The number of Life Cycle Assessment (LCA) studies on foods and diets steadily increases. However, due to lack of data on food products as well as time and resource constraints, many of these studies ignore part of the system (e.g. cooking and waste in the household), which may lead to underestimating impacts greatly. This LCA study compared diets using six methods with different system boundaries; three of these are simplified methods we developed. The aim was to identify which method best optimizes data collection for life cycle inventories from cradle to human mouth of food products and diets. The principle behind the three simplified methods was that, for many foods and impact categories, the farm (or fishery) is the life cycle stage that contributes most to impacts. One average, one healthy and one vegetarian diet, each composed of up to 105 foods, were assessed. Climate change, cumulative energy demand, eutrophication, acidification and land occupation impacts were estimated. Recommendations are given on which methods, depending on study goals, offer the best trade-off among available resources (time, money, and knowledge), while providing the required robustness of results. Compared to a full LCA, simplified LCA methods can yield more accurate results at a lower cost of data collection.