It's not this easy, is it?

It's that easy.

code/__DEFINES/combat.dm

@@ -10,6 +10,10 @@
 #define CLONE		"clone"
 #define STAMINA 	"stamina"
 #define BRAIN		"brain"
+// GS13 EDIT
+#define FAT 		"fat"
+/// What is the rate that one damage is converted to fatness?
+#define FAT_DAMAGE_TO_FATNESS 4
 
 //bitflag damage defines used for suicide_act
 #define BRUTELOSS 		(1<<0)

code/modules/mob/living/carbon/damage_procs.dm

@@ -120,6 +120,13 @@
 	adjustStaminaLoss(diff, updating, forced)
 
 
+/mob/living/carbon/applyFatnessDamage(amount)
+	if(!check_weight_prefs(FATTENING_TYPE_WEAPON)) //Typically, this kind of damage would be applied to people through weapons.
+		return FALSE
+
+	adjust_fatness((amount * FAT_DAMAGE_TO_FATNESS), FATTENING_TYPE_WEAPON)
+
+
 /** adjustOrganLoss
   * inputs: slot (organ slot, like ORGAN_SLOT_HEART), amount (damage to be done), and maximum (currently an arbitrarily large number, can be set so as to limit damage)
   * outputs:

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