well as amino acids (from the muscle, blood, and
liver). Examples of events that use the endurance
pathway include a 1500-m run, marathon, half-
marathon, and all-day cycling or swimming events.
As oxygen becomes more available to the working
muscle, the body begins to switch from anaerobic
systems to more aerobic ones. Only the aerobic
endurance pathway can produce large amounts of
ATP over extended periods of time via the Krebs
cycle and the electron transport system.- The changeover from anaerobic to aerobic pathways
is not abrupt, nor is there ever a time when one path-
way is used exclusively. The intensity, duration, fre-
quency, type of activity, and fitness level of the
participant determines when the crossover from pri-
marily anaerobic to aerobic pathways occurs. After
2 h of activity, most of the energy is derived from the
endurance pathway (~99%) and only a trace from
the anaerobic system (~1%).
- The more energy used in activity, the more calories
need to be consumed in the diet. Energy expenditure
must balance energy intake. - Individuals training for an athletic event will require
more kilocalories than a sedentary individual. The ref-
erence sedentary man weighs 154 lb and expends
2700 to 3500 kcals a day (average 3025) between the
ages of 20 and 29 years. The reference sedentary
woman weighs 125 lb and expends 1890 to 2000 kcal
a day (average 1957) between the ages of 20 and 29
(Briefel et al, 1995). - The cost of the iron man triathlon (consisting of a 2.4 mi
open ocean swim, 112 mi bike race, and 26.2 mi
marathon) is approximately 4800 kcals for a male ath-
lete. The cost of training alone ranges from 3000 to 6000
kcals a day for a male athlete (Erp-Baart et al, 1989).
CONVERSION OFENERGYSOURCESOVERTIME
- Approximately 50–60% of energy during 1 to 4 h of
continuous exercise at 70% of maximal oxygen
capacity is derived from carbohydrates and the remain-
ing energy is derived from fat (Coyle et al, 1986).- As the intensity of exercise decreases, a greater pro-
portion of energy comes from the oxidation of free
fatty acids (Erp-Baart et al, 1989; Coyle et al, 1997;
Martin, III, 1997).
•Training does not alter the total amount of energy
expended (during activity of the same intensity and
duration) but rather changes the proportion of energy
expended from carbohydrates and fat. As a result of
training, the energy derived from fat increases and the
energy from carbohydrates decreases. A trained indi-
vidual uses a higher percentage of fat than an untrained
person at the same workload (Hurley et al, 1986). - Long-chain fatty acids derived from stored muscle
triglycerides are the preferred fuel for aerobic exercise
for individuals involved in mild- to moderate-intensity
exercise (Nicklas, 1997; Turcotte, 1999).
- As the intensity of exercise decreases, a greater pro-
GENERAL DIETARY NEEDS
OF ACTIVE INDIVIDUALSCALORIES- The dietary guidelines are predicated on consumption
of adequate calories to sustain daily energy expendi-
ture and should be provided on an individual basis
(see Table 14-1). - The average endurance athlete should consume
approximately 55 kcal/kg body weight (Houtkooper,
1992). Energy needs for strength trainers and body-
builders depend on their training schedule and gender.
Energy needs range from 33 to 60 kcal/kg a day for
males and 30 to 44 kcal/kg a day for females (Kleiner
et al, 1990; 1994).
PROTEINS- Protein requirements for both endurance and strength
athletes should be individualized to determine adequacy
of intake. The majority of athletes are consuming ade-
quate amounts of protein.
84 SECTION 1 • GENERAL CONSIDERATIONS IN SPORTS MEDICINE
TABLE 14-1 Estimated Energy Expenditure at Various Levels of Physical Activity
LEVEL OF ENERGY EXPENDITURE
INTENSITY TYPE OF ACTIVITY (KCAL/KG/DAY)
Moderate Walking 3.5 to 4 mph, cycling, Male 41
skiing, tennis, dancing Female 37
Heavy Basketball, climbing, football, soccer Male 50
Female 44
Exceptional Training in professional or Male 58
world-class athletic events Female 51
SOURCE: National Academy of Sciences (1989).