The Cairn Terrier diet will need to include animal proteins and carbohydrates, vitamins and minerals, and omega fatty acids--nutrients every dog needs to sustain its health in the long term. This means the best Cairn Terrier food is premium dry kibble, as it has balanced portions of the above-listed ingredients.
Adult Cairns will only need about a cup of premium food per day, divided into two meals. Cairn puppies will need a bit less: depending on age, about ¾ cup per day, divided into three meals (not two) until six months old.
For more info on feeding a Cairn Terrier from puppyhood through maturity, see this chart:
Dog AgeDog WeightFood TypeAmountFrequency7-8 Weeks2 lbsDry (Puppy formula)6-8 pieces3x/day11-12 Weeks4 lbsDry0.15 cups3x/day5 Months8 lbsDry0.25 cups3x/day8 Months11 lbsDry* (Puppy/Adult)0.4 cups2x/day10 Months+13 lbsDry (Adult formula)0.5 cups2x/day*--Around this time, transition to adult food by first mixing in a bit of adult formula with the puppy formula. Over the course of a week, with each meal add a little more adult food to the mixture, until the dog is eating it entirely.
Try if possible to stick to the above-listed portions. If constantly overfed (and under-exercised), these dogs can become overweight--and a fat Cairn Terrier will have numerous health problems and a potentially shortened lifespan. You can help control your Cairn's weight by having consistent feeding and exercise schedules, by not feeding the dog table scraps, and by not leaving food in its bowl all the time.
If you're worried your Cairn Terrier is overweight, try this simple test: run a hand along the dog's side, and if you can't feel any ribs, it's diet time--which means less food and more exercise!