Why is deception acceptable in some psychological investigations?

The answer you're looking for is washing your hands. "at this late date, we all expect our doctors to wash their hands before examining us or performing an operation in order to prevent the spread of infection. surprisingly, physicians did not begin to acknowledge the lifesaving power of this simple act until 1847.it was then that dr. semmelweis began exhorting his fellow physicians at the famed vienna general hospital (allgemeines krankenhaus) to wash up before examining women about to deliver babies. his plea was far more than aesthetic; it was a matter of life and death and to prevent a deadly malady known as “childbed” or puerperal (from the latin words for child and parent) fever." 

True because for most women a body fat of 21-33% is considered healthy.

Health information communicated thru advertisements is usually biased.  these ads are used to further a company's agenda

You can check the labels on the bottle or even ask you doctors