Add the torn kale leaves to a blender. It's helpful to have them on the bottom since they are tougher to blend than the other ingredients.
To prepare the grapefruit, slice in half crosswise and cut out the wedges of each half with a knife (or a grapefruit knife if you have one). Transfer the wedges and whatever leftover juice you can squeeze out of the grapefruit to the blender. You can also segment the grapefruit if that's your preferred method of slicing it. Just make sure there isn't any white membrane on the slices.
Add the bananas, chia seeds, and water. Blend at high speed for about 30 seconds to 1 minute or until smooth. Add more water if needed. If you do not use a frozen banana, consider adding ice to make it colder. Enjoy!
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Notes
Possible additions/variations: Sub baby spinach for kale and frozen pineapple or mango for banana if that's what you have. Some possible flavor boosters include yogurt, vanilla extract, ginger, or coconut flakes.
Storage: The chia seeds in the recipe will gelatinize over time and change the texture and drinkability of the smoothie. I don't really recommend making it ahead of time for that reason. Fortunately, it's easy to prep all the ingredients in advance if you needed to whip it up quickly!