ADJ

balanced, good, healthy, sensible
adequate
People can fight infection more easily if they have an adequate diet
bad, poor, unhealthy
staple
a staple diet of cornmeal and vegetables
vegan, vegetarian

VERB + DIET

eat, have
It is important to eat a balanced diet
live on, survive on
They had to survive on a diet of insects and berries
feed sb on

DIET + VERB

consist of sth
The animal's diet consists mainly of grasses

PREP

in a/the ~
the amount of fat in your diet
on a ~
These animals live on a mainly vegetarian diet
~ of
They were fed on a diet of rice and vegetables

PHRASES

a diet high/rich in sth
a diet rich in vitamins and minerals

ADJ

strict
slimming, weight-loss, weight-reducing
crash, starvation
Crash diets are not the best way to lose weight
calorie-controlled, low-calorie
restricted, special
fibre-rich, gluten-free, high/low-fibre, high/low-protein, low-fat, salt-free, etc.

VERB + DIET

follow, have
If you follow this diet, you're bound to lose weight
go on, start
stick to
I have to stick to a low-fat diet
be on
I'd love some chips, but I'm on a diet

DIET + NOUN

drink, food
plan
Lose pounds with our new diet plan!

PREP

on a ~
They're on a special high-protein diet