ESTIMATION OF FOOD PORTION SIZE USING PHOTOGRAPHS - VALIDITY, STRENGTHS, WEAKNESSES AND RECOMMENDATIONS

Photographs are valuable aids in the estimation of food portion sizes and are easy to use in dietary surveys. As with other methods, employi ng photographs to estimate portion sizes consumed produces errors due to the method itself and errors from other sources such as poor recall of amounts actually eaten. This study was aimed at the first type of error and investigated errors due to the visual perception of food por tions from food photographs in the absence of recall biases. Two hundr ed and seventy women were presented with various amounts of foods and asked to indicate the portion sizes using appropriate sets of photogra phs. The photographs showed three portion sizes (small, medium, large) for each of 45 foods commonly found in the French diet. The portions of real food estimated by subjects were prepared to the same weights a s the portions photographed. Estimates of portion sizes were compared to the weighed amounts. They were found to be accurate within 25% in m ost cases. Except for three foods, errors on estimates were seen for o ne or more portions per food. Two patterns of errors were identified. For 22 foods, the small portion sizes were overestimated and the large portion sizes underestimated, but no error occurred with the medium p ortion. For 20 other foods, the three portion sizes were either all un derestimated or all overestimated. The first pattern of error may be d ue to a general tendency to avoid extreme response categories, while t he second pattern of error may be related to biased visual perception. In general, there was no obvious way to link the patterns of errors w ith the type or physical appearance of the foods. In conclusion, the p hotograph method is recommended for the assessment of portion sizes, p rovided that great care is taken to suggest the volume of the food por tions in the photographs and in selecting a range of portions which en compasses the range of amounts of food actually consumed in the diet.