Lately, your digestion feels off; in fact, you've caught a few more colds than usual and your overall vitality seems a bit compromised. Must be time to detox, right? The truth is that solely focusing on flushing toxins from your digestive system is a bit like cleaning the pool filters but forgetting to add chlorine.

"We tend to think of gut maintenance as only removing poisons and neglect to think of what we need to add to our system to keep it healthy," says Gary Huffnagle, PhD, professor of internal medicine, microbiology, and immunology at the University of Michigan Medical Center in Ann Arbor, and co-author of The Probiotics Revolution (Bantam, 2007). That's where probiotics (or friendly bacteria) come in.