cut down on his diet

It's ambiguous. Diet can either mean what someone eats, or when someone eats less to lose weight. Which meaning do you intend?

The latter.

Vorparor when someone eats less to lose weight

If you mean that he stopped dieting, I would say "He cheated on his diet." or "He's off his diet."

Thanks, Vorpar.

Cut down on his food portions/intake, not diet.

'Cut down on diet' is fine. Please see the following:

New Outlook - Page 165

by Alfred Emanuel Smith - 1892You can probably guess my next move — nearly every " fat " man and woman has
taken it. 1 Ixj- came a follo

Marius HancuCut down on his food portions/intake, not diet.

Why is diet wrong?

Yoong Liat -- one of your examples is from 1892! Even the other one, from 1933, pre-dates the "diet craze" in America, when the word "diet," which originally meant "everything you eat," took on the second meaning of "a regime desinged to help someone lose weight by limiting their food intake." The word was not ambiguous in 1933, or in 1892. Now it is.

Hi Khoff

You're right. Nowadays, 'to diet', 'to go on a diet', or 'to cut down on caffeine, etc' is used.