Experimental design
This experiment used a crossover design with repeated measures
within subjects. On one day a week for five weeks, participants
consumed a mid-day meal consisting of a main course of
pasta accompanied by a large portion of low-energy-dense salad.
Across the meals, the consumption of the salad was varied in
two ways: whether it was served before the main course or together
with the main course, and whether it was consumed in full
(fixed intake) or consumed as desired (ad libitum intake). Specifically,
at two meals the salad was served 20 min before the main
course; one time subjects were instructed to consume the salad
in full and once to consume it as desired. At two other meals the
salad was served along with the main course; again, one was consumed
in full and one was consumed as desired. At the remaining
meal, no salad was served. The pasta was eaten ad libitum at all
meals. The order of presenting the experimental conditions was
counterbalanced across participants.
Subject recruitment and characteristics
Participants for the study were recruited by advertisements in a
local newspaper and by notices sent to electronic mailing lists of
university staff and students. Individuals who responded to the notices
were interviewed by telephone to determine whether they
met the following inclusion criteria: were women between the
ages of 20 and 45 years, had a reported body mass index between
18 and 40 kg/m2, regularly ate three meals per day, and liked the
foods to be served in the test meal. Potential participants were excluded
from participation if they were dieting to gain or to lose
weight, had food allergies or restrictions, were taking medications
known to affect appetite, were smokers, were athletes in training,
or were pregnant or breastfeeding.
Potential participants who met the initial criteria came to the
laboratory for screening. At this time they completed several questionnaires,
including the Zung Self-Rating Scale (Zung, 1986),
which assesses symptoms of depression; the short form of the
Eating Attitudes Test (Garner, Olsted, Bohr, & Garfinkel, 1982),
which evaluates disordered attitudes toward food; and the
Three-factor Eating Questionnaire (TFEQ; Stunkard & Messick,
1985), which assesses dietary restraint, disinhibition, and tendency
toward hunger. Potential participants also rated the taste of six
food samples, including the two foods to be served at the test meal.
The taste of each food was rated on a 100-mm visual analog scale
with a left anchor of ‘‘Not at all pleasant’’ and a right anchor of
‘‘Extremely pleasant’’. The height and weight of potential participants
(without shoes and coats) was measured by trained personnel
using an electronic scale and stadiometer (Seca Corp., Hanover,
MD, USA). Individuals were only enrolled in the study if they
scored 640 on the Zung Self-Rating Scale, scored 620 on the Eating
Attitudes Test, had a measured body mass index between 18 and
40 kg/m2, and rated the taste of both study foods P30 mm.
Individuals gave signed informed consent to participate in the
study. The consent form stated that the purpose of the research
was to investigate perceptions of salad. The research was approved
by the Office for Research Protections of The Pennsylvania State
University, and individuals were financially compensated for their
participation.
The sample size to be enrolled in the study was based on data
from previous single-meal studies conducted among women. A
power analysis was performed using an approximation technique
based on the non-central F-distribution and an exemplary data set
analyzed with a linear mixed model (Littell, Milliken, Stroup,
Wolfinger, & Schabenberger, 2006). The minimum clinically significant
difference in meal energy intake for either experimental
manipulation (timing of serving the salad or fixed versus ad libitum
consumption) was assumed to be 40 kcal [167 kJ], which represents
about 5–10% of typical lunch intakes for women in previous laboratory
studies. It was estimated that a sample of 43 women would
allow the detection of this difference in meal energy intake with
>80% power using a 2-sided test with a significance level of 0.05.
Fifty women were enrolled in the study. During the study, three
women were excluded: two were unable to consume the fixed
amount of salad within the allotted time, and one failed to return
after the third meal. The data of one additional participant was
identified as having undue influence on the outcomes according
to the procedure of Littell et al. (2006). After the first two meals
this participant had extremely low lunch intakes (<170 kcal
[710 kJ]), and her data were excluded from analysis. Thus, 46
women were included in the study; 36 participants (78%) were
normal weight, 8 (17%) were overweight, and 2 (4%) were obese.
Other participant characteristics are shown in Table 1.
Composition of test meals
The composition of the foods served at the test meal was not
varied across experimental conditions. The salad was large
(300 g) and low in energy (100 kcal [418 kJ]) and energy density
(0.33 kcal/g [1.4 kJ/g]). The ingredients of the salad were iceberg
and romaine lettuce, carrots, celery, cucumber, tomatoes, shredded
light cheese (Saputo Cheese USA Inc., Lincolnshire, IL, USA), and
fat-free Italian dressing (T. Marzetti Company, Columbus, OH,
USA). The main course served at all test meals consisted of 700 g
of cheese tortellini (Villa Frizzoni, Houston, TX, USA) with tomato
sauce (Campbell Soup Company, Camden, NJ, USA); the energy
density of the main course was 1.5 kcal/g [6.3 kJ/g].
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