Few studies have specifically addressed this issue. There were some studies in the United States, or perhaps it was in Germany; it does not matter. If I recall correctly, one study in particular showed that adding a health claim to a product influenced the decision for about 10% of consumers. Making a claim can therefore have some effect on a purchasing decision. But we have no specific data on actual consumption. There are almost no studies evaluating the impact that claims or labels have on the actual consumption.
On March 1st, 2011. See this statement in context.