Use 'dieter' when referring to someone who is actively trying to control what they eat. My friend became a dieter last month to prepare for the marathon.
Pro Tip 2/3
Positive Connotation
'Dieter' may suggest a positive change toward healthier habits or better fitness. The dieter now feels more energetic after revising their eating habits.
Pro Tip 3/3
Not Always Weight Loss
Someone can be a 'dieter' even if the goal isn't weight loss, such as for health reasons. As a diabetic, he's a careful dieter, mindful of his sugar intake.