I nearly got run over by some idiot who didn’t use his turn signal last night, and my little ticker was just poundin’ away to beat the band. I thought, gee, exercise gets your heart pumping and is good for your metabolism—burning fat and such.
So, is being scared a good diet tip? Shall I start throwing myself in front of moving traffic and going to horror flicks?
I think it would be a very bad way to lose weight.
I was taught in college that those stress hormones that raise your metabolism and put you in “fight or flight” mode would do massive damage to your body if they stayed in your system too long.
This is why long-term stress causes heart disease, ulcers, and all kinds of ailments.
" . . . long-term stress causes heart disease, ulcers, and all kinds of ailments."
—Yes, yes, darling—but would I be THIN?
In a word, no.
You cannot remain scared often or long enough to notice any significant weight loss.
Additionally, the more often you scare yourself, the more desensitised you become to the experience, until you are totally calm with practically any given situation.
However, if you COULD keep yourself scared often and long enough to lose weight, you would certainly lose a LOT of weight, since your appetite would be affected, your metabolism would make less use of the food you take in, and you likely would spend a lot of time vomiting whatever significant meals you did ingest.
This, coupled with the helath and emotional stress you would suffer, to say nothing of the injuries sustained while trying to find better longer-lasting ways to scare yourself, would negate any weight-reduction benefits.
What would happen in my case, I’d go for “blindly” stepping out in front of that first car, trip, get scared, and break my neck trying to recover from the idea. Then I’d spend time in the hospital, losing weight due to bad food, but when I got home I’d balloon with all the good home food.
So, I’d say it’s probably not a good idea. 
Sure, eventually…
It’s like that gag tombstone says:
“Lose weight now, ask me how!”
Seriously though, I’ve read that stress weakens the immune system, and outright fear is the ultimate stress.
Get yourself a good severe anxiety disorder and the above will come true. You’ll be so panicky you won’t be able to choke down food and what you do eat will be vomited or excreted so fast you won’t get any benefit. As a bonus, if you are able to sleep at all you’ll wake up repeatedly in a cold wave of fear, and sleep deprivation will only make things worse.
I wouldn’t wish it on anyone. It’s not so bad being heavy.