Does reducing the calories of a soft drink make it healthier?
To answer that, imagine a situation in the US where the calories of a Pepsi can are cut from 150 to 100.
According to the 2005-08 National Health and Nutrition Examination Survey, 32% of Americans consume more than 150 calories from sugary drinks a day.
This, however, places them at risk of becoming obese (2009 brief from UCLA Center for Health Policy Research) or diabetic (2010 article in journal Diabetes Care).
If the calories of a Pepsi can are cut from 150 to 100, the proportion of Americans consuming more than 150 calories a day from sugary drinks could go down from 32% to 23%.
But then a fifth of America would still be in danger of becoming obese/diabetic.
Reducing calories will work if your consumer just has a single 'serving', but if they have more than one, a cut won't really make much of a difference.