'Nuts were taboo 10 or 15 years ago,' Dr Ram says. But they're 'not only healthy, but extremely satisfying'.
With a weight-loss coach, dieters 'learn not only how to eat well, but how to deal with food issues', Ms Gonzalez says.
The above sentences could have been written as follows: '... But they're healthy and extremely satisfying' and '...dieters learn how to eat well and how to deal with food issues...'.
The above structure would be simpler. But the slightly more complex structure ('not only ... but') is used for emphasis.
It emphasises the facts that nuts are extremely satisfying (apart from being healthy) and that dieters learn how to deal with food issues (apart from learning how to eat well). The writer assumes that readers are not aware of these facts.