An analysis of dietary survey data from Sweden and Ireland revealed that with a declining ratio of energy intake to estimated basal metabolic rate (EI : BMR), %energy from fat, saturated fat and carbohydrate fell while that of protein increased. However, where the definition of under-reporting is less strict, as in the Irish database (EI : BMR) < (1·27), the quantitative effect on macronutrient balance, while statistically significant, is not of such magnitude as to be of nutritional significance. However, with respect to foods, under-reporting was found to be of considerable significance in using such data to formulate food-based dietary guidelines. The Swedish data show that under-reporting of foods is associated with a decreasing percentage of consumers, decreasing intake per eating occasion and decreasing frequency of eating. The development of food-based dietary guidelines will involve comparisons of patterns of food intake among people with contrasting high or low intakes of target nutrients. The Swedish data compared food intakes for those with low or high intakes of saturated fatty acids (SFA) among under-reporters and acceptable reporters of energy intake. Whereas absolute values were always lower with the lower EI : BMR group (< 1·1) compared to the higher EI : BMR group (> 1·35) the ratio of intakes for the low vs. high SFA groups were broadly significant at either EI : BMR ratio. This paper highlights the difficulties that under-reporting will pose for food-based dietary guidelines.