NUTRITION IN SPORT

tary population or with non-competing athletes
(Linde 1987; Fitzgerald 1991; Brenner et al.1994;
Nieman 1994a; Weidner 1994). For example, in
a study on participants in the Los Angeles
marathon who did not have an infection before
the race, the number of runners who became ill
during the week after the race was almost sixfold
higher than that of the control group. The control
group comprised endurance athletes who had
undergone a similar level of training but who did
not participate in the marathon (Nieman et al.
1990). A high incidence of infections has also
been observed in military personnel undergoing
prolonged and repeated intensive training (Lee
1992; Gray et al.1994).
It has been suggested that moderate, regular
exercise helps to reduce the level of infection in
sedentary individuals but that, in individuals
who undertake intensive or excessive training,
the incidence of infection can increase sharply.
An overall view of this situation has been graphi-
cally described by a ‘J-curve’ (Fig. 11.2) which is
emphasized as being descriptive, rather than
quantitative (Nieman 1994a).

risk factors for
upper respiratory tract infection

Weidner (1994) critically evaluated 10 epi-

demiological studies which have investigated

the incidence of URTI in different sports. The

majority of the studies which showed an

increased incidence of URTI after physical activ-

ity have been performed on runners. A longitudi-

nal study on 530 male and female runners

suggested that an URTI was more likely to occur

with higher training mileage (Heath et al.1991).

Similarly, the risk of illness increased in

endurance runners when training exceeded 97

km · week–1(Nieman 1994b). Another study, on

marathon runners, demonstrated that the stress

of competition more than doubled the risk of

getting an URTI (O’Connor et al.1979). A low

body mass may be another risk factor for infec-

tions (Heath et al.1991).

One problem associated with prolonged exer-

cise is that athletes start at some point to breathe

through the mouth rather than through the

nose, thus bypassing the nasal filter mechanism

(Niinimaet al.1980). This dries up bronchial

secretions, thus impeding the protective activity

of the cilia which cover the cell surface with

mucous (Rylander 1968). The high incidence of

infections after prolonged, exhaustive exercise

suggests therefore that immunodepression may

occur in some athletes due to the stress of hard

training and/or competition.

Immune response to exhaustive exercise

There is evidence that numbers of circulating

white blood cells and subsets, together with

cytokine levels, are markedly altered as a result

of prolonged, exhaustive exercise. A substantial

increase in numbers of circulating white blood

cells, mainly due to a large increase in circulating

neutrophils, was first observed by Larrabee

(1902). Despite earlier reports of leucocytosis

and, particularly, an increase in circulating

numbers of neutrophils, relatively little work has

been undertaken on this phenomenon until the

past few years. Recently, however, several publi-

cations have reported not only that the total

number of white blood cells in the circulation are

substantially increased during the recovery

period immediately after a marathon or inten-

amino acids, fatigue and immunodepression 163

Incidence

of infection

Excessive

training

Moderate

regular

exercise

Sedentary

Fig. 11.2The incidence of infection in sedentary
individuals can be decreased with moderate exercise
but increases sharply in individuals who undertake
excessive amounts of exercise, or who suffer from
over-training. From Nieman (1994a), with permission.