Studies categorised intake by quartiles (McCann et al, 2007; Sieri et al, 2007; Lajous et al, 2008) or quintiles (Augustin et al, 2001; Cho et al, 2003; Jonas et al, 2003; Frazier et al, 2004; Higginbotham et al, 2004; Holmes et al, 2004; Lajous et al, 2005; Silvera et al, 2005).
We categorised food intake during labour as no intake, water only, or calorific drinks/solids.
We categorised food intake in the six hours before labour as no intake, snacks, a light meal, or a large meal.
Each study in the meta-analyses had categorised GI and GL intake differently, and utilised a mixture of glucose and white bread reference values.
In COPSOQ-I alcohol intake was not measured and in Still Working, we were able to categorise alcohol intake only approximately as none, moderate, and heavy.
Quartile distributions among controls were used to categorise dietary intake variables.
The results for alcohol were more variable, which may reflect the greater complexity in measuring and categorising alcohol intake compared with smoking.
The participants were asked to categorise their intake of each listed food as "Never or only occasionally"; "once or twice per week"; or "Three or more times a week".
We categorised daily alcohol intake into five categories: nondrinkers, 0.1 4.9 g per day (∼0.5 drink per day), 5.0 14.9 g per day (∼1 drink per day), 15.0 29.9 g per day (∼2 drinks per day), and ⩾30.0 g per day (2+ drinks per day).
To take advantage of the large number of cases and to examine relative extreme intake categories, we categorised energy-adjusted magnesium intakes into deciles based on the distribution in the study population.
Fruit and vegetable intake was categorised on the basis of daily intake as: five or more portions (reference category), three to less than five portions, one to less than three portions or less than one portion based on current national and international guidelines.
