To describe methodological issues regarding the measurement of food intake of marginal populations in the north-west of Mexico.

Dietary data from three sources were analysed. The Sonora State Food Basket Study (505 individuals from all ages, both sexes, high, medium and low income, and from urban and rural settings), 144 individuals from rural Indian communities, and 152 low-income urban women. Methods of dietary data collection were single 24-hour recalls for the first two studies, and four non-consecutive 24-hour recalls as well as a food-frequency questionnaire (FFQ) for low-income urban women.

The food patterns in the three studies showed similar core foods; however, food diversity resulted in 19, 13 and 57 different foods reported by 90% of the population in the state, Indian communities and urban women, respectively. Mean intakes of selected food components in the sample of urban women vs. a representative sample of the population of the state were similar, although smaller variation in consumption was observed in urban women with repeated 24-hour recalls. Inter-individual variation in intake of food components was similar in the state and urban women samples, but the Indian communities showed lower coefficients of variation (CVs). Use of four 24-hour recalls decreased the inter-individual variation in food component intakes of low-income urban women, especially for vitamin intakes. The FFQ did not show an additional decrease in inter-individual variation for macronutrients, but reduced even more the CVs for vitamins. Intra-individual variation was higher than inter-individual variation for urban women's intakes when estimated by repeated 24-hour recalls. This effect was reversed when estimated by duplicate food frequencies. Ratios of intra-individual to inter-individual variation in food intake were lower for Mexican when compared with US women. These results should be considered, especially for association analysis of low-income women's diets and health outcomes.