A toxin is something that can be harmful to you, but this is about as broad a term as it gets. There's a spectrum; toxicity depends on what it is and how much you take in.
The urge to detoxify your body when a new year starts has nothing to do with a buildup of toxins. Feeling bloated and fatigued is the result of the all the holiday eating.
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There's no evidence that drinking a series of juices, teas, or any of other so-called 'detox' products does anything besides profit the people selling them.
There are no shortcuts to health.
Everything you eat goes to your liver and it determines what to do with the components of what you've ingested:
If it's something useful the liver sends it out into circulation, but if it's not immediately usable or could be harmful, your liver has enzymes to neutralize it and send it off as waste to be removed from the body through urine, mostly. The best you can do to help your liver out is to hydrate and exercise.
The enzymes your liver is using to break down unwanted compounds are made from substances in certain foods you eat—specifically whole fruits, vegetables, and protein.
You should be having whole fruits and vegetables, five to nine servings a day. The juice is not a substitute, as it lacks the fiber your body needs to support waste removal in the liver and in your intestines.
Even if you have a green juice that is mostly veggies and one serving of fruit, you still won't get the full benefit that would result from eating that whole product instead.
That's primarily because super-satiating fiber is mainly located in the skin of fruits and vegetables.