Any associative effects between supplemental dietary fat and nutrient digestibility would have an influence in the calculation of digestible energy and nutrient values of horse diets. However, these effects are inconsistently reported in the literature. The use of meta-analysis, which accounts for variation among studies, could quantify differences in nutrient digestibility caused by fat supplementation. Too few studies have presented data on intake to evaluate the relation between dietary fat intake and nutrient digestibility through a modelling approach. Consequently, unitless standardised effect sizes (Hedges’s g) were calculated within studies for differences in digestibility between supplemented and non-supplemented diets. Twenty-two studies were identified that provided 43 comparisons for apparent total tract digestibility of crude protein (CPD), 44 for neutral detergent fibre digestibility (NDFD), and 30 for acid detergent fibre digestibility (ADFD).