Carbohydrates, in the form of glycogen (a complex sugar), are the primary fuel source for muscles during short-term, high-intensity activities.  Repetitive, vigorous activity can use up most of the carbohydrate stores in the exercised muscles.


Carbohydrates are the primary fuel source for muscles during short-term, high-intensity activities.


The body uses fat to help provide energy for extended activities such as a one-hour run.  Initially, the chief fuel burned is carbohydrates, but as the duration increases, the contribution from fat gradually increases.


The intensity of the exercise also influences whether fats or carbohydrates are used to provide energy.  Very intense activities use more carbohydrates.  Examples include weight training and intensive sit-up and push-up workouts.


Eating foods rich in carbohydrates helps maintain adequate muscle-glycogen reserves while sparing amino acids (critical building-blocks needed for building proteins).  At least 50 percent of the calories in the diet should come from carbohydrates.  Individual caloric requirements vary, depending on body size, sex, age, and training mission.  Foods rich in complex carbohydrates (for example, pasta, rice, whole wheat bread, potatoes) are the best sources of energy for active individuals.


Because foods eaten one to three days before an activity provide part of the fuel for that activity, it is important to eat foods every day that are rich in complex carbohydrates.  It is also important to avoid simple sugars, such as candy, up to 60 minutes before exercising, because they can lead to low blood sugar levels during exercise.


Exercisers often fail to drink enough water, especially when training in the heat.  Water is an essential nutrient that is critical to optimal physical performance.  It plays an important role in maintaining normal body temperature.  The evaporation of sweat helps cool the body during exercise.  As a result, water lost through sweating must be replaced or poor performance, and possibly injury, can result.  Sweat consists primarily of water with small quantities of minerals like sodium.  Cool, plain water is the best drink to use to replace the fluid lost as sweat.  One should drink water before, during, and after exercise to prevent dehydration and help enhance performance.  The table below summarizes recommendations for fluid intake when exercising.


Recommendations for Fluid Intake

  • Drink cool (40 degrees F) water.  This is the best drink to sustain performance.  Fluid also comes from juice, soup, milk, and other beverages.
  • Do not drink coffee, tea, and soft drinks even though they provide fluids.  The caffeine in them acts as a diuretic that can increase urine production and fluid loss.  Avoid alcohol for the same reason.
  • Drink large quantities (20 oz.) of water one or two hours before exercise to promote hyperhydration.  This allows time for adequate hydration and urination.
  • Drink three to six ounces of fluid every 15 to 30 minutes during exercise.
  • Replace fluid sweat losses by monitoring pre- and post-exercise body weights.  Drink two cups of fluid for every pound of weight lost.


Sports drinks, which are usually simple carbohydrates (sugars) and electrolytes dissolved in water, are helpful under certain circumstances.  There is evidence that solutions containing up to 10 percent carbohydrate will enter the blood fast enough to deliver additional glucose to the active muscles.  This can improve endurance.


During prolonged periods of exercise (1.5+ hours) at intensities over 50 percent of heart rate reserve, one can benefit from periodically drinking sports drinks with a concentration of 5 to 10 percent carbohydrate.  During intense training, these beverages can provide a source of carbohydrate for working muscles.  On the other hand, drinks that exceed levels of 10 percent carbohydrate, as do regular soda pops and most fruit juices, can lead to abdominal cramps, nausea, and diarrhea.  Therefore, these drinks should be used with caution during intense endurance training and other similar activities.


Many people believe that body builders need large quantities of protein to promote better muscle growth.  The primary functions of protein are to build and repair body tissue and to form enzymes.  Protein is believed to contribute little, if any, to the total energy requirement of heavy-resistance exercises.  The recommended dietary allowance of protein for adults is 0.8 grams per kilogram of body weight.  Most people meet this level when about 15 percent of their daily caloric intake comes from protein.  During periods of intense aerobic training, one's need for protein might be somewhat higher (for example, 1.0 to 1.5 grams per kilogram of body weight per day).  Weight lifters, who have a high proportion of lean body mass, can easily meet their protein requirement with a well-balanced diet which has 15 to 20 percent of its calories provided by protein.  Recent research suggests that weight trainers may need no more protein per kilogram of body weight than average, nonathletic people.  Most Americans routinely consume these levels of protein, or more.  The body converts protein consumed in excess of caloric needs to fat and stores it in the body.  For information on the protein content for over 6000 food items, visit High Protein Foods


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