diff --git a/src/math_parser_diag.cpp b/src/math_parser_diag.cpp
index b4af37792b643..86ec097c1acc3 100644
--- a/src/math_parser_diag.cpp
+++ b/src/math_parser_diag.cpp
@@ -270,8 +270,15 @@ diag_assign_dbl_f faction_trust_ass( char /* scope */, std::vector<diag_value> c
 diag_eval_dbl_f faction_food_supply_eval( char /* scope */,
         std::vector<diag_value> const &params, diag_kwargs const &/* kwargs */ )
 {
-    return [fac_val = params[0]]( const_dialogue const & d ) {
+    vitamin_id vit_to_check = vitamin_id::NULL_ID();
+    if( params.size() > 1 ) {
+        vit_to_check = vitamin_id( params[1].str() );
+    }
+    return [fac_val = params[0], &vit_to_check]( const_dialogue const & d ) {
         faction *fac = g->faction_manager_ptr->get( faction_id( fac_val.str( d ) ) );
+        if( !vit_to_check->is_null() ) {
+            return static_cast<double>( fac->food_supply.get_vitamin( vit_to_check ) );
+        }
         return static_cast<double>( fac->food_supply.calories );
     };
 }