People with normal weight and higher (exact numbers depend on gender etc.) have a lot of adipose tissue, which absorbs and releases fuel all the time - after every meal there's excess energy, between meals and at nights there's deficit of energy, it actively manages that.

When you eat insufficient amount of food, your body tries to keep your energy in homeostasis by things like:

• increased hunger (strongly countered by ECA)
• decreased energy levels (strongly countered by ECA)
• decreased metabolic rate (somewhat countered by ECA which has minor thermogenic effect)
• increased release of fuel from adipose tissue (what we want, maybe somewhat enhanced by ECA)
• increased breakdown of lean tissue (maybe somewhat countered by ECA, eating relatively higher protein diet may have protective effect here)

Your homeostasis will get what it wants somehow, and ECA tries to prevent it from doing it the way you don't want it to, so it's more or less limited to primarily relying on releasing fuel from adipose tissue. Trying to starve yourself, and willpower the hunger away works much worse than that.

If your adipose tissue cannot do that since it lacks sufficient stored fuel, your body will try the other things harder, and you may have some nasty side effects.

Anyway, who the hell would want to take ECA while being at BMI 18?