Is your diet leaving you sluggish? Make sure you opt for a sugar-free pick-me-up. New Zealand scientists discovered that many popular energy drinks are worse for your gut than soda is, despite claiming to boost your metabolism. Why? They're often packed with sugar. The researchers observed that downing an energy drink containing 80 mg caffeine and 24 g sugar -- about the amount in an 8.3-ounce can of Red Bull -- completely inhibited people's ability to burn fat. While caffeine does help speed metabolism, combining it with sugar negates its short-term fat-burning benefits, says lead author Elaine Rush, Ph.D. See "It Wears On You"  for another reason to avoid energy drinks.