The judge, 79, said the popular show is only concerned with seeking out the best contestants to compete.
Leith said that those who enter the famous tent are not chosen based on their ethnicity, age, or weight, adding that the programme remained "a very inclusive show".
The Channel 4 competition is returning to TV screens, and this year's batch of bakers will be chosen on their culinary skills, according to Leith.
She said: "What's so interesting is when they're choosing the bakers, the main thing they want to know is that they're getting the 12 best bakers in the country.
"They're not choosing, they're not set up to find somebody who's the right ethnic mix, or the right height, or the right age or anything."
The Great British Bake Off trailer
She added that only after the merits of the bakers was assured, the right balance would be found for the show.
Leith said: "It's only when they really know that everybody who's left in the competition is a fantastic baker that they make sure, Channel 4 make sure, they've got the right balance."
Fellow judge Paul Hollywood, 53, added baking is now considered to be “trendy” and is attracting a younger crowd.
He added: "Baking has become such a cool thing to do, and if you're a girl, a young lad, if you can bake, you'll get friends - always."
The pair added that they wished to continue their tenure on the show into old age, or at least to match former judges.
Leith said: "My ambition is not so ambitious. I just want to at least equal Mary Berry, she did seven years. I've got another four years to go."
With reporting from Press Association