Developmental, cardiovascular and neurological effects
Research has shown that consuming RO water deficient in magnesium and calcium is also linked with preterm birth and lower weight at the time of birth. It is also observed that RO water increases your risk of cardiovascular and neurodegenerative diseases. According to epidemiological studies conducted all across the world, water low in magnesium and calcium increases the rate of morbidity and mortality from cardiovascular diseases (CVD).

Chronic kidney disease and gastrointestinal disorders
RO water has a lower pH and is thus acidic. Drinking acidic water for a long period of time can adversely affect your health by increasing your risk of gastrointestinal issues and kidney disorders.

Increased intake of toxic metals
Drinking RO or demineralized water also increases your risk of consumption of toxic metals. This is because of two reasons:

Leaching of metals present in the material that comes in contact with the water. Leaching occurs due to unstability of low-mineralized water. This increases the content of metals in drinking water.
The reduced antitoxic capacity of the water is due to the deficiency of calcium and magnesium. Calcium and magnesium present in food and water have antitoxic activity as they prevent the absorption of toxic minerals like cadmium and lead into the blood.
Increased bacterial contamination
The high temperature of water in the pipe system during distribution also facilitates the re-growth of bacterias. The intact desalination membrane present in the pipe system, can remove almost all bacteria but their 100% effectiveness is still not assured. There have been cases in the past where RO treated water has also been the cause for the spread of bacterial diseases like typhoid

[redcliffelabs.com].