It's not this easy, is it?

It's that easy.

code/modules/mob/living/damage_procs.dm

@@ -25,6 +25,8 @@
 			adjustCloneLoss(damage * hit_percent)
 		if(STAMINA)
 			adjustStaminaLoss(damage * hit_percent)
+		if(FAT)
+			applyFatnessDamage(damage * hit_percent)
 	return 1
 
 /mob/living/proc/apply_damage_type(damage = 0, damagetype = BRUTE) //like apply damage except it always uses the damage procs
@@ -230,6 +232,10 @@
 /mob/living/proc/setStaminaLoss(amount, updating_stamina = TRUE, forced = FALSE)
 	return
 
+/// Apply fatness damage from weapons or items that have the fatness damage type.
+/mob/living/proc/applyFatnessDamage(amount)
+	return
+
 // heal ONE external organ, organ gets randomly selected from damaged ones.
 /mob/living/proc/heal_bodypart_damage(brute = 0, burn = 0, stamina = 0, updating_health = TRUE)
 	adjustBruteLoss(-brute, FALSE) //zero as argument for no instant health update