Beals & Manore 1994) and menstrual dysfunc-
tion (Dueck et al. 1996). Part 4 of this book gives
more specific information about the weight and
nutrition concerns of different sports, particu-
larly weight-category sports.
Energy and nutrient balance
The classic energy balance equation states that if
energy intake equals energy expenditure then
weight is maintained; however, this equation
does not allow for changes in body composition
and energy stores (Ravussin & Swinburn 1993).
The maintenance of body weight and composi-
tion over time requires that energy intake equals
energy expenditure, and that dietary intakes of
protein, carbohydrate, fat and alcohol equal their
oxidation rates (Flatt 1992; Swinburn & Ravussin
1993). If this occurs, an individual is considered
to be in energy balance and body weight and
composition will be maintained.
This approach to energy balance is dynamic
and allows for the effect of changing energy
stores on energy expenditure over time. For
example, after a short period of positive energy
balance, the extra energy would cause weight
gain (of both fat and lean tissues). However,
the larger body size would cause an increase
in energy expenditure that would eventually
balance the extra energy consumed. Thus,
weight gain can be the consequence of an initial
positive energy balance, but can also be a mecha-
nism whereby energy balance is eventually
restored. Conversely, weight loss must reverse
this process. If body fat is to be lost, energy intake
must be less than expenditure, and fat oxidation
must exceed fat intake (Westerterp 1993).
Nutrient balance
The alteration of energy intake and expenditure
is just one part of the energy balance picture.
Changes in the type and amount of nutrients
consumed (i.e. protein, fat, carbohydrate and
alcohol) and the oxidation of these nutrients
within the body must also be considered. Under
normal physiological conditions, carbohydrate,
470 practical issues
protein and alcohol are not easily converted
to body fat (Abbott et al. 1988; Swinburn &
Ravussin 1993). Thus, increases in the intake of
non-fat nutrients stimulate their oxidation rates
proportionally. Conversely, an increase in fat
intake does not immediately stimulate fat oxida-
tion, hence increasing the probability that dietary
fat will be stored as adipose tissue (Abbott et al.
1988; Westerterp 1993). Therefore, the type of
food consumed plays a role in the amount of
energy consumed and expended each day
(Achesonet al. 1984; Swinburn & Ravussin 1993).
Successful weight loss requires that an energy
deficit be produced (either through increased
energy expenditure and/or decreased energy
intake) and diet composition and oxidation be
altered (decreased fat intake and/or increased fat
oxidation through exercise) (Hill et al. 1993).Carbohydrate balance
Carbohydrate balance is proposed to be precisely
regulated (Flatt 1992), with the ingestion of car-
bohydrate stimulating both glycogen storage
and glucose oxidation, and inhibiting fat oxida-
tion (Fig. 35.1). Glucose not stored as glycogen is
thought to be oxidized directly in almost equal
balance to that consumed (Schutz et al. 1989;
Thomaset al. 1992). Thus, the conversion of
excess dietary carbohydrate to triglycerides does
not appear to occur to any large extent in humans
under normal physiological conditions (Acheson
et al. 1988; Hellerstein et al. 1991).Protein balance
Like carbohydrate, the body alters protein oxida-
tion rates to match protein intakes (Thomas et al.
1992). Once anabolic needs are met, the carbon
skeletons of excess amino acids can be used for
energy. The adequacy of both energy and carbo-
hydrate intake dramatically affects this process.
Inadequate energy or carbohydrate intakes will
result in negative protein balance, while excess
intake of energy or carbohydrate will spare
protein (Krempf et al. 1993). Any excess dietary
protein or that made available through protein