Pseudoscience in advertisments

It's very common to see a sleight of hand in pseudoscience advertising where a claim is made like: you will lose 1 lb a day by taking a supplement, but they fail to mention that their plan includes a 500-calorie-a-day diet. Could this be described by a logical fallacy?

I wouldn't call this a fallacy; it's just deceptive advertising (though advertising can contain fallacies). Leaving the important details in a 'fine print' that's deliberately under-emphasised is a standard advertising technique.

When important information is left out to make their argument sound better is called the cherry picking fallacy. 

It could also be a lying with statistics fallacy as well. 

The ‘sleight of hand’ referred to would typically be caught under the applicable truth in advertising regulations which prohibit unsubstantiated claims as well as misleading and deceptive conduct. Any statement that creates a false impression about goods and services can be breaking the law.  It is not uncommon for this type of ad to include ‘before and after’ photos which, in regulated health care professions in Oz, are prohibited under the testimonials prohibitions.  It is common for the type of ads referred to to incorporate fallacies such as Appeal to Celebrity and Appeal to Authority to convince unsuspecting buyers.