First, what I like about smoothies: loads of nutrient-dense foods blended into a convenient, sippable meal. They are refreshing on hot days and conveniently portable for the commute to work or running errands around town.
But here's what can can wrong:
SUGAR
To make blended vegetables palatable, it's easy to add way too much fruit, jacking up the sugar content to the level of a candy bar. So skip the bananas and opt for fresh lemon, ginger root or fresh mint leaves to add zing and a dash of cinnamon or powdered vanilla for sweetness. Better fruits to add include frozen blueberries or cherries.
Think of your smoothie as a smooth salad, not a milkshake.
OXOLATES
Kale is the darling of the vegetable world right now and kale shakes appear on every hipster menu but here's the reality: kale and it's kind (cruciferous vegetables like broccoli, collards and cabbage) are much better for you when lightly steamed. Step aside, raw foodies. Components of these vegetables called oxalates are difficult to digest, cause gas and bloating and are strongly linked to the formation of kidney stones and thyroid disfunction.
no big deal: just lightly steam and discard the water, store in the fridge and add to your smoothie.
Listen to Chris Kresser explain the details.
BUGS
Of course fruits and vegetables rock. But we are not the only species to think so. Bugs and parasites love them too and like to lay their eggs in hard to see places under the leaves.
Do not skip the washing step.
Here's what I do: fill a clean metal bowl or sink with cold water, add a splash of raw cider vinegar and submerge the vegetables. Swirl them around to shake off the dirt and bugs and then leave for 10-15 minutes so the stuff settles at the bottom. Lift the vegetables out and shake off the excess water. You can buy commercial vegetable washes too, but why?
Another tip: add mineral-rich salt to your smoothie.
Not only will the salt provide you with essential mineral action and enhanced flavor, it also helps your stomach to produce HCI (hydrochloric acid) which helps you digest proteins and kill off any unwelcome parasites.
My GO-TO GREEN SMOOTHIE:
1 peeled cucumber (from my garden - not the icky waxy types or the long "English" version OR 1 organic green apple (available year round)
+ 4 -5 organic celery stalks (leaves too)
+ a big handful of lettuce, arugula, parsley or cilantro leaves (or some combination)
+ a 1 inch knob of peeled ginger root
+ juice of a half of lemon
+ 1/2 avocado
+ water and ice