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Fueling the Body

It is crucial that an active individual meet all of his or her daily caloric intake through a nutritious diet. There is no single food or supplement that will enhance “performance” like the consumption of a balanced meal. Whether training a competitive athlete or those who exercise for health and fitness, it is important to understand the role of the body’s energy substrate nutrients known as carbohydrate (CHO), lipids (fats) and protein.

The Role of Energy Substrate Systems

Commonly known as “sugar,” CHO is the most important fuel for the body. For instance, the central nervous system requires “carbohydrate” at all times for proper functioning. In extreme conditions of starvation or improper nourishment, the body will find ways to produce carbohydrates (sugar) from different sources (i.e., breakdown of proteins/fats) with known detrimental side effects (i.e., formation of Ketone bodies).

Carbohydrate (glucose/glycogen) is the main source of energy both during anaerobic and aerobic activity. High intensity activities lasting anywhere from 10 to 90 seconds will oxidize glucose (anaerobic glycolysis) for the production of energy. Low to moderate intensity activities of longer duration (greater than 90 seconds) will also oxidize glucose in addition to fats for energy production (aerobic glycolysis). 

Among many other roles, lipids are an important source of fuel during physical activity and are a key structural component of many cells. Further, most of the body fat in humans is stored in subcutaneous and deep visceral adipose tissue. A small percentage of body fat is stored in the skeletal muscle cells (approximately 300 grams). At the onset of exercise, there will be a shift in the amount of fat/CHO being oxidized, and hence, its contribution to energy production. The rate of fat and CHO oxidation will depend on several factors including exercise intensity, duration and an individual’s level of fitness.

Proteins are made of building blocks known as “amino acids.” There are 20 different amino acids in proteins and a few non-protein amino acids. There is only a very small increase in protein breakdown during exercise. Thus, the oxidation of the amino acids Leucine and Alanine has been shown during moderate exercise.

Now that we understand the role of the body’s energy substrate systems in relation to physical activity, it is imperative to know “when” and “what” to consume to best optimize performance and recovery.

It is well known that CHOs are the main source of energy for both anaerobic and aerobic exercise. The body needs a minimal amount of circulating blood glucose present for proper energy production during activity (i.e., 100 mg.dl-1). In addition, fat oxidation during activity requires a minimal amount of blood glucose present (i.e., >65mg.dl-1) before fatigue sets in. Thus, the “key” to enhanced performance is to know when and what to fuel the body before, during and after physical activity.

Prior to Activity

Endurance Activity

Endurance athletes should consume a medium to high carbohydrate meal (200 to 350 grams) within three to six hours prior to exercise. Keep in mind that it takes approximately three to four hours for the body to absorb 100 grams of CHO. However, at three hours prior to a training session, CHO intake should be limited to about 1.4g/lb BW (i.e., female client= 128 lbs x 1.4g = 179 grams of CHO). Plus, at one hour or less prior to exercise, CHO intake should be limited to 50 grams and/or 0.5 g/lb BW ( i.e., female client = 128 lbs x 0.5 = 64 grams of CHO). The following table provides some ideas on selecting CHO snacks:

Bagel 1/2 85 15
Bread 1 105 15
Cereal 1/2 >60 10
Graham crackers 2 squares 75 15
Oatmeal 1/2 cup 80 15
Rice (brown) 1/2 cup 115 25
Tortilla 1 85 15
Apple 1 medium 80 20
Banana 1 105 25
Dates (dried) 5 115 30
Orange 1 65 15
Raisins 1/3 cup 150 40
Potato (baked/plain) 1 large 220 50
Sweet potato 1 large 120 30

(Source: Duyff, R. ADA: Complete Food and Nutrition Guide. 2nd ED. 2002)

Resistance Training

Athletes who perform resistance training should consume a combination of a CHO and protein snack (i.e., 50 grams of CHO plus 14 grams of protein) approximately one and a half to two hours before the training session (i.e., two slices of whole grain toast and two tbsp peanut butter; one cup yogurt and quarter cup of raisins; whole wheat pita and half cup of canned tuna; whole grain pita with hummus; energy bar and eight-ounce sports drink).

During Activity 

Endurance Activity

For activities lasting more than 90 minutes, it is suggested that CHO be consumed at a rate of 30 to 60 grams per hour throughout the activity (i.e., one energy bar/hour). Also, CHO can be consumed via sports drinks at a rate of 600 to 1,200 milliliters (20 to 40 ounces) per hour.

Resistance Training

To date, the exact amount of CHO intake during resistance training has not been established. As with endurance activity, it is probably safe to suggest that CHO intake in the form of sports drinks at a rate of 20 to 40 ounces per hour could be beneficial for training sessions lasting longer than 90 minutes.

Fluid Intake Before, During and After Activity 

Post Activity

For endurance and resistance training activities, a combination of CHO and protein plus a drink should be consumed within the first two hours after activity. For instance, 50 grams of CHO and 10 to 15 grams of protein with fluid (i.e., sports drink + energy bar – power bar; whole grain pita with hummus + apple juice; low fat yogurt with walnuts and dried apricots).

Energy Bars and Electrolyte Drinks 

Energy Bars: a) total calories (200 to 300 kcal per bar); b) CHO percentage of total calories (50 to 60 percent); c) fat percentage of total calories (20 to 25 percent); d) protein percentage of total calories (15 to 20 percent). Avoid bars with palm kernel oil or partially-hydrogenated fat (saturated) in the first five ingredients on the label. The protein in the bar should come from a quality source such as casein, whey, soy or egg. All energy bars are best tolerated during exercise if ingested with water. Suggested bars: Clif Bar and Power Bar.

Electrolyte Drinks: a) CHO concentration (four to eight percent or nine to 19 grams of CHO per eight ounces); b) sodium (five to seven percent concentration or 110 to 165 mg per eight ounces). Suggested drinks: Gatorade and Power Aid.  

The Body in Balance 

The key for proper body functioning and/or performance is to maintain proper nutritional balance throughout the day. Nothing will enhance performance like the consumption of a well balanced diet. According to several national organizations including the AHA, NIH, ADA and CI, the recommended dietary ranges for most Americans are:


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