Looking on the internet deeply has found these results:

was on a diet is the most popular phrase on the web.

had been dieting 164,100 results on the web	was on a diet 13,450,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
The girls and I have been dieting. Especially since I've been dieting. The girls and I have been dieting today. I've been dieting my whole life for this! So... how long have you been dieting? No extreme dieting and no intensive workout regimen. Alright. I have been dieting all day long. She's been dieting to fit into herjumpsuit.	At first she said she was putting me on a diet for my own good. Or like she was on a liquid diet. He was raised on a diet of western television And airport novels. When Jackie fell on her head during cheerleading, she was on a liquid diet for days. I was on a diet. In my defense, I was on a few diet pills at the time. This one time I was on a diet pill and I stayed up for 67 hours. No! No! Didn't you say you was on a diet, Mom?

had been dieting

164,100 results on the web

was on a diet

13,450,000 results on the web

More popular!

At first she said she was putting me on a diet for my own good.
Or like she was on a liquid diet.
He was raised on a diet of western television And airport novels.
When Jackie fell on her head during cheerleading, she was on a liquid diet for days.
I was on a diet.
In my defense, I was on a few diet pills at the time.
This one time I was on a diet pill and I stayed up for 67 hours.
No! No! Didn't you say you was on a diet, Mom?