I noticed a statement today on the box of the cereal I was eating for breakfast today. It said something to the effect that
1. Eating this cereal may reduce your risk of heart disease.
It was starred, of course, and the fine print said something to the effect that
2. Diets low in saturated fat and high in whole grains may confer a reduced the risk of heart disease.
The problem is this: the first statement makes it appear like the cereal is actively doing something in your body to combat heart disease. That is not what the second statement says (at least based on how I interpreted it). The second statement says that compared with diets with higher levels of saturated fat and (presumably) refined flour instead of whole grains, the risk of heart disease is lower.
Thus if you add the cereal to your diet, all other things being equal, there’s nothing in the second statement that tells you that risk is reduced. Yet the blurb implies it is.
Of course all other things may not remain equal. Eating the cereal will probably reduce your appetite, causing you to eat less of the unhealthy stuff. This implies that eating the cereal does have a causal effect on heart disease.
It all depends on how the majority of people interpret the first statement. Does it make them feel as if they can continue to eat unhealthy food and treat the cereal as an antidote? Or is it obvious that replacing another, less healthy food with this cereal is what will provide benefits?