NUTRITION IN SPORT

1 Mechanical overtraining.
2 Metabolic overtraining or overreaching.
3 Overtraining syndrome or staleness.

Mechanical overtraining

Mechanical overload involves the locomotor
system. An imbalance between exercise and
recovery is usually local and is generally
expressed as an overuse injury. Although little
information is available about the role of nutri-
tion in these injuries, there is some indication
that a low calcium intake increases the risk
for stress injuries to the skeleton. Another type
of mechanical overtraining is exercise-induced
muscle damage. Muscle damage is associated
with inflammatory changes, which are followed
by regeneration. There is some evidence that a
deficit in vitamin E intake may increase the sus-
ceptibility to this type of mechanical damage.
However, athletes who consume a normal mixed
diet are unlikely to have a vitamin E deficiency.
Therefore, supplementation of vitamin E in these
athletes does not provide any protection against
exercise-induced muscle soreness.

Metabolic overtraining or overreaching

Nowadays athletic training includes a high
volume of intensive exercise. Intensive exercise
relies on carbohydrate supply, resulting in a

rapid depletion of glycogen stores. When high-

intensity exercise is done in association with low

glycogen levels, this may lead to an imbalance

between the rates of adenosine triphosphate

(ATP) splitting and ATP generation. This in

turn will lead to an accumulation of adenosine

diphosphate (ADP). In order to restore the

ADP/ATP ratio, 2 ADP form 1 ATP and 1 adeno-

sine monophosphate (AMP), which is further

broken down to inosine monophosphate and

eventually to uric acid (Fig. 37.2), while ammonia

is also formed (Sahlin & Katz 1993). When insuf-

ficient time for recovery is allowed, this may lead

to a decline of the energy-rich phosphate pool.

The metabolic type of overtraining is probably

associated with overreaching.

Data suggest that insufficient carbohydrate

intake may enhance the susceptibility for devel-

oping overreaching. Therefore, adequate carbo-

hydrate intake and quick restoration of glycogen

stores may decrease the risk of developing meta-

bolic overtraining. Studies have shown that a

high carbohydrate intake, starting immediately

after exercise, may restore glycogen stores within

24 h. However, although insufficient carbohy-

drate intake may increase the susceptibility for

metabolic overtraining, a high intake of carbohy-

drate may decrease the risk, but cannot prevent

metabolic overtraining. Therefore, in addition to

proper nutrition, adequate rest and recovery are

of paramount importance.

overtraining: nutritional intervention 493

Overreaching

Training load

Overtraining syndrome

Optimal training

Undertraining

Performance

Fig. 37.1The relationship between training volume
and increase in performance capacity. Courtesy of
C. Foster.

2 ADP 1 ATP +1 AMP

NH 3

IMP

Uric acid

Fig. 37.2Metabolic pathway indicating the

breakdown of ADP to uric acid, under the formation of

ammonia. IMP, inosine monophosphate.