Reducing nitrogen (N) and phosphorus (P) levels and using local feed sources in pig diets can lower the environmental footprint of pig production, but the effectiveness of these strategies largely depends on the production context. We assessed the environmental impacts of fattening pig production in high-density pig farming regions in Switzerland using an attributional life cycle assessment (LCA), comparing different N and P levels in feeds and varying degrees of imported resource use. Four isoenergetic treatments, previously experimentally tested, were compared: control (C), reduced protein content with minimal use of European soybean meal (N-), reduced phosphorus content without mineral phosphate (P-), and a combination of N- and P- (N-P-). The study focused on farm gate impacts for the fattening unit, including emissions up to manure storage. Compared to group C, N- reduced acidification (-2.4%), terrestrial eutrophication (-4.3%) and land use (-33.7%) potentials, but increased water resource depletion (RE: +13.6%) and climate change (2.17 vs. 2.08 kg CO2-eq/kg weight gain); group P- reduced freshwater eutrophication (-1.6%), mineral use (-5.1%) and RE (3.81%); N-P- combined the benefits and trade-offs of N- and P-. Benefits were observed for impacts affected by N emissions in housing and storage and/or by the use of European soybean meal or mineral phosphate. However, trade-offs were observed in RE and climate change impact categories, linked to higher emissions due to greater inclusion of synthetic amino acids. This study confirms that dietary strategies aimed at reducing N and P excretion can effectively lower the environmental impacts, provided they are accompanied using feed ingredients with lower environmental impacts.