I often see patients in my clinic with vague intestinal discomfort, sometimes accompanied by fatigue and joint pain. They may suspect gluten sensitivity, but their blood test for celiac disease (the most severe form of gluten intolerance) was normal. Consequently, they are told they aren't gluten intolerant.
Here is the oversimplified science:
Detoxification involves at least two chemical reactions in the liver, the body's primary detoxifying organ. These reactions (pathways) are called Phase I and Phase II.
Additionally, if there is constipation or undesirable bacteria overgrowing in the gut, toxins exiting the liver can be reabsorbed from the gut into the bloodstream.