, causing a literal flood that flushes all of the nutrients and electrolytes down a path that leads back toward the light, also known as diarrhea.

To avoid this unpleasant reality, make sure that whatever is entering your small intestine has an osmolality that is hypotonic, or less than 260 mOsm per kg. This will allow water to move from the inside of the small intestine back into the body since the osmolality of blood is 260–290 mOsm per kg. You’d probably still be okay if the fluid entering your small intestine is isotonic, or the same as blood, since active transport mechanisms could more easily establish a favorable osmotic gradient for water movement into the body. However, if you are drinking fluids with an osmolality much greater than 290 mOsm per kg, making it hypertonic, make sure you have your diapers on because you might be playing a game of intestinal Russian roulette.

As simple as this sounds, pacing yourself, staying cool, and not becoming dehydrated are three of the most important things you can do to make sure your gut doesn’t fall apart on you when you’re exercising.

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