I learned not to drink distilled water today...
During distillation, water is boiled and evaporated away from its dissolved minerals, and then the vapor is condensed and the resulting water droplets collected. Distilled water is an active absorber, and when it makes contact with air, it quickly absorbs carbon dioxide and becomes acidic.
Since it is free of dissolved minerals and other particles, it has the ability to absorb toxic substances from your body and eliminate them.
However, although drinking distilled water may be helpful when detoxifying for a week or two, the longer you drink it, the more likely you'll develop mineral deficiencies and an acidic state.
You can rapidly lose electrolytes (sodium, potassium, chloride) and trace minerals, which can cause cardiac irregularities, high blood pressure, and cognitive/emotional disturbances.
In a paper by F. Kozisek of the World Health Organization (WHO), water low in calcium and magnesium, such as distilled water, is associated with the following health problems:
Cardiovascular disease
Higher risk of bone fracture in children
Neurodegenerative diseases
Motor neuronal diseases
Pre-term births, low birth weights, and preeclampsia
Various types of cancer
Increased risk of "sudden death"
Acute magnesium and calcium deficiency, weakness, fatigue and muscle cramping
Clearly, changing the water you drink can have profound and potentially disastrous effects on your health, as this research shows.
For these reasons, I've been discouraging people from drinking distilled water for well over a decade now, but there are other far more problematic issues with distilled water that make potential mineral deficiencies pale in comparison.