56 Food Choice
relationship, namely the influence of what we eat on our mood and
cognitive state. This section reviews the effects of caffeine, carbohydrates,
and chocolate.
Caffeine
Over the past few decades there has been much research into the effects of
caffeine, which is mostly consumed through coffee (about 60 mg per cup
of instant coffee) or tea (about 40 mg per cup), although some is also
consumed through cola drinks (34 mg per 12 oz cup). The early literature
produced highly conflicting results, with some studies showing that caf-
feine increased alertness and decreased calmness (e.g., Clubley et al., 1979;
Leathwood and Pollet, 1983), and some showing no effect (e.g., Svensson,
Persson, and Sjoberg, 1980). Other studies showed that caffeine improved
performance on cognitive tasks (Regina et al., 1974; Lieberman et al., 1987),
while some reported no such improvement (Loke and Meliska, 1984). Such
variation has been attributed to methodological differences including the
choice of mood measurement, the performance task used, and the dose of
caffeine give (Lieberman, 1989). More recent work has addressed these
concerns, and it is now generally accepted that caffeine has a positive effect
on mood by increasing vigor and alertness and decreasing anxiety and depres-
sion, particularly when low doses are used (Lieberman et al., 1990). In
addition, it has been shown that caffeine also improves performance in terms
of reaction times and vigilance (Lieberman et al., 1987).
Carbohydrates
There is some evidence that carbohydrate meals can impair performance
on experimental cognitive tasks and induce sleepiness (Lieberman, Spring,
and Garfield, 1986). The research indicates, however, that this is not con-
sistent for all individuals and that the effect it is often small and hard to
detect. Research does show, however, that those individuals who generally
consume a carbohydrate-rich meal and derive most of their calories from
carbohydrates are more likely to show mood and performance changes
following carbohydrate consumption. For example, Lieberman, Wurtman,
and Chew (1986) gave obese participants who were classified as either high
or low carbohydrate eaters a nonsweet, high-carbohydrate, protein-free lunch,
and completed measures of their mood states before and after this interven-
tion. The results showed that the high-carbohydrate eaters reported a decrease
in depression, and that the low-carbohydrate eaters showed increases in
depression, fatigue, sleepiness, and anxiety, and decreases in alertness.