ence Analysis Manual published by the Academy of Nutrition
and Dietetics (14). A flowchart of the included studies is
presented in Figure 1.
Quality assessment
The quality of the included studies was assessed by one researcher
(MNH) on the basis of 5 quality categories according to
criteria assessment. The first category evaluated whether the
selection of study participants was free from bias. There were 4
sub questions under the first category related to the specification
of inclusion and exclusion criteria, whether or not the criteria
were applied equally in all study groups, whether the characteristics
of the subjects were fully described, and finally, whether
the study group was a representative sample of the relevant
population. The second category evaluated the measurement of
exposure/outcome variables. Subvalidity questions to aid the
evaluation were related to how well the exposure and outcome
variables were described; whether the measurements were based
on standard, valid, and reliable data collection procedures; and
whether the measurements were conducted consistently across
groups. The third category evaluated design-specific sources of
bias. This was mainly related to how withdrawals were handled
and described (i.e., dropouts, loss to follow-up, attrition rate, and
missing data). The fourth category evaluated the control for
confounding bias. The studies were evaluated on the basis of
whether adequate adjustments had been made for confounding